添加碳化钛对超细Ti(C,N)-Ni金属陶瓷显微结构和力学性能的影响

用传统粉末冶金法制备了添加碳化钛(TiC)的Ti(C,N)基金属陶瓷.为了得到超细晶粒的显微结构,主要硬质相[Ti(C,N),TiC和TiN]的初始粉末粒度为纳米、亚微米级.通过扫描电子显微镜观察,发现了一种新的白芯/灰壳结构,揭示了其形成机制.此外,通过能谱仪分析,获得了各相的化学成分.用X射线衍射仪并通过计算得出了各相的点阵参数.对室温下该材料的力学性能进行了测试,并尝试把它们与金属陶瓷的原始成分和显微结构的特点联系起来.     

碳化钛粒径对碳氮化钛基金属陶瓷微观结构和力学性能的影响

用粉末冶金真空烧结法制备了超细晶粒碳氮化钛[Ti(C,N)]基金属陶瓷.研究了原始粉末粒径对Ti(C,N)基金属陶瓷微观结构和力学性能的影响.结果表明:在化学成分相同的条件下,晶粒细化使材料的Vickers硬度和抗弯强度上升,但断裂韧性有所下降.在超细晶粒Ti(C,N)基金属陶瓷微观组织中出现了一种新型的白芯/灰壳结构和一种特殊化合物(Ni2Mo2.5W1.3)Cx.初步研究表明:由于原始粉末粒径微小,促进了扩散反应因而生成了这种芯/壳结构.芯/壳结构有利于提高材料的抗弯强度和断裂韧性.(Ni2Mo2.5W1.3)Cx有利于提高材料的Vickers硬度,但是降低了Ti(C,N)基金属陶瓷的抗弯强度和断裂韧性.     

Microstructure evolution and characteristic of Ti(C,N)-based cermets prepared by in situ carbothermal reduction in TiO2

Ti(C,N)-based cermets were prepared by in situ carbothermal reduction in TiO₂ and subsequent liquid phase sintering under vacuum. The prepared cermets were examined using XRD, SEM, TEM, and EDX. During solid-state sintering, fine TiC particles were formed through the carbothermal reduction in TiO₂. A great number of (Ti,W,Mo)C complete solid solutions containing more W and Mo subsequently formed through the counter diffusion of the fine TiC and carbides. The majority of the coarse TiN particles in the raw powders remained undissolved. During liquid phase sintering, Ti-based carbonitride complex solid solutions with less W or Mo precipitated on the coarse TiN particles and fine (Ti,W,Mo)C particles, resulting in black core/gray rim structures and white core/gray rim structures, respectively. Moreover, small amounts of Ti-based carbonitride complex solid solutions precipitated directly from the liquid binder phase in some areas enriched in W and Mo during the cooling stage after sintering, resulting in coreless grains. Ultimately, after being sintered at 1400°C for 1 hour, the present cermets were characterized with white core/gray rim grains, black core/gray rim grains and a few gray grains. In addition, the interfaces between the black core/gray rim grains and binder phase were atomically smooth, exhibiting a orientation relationship with a perfect coherency state.     

Influence of molybdenum addition on the microstructure and mechanical properties of TiC-based cermets with nano-TiN modification

Effect of Mo addition on the microstructure and mechanical properties of TiC–TiN(nm)–WC–Co–Ni–C system cermets was studied in the work. Specimens were fabricated by conventional powder metallurgy techniques. The microstructure was investigated using transmission electron microscope (TEM) and the scanning electron microscope (SEM). Chemical compositions of different phases such as ceramic phase with core/rim structure [the core being TiC and rim being (Ti,W,Mo)(C,N)] and metallic phase were analyzed quantitatively by EDX. Mechanical properties such as flexural strength, fracture toughness and hardness were also measured. Results show that flexural strength and fracture toughness have a trend to decline with increasing Mo addition, but the change of hardness is not apparent with the increase of Mo addition. Results also reveal that finer microstructure and thicker rim phase will be obtained with the increase of Mo addition. The optimal addition of Mo can be estimated to be 4 wt.% with respect to TiC–10TiN(nm)–15WC–5Co–Mo–5Ni–1C system cermets. Fracture micrographs show that main failure mode of the cermets is a mixed one, i.e., trans-granular and inter-granular fractures both exist.     

Effect of WC content on the microstructures and corrosion behavior of Ti(C,N)-based cermets

The influence of WC content on the microstructure and corrosion behavior of Ti(C, N)-based cermets in 2 mol/L nitric acid solution was studied in this paper. There exists typical core/rim structure in the cermets. The cores appear black or white, and the rim is divided into white inner rim and grey outer rim. The undissolved Ti(C, N) particles normally appear as black cores, while the white core, inner rim and outer rim are (Ti, W, Mo) (C, N) solid solution formed at different sintering stages. The inner rim and white core appear brighter atomic contrast than the outer rim and black core, which is attributed to their higher W and Mo content. The thickness of the inner rim increases with WC addition, but the grain size of core/rim phase becomes finer. Meanwhile, the amount of white cores increases and that of black cores decreases. WC is more easily oxidized and dissolved in the nitric acid solution, compared with Ti(C, N). Therefore, the degradation of inner rim phase and the white core becomes more considerable with the increase of WC content. Consequently, the corrosion rate of cermets increases and the corrosion resistance of Ti(C, N)-based cermets is deteriorated with the increase of WC content.     

Effect of Cr3C2 on Valence-Electron Structure and Plasticity of Rim Phase in Ti(C,N)-Based Cermets

Based on the empirical electron theory of solids and molecules, the valence-electron structure (VES) of the rim phase in Ti(C,N)-based cermets was calculated, and the relationship between the VES and plasticity was determined. The results indicated that the plasticity of the rim phase in a Ti(C,N)-based cermet could be defined using the sum of the n _a values for the covalent bonds, and that chromium dissolution in the rim phase improved the plasticity of the rim phase. Moreover, a series of experiments showed that adding Cr₂C₃ to a typical Ti(C,N)-based cermet strengthened the interface. Based on those results, a Ti(C,N)-based cermet with added Cr₃C₂ was manufactured; the new cermet had more than twice the transverse rupture strength of a typical cermet.     

Microstructure and mechanical properties of Ti(C,N)-based cermets fabricated by in situ carbothermal reduction of TiO2 and subsequent liquid phase sintering

Ti(C,N)-based cermets were prepared by in situ carbothermal reduction of TiO₂ and subsequent liquid phase sintering in one single process in vacuum. The densification behavior, phase transformation, and microstructure evolution of the cermets were investigated by DSC, XRD, SEM, and EDX. The results showed that the carbothermal reduction of TiO₂ was completed below 1250 °C, and Ti(C,N)-based cermets with refined grains were obtained after sintered at 1400 °C for 1 h by this method. The hard phase of the cermets mainly exhibited white core/gray rim structure, in great contrast to the typical black core/gray rim structure of hard phase in traditional cermets. Ti(C,N)-based cermets prepared by this novel method showed excellent mechanical properties with a transverse rupture strength of 2516±55 MPa, a Rockwell hardness of 88.6±0.1 HRA, and a fracture toughness of 18.4±0.7 MPa m^1/2, respectively.     

Effect of powder size on microstructure and mechanical properties of Ti(C,N) based cermets

Abstract

Effect of the particle size of TiC and TiN on the microstructure and mechanical properties of Ti(C,N) based cermets has been evaluated. Ti(C,N)–WC–Co cermets made from four groups of mixed raw powders of different sizes were manufactured by vacuum sintering. The microstructure and composition were studied using X-ray diffraction (XRD), scanning electron microscopy (SEM) and energy dispersive X-ray spectrum (EDX). The result shows that the four samples have the typical microstructures of 'black core/grey rim'. The mechanical properties of the cermet manufactured from submicron TiC and nano TiN are the best among the four samples.     

WC含量对Ti(C,N)基金属陶瓷组织和性能的影响研究

用传统的粉末冶金方法制备了不同WC含量的超细Ti(C,N)基金属陶瓷试样,运用SEM,EDX等手段对材料的显微组织进行了表征分析,并用这些显微组织的特征和差异解释了材料宏观力学性能的特点.结果表明,金属陶瓷的组织为典型的两相结构特征,其中陶瓷相的芯、壳结构(core/timstructure)与溶解析出机制有关.少量WC的加入能提高材料的力学性能.断口SEM分析表明:断裂机理为典型的混合型断裂(穿晶断裂和沿晶断裂),金属相存在着明显撕裂的痕迹.     

Effect of Mo addition mode on the microstructure and mechanical properties of TiC–high Mn steel cermets

In TiC- and Ti(C,N)-based cermets, the wettability of the ceramic phase with the metallic binder is commonly increased through supplementation with Mo in the form of pure Mo powder or Mo₂C. Herein, TiC–high Mn steel cermets were fabricated by conventional powder metallurgy techniques using Fe–Mo pre-alloyed powders as binders to guarantee uniform Mo distribution, and the cermet preparation process was optimized and investigated in detail. The microstructures of the thus obtained cermets were observed by scanning electron microscopy and compared to those of a Mo-free cermet and a cermet prepared using pure Mo powder. The grain size of Fe–Mo powder cermets exceeded that of the Mo-free cermet but was much smaller and more homogeneous than that of the Mo powder cermet. For Fe–Mo powder cermets, angular and tetragonal TiC particles were observed at Mo contents of <1.2 wt%, while round shapes became dominant at higher Mo contents. The hardness of Fe–Mo powder cermets increased with increasing Mo content, as did transverse rupture strength, which was maximal (2264 MPa) at a Mo content of 2.4 wt%, while impact toughness was maximal (11.2 J/cm²) at a Mo content of 1.2 wt%. The above values exceeded those reported for similar conventional cermets, and the use of Fe–Mo pre-alloyed powder as a metallic binder was therefore concluded to be an attractive strategy of increasing the strength and toughness of TiC–high Mn steel cermets.     

Strengthened Ti(C,N)-based cermets using high-energy ball-milled NiTiC binders: Microstructure and mechanical properties

High-energy ball-milled NiTiC powders were used for preparing Ti(C, N)-based cermets. Effect of NiTiC content on the morphology, composition, interface structure and mechanical properties of cermets were investigated. NiTiC binders promoted the formation of inner rims on Ti(C, N) cores and hindered their coalescence, leading to well-distributed microstructure. Binder had little effect on the composition of rims, but greatly affected the interface structure of core-rim and rim-binder. Complete inner rim could decrease the lattice mismatch between outer rim and core, forming highly coherent interface. With increasing the Ti-C in Ni, the rim-binder boundaries evolved from semi-coherent to coherent interface, due to the decreased lattice mismatch. Small difference in Vickers hardness of cermets was found, with the values ranging from 1622 to 1684 N/mm². Bending strength of cermets increased from 1330 to 2073?MPa, with the Ti-C content from 0 to 20?wt%. Further increasing the Ti-C could lead to thick rims, resulting in decreased strength and toughness. This work showed us a way to strengthen the Ti(C, N)-based cermets via modifying the interface structure.     

Effect of short carbon fibre concentration on microstructure and mechanical properties of TiCN-based cermets

Short carbon fibre (C_f) reinforced TiCN-based cermets (C_f/TiCN composites) were produced by powder metallurgy method with pressureless sintering technology. The phase evolution, microstructure and fracture morphology of C_f/TiCN composites were investigated. The results showed that TiC, TiN, WC, Cr₃C₂ and Mo phases disappeared gradually and diffused into core and rim phases by dissolution–reprecipitation process, finally formed new hard TiCN core phases and complex compound (Cr, W, Mo, Ti)(CN) rim phases, with the sintering temperature increasing. The added C_f did not change the ‘core–rim’ microstructure but improved the mechanical properties of TiCN-based cermets. The C_f/TiCN composite containing 3?wt-% C_f achieved the best comprehensive mechanical properties, with fracture toughness and bending strength increasing by about 14.4% and 30.8%, respectively, when compared with the composite without C_f. Toughening and strengthening mechanisms of C_f/TiCN composite were concluded as crack deflection and branch, as well as the pull-out, fracture and bridging of carbon fibres.     

Evolution of microstructure and interfacial characteristics of complete solid-solution Ti(C,N)-based cermets fabricated by mechanical activation and subsequent in situ carbothermal reduction

Complete solid-solution Ti(C,N)-based cermet, with no typical core-rim structure, was synthesized through mechanical activation and subsequent in situ carbothermal reduction method. XRD, SEM, TEM, and C/N analysis were used to investigate the microstructure, phase transformation, and the interfacial characteristics of the present cermets. During solid-state sintering, the (Ti,Mo)C/(Ti,Mo)(C,N) phases formed through the transformation of Mo-based solid solution which generated by mechanical activation. Then, the formed (Ti,Mo)C/(Ti,Mo)(C,N) continuously dissolved into the nickel-based binder above 1100 °C. It was found that in the subsequent stage of liquid sintering, the mechanical activation and also the presence of extremely fine TiC/Ti(C,N) particles accelerated the Mo diffusion into the hard phase, resulting in a large quantity of (Ti,Mo)(C,N) solid solutions formed in the nickel-based binder. Finally, complete (Ti,Mo)(C,N) solid-solution phase was obtained via dissolution and re-precipitation. The higher toughness and transverse rupture strength (TRS) of the synthesized new cermet, as compared with traditional cermets, were mainly caused by the increased crack deflection and transgranular fracture of the novel cermets. Moreover, the interface among the Ni-based binder phase and complete solid solution hard phase exhibited a semi-coherency state with high-density dislocations, which also significantly improved the TRS and toughness of the synthesized cermets.     

Morphology evolution of TiC-based cermets via different sintering schedules

Solid state sintering, liquid phase and cooling stages play different roles in determining the final morphology and composition of cermets, especially the well-known core-rim structure. In this work, TiC-(5–25 wt%)WC-11Mo₂C-18(Ni-Co) cermets were prepared and sintered by different sintering schedules. Morphology evolution and rim phase composition during sintering from 1250 °C to 1600 °C were investigated. Effects of sintering stages on the final morphology of cermets were also studied. It was shown that submicron (Ti, W, Mo)C grains tend to precipitate in binder during the cooling for cermets with high WC content. After the formation of outer rims during liquid sintering stage, interface reaction began to take effect between the rims and core. Coreless (Ti_0.76, W_0.13, Mo_0.11)C ceramic grains would be formed under high temperature (1600 °C) for TiC cermets with 25% WC. Long time sintering at solid state favored the formation of black core-thick inner rim and bright core-grey rim phases, while cooling near the melting point could result in submicron bright particles. This study provided not only a better view of the formation of rim-core structure but also an easier way to control the final morphology of cermets via reasonable changing the sintering cycle.     

Effect of WC-Co granules on mechanical properties and microstructure of Ti(C,N)-based cermets

The effect of WC-Co granules addition on the microstructure and mechanical properties of TiC(nm)-TiN(nm)-Co-Mo-C cermets (shown as Ti(C,N)-based cermets in other part of the paper) was studied in this paper. The results show that the WC-Co granules distribute homogeneously in the matrix of Ti(C,N) based cermets. There was a transitional layer containing intermetallic compound between the WC-Co granules and the matrix of Ti(C,N) based cermets. Transverse rupture strength(TRS) and fracture toughness increase with the increase of WC-Co granules, reach a peak value at 15 vol% addition in comparing with that of without WC-Co granules addition. The toughening mechanisms were crack deflection, branching and trapping. However, when the content of WC-Co granules was higher than 15 vol%, the excessive content of WC-Co granules leaded to voids in the cermets, which decreased the mechanical properties of the cermets.     

Cr3C2和VC对Ti(C,N)基金属陶瓷中环形相的价电子结构和性能的影响

根据固体与分子经验电子理论，计算了Ti(C，N)基金属陶瓷中界面环形相的价电子结构，讨论了其价电子结构与塑性间的关系。当材料晶体结构相同时，Σna可用来比较其塑性的相对高低。Cr在环形相(Ti，Mo)(C，N)中的固溶，可使其塑性增强，V在环形相中的固溶将使其塑性变差。在计算的基础上进行实验，实验结果表明：Cr3C2的适量加入确实有利于提高金属陶瓷的强度，最终所制备出金属陶瓷的强度比典型成分体系材料的提高了1倍以上；尽管VC的加入能使材料的晶粒得到有效地细化，但它使环形相塑性降低，使金属陶瓷的抗弯强度略有增加。     

Fabrication of Ti(C,N)‐based cermets by in situ carbothermal reduction of MoO3 and subsequent liquid sintering

Ti(C,N)‐based cermets were fabricated by in situ carbothermal reduction of MoO₃ and subsequent liquid sintering in a single heating process. The densification behavior, phase formation, and microstructure evolution of the cermets were characterized by DSC, XRD, SEM, and TEM. The results showed that near‐fully dense Ti(C,N)‐based cermets with fine carbonitride grains could be obtained by the above‐mentioned method. The carbonitride grains of the cermets still exhibited typical core/rim structures and evenly distributed in the binder phase, but the rim phase was more complete and thinner compared with traditional cermets. In addition, the interfaces between the ceramic phase and binder phase of the cermets were atomically smooth, having the orientation relationship of ()_R//(110)_B with a perfect coherency state. The prepared Ti(C,N)‐based cermets produced with MoO₃ showed excellent comprehensive mechanical properties having a transverse rupture strength of 2461±62 MPa, a Rockwell hardness of 88.0±0.1 HRA, and a fracture toughness of 22.3±0.4 MPa·m^1/2, respectively.     

Microstructural evolution and phase transition mechanism of Ti(C,N)-based cermets during vacuum sintering process

Ti(C,N)-based cermets were fabricated using section vacuum sintering. The density and shrinkage ratio of sintered samples were recorded and analyzed. Phase transition, microstructure and interface behaviors of cermets were examined by transmission electron microscope, X-ray diffractometer and scanning electron microscope. As the sintering temperature rising, the number of pores in cermets was decreased and the density of samples was increased. Moreover, with the progress of sintering, atoms of various elements were constantly diffusing and gathering, affecting the formation of different microstructure, and the angle and intensity of each XRD peak were also constantly changing slightly, or disappearing at a certain temperature. These relevant results indicated that the proportion of heavy element to Ti in the hard phase would affect its lattice parameter. After the appearance of liquid phase, the densification mechanism changed from surface diffusion to grain boundary diffusion, thus accelerating the densification. With the proceeding of sintering process, various microstructures were formed such as core - rim/rimless structure. Finally, the process of interface change in core rimless phase transition was discussed. This rim phase, formed in solid state sintering process while failing to develop into core-rim structure after liquid sintering, was formed by some heavy elements short-range diffusion into core phase. The orientation different and the misfit between this core and rim were relatively large. While after liquid sintering, the semi-coherency or even coherency state at the interface of core-rimless and binder was formed.     

Ti(C,N)-based cermets with fine grains and uniformly dispersed binders: Effect of the Ni–Co based binders

Metallic binder is a key factor affecting the microstructure and mechanical properties of Ti(C,N)-based cermets. To optimize the overall performances, cermets with various weight ratios of Ni/(Co + Ni) ranging from 0 to 1 were fabricated by gas pressure sintering. Microstructure, phase formation, interface structure and related mechanical properties of the sintered cermets were investigated. With the increase of the Ni/(Co + Ni) ratios, the black cores became smaller and grains of Ti(C,N) dispersed uniformly. Compared to the pure Ni or Co, Ni–Co binders accelerated the formation of rim phases, and avoided the nonuniform dispersed binder pools. When the ratio was 0.5, the cermets showed fine grains, uniformly dispersed binders and small lattice misfit of the core-rim interface, exhibiting the optimal mechanical properties, i.e. satisfactory Vickers hardness of 1670 (HV30) Kgf/mm², bending strength of 1970 MPa and Fracture toughness of 8.94 MPa m^0.5. This work sheds light on constructing the relationship between the microstructure, mechanical performance of Ti(C,N)-based cermets and the Ni/Co-based binders.     

Ti(C,N)基金属陶瓷中陶瓷相芯/壳组织的观察与分析

用ＸＲＤ，ＳＥＭ，ＴＥＭ和ＨＲＥＭ观察、分析和方法研究了Ｔｉ（Ｃ，Ｎ）基金属陶瓷中陶瓷相的芯、壳组织。结果表明，在ＳＥＭ，ＴＦＭ观察中，芯、壳之间存在相界面，而ＨＲＥＭ观察显示陶瓷相的芯／壳组织具有连续相同的点阵结构。