In Situ Formation of Tib2 in Fe-B System with Titanium Addition and Its Influence on Phase Composition, Sintering Process and Me

materials

Article In Situ Formation of TiB2 in Fe-B System with Titanium Addition and Its Inﬂuence on Phase Composition, Sintering Process and Mechanical Properties

Mateusz Skało ´n 1,2, Marek Hebda 2 , Benedikt Schrode 3 , Roland Resel 3 , Jan Kazior 2 and Christof Sommitsch 1,*

1 IMAT Institute of Materials Science, Joining and Forming, Graz University of Technology, Kopernikusgasse 24/1, 8010 Graz, Austria; [email protected] 2 Institute of Materials Engineering, Cracow University of Technology, 24 Warszawska ave, 31-155 Cracow, Poland; [email protected] (M.H.); [email protected] (J.K.) 3 Institute of Solid State Physics, Graz University of Technology, Petersgasse 16/II, 8010 Graz, Austria; [email protected] (B.S.); [email protected] (R.R.) * Correspondence: [email protected]; Tel.: +43-316-873-7180

Received: 20 July 2019; Accepted: 11 December 2019; Published: 13 December 2019

Abstract: Interaction of iron and boron at elevated temperatures results in the formation of an E (Fe + Fe2B) eutectic phase that plays a great role in enhancing mass transport phenomena during thermal annealing and therefore in the densification of sintered compacts. When cooled down, this phase solidifies as interconnected hard and brittle material consisting of a continuous network of Fe2B borides formed at the grain boundaries. To increase ductile behaviour, a change in precipitates’ stoichiometry was investigated by partially replacing iron borides by titanium borides. The powder of elemental titanium was introduced to blend of iron and boron powders in order to induce TiB2 in situ formation. Titanium addition influence on microstructure, phase composition, density and mechanical properties was investigated. The observations were supported with thermodynamic calculations. The change in phase composition was analysed by means of dilatometry and X-ray diffraction (XRD) coupled with thermodynamic calculations.

Keywords: titanium diboride; titanium; boron; iron; dilatometry; liquid phase; phase composition; sintering

1. Introduction Due to a stable microstructure and mechanical properties under exposure to long-term thermal neutron irradiation, borated stainless steels find extensive applications in the nuclear industry [1,2]. Potential applications of boron steels in nuclear industry are, e.g., control/shutoff rods in nuclear reactors, sensor for neutron counting, shapes for neutron shielding, dry transportation casks, spent fuel rod storage racks. The advantage of borated stainless steel produced by powder metallurgy technology in comparison to the conventionally produced cast/wrought products is that they contain much smaller and more uniformly distributed boride particles [1,2]. Moreover, it is possible to obtain a material whose properties have lesser decrease in ductility and impact toughness as the boron content increases compared with similar boron-containing cast/wrought borated stainless steels. The application of boron in powder metallurgy as addition to ferrous alloys has been of interest to numerous researchers over the last years [3–12]. The main cause of this interest is the eutectic reaction between iron and boron, resulting in the appearance of a liquid phase which efficiently improves mass transport by intensifying a diffusion process [7]. Even small amounts of boron (0.4–0.6 wt %) added to ferrous powder may result

Materials 2019, 12, 4188; doi:10.3390/ma12244188 www.mdpi.com/journal/materials Materials 2019, 12, 4188 2 of 12 in great densification of sintered compacts reaching almost full relative density [7,9]. In accordance with a series of experiments [9,13–16], the sintering process is activated through the presence of a liquid phase by acting in three stages: (i) rearrangement (liquid spreading), (ii) solution-reprecipitation of a base material and (iii) microstructural coarsening [9]. Unfortunately, boron is almost non-soluble in an iron-based solid matrix, so it remains at the grain boundaries after the sintering process in the form of hard and brittle borides creating an almost continuous network surrounding the individual grains, therefore significantly influencing the material properties. Controlling the borides’ morphology and/or crystallographic network could considerably broaden the application field of steel parts manufactured by Powder Metallurgy (PM). It was already proved that the addition of titanium to iron-based cast alloys may result in the formation of titanium diboride (TiB2)[17,18]. Such an interaction, however, was not yet investigated in sintered particular materials. The objective of the present paper was to investigate the influence of titanium addition on phase changes and sintering behaviour in the Fe-B system. This topic is of high importance due to the possibility of improving the ductility of the particulate borated steels.

2. Materials and Methods Water atomised iron powder of >99% purity delivered by Sigma Aldrich (St. Louis, MO, USA) was used as a base powder. Boron of >99.7% purity (grain size < 1 µm) delivered by Sigma Aldrich was added to the blend in an amount of 0.8 wt % in order to induce a eutectic liquid appearance during the sintering process and consequently the appearance of Fe2B borides. The titanium of 99.9% purity delivered by VWR company (Radnor, PA, USA) was also added to the blend as a powder in a stepwise increasing amount in two diﬀerent grain size classes as listed in Table1. The varying grain size distribution of titanium allowed for control of the initial reaction surface between titanium and the eutectic liquid. Blends were mixed using a Turbula-type mixer for 12 h in order to assure homogeneity of blends.

Table 1. Composition of utilised powder blends.

Sample Description Titanium Grain Size Class/µm Titanium/wt % Boron/wt % Iron/wt % REF 0 0.00 A63 45–63 0.47 B63 45–63 0.93 C63 45–63 1.40 0.80 balance A140 100–140 0.47 B140 100–140 0.93 C140 100–140 1.40

The mixed powders were single-axially compacted under a pressure of 600 MPa informs of (a) 4 mm 4 mm 20 mm cuboid (dilatometric samples); (b) a 5 mm 20 mm cylinder (microstructure × × × and density check) and (c) a cuboid 6 mm 12 mm 30 mm for Transverse Rupture Strength test × × (TRS). During the consolidation of the samples, no lubricant was used. Dilatometric tests were carried out in a Netzsch DIL 402C dilatometer (NETZSCH-Gerätebau GmbH, Selb, Germany) under hydrogen atmosphere of 99.9999% purity and a flow rate of 100 mL/min. The heating rate was equal to 20 ◦C/min, the cooling rate was 10 ◦C/min while the isothermal temperature and time were 1240 ◦C and 30 min, respectively. Nabertherm P330 tubular (Nabertherm, Lilienthal, Germany) furnace was used for sintering both the TRS and the cylindrical samples under the same conditions as dilatometric samples. Material porosity was checked using the standard water displacement method. Scanning Electron Microscope (SEM, TESCAN Brno s.r.o., Brno, Czech Republic) TESCAN Mira3-SEM equipped with an Energy Dispersive X-Ray (EDX) spectrometer was used to evaluate the localisation of titanium in the microstructure. Specular X-ray diffraction patterns were collected on a PANalytical Empyrean diffractometer (Malvern Panalytical, Herrenberg, Germany) using a wavelength of 0.154 nm. On the primary side, the machine was equipped with a 1/8◦ receiving slit, a 10 mm beam mask and a multilayer Materials 2019, 12, 4188 3 of 12

mirror for monochromatisation and formation of a parallel beam. On the secondary side, an 8 mm Materials 2020, 13, x FOR PEER REVIEW 3 of 13 anti-scatter slit, 0.02 Rad Soller slits and a PANalytical PIXcel 3D detector (Malvern Panalytical, 85 Herrenberg,detector (Malvern Germany) Panalytical in scanning, Herrenberg, line mode Germany were used.) in scanning All thermodynamic line mode calculations were used. were All 86 performedthermodynamic using Thermo-Calc calculations weresoftware performed v.3.0 (Thermo-Calc using Thermo Software,-Calc Solna, software Sweden) v.3.0 with(Thermo the TCFE6-Calc 87 databaseSoftware, appended. Solna, Sweden State) with of full the equilibrium TCFE6 database was assumed. appended. State of full equilibrium was assumed.

88 3.3. ResultsResults and Discussion 3.1. Thermodynamic Calculations 89 3.1. Thermodynamic Calculations Figure1a presents the calculated phase diagram in the Fe-B-Ti system along with the temperature 90 Figure 1a presents the calculated phase diagram in the Fe-B-Ti system along with the increase for various amounts of titanium addition. The calculations indicate that an increasing amount 91 temperature increase for various amounts of titanium addition. The calculations indicate that an of titanium results in disappearance of Fe2B boride which gets replaced by TiB2 (Figure1c). According to 92 increasing amount of titanium results in disappearance of Fe2B boride which gets replaced by TiB2 93 literature,(Figure 1c). the According reason for to literature, this replacement the reason is thefor this relatively replacement high diis ﬀtheerence relatively in Gibbs high freedifference energy in of 94 formationGibbs free betweenenergy of the formation two borides between reaching the two 150 borides kJ/mol reaching throughout 150 kJ/mol the whole throughout temperature the whole range 95 betweentemperature 400–1800 range◦ C[between18]. Such 400 a–1800 replacement °C [18]. Such should a replacement be also accompanied should be by also a decreasing accompanied amount by a of 96 adecreasing eutectic liquid amount at sintering of a eutectic temperature liquid at sintering of 1240 ◦ Ctemperature as presented of in1240 Figure °C as1b. presented in Figure 1b.

97 98 FigureFigure 1.1. ((a) CalculatedCalculated phase diagram diagram of of the the system system (Iron (Iron ++ 0.8B0.8B wt wt %) %) ++ Ti,Ti, where: where: α,α Ferrite, Ferrite;; γ,γ , 99 Austenite;Austenite; δδ,, DeltaDelta Ferrite;Ferrite; L,L, Liquid; ( (b)) calculated calculated phase phase fraction fraction of of a a liquid liquid phase phase at at the the sintering sintering

100 temperaturetemperature ofof 12401240◦ °C;C; (c(c)) calculated calculated mole mole fraction fraction of of borides borides as as a a function function of of titanium titanium addition addition at at the 101 roomthe room temperature. temperature.

102 FigureFigure1 1bb presents presents thethe molemole fractionfraction ofof aa liquidliquid phase for the constant sintering sintering temperature temperature of of 103 12401240◦ °C,C, showingshowing aa decreasingdecreasing amount of liquid phase phase with with an an increasing increasing titanium titanium addition. addition. This This is is 104 connectedconnected toto changeschanges in in borides borides composition composition due due to to the the replacement replacement of of Fe Fe2B2B by by TiB TiB2 borides2 borides (Figure (Figure1c). 3 3 105 As1c). a As result a result of the of the diﬀ erencedifference in molarin molar volume volume (Fe (Fe2B:2B: 16.76 16.76 mol mol/cm/cm ,3, TiB TiB22:: 15.37 mol/cm mol/cm3) )and and the the 106 didifferenceﬀerence inin metal-boron metal-boron stoichiometry stoichiometry (each (each mole mole of of TiB TiB2 contains2 contains two two times times more more boron boron than than a mole a 107 ofmole Fe2 ofB), Fe titanium2B), titanium diboride diboride is capable is capable of bindingof binding the the same same amount amount ofof boronboron in lesser lesser volume volume as as 108 comparedcompared to to Fe Fe2B2B (Figure (Figure1c). 1c). Moreover, Moreover, due due to the to high the high melting melting point point of TiB of2 it TiB is solid2 it is in solid the sintering in the 109 temperature;sintering temperature therefore,; therefore, calculations calculations indicate theindicate decreasing the decreasing amount ofamount a liquid of phasea liquid in phase Figure in1b. 110 ForFigure the experimental1b. For the experimental studies, the studies, compositions the compositions of blends (Table of blends1) were (Table selected 1) wer basede selected on the based presented on 111 thermodynamicthe presented thermodynamic calculations to calculations assure a considerable to assure a but considerable linear decrease but linear of the decrease eutectic of liquid the eutectic amount 112 atliquid the sintering amount at temperature the sintering (Figure temperature1a,b). (Figure 1a,b).

113 3.2. Dilatometric Analysis and Density Changes 114 To study the influence of the particle size of the additional titanium on the dilatometric 115 behaviour, length changes of the investigated samples were recorded over a broad temperature range 116 (Figure 2a). Figure 2b,c shows the results for the different particle sizes. Quantification of the

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3.2. Dilatometric Analysis and Density Changes To study the influence of the particle size of the additional titanium on the dilatometric behaviour, length changes of the investigated samples were recorded over a broad temperature range (Figure2a). Figure2b,c shows the results for the different particle sizes. Quantification of the dilatometric changes Materials 2020, 13, x FOR PEER REVIEW 4 of 13 was performed by calculating the length changes in specific regions (measurement scheme and detailed 117values indilatometric supplementary). changes Independently was performed of bythe calculatingtitanium particle the length size, the changes measurements in specific showed regions rapid 118swelling (measurement of the sample scheme at the and temperature detailed values of 478 in supplementary).2.3 C and ends Independently up at the temperature of the titanium of 509particle1.9 C. ± ◦ ± ◦ 119It is thensize, followed the measurements by a rapid showed shrinkage rapid swelling starting of at the 646 sample0.3 at◦C. the The temperature former reflectsof 478 ± the2.3 °C temporary and 120 ends up at the temperature of 509 ± 1.9 °C . It is then followed± by a rapid shrinkage starting at 646 ± formation of titanium hydride (TiHx) due to interaction of elemental titanium and hydrogen derived from 121 x the atmosphere.0.3 °C. The With former an reflects increase the of temporary titanium formation content, theof titanium swelling hydride effect was(TiH more) due pronounced.to interaction of On the 122 elemental titanium and hydrogen derived from the atmosphere. With an increase of titanium content, other hand, the shrinkage observed at around 646 0.3 C reflects the intensive release of hydrogen due 123 the swelling effect was more pronounced. On the± other◦ hand, the shrinkage observed at around 646 124to thermally ± 0.3 °C induced reflects dehydrogenation, the intensive release i.e., of decomposition hydrogen due to of ther TiHmallyx. This induced decomposition dehydrogenation, is finished i.e. before, 125the temperaturedecomposition of 905 of TiH◦Cx is. This reached decomposition because TiHis finishedx becomes before unstable the temperature below this of 905 temperature °C is reached [19 ,20]. 126This alsobecause means TiH thatx becomes above 905unstable◦C, titanium below this is temperature available in [1 elemental9,20]. This form.also means These that observations above 905 °C, stay in 127a good titanium agreement is available with previous in elemental studies form. [19 These–21]. observations The swelling stay of in titanium a good agreement particles originated with previous from its 128hydrogenation studies [19 can–21 introduce]. The swelling large of tensiletitanium stress particles on neighbouringoriginated from ironits hydrogenation particles and can therefore introduce can be 129 a potentiallarge source tensile of stress an additional on neighbo porosityuring iron in the particles sintered and compact. therefore After can be the a potential dehydrogenation source of process, an 130 additional porosity in the sintered compact. After the dehydrogenation process, the transition of the the transition of the base iron powder from the crystallographic form of BCC to FCC starts at 905 4.4 C 131 base iron powder from the crystallographic form of BCC to FCC starts at 905 ± 4.4 °C and ends at 934± ◦ and ends at 934 0.3 C (Figure2b,c). 132 ± 0.3 °C (Figure± 2b◦ ,c).

133 Figure 2. (a) Temperature proﬁle of dilatometric tests; (b) dilatometric curves of samples A63, B63 and 134 C63; (c)Figure dilatometric 2. (a) Temperature curves of samplesprofile of A140,dilatometric B140 andtests; C140. (b) dilatometric curves of samples A63, B63 135 and C63; (c) dilatometric curves of samples A140, B140 and C140.

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136 Consequent heating heating results results in the in eutectic the eutectic reaction reaction between between iron and iron boron and starting boron at a starting temperature at a 137 aroundtemperature 1172 ◦aroundC, which 1172 is in °C, good which agreement is in good with agreement previous with studies previous [8,10]. studies The presence [8,10]. of The the presence eutectic 138 liquidof the eutectic enhances liquid mass enhances transport mass causing transport densification causing densification of the material. of the As material. a consequence, As a consequence, shrinking 139 becomesshrinking a morebecomes dominant a more e ff dominantect than thermal effect thanexpansion. thermal This expansion. is observed This as a is length observed reduction as a length of the 140 samplereduction in theof the dilatometric sample in curves. the dilatometric This effect curves. is presented This effect in Figure is presented3a,b in detail. in Figure As can 3a, beb in easily detail. seen As 141 incan Figure be easily3a,b the seen more in Figure titanium 3a was,b the introduced more titanium the higher was was introduced swelling the at temperatures higher was swelling of eutectic at 142 liquidtemperatures occurrence. of eutectic This shows liquid that occurrence. the more This titanium shows was that added the more the less titanium effectively was massadded transport the less 143 occurredeffectively at mass a temperature transport occurred above 1172 at ◦aC. temperature Moreover, theabove total 1172 dimensional °C. Moreover, changes the tota (Figurel dimensional3c) show 144 thechanges same ( trend.Figure The3c) show dilatometric the same results trend. suggest The dilatometric that fine titanium results suggest particles that reacted fine titanium more eff particlesectively 145 withreacted the more eutectic effectively liquid rather with the than eutectic coarse liquid ones. rather than coarse ones.

146 147 Figure 3.3. Dilatometric changes: ( (aa)) in in the region between the occurrence of the eutecticeutectic phasephase andand 148 isothermalisothermal step;step; ((bb)) atat anan isothermalisothermal stepstep andand ((cc)) totaltotal changes.changes.

Such an observation indicates a decreasing amount of the liquid phase along with the increasing 149 Such an observation indicates a decreasing amount of the liquid phase along with the increasing addition of the titanium. The smaller was the particle size of titanium the stronger was its influence 150 addition of the titanium. The smaller was the particle size of titanium the stronger was its influence dilatometric behaviour. As presented in Figure3c, use of high additions of titanium of fine particles 151 dilatometric behaviour. As presented in Figure 3c, use of high additions of titanium of fine particles (C63) may lead even to swelling in respect to the compacted sample. This may a ect negatively the 152 (C63) may lead even to swelling in respect to the compacted sample. This may affectff negatively the final relative density of the sintered compacts. The cooling part of dilatometric curves (Figure2a–c) 153 final relative density of the sintered compacts. The cooling part of dilatometric curves (Figure 2a–c) 154 showsshows nono evidenceevidence of elementalelemental titaniumtitanium presence asas itit waswas observedobserved inin thethe heatingheating stage.stage. The only 155 visible effects are a solidification of borides at 1147 ± 2.1 °C and transformation of austenite back to 156 ferrite starting at 893 ± 2.4 °C.

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Materials 2020, 13, x FOR PEER REVIEW 6 of 13 visible eﬀects are a solidiﬁcation of borides at 1147 2.1 C and transformation of austenite back to ± ◦ 157 ferriteThe starting results at of 893 dilatometri2.4 ◦C. c tests (Figure 3c) correspond well with the density of sintered samples ± 158 (FigureThe 4 results) in respect of dilatometric to the direction tests (Figure of changes3c) correspond. There is well, however, with the no density agreement of sintered between samples these 159 (Figureresults4 in) in respect respect to to the the magnitude direction of of changes. changes. There The shrinkage is, however, observed no agreement in dilatometric between thesesamples results was 160 inheavily respect impaired to the magnitude by the evaporation of changes. of The the shrinkage boron from observed the sample in dilatometric surface samples[22]. Missing was heavily boron 161 impairedresulted in by creation the evaporation of smaller of theamount boron of from eyectic the samplephase. This surface effect [22 is]. Missingthe more boron pronounced resulted the in creation smaller 162 ofis smallerthe sample amount and ofthe eyectic larger phase. is its surface This effect. As iscan the be more easily pronounced seen, the thehigher smaller the addition is the sample of titanium and the, 163 largerthe lower is its the surface. density As after can be the easily sintering seen, theprocess. higher This the effect addition was of more titanium, pronounced the lower for the the density 45–63 after µm 164 thetitanium sintering particle process. size This compared effect was to the more size pronounced of 100–140 for µm. the This 45–63 showsµm titanium that the particle reaction size between compared the 165 totitanium the size and of 100–140 the eutecticµm. Thisphase shows was thatmore the effective reaction when between fine the particles titanium of andhigh the specific eutectic surface phase area was 166 morewere effectiveadded. when fine particles of high specific surface area were added.

167 Figure 4. (a) Density changes and (b) porosity in function of the added titanium of Ø20 5 mm samples. 168 Figure 4. (a) Density changes and (b) porosity in function of the added titanium× of Ø 20 × 5 mm 169 samples. 3.3. Phase Identiﬁcation

170 3.3. PhaseResults Identification of specular X-ray diffraction experiments are shown in Figure5a together with peak positions from literature in Figure5b. As a measure for expected intensity, the absolute value of the 171 Results of specular X-ray diffraction experiments are shown in Figure 5a together with peak squared structure factor is used. For clarity, the experimental curves are shifted vertically. A major 172 positions from literature in Figure 5b. As a measure for expected intensity, the absolute value of the Bragg peak can be observed at 2 Theta = 44.7 , corresponding to the 110 reflex of the cubic iron phase 173 squared structure factor is used. For clarity,◦ the experimental curves are shifted vertically. A major (a = 2.8665 Å) [23]. Additional peaks of this phase can be found at 65.1 , 82.4 , 99.0 and 116.5 , 174 Bragg peak can be observed at 2 Theta = 44.7°, corresponding to the 110 reflex◦ of the◦ cubic◦ iron phase◦ originating from the (200), (211), (220) and (310) planes (Figure S1 in supplementary data). In the 175 (a = 2.8665 Å ) [23]. Additional peaks of this phase can be found at 65.1°, 82.4°, 99.0° and 116.5°, reference sample as well as all samples with titanium addition, additional peaks due to the presence of 176 originating from the (200), (211), (220) and (310) planes (Figure S1 in supplementary data). In the Fe B can be observed (35.1 and 42.6 , corresponding to the 200 and 002 plane) [24]. At high titanium 177 reference2 sample as well as◦ all samples◦ with titanium addition, additional peaks due to the presence concentrations, a simultaneous presence of these peaks and the peak of the 100 reflex of TiB2 [25] 178 of Fe2B can be observed (35.1° and 42.6°, corresponding to the 200 and 002 plane) [24]. At high can be found. Such observations stay in a good agreement with results of [17,18] who observed the 179 titanium concentrations, a simultaneous presence of these peaks and the peak of the 100 reflex of TiB2 formation of TiB at the expense of Fe B borides in cast iron-based alloys during the solidification 180 [25] can be found.2 Such observations stay2 in a good agreement with results of [17,18] who observed process. Moreover, a weak presence of crystalline titanium (40.2◦) was noticed, informing that the 181 the formation of TiB2 at the expense of Fe2B borides in cast iron-based alloys during the solidification reaction between titanium and boron was not complete [23]. 182 process. Moreover, a weak presence of crystalline titanium (40.2°) was noticed, informing that the Comparison of observed borides (REF and C63 samples) was presented in Figure6a,b. Figure6a 183 reaction between titanium and boron was not complete [23]. shows borides in a network of rib-like structures observed in REF sample, whereas Figure6b shows interconnected globular structures observed in titanium modified sintered compacts. This change in morphology was caused by crystallization of TiB2 as a result of the reaction of titanium and boron. The crystallisation process requires binding boron from a eutectic liquid resulting in precipitation of iron atoms in surrounding space. As the TiB2 is solid at sintering temperature it consisted a barrier for grain growth. This can be seen as uneven grain boundaries in samples with titanium addition (Figure6b). Without the presence of titanium, Fe 2B crystallizes at lower temperatures as the last phase (Figure1a), therefore its morphology is determined by already existing spherical grains. Ex situ SEM EDX measurements (Figure6c,d) after the sintering process showed the presence of small and nearly spherical titanium based particles in all samples with titanium additions. Moreover, the addition of titanium causes separation of previously (REF) connected borides, which enhances the connection among neighbouring grains (figures available in supplementary data in the folder: Eutectics distribution).

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184 Figure 5. (a) Specular X-ray diﬀraction patterns of the Fe-B system with titanium additions; (b) expected 185 Figure 5. (a) Specular X-ray diffraction patterns of the Fe-B system with titanium additions; (b) peak positions from known crystal structures and their square of the absolute value of the structure 186 expected peak positions from known crystal structures and their square of the absolute value of the factor SF as a measure for expected intensity. 187 structure factor SF as a measure for expected intensity. Materials 2020, 13, x FOR PEER REVIEW 8 of 13 188 Comparison of observed borides (REF and C63 samples) was presented in Figure 6a,b. Figure 6a 189 shows borides in a network of rib-like structures observed in REF sample, whereas Figure 6b shows 190 interconnected globular structures observed in titanium modified sintered compacts. This change in 191 morphology was caused by crystallization of TiB2 as a result of the reaction of titanium and boron. 192 The crystallisation process requires binding boron from a eutectic liquid resulting in precipitation of 193 iron atoms in surrounding space. As the TiB2 is solid at sintering temperature it consisted a barrier 194 for grain growth. This can be seen as uneven grain boundaries in samples with titanium addition 195 (Figure 6b). Without the presence of titanium, Fe2B crystallizes at lower temperatures as the last phase 196 (Figure 1a), therefore its morphology is determined by already existing spherical grains. Ex situ SEM 197 EDX measurements (Figure 6c,d) after the sintering process showed the presence of small and nearly 198 spherical titanium based particles in all samples with titanium additions. Moreover, the addition of 199 titanium causes separation of previously (REF) connected borides, which enhances the connection 200 among neighbouring grains (figures available in supplementary data in the folder: Eutectics 201 distribution).

202

203 Figure 6. SEM pictures of borides: (a) REF sample; (b) C63 sample; (c) TiB2 neighbouring the Fe2B; (d) Figure 6. SEM pictures of borides: (a) REF sample; (b) C63 sample; (c) TiB2 neighbouring the Fe2B; 204 (dEDS) EDS line line scan scan of sample of sample C63 C63 presented presented in Figure in Figure 6c. 6c.

205 Furthermore,Furthermore, thethe titanium addition addition influences inﬂuences also also thethe microstructure microstructure of the of sintered the sintered material material as 206 aspresented presented in inFigure Figure 7a7–a–e.e. The The reference reference sample sample characterizes characterizes with with a nearly a nearly continuous continuous network network of 207 ofborides borides located located on on grain grain boundaries boundaries and and uniaxial uniaxial grains. grains. Cont Contraryrary to the to the REF REF sample sample,, the the more more 208 titanium was introduced, the more developed the grains’ shapes became. The presence of the TiB2 in titanium was introduced, the more developed the grains’ shapes became. The presence of the TiB2 in 209 the matrix of the material may effectively hinder the grain growth, as was demonstrated in research the matrix of the material may eﬀectively hinder the grain growth, as was demonstrated in research 210 carried out, e.g., by Namini et al., Sobhani et al. and Gan et al. [26–28]. Moreover, the more titanium carried out, e.g., by Namini et al., Sobhani et al. and Gan et al. [26–28]. Moreover, the more titanium 211 was added, the more and the larger pores were observed. This is believed was a side effect of limiting 212 the amount of liquid phase during the sintering process.

213 214 Figure 7. Representative microstructure micrographs of the tested samples: (a) REF; (b) A63; (c) A140; 215 (d) C63; (e) C140 (detailed micrographs in supplementary data).

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202

203 Figure 6. SEM pictures of borides: (a) REF sample; (b) C63 sample; (c) TiB2 neighbouring the Fe2B; (d) 204 EDS line scan of sample C63 presented in Figure 6c.

205 Furthermore, the titanium addition influences also the microstructure of the sintered material as 206 presented in Figure 7a–e. The reference sample characterizes with a nearly continuous network of 207 borides located on grain boundaries and uniaxial grains. Contrary to the REF sample, the more 208 Materialstitanium2019 was, 12 introduced, 4188 , the more developed the grains’ shapes became. The presence of the TiB8 of2 in 12 209 the matrix of the material may effectively hinder the grain growth, as was demonstrated in research 210 carried out, e.g., by Namini et al., Sobhani et al. and Gan et al. [26–28]. Moreover, the more titanium 211 was added,added, the more and the larger pores were observed. This is believed was a side effect eﬀect of limiting 212 the amount of liquid phase duringduring thethe sinteringsintering process.process.

213 214 Figure 7. Representative microstructure micrographs of the tested samples: ( a) REF; ( b) A63; ( (c)) A140; A140; 215 Materials((d)) C63;C63;2020, ((1ee3)), C140xC140 FOR (detailed (detailedPEER REVIEW micrographsmicrographs inin supplementarysupplementary data).data). 9 of 13

216 AA comparisoncomparison ofof aa graingrain sizesize presentedpresented inin FigureFigure8 8shows shows that that the the introduction introduction of of titanium titanium of of

217 particleparticle sizesize ofof 6363 µμmm (Figure(Figure8 a)8a) results results in in grain grain refinement, refinement which, which stays stays in in good good agreement agreement with with 218 otherother researchers’researchers’ findingsfindings [[2266––2828].]. AA similarsimilar trendtrend waswas observedobserved whenwhen titaniumtitanium waswas introducedintroduced inin 219 thethe formform ofof biggerbigger particlesparticles (140(140 µμm).m). TheThe graingrain sizesize refinementrefinement isis causedcaused byby thethe presencepresence ofof aa TiBTiB22,, 220 whichwhich consistsconsists ofof anan eeffectiveffective barrier barrier for for grain grain growth. growth.

221 222 FigureFigure 8.8. HistogramsHistograms ofof graingrain sizesize distributiondistribution forfor seriesseries ((aa)) 6363 andand ((bb)) 140.140.

The presence of TiB2 (induced by addition of Ti) changed also the grain shape, i.e., grains in REF 223 The presence of TiB2 (induced by addition of Ti) changed also the grain shape, i.e., grains in REF sample were of polygonal shape with rounded corners due to presence of large amount of borides. 224 sample were of polygonal shape with rounded corners due to presence of large amount of borides. Moreover, the more titanium was added, the more irregular was the shape of the grains and the more 225 Moreover, the more titanium was added, the more irregular was the shape of the grains and the more dispersed were the borides. Furthermore, as appears from the analysis of grain size distribution a large 226 dispersed were the borides. Furthermore, as appears from the analysis of grain size distribution a amount of titanium (1.40 wt % – C) promotes also the appearance of large grains (diameter 100–150 µm) 227 large amount of titanium (1.40 wt % – C) promotes also the appearance of large grains (diameter 100– in the case of series 140. The medium-sized (50–100 µm) grains are promoted when titanium is added 228 150 μm) in the case of series 140. The medium-sized (50–100 μm) grains are promoted when titanium in the form of a particle of size 63 µm. 229 is added in the form of a particle of size 63 μm. 3.4. Mechanical Properties 230 3.4. Mechanical Properties Transverse Rupture Strength (TRS) tests were performed in order to estimate the influence of the 231 Transverse Rupture Strength (TRS) tests were performed in order to estimate the influence of changed phase composition on the mechanical behaviour of the samples. The results were presented in 232 the changed phase composition on the mechanical behaviour of the samples. The results were 233 presented in a way to highlight the influence of the increasing amount of titanium addition on 234 mechanical behaviour (Figure 9a–c). The results bring the conclusion that high addition of titanium 235 (C-series, 1.40 wt % Ti) causes an increase in the maximum observed deformation on the expense of 236 maximum force (Figure 9a,b) and hence, the TRS as well (Figure 9c). Other additions of titanium (A 237 and B) show intermediate states—samples with 0.47 wt % titanium addition (A) show a decrease of 238 both TRS and maximum deformation, whereas the B-series (0.93 wt % titanium addition) is 239 characterized by lower peak force and higher maximum deformation with respect to the reference 240 sample (Figure 9). The increase of ductility remains in the agreement of the other works [17,18]. The 241 drop of the mechanical properties, however, stays in opposition to the recent findings [17,18]. The 242 increase in ductility of sample (C series) can be attributed to joint effect of few phenomena: (i) the 243 grains are not anymore separated with a network of brittle borides; therefore, grains are connected 244 with each other; (ii) formation of TiB2 reinforces the matrix; (iii) grains are not uniaxial anymore and 245 their shape is irregular; therefore, mutual interlocking is possible. The irregular shape of grains is 246 attributed to the joint effect of grain growth in conditions of solid-state sintering (induced by eutectic 247 phase disappearance) and liquid phase sintering. These changes were possible mainly due to the 248 change of manufacturing process change—in previous works, the casting was used whereas is the 249 present work the liquid phase sintering process was applied. For the densification of the sample, 250 effective mass transport is required, and when the titanium is added, it is impeded as presented in 251 Figure 4, which leads to the drop of the density (in relation to the REF sample).

Materials 2019, 12, 4188 9 of 12 a way to highlight the influence of the increasing amount of titanium addition on mechanical behaviour (Figure9a–c). The results bring the conclusion that high addition of titanium (C-series, 1.40 wt % Ti) causes an increase in the maximum observed deformation on the expense of maximum force (Figure9a,b) and hence, the TRS as well (Figure9c). Other additions of titanium (A and B) show intermediate states—samples with 0.47 wt % titanium addition (A) show a decrease of both TRS and maximum deformation, whereas the B-series (0.93 wt % titanium addition) is characterized by lower peak force and higher maximum deformation with respect to the reference sample (Figure9). The increase of ductility remains in the agreement of the other works [17,18]. The drop of the mechanical properties, however, stays in opposition to the recent findings [17,18]. The increase in ductility of sample (C series) can be attributed to joint effect of few phenomena: (i) the grains are not anymore separated with a network of brittle borides; therefore, grains are connected with each other; (ii) formation of TiB2 reinforces the matrix; (iii) grains are not uniaxial anymore and their shape is irregular; therefore, mutual interlocking is possible. The irregular shape of grains is attributed to the joint effect of grain growth in conditions of solid-state sintering (induced by eutectic phase disappearance) and liquid phase sintering. These changes were possible mainly due to the change of manufacturing process change—in previous works, the casting was used whereas is the present work the liquid phase sintering process was applied. For the densification of the sample, effective mass transport is required, and when the titanium is added, it is impeded as presentedMaterials in Figure 2020, 134, x, FOR which PEER leads REVIEW to the drop of the density (in relation to the REF sample).10 of 13

252 Figure 9. Representative Transverse Rupture Strength (TRS) curve for samples from series (a) 63 and 253 Figure 9. Representative Transverse Rupture Strength (TRS) curve for samples from series (a) 63 and 254 (b) 140;(b (c) )140; comparison (c) comparison of TRS of TRS values values of of tested tested samples. samples.

255 Fracture surfaces investigated using SEM (Figure 10) reveal that the more titanium was added 256 the more ductile was the behaviour of the fracture surfaces, which can be seen by the development 257 of the fracture surface in Figure 10a–e. The more titanium was introduced the less flat intra-granular 258 fractures were observed and the more signs of ductility were present, i.e., “mountain-chain shapes 259 and valleys”. The extent of the changes was relatively small. This, however, suggests a lesser amount 260 of precipitated brittle borides at the grains boundaries compared to the material without titanium 261 addition (REF).

Materials 2019, 12, 4188 10 of 12

Fracture surfaces investigated using SEM (Figure 10) reveal that the more titanium was added the more ductile was the behaviour of the fracture surfaces, which can be seen by the development of the fracture surface in Figure 10a–e. The more titanium was introduced the less ﬂat intra-granular fractures were observed and the more signs of ductility were present, i.e., “mountain-chain shapes and valleys”. The extent of the changes was relatively small. This, however, suggests a lesser amount of precipitated brittle borides at the grains boundaries compared to the material without titanium addition (REF). Materials 2020, 13, x FOR PEER REVIEW 11 of 13

262 263 FigureFigure 10.10. RepresentativeRepresentative fracture fracture surfaces surfaces of of the the tested tested samples: samples: (a) (REF;a) REF; (b) (A63;b) A63; (c) A140; (c) A140; (d) C63; (d) C63; 264 (e()e) C140. C140.

265 4.4. Conclusions Conclusions 266 TheThe influence influence of increasing titanium titanium addition addition on on the the iron iron-boron-boron system system was was under under investigation. investigation. 267 TitaniumTitanium was was introduced introduced in in three three various various amounts amounts of of two two different different grain grain size size distributions distributions each. each. ItIt was 268 shownwas shown that the that addition the addition of titanium of titanium partially partially binds binds the boronthe boron from from the Fethe2 BFe+2BFe + eutecticFe eutectic liquid liquid phase 269 and,phase therefore, and, therefore reduces, reduces the shrinking the shrinking rate by influencing rate by influencing the amount the of amount the eutectic of the liquid eutectic phase. liquid It was 270 alsophase. shown It was that also the shown addition that of titaniumthe addition changes of titanium the phase changes composition the phase of thecomposition final material of the by final creating 271 material by creating crystalline TiB2 borides besides crystalline Fe2B borides. Changes in the crystalline TiB2 borides besides crystalline Fe2B borides. Changes in the morphology from a network 272 ofmorphology rib-like structures from a network for the pure of rib Fe-B-like systemstructures to interconnectedfor the pure Fe-B globular system to structures interconnected in case globu of titaniumlar 273 additionsstructures were in case also of observed titaniumdue additions to TiB wereappearance. also observed Both, due increasing to TiB2 amountappearance. of titanium Both, increasing addition and 274 2 itsamount fine grain of titanium size distribution, addition and impede its fine the grain shrinking size distribution, of Fe-B system impede by decreasingthe shrinking the of amount Fe-B system of liquid 275 by decreasing the amount of liquid phase. The addition of titanium enhances the plastic behaviour phase. The addition of titanium enhances the plastic behaviour of a sintered material at the expense of 276 of a sintered material at the expense of transverse rupture strength. This change was attributed to (i) transverse rupture strength. This change was attributed to (i) grain refinement; (b) irregular shape of 277 grain refinement; (b) irregular shape of grains, which allowed for mutual interlocking, and (iii) grains, which allowed for mutual interlocking, and (iii) decreasing amount of brittle borides on the grain 278 decreasing amount of brittle borides on the grain boundaries, which enhanced the connection among boundaries, which enhanced the connection among the neighbouring grains. 279 the neighbouring grains. 280 ContraryContrary to to the the casting casting process, process the, sinteringthe sintering process process was found was to found be not to suitable be not for suitable strengthening for iron-based sintered compacts via TiB in situ formation due to a negative inﬂuence on the densiﬁcation 281 strengthening iron-based sintered compacts2 via TiB2 in situ formation due to a negative influence on 282 behaviourthe densification of the sinteredbehaviour material. of the sintered material.

283 SupplementarySupplementary Materials:Materials: The following following are are available available online online at atwww.mdpi.com/xxx/s1, http://www.mdpi.com Figure/1996-1944 S1: Specular/12/24/4188 X- /s1, 284 Figureray diffraction S1: Specular patterns X-ray of di theﬀraction Fe-B system patterns with of thetitanium Fe-B systemadditions with over titanium the full additions measured over range. the Grey full measured lines 285 range.denote Grey expected lines peak denote positions expected of the peak cubic positions iron phase. of the cubic iron phase. Author Contributions: Conceptualization, M.S., M.H. and J.K.; Data Curation, M.S., M.H., B.S. and R.R.; Formal 286 Analysis,Author Contributions: M.H., R.R. and Conceptualization, C.S.; Funding Acquisition,M.S., M.H. and M.S., J.K.; J.K.Data and Curation C.S.;, Investigation,M.S., M.H., B.S. M.S., and R.R. M.H.; Formal and B.S.; 287 Methodology,Analysis, M.H. M.S.,, R.R. M.H. and andC.S.; B.S.; Funding Project Acquisition Administration,, M.S., J.K. M.S. and and C.S. J.K.;; Investigation, Resources, M.S.,M.S., R.R.,M.H. J.K.and andB.S.;C.S.; 288 Methodology, M.S., M.H. and B.S.; Project Administration, M.S. and J.K.; Resources, M.S., R.R., J.K. and C.S.; 289 Supervision, R.R., J.K. and C.S.; Validation, M.H., R.R. and C.S.; Visualization, M.S. and B.S.; Writing—Original 290 Draft, M.S., M.H. and B.S.; Writing—Review & Editing, M.H., R.R. and C.S.

291 Funding: This research was supported by grant no. UMO-2014/13/N/ST8/00585, financed by the National 292 Science Centre, Poland. This research was also supported by internal funds of Graz University of Technology. 293 Open Access Funding by the Graz University of Technology.

294 Conflicts of Interest: The authors declare no conflicts of interest

Materials 2019, 12, 4188 11 of 12

Supervision, R.R., J.K. and C.S.; Validation, M.H., R.R. and C.S.; Visualization, M.S. and B.S.; Writing—Original Draft, M.S., M.H. and B.S.; Writing—Review & Editing, M.H., R.R. and C.S. Funding: This research was supported by grant no. UMO-2014/13/N/ST8/00585, financed by the National Science Centre, Poland. This research was also supported by internal funds of Graz University of Technology. Open Access Funding by the Graz University of Technology. Conflicts of Interest: The authors declare no conflicts of interest

References

1. Subramanian, C.; Suri, A.K.; Murthy,T.S.R.C. Development of Boron-based Materials for Nuclear Applications. Barc Newsl. 2010, 313, 14–22. 2. Lherbier, L. Solving Nuclear Storage Issues with Powder Alloys. 2019. Available online: https://www.carpentertechnology.com/en/alloy-techzone/technical-information/technical-articles/ solving-nuclear-storage-issues (accessed on 20 July 2019). 3. Hye-Jin, K.; Soon-Hyeok, J.; Won-Seog, Y.; Byung-Gil, Y.; Yoo-Dong, C.; Heon-Young, H.; Hyun-Young, C. Eﬀects of titanium content on hydrogen embrittlement susceptibility of hot-stamped boron steels. J. Alloys Compd. 2018, 735, 2067–2080.

4. Fedorova, I.; Liu, F.; Grumsen, F.B.; Cao, Y.; Mishin, O.V.; Hald, J. Fine (Cr,Fe)2B borides on grain boundaries in a 10Cr-0.01B martensitic steel. Scr. Mater. 2018, 156, 124–128. [CrossRef] 5. Yang, X.; Guo, S. Fe-Mo-B Enhanced Sintering of P/M 316 L Stainless Steel. J. Iron Steel Res. Int. 2008, 15, 10–14. [CrossRef] 6. Uzunsoy, D. Investigation of dry sliding wear properties of boron doped powder metallurgy 316 L stainless steel. Mater. Des. 2010, 31, 3896–3900. [CrossRef] 7. Wu, M.W.; Cai, W.Z.; Lin, Z.J.; Chang, S.H. Liquid phase sintering mechanism and densification behavior of boron-alloyed Fe-Ni-Mo-C-B powder metallurgy steel. Mater. Des. 2017, 133, 536–548. [CrossRef] 8. Sulikowska, K.; Skałoń,M.; Kozub, B.; Kazior, J. Influence of mechanical alloying on surface layer of P/M AISI 316 L powder with addition of FeNiMnSiB master alloy. Surf. Coat. Technol. 2016, 302, 142–149. [CrossRef] 9. Molinari, A.; Kazior, J.; Straffelini, G. Investigation of liquid-phase sintering by image analysis. Mater. Charact. 1995, 34, 271–276. [CrossRef] 10. Luitjohan, K.E.; Krane, M.J.M.; Ortalan, V.; Johnson, D.R. Investigation of the metatectic reaction in iron-boron binary alloys. J. Alloys Compd. 2018, 732, 498–505. [CrossRef] 11. Skałoń,M.; Buzolin, R.; Kazior, J.; Sommitsch, C.; Hebda, M. Improving the dimensional stability and

mechanical properties of AISI 316L + B sinters by Si3N4 addition. Materials 2019, 12, 1798. [CrossRef] 12. Skałoń,M.; Hebda, M.; Sulikowska, K.; Kazior, J. Influence of FeNiMnSiB master alloy on the structure and mechanical properties of P/M AISI 316L. Mater. Des. 2016, 108, 462–469. [CrossRef] 13. Kwon, O.J.; Yoon, D.N. Closure of isolated pores in liquid phase sintering of W-Ni. Int. J. Powder Metall. 1981, 17, 127–133. 14. Kang, S.J.L.; Kaysser, W.A.; Petzow, G.; Yoon, D.N. Elimination of pores during liquid phase sintering of Mo-Ni. Powder Metall. 1984, 27, 97–100. [CrossRef] 15. Lee, D.D.; Kang, S.G.L.; Yoon, D.N. A direct observation of the grain shape accommodation during liquid phase sintering of Mo-Ni alloy. Scr. Metall. Mater. 1990, 24, 927–930. [CrossRef] 16. Kang, S.J.L.; Azou, P. Trapping of pores and liquid pockets during liquid phase sintering. Powder Metall. 1985, 28, 90–92. [CrossRef] 17. Lin, H.; Ying, L.; Jun, L.; Binghong, L. Effects of hot rolling and titanium content on the microstructure and mechanical properties of high boron Fe-B alloys. Mater. Des. 2012, 36, 88–93. 18. Liu, Y.; Li, B.; Li, J.; He, L.; Gao, S.; Nieh, T.G. Effect of titanium on the ductilization of Fe-B alloys with high boron content. Mater. Lett. 2010, 64, 1299–1301. [CrossRef] 19. Ma, M.; Wang, L.; Wang, Y.; Xiang, W.; Lyu, P.; Tang, B.; Tan, X. Effect of hydrogen content on hydrogen desorption kinetics of titanium hydride. J. Alloys Compd. 2017, 709, 445–452. [CrossRef] 20. Ma, M.; Liang, L.; Wang, L.; Wang, Y.; Cheng, Y.; Tang, B.; Tan, X. Phase transformations of titanium hydride in thermal desorption process with different heating rates. Int. J. Hydrogen Energy 2015, 40, 8926–8934. [CrossRef] Materials 2019, 12, 4188 12 of 12

21. Baymakov, Y.V.; Lebedev, O.A. Metallurgy of Nonferrous Metals Transactions. Leningrad Polytechnic. Inst. 1963, 223, 25–34. 22. Karwan-Baczewska, J. The properties of Fe-Ni-Mo-Cu-B materials produced via liquid phase sintering. Arch. Metall. Mater. 2011, 56, 789–796. [CrossRef] 23. Wyckoff, R.W.G. Crystal Structures, Second edition; Interscience Publishers: New York, NY, USA, 1963; pp. 7–83. 24. Bjurstroem, T. Arkiv for Kemi, Mineralogi Och Geologi; Almqvist & Wiksell: Stockholm, Sweden, 1933; pp. 1–12. 25. Moehr, S.; Mueller-Buschbaum, H.K.; Grin, Y.; Schnering, H.G. Zeitschrift für anorganische und allgemeine Chemie 5/2003. Z. Fur Anorg. Allg. Chem. 1996, 622, 1035–1037. 26. Namini, A.S.; Azadbeh, M.; Asl, M.S. Effect of TiB2 content on the characteristics of spark plasma sintered Ti–TiBw composites. Adv. Powder Technol. 2017, 28, 1564–1572. [CrossRef] 27. Sobhani, M.; Arabi, H.; Mirhabibi, A.; Brydson, R.M.D. Microstructural evolution of copper-titanium alloy during in-situ formation of TiB2 particles. Trans. Nonferr. Met. Soc. 2013, 23, 2994–3001. [CrossRef] 28. Gan, G.; Yang, B.; Zhang, B.; Jiang, X.; Shi, Y.; Wu, Y. Refining Mechanism of 7075 Al Alloy by In-Situ TiB2 Particles. Materials 2017, 10, 132. [CrossRef]

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