Al Matrix Composites Reinforced by Ti and C Dedicated to Work at Elevated Temperature

materials

Article Al Matrix Composites Reinforced by Ti and C Dedicated to Work at Elevated Temperature

Bartosz Hekner *, Jerzy Myalski, Patryk Wrze´sniowski and Tomasz Maci ˛ag

Faculty of Materials Engineering Krasi´nskiego8, Silesian University of Technology, 40-019 Katowice, Poland; [email protected] (J.M.); [email protected] (P.W.); [email protected] (T.M.) * Correspondence: [email protected]

Abstract: In this paper, the applicability of aluminium matrix composites to high-temperature working conditions (not exceeding the Al melting point) was evaluated. The behaviour of Al-Ti-C composites at elevated temperatures was described based on microstructural and phase composition observations for composites heated at temperatures of 540 and 600 ◦C over differing time intervals from 2 to 72 h. The materials investigated were aluminium matrix composites (AMC) reinforced with a spatial carbon (C) structure covered by a titanium (Ti) layer. This layer protected the carbon surface against contact with the aluminium during processing, protection which was maintained for

the material’s lifetime and ensured the required phase compositions of Al4C3 phase limitation and AlTi3 phase creation. It was also proved that heat treatment inﬂuenced not only phase compositions but also the microstructure of the material, and, as a consequence, the properties of the composite.

Keywords: aluminium matrix composites; metal matrix composites; heterophase composites; Al-Ti-C system; carbon compounds

Citation: Hekner, B.; Myalski, J.; Wrze´sniowski,P.; Maci ˛ag,T. Al 1. Introduction Matrix Composites Reinforced by Ti Aluminium matrix composites (AMCs) are among the most commonly used material and C Dedicated to Work at Elevated Temperature. Materials 2021, 14, 3114. solutions in many industries. Their applicability depends on the speciﬁc properties of the https://doi.org/10.3390/ma14113114 composite. When aluminium is used as a matrix, chemical resistance, thermal conductivity, and elasticity can be achieved in the material created. The low density of Al (2.7 g/cm3)

Academic Editor: Theodore Matikas leads to a reduction in composite mass compared to, e.g., Fe-based composites. However, the mechanical properties of pure aluminium are not sufficient for many of real working Received: 11 April 2021 conditions. To achieve the required parameters, aluminium can be reinforced using a Accepted: 2 June 2021 variety of ceramic materials, such as oxides (e.g., Al2O3)[1,2], carbides (e.g., SiC and Published: 6 June 2021 WC) [2–4] or nitrides (e.g., Si3N4)[5,6]. These ceramic phases are commonly used to improve a material’s mechanical properties, e.g., hardness or stiffness [2–6]. Moreover, Publisher’s Note: MDPI stays neutral the applied reinforcement can also provide additional benefits. For example, carbon and with regard to jurisdictional claims in its compounds are promising components for the improvement of AMC’s mechanical published maps and institutional affil- properties (e.g., carbon nanotubes [7] or graphene [8]) and also for tribological properties iations. (e.g., graphite [9] or glassy carbon [10]). In highly developed materials solutions, additional reinforcements such as carbon are present around the main reinforcement phase (such as Al2O3 or SiC). These types of composites belong to the heterophase group of reinforced composites, where more than one compound affects the material’s properties [11–13]. Copyright: © 2021 by the authors. The application of carbon in AMC is very limited due to the high reactivity between Licensee MDPI, Basel, Switzerland. aluminium and carbon components. High-temperature processing leads to the creation This article is an open access article of an aluminium carbide (Al4C3) phase, which is metastable in ambient conditions and distributed under the terms and strongly hydrophilic. This can result in a reduction in properties and, in extreme cases, conditions of the Creative Commons destruction of the material. Moreover, this type of undesired phase can occur during the life- Attribution (CC BY) license (https:// time of AMC, which is especially dangerous for composites working at high temperatures creativecommons.org/licenses/by/ under high load for long periods of time [14,15]. 4.0/).

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In this paper, innovative composites with heterophase reinforcements Ti-C and Al matrix were evaluated. The applied carbon consisted of an amorphous phase of carbon named glassy carbon, which was produced using our own method [16] in the shape of spatial, open-cell foam [17]. The titanium was in the form of a layer on the carbon surface used to prevent contact of C with melted Al during infiltration and subsequently through the material’s lifetime. Specific phases were used to increase the properties of the composite and also for their high potential for creating new phases between aluminium and titanium or titanium and carbon. The presence of these new phases led to improved mechanical properties and also provided high chemical bonding between phases, additionally increasing the properties of the created material. The potential for creation of new phases in an Al-Ti system was analysed based on the phase diagram for these compounds presented in Figure1a [ 18,19]. According to this survey, the phase composition in Al-Ti strongly depends on component proportion. Al3Ti (tetragonal) is created when the minimum amount of Ti is about 24 mass.%. Increasing the amount of Ti leads to the creation of Al2Ti (orthorhombic), AlTi (tetragonal) and AlTi3 (hexagonal), respectively. The experimental investigation presented in [20] confirms the relationship between Ti-Al ratio and phase composition of the created materials. The presence of these intermediate phases increases material properties, such as melting point, chemical resistance and mechanical strength; however, these phases limit the material ductility and also increase fragility. Another area suitable for the creation of new phases is the border between Ti and C compounds. Analysis was based on the Ti-C phase diagram (Figure1b) [ 21,22], which showed the Ti-C tendency to create titanium carbide phases in a whole range of compound content. However, the analysis of the crystal structure data showed that the aluminium carbide phase consisted of 32–48.8 at.% and 32–36 at.% for TiC and Ti2C, respectively [21], which approximates to 10–20 mass.% of carbon. The phase diagram obtained at equilibrium conditions for an Al–C system revealed a tendency to create three types of phases, namely, aluminium, graphite and Al4C3, as the only intermediate compounds (Figure1c) [ 23]. It is known that the aluminium carbide phase is metastable in ambient conditions, which limits the detectability of this phase in investigation conducted in stable conditions [24]. The supporting diagrams for the three-compounds system Al-Ti-C is presented in [25]. The authors took into account the phase composition of this system at 1000, 1100 and 1300 ◦C. This examination demonstrated the possibility of creating not only two-component phases (e.g., TiAl2 and Al3Ti) but also three-component phases, such as Ti2AlC. Moreover, the findings revealed a strong tendency to reaction of both components with melted aluminium. The isothermal sections of the Al-Ti-C system [25] showed that in a whole range of compounds, the new phases could be created at elevated temperatures. The thermodynamic analyses presented in [26,27] and confirmed by experimental analyses [28] revealed a tendency to creation of inter-metallic phases in an Al-Ti-C system. A further thermodynamic analysis of an Al-Ti-C system [29] revealed that the most stable phase in this system is TiC. However, in some cases, this reaction is more complicated because additional, inter-metallic phases can be obtained. These results were also confirmed in experiments by the authors. Based on the analysed diagrams and thermodynamic assumptions, it can be noted that component ratio influences the chemical composition of the analysed composite materials. However, high temperature is needed to obtain the expected new phases. In this paper, the possibility of creating new phases in an Al-Ti-C system was evaluated. To obtain the required reaction between phases, heat treatment over time was conducted. The parameters for the heat treatment investigations were comparable to actual working conditions, which provided an opportunity to predict material behaviour during its lifetime. Materials 2021, 14, 3114 3 of 14

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(a) (b)

(c)

FigureFigure 1. Phase 1. Phase diagram diagram for for(a) (Ti-Ala) Ti-Al system system [19]; [19 (];b ()b Ti-C) Ti-C system system [22]; [22]; (c (c) )Al-C Al-C system system [23]; [23]; L—liquid, G—gas,G—gas, C—carbon. C—carbon.

2.The Materials thermodynamic and Methods analyses presented in [26,27] and confirmed by experimental analysesThe [28] evaluated revealed materialsa tendency were to creation aluminium of inter-metallic matrix composites phases (AMCs) in an Al-Ti-C reinforced system. by A furtherglassy carbonthermodynamic (GC) and titanium analysis (Ti) of an phases. Al-Ti-C The system glassy carbon[29] revealed with spatial that the structure most stable was phasemanufactured in this system using is our TiC. own However, production in methodsome cases, [30]. Thethis structure,reaction similaris more to complicated foam, was becauseachieved additional, by application inter-metallic of polyurethane phases ca (PU)n be foam obtained. as a precursor These results of shape were (Figure also2 con-a). firmedThis in type experiments of PU foam by is the widely authors. used in industry, for example, as a ﬁlter for liquids. The mainBased factor on forthe the analysed selection diagrams of foam was and its ther structure—poresmodynamic perassumptions, inch (ppi)—and it can its be impact noted thaton component the glassy carbonratio influences amount and the size chemical of the singlecomposition carbon of area the in analysed the composite composite [31]. For ma- evaluation, foam with a ppi of 45 was chosen as a result of own prior research. terials. However, high temperature is needed to obtain the expected new phases. In this In the next step, a thin layer of suspension of titanium particles in phenol–formaldehyde paper, the possibility of creating new phases in an Al-Ti-C system was evaluated. To ob- (P-F) resin was applied to PU foam walls via the immersion method. A high-purity titanium tain the required reaction between phases, heat treatment over time was conducted. The powder (>99%), with a particle size < 45 µm was applied. The microstructural view of Ti parameterspowder is for presented the heat in treatment Figure2b. investigatio A range of Tins particle were comparable suspensions to were actual prepared working (5, 10,conditions,15, 20, which 25, 30, provided 35 and 40 vol.%)an opportunity in a relation to to predi the resinct material (Table1 behaviour). All suspensions during were its lifetime. cured adequately using a P-F resin curing process. Prepared 3D C-Ti structures were pyrolyzed 2. Materialsat non-equilibrium and Methods conditions—1000 ◦C for 1 h in inert gas. Both polymer materials—PU andThe P-F evaluated resin—were materials carbonised were to aluminium achieve glassy matrix carbon—a composites pure carbon(AMCs) structure reinforced with by glassya low carbon level (GC) of additional and titanium components (Ti) phases. and anThe amorphous glassy carbon structure. with spatial The P-F structure resin, as was a manufacturedduroplastic material,using our prevents own production deformation method of the [30]. PU foamThe structure, during the similar pyrolysis to foam, process, was achievedwhich allowedby application for the 3Dof structurepolyurethane of the (PU) PU precursor foam as toa precursor be preserved of (Figureshape (Figure2a). 2a). This type of PU foam is widely used in industry, for example, as a filter for liquids. The main factor for the selection of foam was its structure—pores per inch (ppi)—and its impact on the glassy carbon amount and size of the single carbon area in the composite [31]. For evaluation, foam with a ppi of 45 was chosen as a result of own prior research. In the next step, a thin layer of suspension of titanium particles in phenol–formaldehyde (P-F) resin was applied to PU foam walls via the immersion method. A high-purity titanium powder (>99%), with a particle size < 45 µm was applied. The microstructural

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view of Ti powder is presented in Figure 2b. A range of Ti particle suspensions were prepared (5, 10, 15, 20, 25, 30, 35 and 40 vol.%) in a relation to the resin (Table 1). All suspensions were cured adequately using a P-F resin curing process. Prepared 3D C-Ti structures were pyrolyzed at non-equilibrium conditions—1000 °C for 1 h in inert gas. Both polymer materials—PU and P-F resin—were carbonised to achieve glassy carbon—a pure carbon structure with a low level of additional components and an amorphous structure. The P-

Materials 2021, 14, 3114 F resin, as a duroplastic material, prevents deformation of the PU foam during the4 ofpyrol- 14 ysis process, which allowed for the 3D structure of the PU precursor to be preserved (Fig- ure 2a).

(a) (b)

FigureFigure 2. 2.MicrostructureMicrostructure of of (a (a) )spatial spatial structure structure ofof PUPU foam;foam; ( b(b)) Ti Ti powder powder (SEM). (SEM).

TableTable 1. 1.ChemicalChemical compositions compositions of of suspension suspension used to produceproduce spatial spatial Ti-C Ti-C reinforcement. reinforcement.

TiTi 5 5 10 10 1515 2020 2525 3030 40 40 VolumeVolume Ratio (%)(%) P-FP-F resin resin 9595 9090 8585 8080 7575 7070 60 60 TiTi 8.988.98 17.9617.96 26.9426.94 35.9235.92 44.9044.90 53.8853.88 71.8471.84 massmass (g) P-FP-F resin resin 45.60 45.60 43.20 43.20 40.80 40.80 38.4038.40 36.0036.00 33.6033.60 28.8028.80

TheThe prepared prepared 3D 3D C-Ti C-Ti structures structures were were infiltrated inﬁltrated by by high high purity purity aluminium aluminium in in a aDe- ◦ ◦ gussaDegussa press press at 720 at °C 720 forC 15 for min, 15 with min, a with break a breakof 15 min of 15 at min 450 at°C. 450 UnderC. Underexternal external pressure ofpressure 10 MPa of under 10 MPa vacuum under vacuumconditions, conditions, the optimum the optimum infiltration inﬁltration of spatial of spatial structures structures was achieved.was achieved. The next The part next of our part investigation of our investigation was conducted was conducted to evaluate to evaluate material materialbehaviour behaviour in real-life, real-time conditions. During the heat treatment investigations, in real-life, real-time conditions. During the heat treatment investigations, materials were materials were heated for a range of times: 2, 8, 24, and 72 h, and at temperatures of heated for a range of times: 2, 8, 24, and 72 h, and at temperatures of 540 and 600 °C, under 540 and 600 ◦C, under air conditions. The schematic diagram of processes performed is airpresented conditions. in Figure The schematic3. diagram of processes performed is presented in Figure 3. Materials 2021, 14, x FOR PEER REVIEW As part of the evaluation, the materials were inspected with scanning electron5 of 14mi- croscopy (SEM) (JEOL JCM-6000 Neoscope II, Tokyo, Japan and Hitachi S-3400, Krefeld, Germany) using the EDS technique. The thermogravimetric (TG) evaluation of Al-Ti-C composition was conducted in ambient gas at temperatures up to 1200 °C. All components were mechanically mixed in an initial powder form. The powders consisted of equal proportions of each component (Al-Ti-C). The phase compositions were determined using a PANalytical Empyrean X-Ray Diffractometer (Almelo, Netherlands) for the powder form and XRD X’Pert 3 (Almelo, Netherlands) for solid materials. Mechanical properties were described based on hardness measurement using the Vickers method HV 0.2.

Figure 3.3. Schematic diagram of processes performed.performed.

3. Results An initial material evaluation using thermodynamic analysis was performed for all components used in composite creation (Figure 4). This analysis revealed that the most likely phase creations were AlTi, TiAl3 and Al3Ti. The free energy of formation was lower for phases with an amorphous structure than those with a crystal structure.

Figure 4. Free energy of formation in an Al-Ti-C (amorphous) system.

In the next step, the prepared composition of Al-Ti-C was evaluated in a powdered, initial (non-processed) form in a TG test. Based on the results presented in Figure 5, the potential reaction in the analysed system was determined. A temperature of 1200 °C was applied, which is higher than the processing temperature of carbon, to determine whether any additional components (non-pyrolysed) appeared in these conditions. Under the influence of temperature, two reactions appeared: firstly an endothermic reaction at about 670 °C; secondly an exothermic reaction, which manifested immediately after the first one at about 700 °C. A mass changes analysis revealed a decrease of about 0.8% at the start of heating, followed by an increase of about 3%.

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As part of the evaluation, the materials were inspected with scanning electron mi- croscopy (SEM) (JEOL JCM-6000 Neoscope II, Tokyo, Japan and Hitachi S-3400, Krefeld, Germany) using the EDS technique. The thermogravimetric (TG) evaluation of Al-Ti-C composition was conducted in ambient gas at temperatures up to 1200 ◦C. All components were mechanically mixed in an initial powder form. The powders consisted of equal proportions of each component (Al-Ti-C). The phase compositions were determined using Figure 3. Schematic diagrama PANalytical of processes Empyrean performed. X-Ray Diffractometer (Almelo, Netherlands) for the powder form and XRD X’Pert 3 (Almelo, Netherlands) for solid materials. Mechanical properties were 3. Results described based on hardness measurement using the Vickers method HV 0.2. An initial material3. Resultsevaluation using thermodynamic analysis was performed for all components used in compositeAn initial creation material evaluation(Figure 4). using This thermodynamic analysis revealed analysis was that performed the most for all components used in composite creation (Figure4). This analysis revealed that the most likely phase creations were AlTi, TiAl3 and Al3Ti. The free energy of formation was lower likely phase creations were AlTi, TiAl3 and Al3Ti. The free energy of formation was lower for phases with an amorphousfor phases withstructure an amorphous than those structure with than a those crystal with astructure. crystal structure.

Figure 4. Free energy ofFigure formation 4. Free energy in an ofAl-Ti-C formation (amorphous) in an Al-Ti-C (amorphous) system. system. In the next step, the prepared composition of Al-Ti-C was evaluated in a powdered, In the next step, theinitial prepared (non-processed) compositio form inn a TGof Al-Ti-C test. Based was on the evaluated results presented in a powdered, in Figure5, the ◦ initial (non-processed)potential form in reaction a TG in test. the analysed Based systemon the was results determined. presented A temperature in Figure of 1200 5, Cthe was applied, which is higher than the processing temperature of carbon, to determine whether potential reaction in theany analysed additional system components was (non-pyrolysed) determined. appeared A temperature in these conditions. of 1200 °C Under was the applied, which is higherinfluence than the of temperature, processing two temperature reactions appeared: of carbon, firstly an to endothermic determine reaction whether at about ◦ any additional components670 C; secondly(non-pyrolysed) an exothermic appeared reaction, which in these manifested conditions. immediately Under after the the first in- one at about 700 ◦C. A mass changes analysis revealed a decrease of about 0.8% at the start of fluence of temperature,heating, two reactions followed by anappeared: increase of firstly about 3%. an endothermic reaction at about 670 °C; secondly an exothermic reaction, which manifested immediately after the first one at about 700 °C. A mass changes analysis revealed a decrease of about 0.8% at the start of heating, followed by an increase of about 3%.

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Figure 5. Thermogravimetric analysis of an Al-Ti-C system (TG). FigureFigure 5. 5.Thermogravimetric Thermogravimetric analysis analysis of an Al-Ti-C of an system Al-Ti-C (TG). system (TG). For material after TG investigation, phase analyses were conducted to determine new phases Forcreated material in the after Al-Ti-C TG investigation, system at a phasetemperature analyses of were 1200 conducted °C. Figure to 6 determinepresents the new For material after TG investigation, phase analyses◦ were conducted to determine new resultphases of createdthe examination. in the Al-Ti-C A reaction system atbetw a temperatureeen the aluminium of 1200 C. and Figure titanium6 presents led to the AlTi result3 phaseofphases the creation, examination. created whilst A inthe reaction the aluminium–carbon Al-Ti-C between system the aluminium syst emat revealeda te andmperature titanium a tendency led of to to AlTi 1200creation3 phase °C. of Figure cre-a 6 presents the ation, whilst the aluminium–carbon system revealed a tendency to creation of a metastable metastableresult of Al 4theC3 phase. examination. There was noA reactionreaction between betw theeen titanium the aluminium and carbon compo-and titanium led to AlTi3 Al C phase. There was no reaction between the titanium and carbon components. nents.phase4 3 creation, whilst the aluminium–carbon system revealed a tendency to creation of a metastable Al4C3 phase. There was no reaction between the titanium and carbon components.

FigureFigure 6. Phase 6. Phase composition composition of ofAl-Ti-C Al-Ti-C system system after after thermogravimetric thermogravimetric analyses analyses at at1200 1200 °C◦ C(XRD). (XRD).

The microstructure observations of the created spatial forms of heterophase reinforcements revealed the influence of the amount of titanium on the quality of the created layer on GC foam. With lower volumes of Ti particle suspension, the created layer was discontinuous and did not cover the whole carbon surface. This phenomenon is presented

Figure 6. Phase composition of Al-Ti-C system after thermogravimetric analyses at 1200 °C (XRD).

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The microstructure observations of the created spatial forms of heterophase reinforcements revealed the inﬂuence of the amount of titanium on the quality of the created Materials 2021, 14, x FOR PEER REVIEWlayer on GC foam. With lower volumes of Ti particle suspension, the created layer was7 of 14 discontinuous and did not cover the whole carbon surface. This phenomenon is presented inin FigureFigure 7a7a andand its its inset. inset. Increasing Increasing the volumevolumeof of Ti Ti particle particle suspension suspension led led to to improve- improve- mentsments in in the the continuity continuity of of the Ti layer.layer. TheThe expected expected microstructure microstructure was was achieved achieved with with a a suspensionsuspension of of 25 25 vol.% vol.% of of titanium particlesparticles inin P-F P-F resin. resin. The The created created layer layer fully fully covered covered thethe carbon carbon surface, surface, as as shown shown in Figure7 7b.b. The The thickness thickness of of the the Ti Ti layer layer was was approximately approximately thethe size size of of a a single single Ti Ti particle (Figure(Figure7 b7b inset). inset). With With reinforcements reinforcements with with a Ti a particleTi particle suspensionsuspension volume volume higher higher than than 25 vol.%,vol.%, undesiredundesired microstructural microstructural aspects aspects were were noted. noted. OnOn the the surface surface of thethe createdcreated layer, layer, some some discontinuity discontinuity was was noted noted (Figure (Figure7c). However, 7c). However, the themicrostructural microstructural defects defects relate relate to a to high a high volume volu ofme brittle of brittle particles. particles. The size The of size the of created the cre- atedlayer layer was was more more than than 100µ m,100µm, which which was greater was greater than the than Ti particle the Ti particle size (Figure size7 c(Figure inset). 7c ◦ inset).Moreover, Moreover, at the processingat the processing temperatures temperat appliedures (1000applied and (1000 720 Cand for 720 pyrolysis °C for and pyrolysis the andinﬁltration the infiltration process, process, respectively), respectively), the titanium the titanium particles particles were not were melted not (melting melted (melting point of Ti: 1668 ◦C). In the created layer, adhesion bonding of Ti was supported by the carbon point of Ti: 1668 °C). In the created layer, adhesion bonding of Ti was supported by the bonding obtained as a result of the P-F resin carbonisation process. carbon bonding obtained as a result of the P-F resin carbonisation process.

(a) (b)

(c)

FigureFigure 7. The 7. The microstructure microstructure of of the the obtained obtained reinforcement reinforcement layerslayers variedvaried accordingaccording to to Ti Ti particle particle suspension: suspension: (a )( 10a) 10 vol.%; vol.%; (b)( b25) 25vol.%; vol.%; (c) ( c40) 40 vol.% vol.% in in PF PF resin resin suspension. suspension. In In insets, thethe cross-sectionscross-sections of of foam foam are are presented presented (SEM). (SEM).

The analysis of the microstructures of the composites after infiltration by aluminium (Figure 8) confirmed observations for Ti-C spatial reinforcement. The quality of the composite depends on the amount of titanium particles in suspension. With low Ti particle suspension, the discontinuity in the Ti layer led to direct contact of melted aluminium with the carbon reinforcement. This phenomenon is presented in Figure 8a. For composites with the highest Ti particle suspension (more than 30 vol.%), the created layer revealed high porosity due to poor bonding with unmelted Ti particles. In material with 40 vol.% of Ti particles in suspension, the size of the created layer was in the range of 50–200 µm. The optimal areas for void creations were Ti areas with high width, which can be seen in Figure 8c. The required quality of the microstructure was achieved for materials with

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The analysis of the microstructures of the composites after infiltration by aluminium (Figure8) confirmed observations for Ti-C spatial reinforcement. The quality of the composite depends on the amount of titanium particles in suspension. With low Ti particle suspension, the discontinuity in the Ti layer led to direct contact of melted aluminium with the carbon reinforcement. This phenomenon is presented in Figure8a. For composites with the highest Ti particle suspension (more than 30 vol.%), the created layer revealed high porosity due to poor bonding with unmelted Ti particles. In material with 40 vol.% of Ti particles in suspension, the size of the created layer was in the range of 50–200 µm. Materials 2021, 14, x FOR PEER REVIEWThe optimal areas for void creations were Ti areas with high width, which can be seen8 in of 14 Figure8c . The required quality of the microstructure was achieved for materials with Ti Tiparticle particle suspensions suspensions in ain range a range of 20–30 of 20–30 vol.%. vol.%. The titanium The titanium particles particles created created a consistent a con- sistentlayer onlayer the on carbon the carbon surface. surface. The microstructure The microstructure is shown is shown in Figure in 8Figureb. Moreover, 8b. Moreover, the thepresence presence of looseof loose Ti particles Ti particles (not (not bonded bonded to spatial to spatial structure) structure) was observed was observed (Figure (Figure9c). 9c).These These particles particles are probablyare probably a result a result of turbulent of turbulent liquid aluminium liquid aluminium flow during flow the during infiltra- the infiltrationtion process. process. The distribution The distribution of reinforcement of reinforcement phases presented phases presented in Figure9 in confirms Figure the9 con- firmsrequired the required spatial structure spatial ofstructure reinforcement of reinforcement (shape of foam) (shape but of also foam) reveals but that also the reveals area of that thereinforcement area of reinforcement present is higher present than is higher the theoretical than the assumption theoretical (loose assumption particles (loose of Ti andparticles C ofin Ti Al and matrix). C in Al matrix).

(a) (b)

(c)

FigureFigure 8. The 8. The microstructure microstructure of composites of composites obtained obtained by infiltration by inﬁltration of spatial of spatial structures structures withwith a range a range of Ti particle of Ti particle suspensions:suspensions: (a) 5 vol.%; (a) ( 5b) vol.%; 20 vol.%; (b) 20 (c vol.%;) 40 vol.% (c) 40 used vol.% for used Ti layer for Ti creation layer creation (SEM). (SEM).

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(a) (b)

FigureFigure 9. ( 9.a) The(a) The microstructure microstructure and and chemical chemical distribution distribution of ( ofb) (aluminium;b) aluminium; (c) (titanium;c) titanium; (d) (carbond) carbon compounds compounds in com- in positecomposite with layers with layerscreated created by a suspension by a suspension of 20 ofvol.% 20 vol.% of Ti of (SEM Ti (SEM + EDS). + EDS).

BasedBased on on the the knowledge knowledge gained gained on on microstr microstructuraluctural aspects aspects and and their their influence influence on on the finalthe quality final quality of composite of composite material, material, AMC AMC with with a 20 avol.% 20 vol.% of Ti of was Ti was selected selected for forheat heat treat- ◦ menttreatment investigations. investigations. The applied The applied temperatures, temperatures, 540 and 540 600 and °C, 600 wereC, wereadequate adequate for 0.8 for and 0.90.8 of and the 0.9aluminium of the aluminium melting point, melting respectively. point, respectively. Under heating Under heatingconditions, conditions, the structure the ofstructure the analysed of the composites analysed composites changed, especially changed, in especially the carbon in theareas, carbon as shown areas, in as Figure shown 10. Thein main Figure influence 10. The on main the influencereinforcement on the struct reinforcementure was the structure duration was of processing. the duration At of the processing. At the start of the heating process (2 h), the characteristic geometric shape start of the heating process (2 h), the characteristic geometric shape of glassy carbon foam of glassy carbon foam was found in some areas, dependent on C width. The larger was found in some areas, dependent on C width. The larger size of carbon reinforcement size of carbon reinforcement affected the presence of pure carbon in the microstructure affected the presence of pure carbon in the microstructure (Figure 10a). After a period of (Figure 10a). After a period of time at high temperature (8–72 h), the Ti-C areas did not timereveal at high a clear temperature boundary between (8–72 h), phases the Ti-C in comparisonareas did not to reveal the initial a clear stage. boundary Moreover, between the phasesheat treatment in comparison led to increasedto the initial porosity. stage. The Moreover, optimal areasthe heat for voidtreatment creation led were to increased those porosity. The optimal areas for void creation were those with sufficient carbon presence and the boundaries between titanium reinforcement and aluminium matrix. Similar microstructural changes were seen at both 540 and 600 °C.

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with sufﬁcient carbon presence and the boundaries between titanium reinforcement and aluminium matrix. Similar microstructural changes were seen at both 540 and 600 ◦C. Materials 2021, 14, x FOR PEER REVIEW 10 of 14

(a) (b)

FigureFigure 10. 10.TheThe microstructures microstructures of composites of composites after after heat heat treatment treatment at 600 at 600 °C ◦atC varying at varying times: times: (a) (2a )h; 2 ( h;b) (8b h;) 8 (c h;) 24 (c) h; 24 (d h;) 72 h (SEM).(d) 72 h (SEM).

TheThe microstructural microstructural changes observedobserved after afte heatingr heating were were evaluated evaluated by phaseby phase analysis analy- sisin in the the areas areas of of composite composite reinforcement. reinforcement. The The result result of of the the XRD XRD examination examination is shownis shown inin Figure Figure 11. 11 .Firstly, Firstly, an an absence absence ofof thethe aluminiumaluminium carbide carbide phase phase (Al (Al4C43C)3 was) was observed, observed, ◦ eveneven after after 72 72 h h of of heating heating at 600 °C.C. TheThe compositescomposites at at the the start start of of heat heat treatment treatment (>8 (>8 h) h) revealedrevealed the the same same phase phase composition asas materialmaterial without without additional additional processing, processing, which which furtherfurther revealed revealed a a low low tendency tendency to creationcreation ofof newnew phases phases in in Al-Ti-C Al-Ti-C systems systems in in these these conditions. After more prolonged heating (≥24 h), new phases were created. The creation conditions. After more prolonged heating (≥24 h), new phases◦ were created. The creation of AlTi3 was noted for composites heated at both 540 and 600 C. of AlTi3 was noted for composites heated at both 540 and 600 °C. The changes in material properties were described based on the hardness measurement in areas of the aluminium matrix. Hardness results are presented in Figure 12. The signiﬁcant inﬂuence of heat treatment on hardness was noted. After short periods of time (2 h at 600 ◦C and 8 h at 540 ◦C), a decrease in hardness was noted. Over longer periods (>24 h) at both temperatures, the hardness increased up to a level similar to the hardness of the matrix in composites without heat treatment.

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(a) (b)

(c) (d) Figure 10. The microstructures of composites after heat treatment at 600 °C at varying times: (a) 2 h; (b) 8 h; (c) 24 h; (d) 72 h (SEM).

The microstructural changes observed after heating were evaluated by phase analysis in the areas of composite reinforcement. The result of the XRD examination is shown in Figure 11. Firstly, an absence of the aluminium carbide phase (Al4C3) was observed, even after 72 h of heating at 600 °C. The composites at the start of heat treatment (>8 h) revealed the same phase composition as material without additional processing, which further revealed a low tendency to creation of new phases in Al-Ti-C systems in these conditions. After more prolonged heating (≥24 h), new phases were created. The creation Materials 2021, 14, 3114 11 of 14 of AlTi3 was noted for composites heated at both 540 and 600 °C.

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Figure 11. Phase composition for AMC-Ti-C composite after heat treatment at 540 °C, over 8 and 72 h durations (XRD).

The changes in material properties were described based on the hardness measurement in areas of the aluminium matrix. Hardness results are presented in Figure 12. The significant influence of heat treatment on hardness was noted. After short periods of time (2 h at 600 °C and 8 h at 540 °C), a decrease in hardness was noted. Over longer periods (>24 h) at both temperatures, the hardness increased up to a level similar to the hardness of theFigure matrix 11. Phase in composition composites for AMC-Ti-C without composite heat treatment. after heat treatment at 540 ◦C, over 8 and 72 h durations (XRD).

FigureFigure 12. 12. HardnessHardness and and porosity porosity of Al-Ti-C of Al-Ti-C composites composites after heat treatment after heat at different treatment times (2–72 at different h) and temperatures times (2– ◦ 72 (540h) and and 600temperaturesC). (540 and 600 °C). 4. Discussion 4. Discussion The initial step of the examination was focused on the reactivity of an Al-Ti-C system. Based on knowledge gained via TG testing and subsequent XRD analysis, it was noted The initial step ofthat the the examination ﬁrst chemical reactionwas focuse appearedd on at the temperatures reactivity higher of an than Al-Ti-C the melting system. point of Based on knowledge aluminiumgained via (Figure TG5 testing). The exothermic and subsequent reaction noted XRD at about analysis, 700 ◦C relatesit was to noted an Al-Ti that the first chemical systemreaction and appeared corresponds at to AlTitemperatures3 phase creation higher (Figure than6). A the mechanism melting of newpoint phase of creation was presented in [32]. The inﬂuence of new phases on mechanical properties as a aluminium (Figure 5). The exothermic reaction noted at about 700 °C relates to an Al-Ti system and corresponds to AlTi3 phase creation (Figure 6). A mechanism of new phase creation was presented in [32]. The influence of new phases on mechanical properties as a result of precipitation hardening and alloying mechanisms is presented in [33–36]. How- ever, XRD investigation revealed a tendency to undesired reactions between aluminium and carbon after heating to 1200 °C (Figure 6). A first finding was that a limitation in the contact area between aluminium and carbon is needed in order to control the phase composition in the material. In the literature, many manufacturing solutions for aluminium matrix composites reinforced by carbon are described [33–37]. However, no solution for avoiding contact between carbon and aluminium has been presented. Other authors [32,33,37] present methods for obtaining phase composition through the control of the chemical compositions of material, but this solution limits the C component content in the finished material. This does not support composite design, e.g., for tribological properties, which strongly depend on carbon content in Al matrix composites [38,39]. In presenting this paper, an innovative spatial form of reinforcement was proposed in order to control the phase composition during processing and the subsequent lifetime of the material (Figure 2a). The reinforcement comprised glassy carbon foam covered by titanium particles (Figure 7). The limitation of contact between C and the Al matrix was achieved through selection of the optimum Ti amount (Table 1, Figure 7b). Based on our investigations, it can be noted that the Ti layer should be equal to the Ti particle size. This was achieved for composites with a 20 vol.% of Ti in suspension. This solution gave an

Materials 2021, 14, 3114 12 of 14

result of precipitation hardening and alloying mechanisms is presented in [33–36]. How- ever, XRD investigation revealed a tendency to undesired reactions between aluminium and carbon after heating to 1200 ◦C (Figure6). A first finding was that a limitation in the contact area between aluminium and carbon is needed in order to control the phase composition in the material. In the literature, many manufacturing solutions for aluminium matrix composites reinforced by carbon are described [33–37]. However, no solution for avoiding contact between carbon and aluminium has been presented. Other authors [32,33,37] present methods for obtaining phase composition through the control of the chemical compositions of material, but this solution limits the C component content in the finished material. This does not support composite design, e.g., for tribological properties, which strongly depend on carbon content in Al matrix composites [38,39]. In presenting this paper, an innovative spatial form of reinforcement was proposed in order to control the phase composition during processing and the subsequent lifetime of the material (Figure2a). The reinforcement comprised glassy carbon foam covered by titanium particles (Figure7). The limitation of contact between C and the Al matrix was achieved through selection of the optimum Ti amount (Table1, Figure7b). Based on our investigations, it can be noted that the Ti layer should be equal to the Ti particle size. This was achieved for composites with a 20 vol.% of Ti in suspension. This solution gave an opportunity to produce Al matrix composites with high C content without creating unwanted phases in the Al-C system. The composites with spatial Al-Ti-C structure were heat treated at a temperature equal to between 0.8 and 0.9 of the Al melting point. These parameters correspond to normal working conditions. This part of the investigation proved that Al-Ti phases seen at temperatures of about 700 ◦C (Figures5 and6) can also be created at lower temperatures, over a longer time (Figure 11). Another achievement was the avoidance of theAl4C3 phase even with high-temperature processing (0.9 melting point of aluminium) and a long period of heat treatment (72 h). The presence of the Ti layer prevented unwanted reactions in the Al-C system but supported the Al-Ti system reaction (Figure 11). The treatment we carried out also influenced the microstructure and properties of the material. The changes in GC at the start of heat treatment led to higher porosity and decreased hardness (8 h) (Figures 10 and 12). A longer heating time (24 h) resulted in a new AlTi3 phase creation and improvement in the mechanical properties as a consequence (Figure 12). The observed alloying mechanisms are described in many publications. Other authors [40,41] describe the influence of Ti and the amount of compound used on the mechanical properties of Al matrix composites. A further investigation showed the strong influence of manufacturing technique on the composite properties [42]. The hardness achieved corresponds to results achieved with Al matrix composites reinforced by Ti or its compound, obtained by conventional methods, as documented in the literature. Additionally, in this study, it was found that after longer heating times, the boundaries between C and Ti phases were unclear and the porosity decreased (Figures 10 and 12). These microstructural changes also correspond to the increase in material properties (Figure 12). The changes in phase composition and microstructure proved that the material created can be successfully applied in elevated temperatures. Based on these results, it can be seen that Al matrix composites with spatial C-Ti reinforcement can be successfully applied at high temperature and/or in high-load application (e.g., friction points), which will provide a basis for the authors’ next investigation.

5. Conclusions For the manufacturing of AMC reinforced by Ti and C, the following should be taken into account: - The Al-Ti-C system revealed a tendency for the creation of new phases, especially in Al-Ti and Al-C systems. AlTi3 and Al4C3 phases were detected after TG investigation in the evaluated composites. Materials 2021, 14, 3114 13 of 14

- To control the phase composition of the material, a thin layer of Ti particles was applied on the surface of a C spatial structure. This solution limits contact between Al and C compounds and avoids the presence of an unwanted Al4C3 phase as a result. - Using titanium as a material for the creation of a protective layer, reactions (AlTi3) with the aluminium matrix, which beneﬁt the material’s properties, are possible. - The optimal quality of protective Ti layer (tightness, continuity) can be achieved by adjustment of the compound’s ratio. The desired structure was achieved with reinforcement foam covered by suspension of 20 vol.% of Ti in PA resin. - The optimum conditions for new phase creation in Al-Ti systems are high temperature (higher than 0.8 of the melting point of aluminium) and prolonged treatment duration (minimum 24 hours). With these parameters, the behaviour of Al-Ti-C composites in real-time working conditions (high temperature and load) can be better understood.

Author Contributions: Conceptualization, B.H. and J.M.; methodology, B.H. and P.W.; investigation, P.W., and T.M.; formal analysis, J.M. and B.H. resources, B.H.; data curation, B.H. All authors have read and agreed to the published version of the manuscript. Funding: This research was funded by NCN (Narodowe Centrum Nauki), grant number UMO- 2016/23/N/ST8/00994. Institutional Review Board Statement: Not applicable. Informed Consent Statement: Not applicable. Data Availability Statement: Data is contained within the article. Conﬂicts of Interest: The authors declare no conﬂict of interest.

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