Article Simultaneously Enhanced Strength, Toughness and Ductility of Cast 40Cr Steels Strengthened by Trace Biphase TiCx-TiB2 Nanoparticles
Chuan-Lu Li 1,2, Feng Qiu 1,2,3,* , Fang Chang 1,2, Xu-Min Zhao 1,2, Run Geng 1,2, Hong-Yu Yang 4, Qing-Long Zhao 1,2 and Qi-Chuan Jiang 1,2,*
1 State Key Laboratory of Automotive Simulation and Control, Jilin University, Renmin Street NO. 5988, Changchun 130025, China; [email protected] (C.-L.L.); [email protected] (F.C.); [email protected] (X.-M.Z.); [email protected] (R.G.); [email protected] (Q.-L.Z.) 2 Key Laboratory of Automobile Materials, Ministry of Education and Department of Materials Science and Engineering, Jilin University, Renmin Street NO. 5988, Changchun 130025, China 3 Qingdao Automotive Research Institute of Jilin University, Renmin Street NO. 5988, Changchun 130025, China 4 School of Materials Science and Engineering, Jiangsu University of Science and Technology, Zhenjiang 212003, China; [email protected] * Correspondence: [email protected] (F.Q.); [email protected] (Q.-C.J.); Tel.: +86-431-8509-5592 (F.Q.); +86-431-8509-4699 (Q.-C.J.) Received: 10 August 2018; Accepted: 4 September 2018; Published: 9 September 2018
Abstract: Simultaneously improving the strength, toughness, and ductility of cast steels has always been a difficult problem for researchers. Biphase TiCx-TiB2 nanoparticle-reinforced cast steels are prepared by adding in situ nanosized biphase TiCx-TiB2/Al master alloy during the casting process. The experimental results show that a series of significant changes take place in the microstructure of the steel: the ferrite-pearlite structure of the as-cast steels and the bainite structure of the steels after heat treatment are refined, the grain size is reduced, and the content of nanoparticles is increased. Promotion of nucleation and inhibition of dendrite growth by biphase TiCx-TiB2 nanoparticles leads to a refinement of the microstructure. The fine microstructure with evenly dispersed nanoparticles offers better properties [yield strength (1246 MPa), tensile strength (1469 MPa), fracture strain (9.4%), 2 impact toughness (20.3 J/cm ) and hardness (41 HRC)] for the steel with 0.018 wt.% biphase TiCx-TiB2 nanoparticles, which are increased by 15.4%, 31.2%, 4.4%, 11.5%, and 7.9% compared with the 40Cr steels. The higher content of nanoparticles provides higher strengths and hardness of the steel but are detrimental to ductility. The improved properties may be attributed to fine grain strengthening and the pinning effect of nanosized carbide on dislocations and grain boundaries. Through this work, it is known that the method of adding trace (0.018 wt.%) biphase TiCx-TiB2 nanoparticles during casting process can simultaneously improve the strength, toughness, as well as ductility of the cast steel.
Keywords: TiCx-TiB2 nanoparticles; grain refinement; mechanical property; strengthening mechanism
1. Introduction Solving the problem of simultaneously improving the strength and toughness of cast steels is a long-term goal for researchers. In recent years, ceramic particles, such as TiC [1,2], TiB2 [3,4], (NbTi)C [5] and other ceramics [6,7], have been used to enhance metallic materials because of their good thermal stability, high modulus, and good wettability with the metal matrix [6,8–11]. The outstanding role of ceramic particles in enhancing metal properties has aroused researchers’ interest [12–15].
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Sung-Mo Hong et al. found that nanosized TiC particles can refine the structure and increase the strength of SA-106B carbon steel [16]. S.W. Hu et al. found that TiC particles 2 µm in size can strengthen the hardness and wear resistance of manganese steel [17]. TiB2 and TiC-reinforced steel matrix composites were produced through powder metallurgy, improving the wear properties of the composites [18–20]. Improved steel properties were expected to be obtained through the combination of TiB2 and TiC particles. Spark plasma sintering was used to produce TiB2-TiC-reinforced steel matrix composites [21]. Among the various methods, traditional casting is economical and convenient, with its high production efficiency and flexible product shape [22,23]. However, due to the large difference in specific gravity, agglomeration of nanoparticles and surface pollution of nanoparticles, it seems that the traditional casting method is not suitable for producing nanoceramic-reinforced steel. Because of these constraints, there have been few studies on enhancing the strength and toughness of cast steel with dual-phase nanosized ceramic particles (TiCx-TiB2). Therefore, it is meaningful to prepare TiCx-TiB2 nanoparticle-reinforced steel through traditional casting method. In this work, we have innovatively added the in situ nanosized biphase TiCx-TiB2/Al master alloy into the casting process to make trace (TiCx-TiB2) nanoparticle-reinforced cast steel. The master alloy, in which nanosized ceramic particles are uniformly dispersed, is prepared by the SHS (self-propagating high temperature synthesis) method. This work helps to solve the problem of floating, agglomerating, and surface contamination of ceramic particles in the steel and makes full use of the benefits of the dual-phase ceramic particles (TiCx-TiB2) and conventional casting methods. The experimental results show that the strength and toughness of the dual-phase ceramic particles-reinforced cast steel are simultaneously improved. The influence mechanisms of the ceramic particles on both microstructural evolution and properties improvement of steels have been fully analyzed and discussed. In this paper, the preparation methods show a large potential application value, and the analysis and discussion provide a valuable reference for subsequent researchers.
2. Materials and Methods The main chemical composition of investigated 40Cr steel is displayed in Table1. The nanosized TiCx-TiB2/Al master alloy was prepared through self-propagating high-temperature synthesis. First, Al (~13 µm), Ti (~25 µm), and B4C (~1.5 µm) powders at a ratio corresponding to that of stoichiometric TiC and TiB2 were sufficiently mixed in a ball mill at a speed of 50 rpm for 18 h. Second, the sufficiently mixed powders were cold pressed into cylindrical compacts (Φ = 28 mm; h = 30 mm). Finally, a self-made vacuum thermal explosion furnace was used to initiate the self-propagating high-temperature synthesis (SHS) reaction in the compacts at a heating speed of about 30 K/min. ◦ and then at 900 C for 10 min. After the reaction was completed, the nanosized TiCx-TiB2/Al master alloy was successfully produced. The molten steel melted in 5 kg-capacity medium frequency furnace (GW-0.3t, NHHY, Foshan, China) was poured into a ladle, in which the master alloy had been placed. The molten steel with the ceramic particles dispersed throughout was cast into the sand mold and then cooled to room temperature. The steels were kept at 950 ◦C for 90 min and oil quenched. Afterwards, the steels were kept at 200 ◦C for 480 min and air cooled to room temperature, as shown in Figure1.
Table 1. Chemical composition of the 40Cr steel.
Element C Si Mn Cr S P Fe wt.% 0.41 0.25 0.65 0.95 0.004 0.018 Bal. Metals 2018, 8, 707 3 of 11 Metals 2018, 8, x FOR PEER REVIEW 3 of 11
Figure 1. Heat treatment process diagram for cast steels. Figure 1. Heat treatment process diagram for cast steels. The chemical composition of the investigated alloy was determined by direct-reading spectrometer The chemical composition of the investigated alloy was determined by direct-reading (CX-9000, GLMY, WuXi, China). The ceramic particles extracted from the master alloy were spectrometer (CX-9000, GLMY, WuXi, China). The ceramic particles extracted from the master alloy characterized by X-ray diffraction (XRD, D/Max 2500PC Rigaku, Japan) and by field emission scanning were characterized by X-ray diffraction (XRD, D/Max 2500PC Rigaku, Japan) and by field emission electron microscopy (FESEM, JSM-6700F, JEOL, Tokyo, Japan). Samples after heat treatment were cut scanning electron microscopy (FESEM, JSM-6700F, JEOL, Tokyo, Japan). Samples after heat treatment into dog-bone shaped tensile specimens (the specimens had a gauge length of 8 mm and cross-sectional were cut into dog-bone shaped tensile specimens (the specimens had a gauge length of 8 mm and area of 2.5 mm × 2 mm) and Charpy impact specimens (10 × 10 × 55 mm, U-notch). The samples cross-sectional area of 2.5 mm × 2 mm) and Charpy impact specimens (10 × 10 × 55 mm, U-notch). used for optical microscopy (OM, Olympus PMG3, Tokyo, Japan) were polished and etched with The samples used for optical microscopy (OM, Olympus PMG3, Tokyo, Japan) were polished and a 5% nitric acid and 95% alcohol mixture. The samples designated for electron backscatter diffraction etched with a 5% nitric acid and 95% alcohol mixture. The samples designated for electron backscatter (EBSD, Oxford NordlysMax EBSD, Oxford, UK) analysis were electropolished in a mixture of 20% diffraction (EBSD, Oxford NordlysMax EBSD, Oxford, UK) analysis were electropolished in a perchloric acid (Beijing Chemical Works, Beijing, China) and 80% acetic acid (Beijing Chemical Works, mixture of 20% perchloric acid (Beijing Chemical Works, Beijing, China) and 80% acetic acid (Beijing Beijing, China) at 30 V for 15 s. 0.3 µm was chosen as step size of EBSD scans. The TEM microstructure Chemical Works, Beijing, China) at 30 V for 15 s. 0.3 µm was chosen as step size of EBSD scans. The was acquired by using a JEOL microscope (JEM 2100F, JEOL, Tokyo, Japan). The tensile tests were TEM microstructure was acquired by using a JEOL microscope (JEM 2100F, JEOL, Tokyo, Japan). The performed on a servo hydraulic materials testing system (MTS 810, MTS Systems Corporation, tensile tests were performed on a servo hydraulic materials testing system (MTS 810, MTS Systems Minneapolis, MN, USA). The standard Charpy impact test was conducted using a semiautomatic Corporation, Minneapolis, MN, USA). The standard Charpy impact test was conducted using a impact tester (JBW-300B, JNKH, Jinan, China). The fracture surfaces of the tensile and impact specimens semiautomatic impact tester (JBW-300B, JNKH, Jinan, China). The fracture surfaces of the tensile and were observed by means of scanning electron microscopy (SEM, VEGA 3 XMU, TESCAN, Brno, impact specimens were observed by means of scanning electron microscopy (SEM, VEGA 3 XMU, Czech Republic). TESCAN, Brno, Czech Republic). 3. Results and Discussion 3. Results and Discussion Figure2a presents the XRD pattern of the nanosized biphase TiC x-TiB2/Al master alloy, which Figure 2a presents the XRD pattern of the nanosized biphase TiCx-TiB2/Al master alloy, which shows that the master alloy mainly contains α-Al, TiC, TiB2 and a small amount of Al3Ti. It can be shows that the master alloy mainly contains α-Al, TiC, TiB2 and a small amount of Al3Ti. It can be determined that the SHS reaction is relatively complete, with only a small amount of Al3Ti remaining. determined that the SHS reaction is relatively complete, with only a small amount of Al3Ti remaining. Figure2b is a FESEM image of the mixed particles of TiC and TiB 2 obtained by extracting the nanosized Figure 2b is a FESEM image of the mixed particles of TiC and TiB2 obtained by extracting the TiCx-TiB2/Al master alloy with hydrochloric acid. It can be seen that in addition to the spherical TiC nanosized TiCx-TiB2/Al master alloy with hydrochloric acid. It can be seen that in addition to the particles, there are also hexagonal prisms or lamellar TiB2 particles. The size statistics of the TiCx and spherical TiC particles, there are also hexagonal prisms or lamellar TiB2 particles. The size statistics TiB2 mixed particles in Figure2b shows that the average grain size of the mixed particles is 190 nm. ofThe the nanoparticles TiCx and TiB are2 mixed separated particles by aluminum, in Figure with2b shows no significant that the averag agglomeratione grain size with of one the anothermixed particles is 190 nm. The nanoparticles are separated by aluminum, with no significant agglomeration (Figure2c,d). Then, the nanosized TiC x-TiB2/Al master alloy was melted and gradually dispersed within the one molten another steel. (Fig Theure aluminum 2c,d). Then, in the the master nanosized alloy can TiC alsox-TiB be2/Al simultaneously master alloy used was as melted an oxygen and gradually dispersed in the molten steel. The aluminum in the master alloy can also be simultaneously scavenger to react with oxygen at a high temperature to form Al2O3. Then, the Al2O3 is removed usedduring as thean oxygen smelting scavenger process. to This react method with oxygen not only at successfullya high temperature introduces to form nanoparticles Al2O3. Then, into the the Almolten2O3 is steel removed but also during contributes the smelting to the uniform process.dispersion This method of nanoparticles not only successfully and to the introducesremoval of nanopoxygenarticles from theinto steel. the molten steel but also contributes to the uniform dispersion of nanoparticles and to the removal of oxygen from the steel.
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Figure 2. ((aa)) XRD XRD pattern of nano nano-sized-sized TiC xx--TiBTiB2/Al master master alloy, alloy, ( (bb)) morphologies morphologies and and sizes analysis of the nano-sizednano-sized TiCxx-TiB-TiB22 particles in the nano-sizednano-sized TiCTiCxx-TiB-TiB22/Al master master alloy. alloy. TEM images ( c) and electron diffraction patterns (d)) ofof thethe nano-sizednano-sized TiCTiCxx-TiB-TiB22/Al master master alloy. alloy.
The typical ferrite ferrite-pearlite-pearlite hybrid hybrid structure structure of of the the medium medium-carbon-carbon steel steel can can be be seen seen i inn Figure Figure 33a.a. The bright bright area area is is pro pro-eutectoid-eutectoid ferrite, ferrite, and and the thedark dark area areais pearlite is pearlite [24–26]. [24 Because–26]. Because the carbon the content carbon contentof the steel of the is less steel than is less 0.77 than wt.%, 0.77 ferrite wt.%, will ferrite be precipitated will be precipitated along the along austenite the austenite grain boundary grain boundary before beforethe pearlite the pearlite transformation transformation.. Therefore, Therefore, the long strip the long ferrite strip is distributed ferrite is distributed on the boundary on the of boundary the coarser of thepearlite coarser group. pearlite After group. adding After the 0.018 adding wt.% the nanoceramic 0.018 wt.% particles, nanoceramic it can particles, clearly be it seen can clearlythat instead be seen of thatthe strip instead-shaped of the pro strip-shaped-eutectoid ferrite, pro-eutectoid the small ferrite, pieces the of smallferrite pieces dispersed of ferrite inside dispersed the pearlite inside group the pearliteincrease, group and the increase, size of the and pearlite the size group of the is pearlite also reduced group (Figure is also reduced3b). When (Figure the content3b). When of nanoparticles the content ofin the nanoparticles steel is increased in the steelto 0.054 is increased wt.%, smaller to 0.054 ferrite wt.%, in the smaller structure ferrite is distributed in the structure uniformly, is distributed and the uniformly,pearlite structure and the is further pearlite refined structure Figure is further3c. It is known refined from Figure the3 structurec. It is known observed from under the the structure optical observedmicrographs under that the the optical microstructure micrographs of the that steel the after microstructure heat treatment of the mainly steel aftercontains heat low treatmenter bainite mainly and containsa small amount lower bainiteof residual and ferrite. a small The amount uneven ofly residual distributed ferrite. particles The unevenly with a large distributed number particlesof large white with ablocks large can number be seen of in large Figure white 3d. blocks In contrast, can be the seen lath in ferrite Figure and3d. carbide In contrast, in the the microstructure lath ferrite and of the carbide steel, into thewhich microstructure the 0.018 wt.% of nanoparticles the steel, to which are added, the 0.018 are wt.%significantly nanoparticles finer and are more added, uniform. are significantly More lath finerferrites and and more carbides uniform. were More observed lath ferrites in Figure and 3f, carbides but the werelath ferrite observed is thicker in Figure than3 f,that but in the Figure lath ferrite3e. is thicker than that in Figure3e.
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FigureFigure 3. 3. OOMM images images of of the the as as-cast-cast steels steels ( (aa––cc),), and and steels steels after after heat heat treatment treatment ( d–f). Figure 3. OM images of the as-cast steels (a–c), and steels after heat treatment (d–f). TheThe EBSD EBSD method method was was used used to to clarify clarify the the quantitative quantitative degree degree of of grain grain refinement, refinement, as as shown shown in in The EBSD method was used to clarify the quantitative degree of grain refinement, as shown in FigureFigure 44.. The average grain size is refinedrefined from 9.12 μmµm in in the the 40Cr 40Cr steel steel to to 3.59 3.59 μmµm in in the the steel steel with with Figure 4. The average grain size is refined from 9.12 μm in the 40Cr steel to 3.59 μm in the steel with 0.0180.018 wt.% wt.% TiC TiCx-TiBx-TiB2 2nanoparticles. nanoparticles. The The grain grain size size of the of steels the steelswith trace with nanosized trace nanosized TiCx-TiB 2 TiCparticlesx-TiB2 0.018 wt.% TiCx-TiB2 nanoparticles. The grain size of the steels with trace nanosized TiCx-TiB2 particleswas refined was by refined 60.6%. More by 60.6%. importantly, More importantly,the nanoparticles the notnanoparticles only changed not the only cast changed microstructure the cast of particles was refined by 60.6%. More importantly, the nanoparticles not only changed the cast microstructurethemicrostructure steel but also of of refinedthe the steel steel the but but grain also also sizerefined refined of the thethe steel graingrain after size heat ofof thethe treatment. steel steel after after heat heat treatment. treatment.
Figure 4. a b Figure 4.EBSD EBSD images images of of steels steels withwith TiCTiCxx-TiB-TiB2 particlesparticles contents contents of of 0 0wt. wt.%% (a () and) and 0.018 0.018 wt. wt.%% (b) ( after) after Figureheatheat treatment. 4.treatment. EBSD images (c ()c shows) shows of steels the the average average with TiC graingrainx-TiB sizesize2 particles of ( a,,b).). contents of 0 wt.% (a) and 0.018 wt.% (b) after heat treatment. (c) shows the average grain size of (a,b). TheThe TEM TEM images images in in Figure Figure5 5show show the the differences differences inin microstructuresmicrostructures between between the the steels steels without without nanosizednanosizThe TEMed TiC TiC imagesx-TiBx-TiB2 2 particlesinparticles Figure and5 and show the the the 0.018 0.018 differences wt.% wt.% TiC TiCin xmicrostructuresx-TiB-TiB2 nanoparticlenanoparticle-reinforced between-reinforced the steels steel. steel. without It isIt is nanosizdetermineddetermineded TiC that thatx-TiB a a large 2large particles number number and ofof carbide carbide the 0.018 particles, wt.% of of TiCwhich whichx-TiB the the2 sizenanoparticle size is isless less than than-reinforced 100 100 nm, nm, are steel. are evenly evenly It is determineddistributeddistributed inthat in the the a microstructurelarge microstructure number of ofof carbide thethe 0.0180.018 particles, wt.% TiC TiC ofxx- -TiBTiBwhich2 2nanoparticlenanoparticle-reinforced the size is-reinforced less than steel100 steel nm, in Figure in are Figure evenly 5b.5 b. distributedOnOn the the contrary, contrary, in the microstructure a asmaller smaller amount amount of the of of 0.018 carbide carbide wt.% particles TiCx-TiB is is 2found foundnanoparticle in in the the microstructure-reinforced microstructure steel of in of the Figure the steels steels 5b. Onwithoutwithout the contrary, nanosized nanosized a TiC smaller TiCx-TiBx-TiB amount22 particles of (Figure (Figure carbide 5a).5 a). particles is found in the microstructure of the steels without nanosized TiCx-TiB2 particles (Figure 5a).
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Figure 5. TEM images of steels without nano-sized TiCx-TiB2 particles (a) and steels with 0.018 wt.% TiCx-TiB2 nanoparticles (b). Figure 5 5.. TEM images of steels without nano nano-sized-sized TiC x--TiBTiB22 particlesparticles ( (aa)) and and steels steels with with 0.018 0.018 wt. wt.%% TiCx--TiBTiB2 nanoparticlesnanoparticles ( (bb)).. TiCx and TiB2 have been proven to be effective austenitic heterogeneous nucleation sites
[2,10,12].TiCx Nanoparticlesandand TiB TiB2 2have have beenare been successfully proven proven to be tointroduced effective be effective austenitic and austeniticdispersed heterogeneous heterogeneousinto the nucleation liquid nucleationsteel sites herein. [2,10 sites ,12As]. [2,10,12].shownNanoparticles in Nanoparticles Figure are 6, successfully because are successfully structural introduced undulations introduced and dispersed andand energydispersed into the fluctuations liquid into the steel liquid of herein. the steel heterogeneous As herein. shown As in shownnucleationFigure6 in, because Figure are small, structural 6, because nucleation undulations structural occurs undulations and preferentially energy fluctuations and on energy the nanoparticles. of fluctuations the heterogeneous of Usually, the heterogeneous nucleation larger-sized are nucleationnanoparticlessmall, nucleation are act small, as occurs the nucleation nucleation preferentially occurssite, while on preferentially the smaller nanoparticles.-sized on nanoparticles the Usually, nanoparticles. larger-sized adsorb Usually, onto nanoparticles the largersolid--liquidsized act nanoparticlesinterfaceas the nucleation and acthinder as site, the the whilenucleation transition smaller-sized site, of thewhile interface, nanoparticles smaller thus-sized inhibiting adsorb nanoparticles onto the the growth adsorb solid-liquid ofonto the the dendrite interface solid-liquid [27]. and interfaceThehinder primary the and transition dendriteshinder the of gradually thetransition interface, grow, of the thus and interface, inhibiting finally ,thus the the materialinhibiting growth becomes of the the growth dendrite completely of [the27]. dendrite austenitized. The primary [27]. Thedendrites sizeprimary of gradually the dendrites prior austenite grow, gradually and grains finally, grow, seriously the and material finallyaffects becomes, thethe later material completelyferrite becomes-pearlite austenitized. completely structure and The austenitized. the size bainite of the Thestructureprior size austenite of [28]. the priorgrains The austenite increased seriously grains grain affects seriously boundaries the later affects ferrite-pearlite due the to later austenite ferrite structure - grainpearlite and refinement thestructure bainite and structure the dispersed bainite [28]. structurenanoparticlesThe increased [28]. providegrain The boundaries increased more precipitation grain due to boundaries austenite sites for grain duepro refinement- toeutectoid austenite ferrite, and grain dispersed which refinement avoid nanoparticles the and formation dispersed provide of nanoparticleslongmore precipitationferrite andprovide limit sites more for the pro-eutectoid precipitation growth of ferrite, pearsiteslite. for which pro The-eutectoid avoid mostthe important formationferrite, which process of long avoid ferrite when the formation and the limit bainite the of longtransformationgrowth ferrite of pearlite. and occurs limit The most isthe the important growth diffusion of process pear of lite. carbon. when Thethe The most bainite rapid important transformation diffusion process channels occurs when is and the the diffusion thermal bainite transformationmismatchesof carbon. The produced rapid occurs diffusion by is the the grain channels diffusion boundaries and of thermal carbon. and nanoparticles mismatches The rapid produced diffusion promote bythe channels the diffusion grain and boundaries of thermal carbon mismatchesand nanoparticlesthe nucleation produced promote of carbides,by the the grain diffusion so thereboundaries ofare carbon a large and and nanoparticlesnumber the nucleation of dispersed promote of carbides, carbides the diffusion so therethroughout areof carbon a large the andstructure.number the nucleation of dispersed of carbides carbides, throughout so there are the a structure. large number of dispersed carbides throughout the structure.
Figure 6.6. The effect of TiCxx--TiBTiB2 nanoparticles on the refinement refinement of austenite. ( a) Nanoparticles in Figuremolten 6steel steel;. The; ( beffect) Nucleation of TiCx -and TiB2 growth nanoparticles of austenite;austenite on the; (c )refinement Nanoparticles of austenite. in austenite.austenite (a). Nanoparticles in molten steel; (b) Nucleation and growth of austenite; (c) Nanoparticles in austenite. FigFigureure7 7 shows shows the the tensile tensile engineering engineering stress-strain stress-strain curves curves and and fracture fracture surfaces surfaces of the of 40Cr the steels 40Cr steelswithFig different withure different7 shows TiCx-TiB TiC the2 nanoparticlex- tensileTiB2 nanoparticle engineering contents. contents. stress It is- notstrain It difficultis curvesnot difficult to and determine fractureto determine from surfaces Figure from of7 a Fig the thature 40Cr the 7a steelsthatyield the strengthwith yield different strength and tensile TiC andx-TiB strength tensile2 nanoparticle strength of the steel ofcontents. the with steel nanoparticles It withis not nanoparticles difficult are greatly to determine are improved. greatly from improved. In Fig additionure 7Ina thatadditionto a largethe yield to increase a large strength inincrease the and yield intensile the strength yield strength strength and of tensile the and steel strength tensile with strength comparednanoparticles compared to the are 40Cr greatlyto the steel, 40Cr improved. the steel, fracture the In additionfracturestrain of strain to the a large steel of the containingincrease steel containing in the 0.018 yield wt.% 0.018 strength TiC wt.x%-TiB andTiC2 nanoparticlesxtensile-TiB2 nanoparticles strength is compared also is improved. also toimproved. the 40Cr The yieldThesteel, yield andthe fractureand tensile strain strengths of the steelof the containing steel with 0.018 0.054 wt. wt.%% TiC nanoparticlesx-TiB2 nanoparticles added are is furtheralso improved. enhanced, The but yield the and tensile strengths of the steel with 0.054 wt.% nanoparticles added are further enhanced, but the
Metals 2018, 8, 707 7 of 11 tensileMetals strengths2018, 8, x FOR of PEER the steel REVIEW with 0.054 wt.% nanoparticles added are further enhanced, but the7 fractureof 11 strain is obviously decreased. From Table2 one can observe that the yield strength, tensile strength, fracture strain is obviously decreased. From Table 2 one can observe that the yield strength, tensile and fracture strain of the 40Cr steel are 1080 MPa, 1120 MPa and 9%, respectively. Compared with strength, and fracture strain of the 40Cr steel are 1080 MPa, 1120 MPa and 9%, respectively. the 40Cr steel, the yield strength (1246 MPa), tensile strength (1469 MPa), and fracture strain (9.4%) Compared with the 40Cr steel, the yield strength (1246 MPa), tensile strength (1469 MPa), and of the steel containing 0.018 wt.% TiCx-TiB2 nanoparticles are increased by 15.4%, 31.2% and 4.4%, fracture strain (9.4%) of the steel containing 0.018 wt.% TiCx-TiB2 nanoparticles are increased by respectively. The yield strength (1415 MPa) and tensile strength (1554 MPa) of the steel with 0.054 wt.% 15.4%, 31.2% and 4.4%, respectively. The yield strength (1415 MPa) and tensile strength (1554 MPa) TiCx-TiB nanoparticles are increased by 31% and 38.8% compared with the 40Cr steel, respectively. of the steel2 with 0.054 wt.% TiCx-TiB2 nanoparticles are increased by 31% and 38.8% compared with Thethe fracture 40Cr steel, surfaces respectively (Figure. The7b–d) fracture also reflect surfaces the (Fig tensileure 7 propertiesb–d) also reflect of the the material tensile toproperties some extent. of Therethe material are some to dimplessome extent. with There uneven are some sizes dimples on the tensilewith uneven fracture sizes of on the the 40Cr tensile steel, fracture which of showsthe a ductile40Cr steel, fracture. which Theshows dimples a ductile on fracture the fracture. The dimples surface ofon thethe steelfracture with surfac 0.018e of wt.% the steel nanoparticles with 0.018 are morewt.% obvious, nanoparticles and the are size more is more obvious, uniform, and the but size it is is also more a ductileuniform, fracture. but it is However,also a ductile there fracture. are fewer dimplesHowever, on thethere fracture are fewer surface dimples of the on steelthe fracture with 0.054 surface wt.% of nanoparticles,the steel with 0.054 and mostwt.% ofnanoparticles the areas have, a rockand ormost sugar-like of the areas morphology, have a rock which or showssugar-like a brittle morphology, intergranular which fracture. shows a Owing brittleto intergranular the increase of carbidesfracture. inside Owing and to the increase refinement of carbides of grain inside size, hardnessand the refinement of steels with of grain nanoparticles size, hardness has of increased, steels aswith Table nanoparticles2 lists. has increased, as Table 2 lists.
FigureFigure 7. 7.The The tensile tensile engineering engineering stress-strainstress-strain curves of of steels steels with with different different TiC TiCx-xTiB-TiB2 nanoparticles2 nanoparticles contentcontent in in (a (),a), and and the the corresponding corresponding fracturefracture surfaces in in ( (bb––dd).).
Table 2. Tensile properties, impact toughness and hardness of the steels with different content of Table 2. Tensile properties, impact toughness and hardness of the steels with different content of TiCx-TiB2 nanoparticles. TiCx-TiB2 nanoparticles. σ σ 0.2σ0.2 σUTSUTS FractureFracture S Straintrain 2 Hardness Samples ak (J/cm 2) Hardness HRC Samples (MPa) (MPa) εf (%) ak (J/cm ) (MPa) (MPa) εf (%) HRC 40Cr steels 1080+10+10 1120++2020 9.0++1.61.6 18.2 ± 0.85 38+1.5+1.5 40Cr steels 1080−70−70 1120−−10 9.0−2.02.0 18.2 ± 0.85 38−−1.01.0 +8 +20 +1.7 +1.1 0.018 wt.% TiCx + TiB2 1246− +8 1469−+20 9.4−+1.7 20.3 ± 1.66 41+1.1− 0.018 wt.% TiCx + TiB2 12466 −6 1469−10 9.4−11 20.3 ± 1.66 41− 0.8 +5 +8 +0.8 ± +0.82.2 0.054 wt.% TiCx + TiB2 1415−2 +5 1554−+835 5.5.6+0.8− 1.3 13.8 0.80 47+2.2−2.1 0.054 wt.% TiCx + TiB2 1415−2 1554−35 5.6−1.3 13.8 ± 0.80 47−2.1
FigureFigure8 reflects8 reflects the the work-hardening work-hardening abilityability ofof thethe steelssteels with different TiC TiCx-xTiB-TiB2 nanoparticles2 nanoparticles contents.contents. Combined Combined with with thethe analysisanalysis of Figure 77,, it is determined that that the the yield yield strength strength of ofthe the 40Cr 40Cr steelsteel is is low, low, the the yielding yielding occurs occurs veryvery quickly,quickly, and the the work work-hardening-hardening rate rate rapidly rapidly decreases decreases after after yielding.yielding. Compared Compared with with the the 40Cr 40Cr steel,steel, thethe steel with 0.018 0.018 wt.% wt.% TiC TiCx-xTiB-TiB2 nanoparticles2 nanoparticles has has a higher a higher yieldyield strength, strength, and and a a higher higher work-hardeningwork-hardening rate rate with with the the same same plastic plastic deformation deformation after after yielding. yielding. TheThe steel steel with with 0.054 0.054 wt.% wt.% TiCTiCxx-TiB-TiB22 nanoparticlesnanoparticles has has the the highest highest yield yield strength, strength, and and the the hig highesthest work-hardening rate, but its work-hardening rate is less than the steel with 0.018 wt.% nanoparticles
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MetalsMetals 2018 2018, 8,, 8x, xFOR FOR PEER PEER REVIEW REVIEW 8 of8 11of 11 work-hardening rate, but its work-hardening rate is less than the steel with 0.018 wt.% nanoparticles whenwhenwhen thethe the truetrue true strainstrain strain atat at a a valuea value value exceeding exceedingexceeding 4.5% 4.5% as asas shown shownshown inin in FigureFigure Figure8 8,, and8, and and there there there isis noisno no bufferingbuffering buffering stagestage stage of theofof the work-hardening the work work-hardening-hardening rate rate rate similar similar similar to that to that of theof thth steelee steelsteel with with with 0.018 0.018 0.018 wt.% wt.% wt.% TiC TiC TiCx-TiBx-TiBx-TiB2 2nanoparticles. nanoparticles.2 nanoparticles.
Figure 8. The corresponding work-hardening rate-true strain curves that were obtained from the Figure 8. The corresponding work-hardening rate-true strain curves that were obtained from the Figuretensile 8. curve. The corresponding work-hardening rate-true strain curves that were obtained from the tensiletensile curve.curve. Figure 9 shows the impact properties and impact fracture morphologies of the steels with Figure9 shows the impact properties and impact fracture morphologies of the steels with different differentFigure contents 9 shows of TiC thex - impactTiB2 nanoparticles properties at and room impact temperature. fracture The morphologies impact toughness of the of steel steels with with contents of TiCx-TiB2 nanoparticles at room temperature. The impact toughness of steel with 0.018 wt.% different0.018 wt.% contents TiCx -ofTiB TiC2 nanoparticlesx-TiB2 nanoparticles is 11.5% at room higher temperature. than that of The the impact40Cr steel toughness, but the of impactsteel with TiCx-TiB2 nanoparticles is 11.5% higher than that of the 40Cr steel, but the impact toughness of the 0.018toughness wt.% of TiC thex-TiB steel2 nanoparticles with 0.054 wt.% is nanoparticles 11.5% higher is thanreduced. that Some of the of the40Cr reasons steel, for but this the can impact be steel with 0.054 wt.% nanoparticles is reduced. Some of the reasons for this can be seen from the toughnessseen from of the the impact steel withfracture 0.054 surfaces wt.% nanoparticlesin Figure 9. In isthe red impactuced. fracture Some of surfaces the reasons of the for 40Cr this steel can, be impact fracture surfaces in Figure9. In the impact fracture surfaces of the 40Cr steel, the quasi-cleavage seenthe from quasi the-cleavage impact transgranular fracture surfaces fracture in Figure is dominant, 9. In the and impact there fracture is a small surfaces intergranular of the fracture40Cr steel , transgranular fracture is dominant, and there is a small intergranular fracture section. Except for thesection. quasi -Exccleavageept for a transgranular small amount fractureof cleavage is fracture, dominant, there and are there many is dimples a small on intergranular the impact fracture fracture a small amount of cleavage fracture, there are many dimples on the impact fracture of the steel with section.of the Excsteelept with for 0.018a small wt.% amount TiCx -ofTiB cleavage2 nanoparticles, fracture, indicating there are thatmany the dimples impact ontoughness the impact is better fracture 0.018 wt.% TiCx-TiB2 nanoparticles, indicating that the impact toughness isx better2 than that of the ofthan the steel that ofwith the 0.018 40Cr wt.% steel .TiC Thex- TiB fracture2 nanoparticles, of the steel indicating with 0.05 4that wt.% the TiC impact-TiB toughnessnanoparticles is better is 40Crdominated steel. The by fracturebrittle intergranular of the steel fracture with 0.054 and wt.%some TiCcracks,x-TiB which2 nanoparticles are unfavorable is dominated for the toughness by brittle than that of the 40Cr steel. The fracture of the steel with 0.054 wt.% TiCx-TiB2 nanoparticles is intergranular fracture and some cracks, which are unfavorable for the toughness of the material. dominatedof the material. by brittle intergranular fracture and some cracks, which are unfavorable for the toughness of the material.
FigureFigure 9. 9.Impact Impact toughness toughness of of steels steels with different contents contents of of Ti TiCCx-xTi-TiBB2 2nanoparticlesnanoparticles and and the the correspondingcorresponding impact impact fracture fracture surfaces. surfaces.
Figure 9. Impact toughness of steels with different contents of TiCx-TiB2 nanoparticles and the
corresponding impact fracture surfaces.
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According to the Hall-Petch formula, a smaller average grain size can result in a larger yield strength [29]. The difference in grain orientation on both sides of the grain boundary increases the slip resistance of the dislocation near the grain boundary and causes dislocations to accumulate at the grain boundary. According to the Orowan mechanism, a large number of dispersed carbides can hinder the movement of dislocations and cause an entanglement of the dislocations [30–33]. The dislocation loop will be left behind when dislocations pass through the carbides and nanoparticles, which produces a reaction force to the dislocation source. All of the above factors can improve the tensile strength of the steels. In a certain volume, the smaller grains size means a greater number of grains. The deformation is more uniform because it can be dispersed in more grains with the same amount of plastic deformation. On the other hand, when there is stress concentration in the material, voids tend to form in the weaker part. With the strain increasing, voids grow up, and adjacent voids gather together to form dimples. However, materials with uniform internal structure such as steels with 0.018 wt.% nanoparticles have more uniform distribution of vacancies when strain occurs, so the dimples on the fracture surface are more uniform and the size of dimples is larger before fracture. On the contrary, materials with less uniform internal structure, such as the matrix, tend to concentrate a large number of voids to form crack sources when strain occurs. Therefore, the ductility of steels with 0.018 wt.% nanoparticles is better than that of the matrix. Materials can withstand a large amount of deformation before breaking when fewer cracks caused by a stress concentration occur. It is difficult for many nanoparticles to disperse uniformly in the steel. In the case of a large deformation, the second phase of agglomeration often becomes the source of cracks. This is why the strength and plasticity of the steel with 0.018 wt.% TiCx-TiB2 nanoparticles are simultaneously improved, while the strength of the steel with 0.054 wt.% TiCx-TiB2 nanoparticles is greatly increased but the fracture strain drops. The smaller grain size means more grain boundary area and more zigzag grain boundary, which is not conducive to the crack propagation. Therefore, the impact performance of the steel with 0.018 wt.% TiCx-TiB2 nanoparticles is improved. For the steels with 0.054 wt.% TiCx-TiB2 nanoparticles, the enhancement caused by grain refinement does not offset the damage of the poorly dispersed particles to impact toughness.
4. Conclusions
Trace biphase TiCx-TiB2 nanoparticle-reinforced steels are innovatively produced by adding nanosized TiCx-TiB2/Al master alloy during the casting process. The pearlitic-ferrite structure of the as-cast steels and the bainite structure of the steels after heat treatment have been refined and the dispersed nanosized carbides are increased. The average grain size is refined from 9.12 µm for the 40Cr steel to 3.59 µm for the steel with 0.018 wt.% TiCx-TiB2 nanoparticles. Microstructural refinement benefits from the promotion of nucleation and inhibition of dendrite growth by TiCx-TiB2 nanoparticles. 2 Compared with the 40Cr steel (σ0.2 = 1080 MPa, σUTS = 1120 MPa, εf = 9%, ak = 18.2 J/cm , HRC = 38), the yield strength (1246 MPa), tensile strength (1469 MPa), fracture strain (9.4%), impact toughness 2 (20.3 J/cm ), and hardness of the steel containing 0.018 wt.% TiCx-TiB2 nanoparticles are increased by 15.4%, 31.2%, 4.4%, 11.5%, and 7.9%, respectively. The yield strength (1415 MPa) and tensile strength (1554 MPa) of the steel with 0.054 wt.% TiCx-TiB2 nanoparticles are increased by 31% and 38.8% compared with the 40Cr steel, respectively. The improved tensile properties may be attributed to fine grain strengthening and the pinning effect of nanosized carbides on dislocations and grain boundaries. The results indicate that the trace TiCx-TiB2 nanoparticles are successfully introduced into the cast steel through the method of adding nanosized TiCx-TiB2/Al master alloy during the casting process and that trace (0.018 wt.%) TiCx-TiB2 nanoparticles can simultaneously improve the strength, toughness, as well as ductility of the cast steel.
Author Contributions: Conceptualization, C.-L.L., F.Q. and Q.-C.J.; Data curation, C.-L.L. and F.Q.; Formal analysis, C.-L.L., F.Q., F.C., X.-M.Z., R.G., H.-Y.Y. and Q.-L.Z.; Investigation, C.-L.L., F.Q., F.C., X.-M.Z. and R.G.; Supervision, C.-L.L. and F.Q.; Writing—review & editing, C.-L.L., F.Q. and Q.-C.J. Funding: This research received no external funding. Metals 2018, 8, 707 10 of 11
Acknowledgments: This work is supported by the National Natural Science Foundation of China (NNSFC, No. 51571101 and No. 51771081), the Source Innovation Plan of Qingdao City, China (No. 18-2-2-1-jch), the Science and Technology Development Program of Jilin Province, China (20170101178JC) and the Project 985-High Properties Materials of Jilin University. Conflicts of Interest: The authors declare no conflict of interest.
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