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.     

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.     

Effects of TiC/TiN addition on the microstructure and mechanical properties of ultra-fine grade Ti (C, N)–Ni cermets

Two series of Ti (C, N)-based cermets, one with TiC addition and the other with TiN addition, were fabricated by conventional powder metallurgy technique. The initial powder particle size of the main hard phase components (Ti (C, N), TiC and TiN) was nano/submicron-sized, in order to achieve an ultra-fine grade final microstructure. The TiC and TiN addition can improve the mechanical properties of Ti (C, N)-based cermets to some degree. Ultra-fine grade Ti (C, N)-based cermets present a typical core/rim (black core and grayish rim) as well as a new kind of bright core and grayish rim structure. The average metallic constituent of this bright core is determined to be 62 at% Ti, 25 at% Mo, and 13 at% W by SEM–EDX. The bright core structure is believed to be formed during the solid state sintering stage, as extremely small Ti (C, N)/TiC/TiN particles are completely consumed by surrounding large WC and Mo₂C particles. Low carbon activity in the binder phase will result in the formation (Ni₂Mo₂W)C_x intermetallic phase, and the presence of this phase plays a very important role in determining the mechanical properties of TiN addition cermets.     

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.     

Fabrication and properties of Ti(C,N) based cermets reinforced by nano-CBN particles

Titanium carbontride (Ti(C,N)) based cermets with and without nano-cubic boron nitride (CBN) particles were prepared by microwave sintering in argon and nitrogen environment, respectively. Two kinds of core–rim microstructure, black core–grey rim and white core–grey rim, are shown in the cermets by scanning electron microscopy (SEM) in combination with energy dispersive X-ray analysis (EDX). It is found that, for the cermet with 1.5% nano-CBN particles sintered at 1500 °C for 30 min in argon, its transverse rupture strength (TRS) and hardness are improved to about 25.9% and 1.4%, respectively. The SEM analysis shows that the inhibition effect of nano-CBN particles on the dissolution of Ti(C,N) is weakened with the increase of content of nano-CBN particles. Moreover, for the cermet sintered in argon reinforced by 1.5% nano-CBN particles, more fine black core–grey rims are found in the microstructure compared to the others. For the material sintered in nitrogen, its microstructure accompanied with many white core–grey rims in number and big black core and thin outer rim in size, results in high hardness and low TRS.     

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 Nano-Size Powders on the Microstructure of Ti(C,N)-xWC-Ni Cermets

Microstructural and compositional analyses of Ti(C,N)– x WC–20Ni cermets were performed to understand the dissolution behavior of nano-size Ti(C,N) and WC, compared with an ultra-fine and a micro-sized system. The WC content was varied from 10 wt% to 70 wt%. The rapid dissolution and reprecipitation of nano-Ti(C,N) powders induced the formation of a coreless microstructure. With an increase in the amount of WC, the concentration of W in the rims increased but was low compared with the ultra-fine or micrometer system. An inversion in W concentration between the inner rim and the outer rim occurred at ∼40 wt% added WC. That is, the outer rim was richer in W than the inner rim. Powder size strongly affected the dissolution and precipitation of particles and was a major determinant of the final microstructure and compositions of the rims. Furthermore, a significant improvement in toughness in the case of simple nano systems was found, compared with that of the corresponding micrometer and ultra-fine systems ( K _{I C} 10–12 vs. 6–10 MPa·m^1/2). The coreless microstructure and the high fraction of rim phase in the nano system are responsible for this difference.     

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 WC or NbC addition on lattice parameter of surrounding structure in Ti(C0.7N0.3)–Ni cermets investigated by TEM/CBED

The changes in the lattice parameters of the solid solutions in the Ti(C_0.7N_0.3)–WC–Ni and Ti(C_0.7N_0.3)–NbC–Ni systems were first shown quantitatively by the CBED (Convergent Beam Electron Diffraction) technique together with TEM (Transmission Electron Microscopy) microstructure characterization. The extent of the changes in the lattice parameters between core and rim differs in the case of WC and NbC additions. No change in the lattice parameters is observed in the Ti(C_0.7N_0.3)–WC–Ni cermets, in contrast to the Ti(C,N)–NbC–Ni cermets where significant changes in the lattice parameters are observed. The difference in the parameters is correlated with the core/rim structure, which disappears in the Ti(C,N)–NbC–Ni cermets when a large amount of NbC is added, and is discussed based on thermodynamic arguments. Large strain in the core and rim structure, especially near the core/rim interface, is also observed from the HOLZ (High Order Laue Zone) line splitting.     

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.     

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.     

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.     

Submicron Ti(C,N)-based cermets with improved microstructure using high-energy milled and subsequent heat-treated ultrafine Ti(C,N) powders

Ti(C,N)-based cermets with ultrafine or submicron black core-rim grains are attracting candidates for high-quality tools and dies, due to their high hardness and strength. However, high chemical activity of ultrafine Ti(C,N) powders lead to the increased instability and difficulty to control the sintering process, since the denitrification and interface diffusion are accelerated during the solid-state reaction. Based on this, owing to the unrealized commercial ultrafine-grade powders, ultrafine Ti(C,N) powders with an average grain size around 150 nm, low oxygen content and few dislocations are fabricated via the high-energy ball milling and subsequent heat treatment of commercial micron Ti(C,N) powders. Related morphology evolution, microstructure and composition of the ultrafine Ti(C,N) powders are investigated. Dense submicron Ti(C,N)-based cermets with grain size of 0.62 μm and uniform core-rim phases are successfully prepared by using the as-fabricated ultrafine Ti(C,N) powders. Compared to cermets via the conventional high-energy milling route, submicron Ti(C,N)-based cermets exhibit higher hardness of 1750 ± 40 N/mm², bending strength of 1960 ± 135 MPa, and satisfactory fracture toughness of 9.2 MPa m^1/2, owing to smaller grain size, uniform microstructure and partial black core-rim grains.     

Effect of WC content on microstructure and mechanical properties of Ni3Al-bonded cermets

The effect of WC content on microstructure and mechanical properties of the TiC–Ni₃Al system cermets was investigated. Ni₃Al-bonded cermets showed a core–rim structure with carbide particle coupled with rim embedded in Ni₃Al binder. With WC content increasing, TiC grains were refined and the white rim became complete and got thicker gradually. Interface between core and rim showed a completely coherent relationship. The rim enriched in W constituted an ideal coherence between hard phase and Ni₃Al binder phase. With WC content increasing, the densification of cermets was enhanced, and hardness and TRS were increased firstly and then reduced, reaching peak values 90.9 HRA (HV₃₀ 15 GPa) and 1629 MPa, respectively in cermet N5 (25 wt% WC). Similarly, fracture toughness got a peak value (11.6 MPa m^1/2), at the composition with 20 wt% WC.     

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。     

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 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.     

Formation of Core/Rim Structures in Ti(C,N)-WC-Ni Cermets via a Dissolution and Precipitation Process

The core/rim structures of compounds in the Ti(C,N)- x WC-20Ni system have been investigated to determine the effect of WC and nitrogen content on the microstructure of the system. In addition, the relative dissolution rate of WC to Ti(C,N) in the system was studied by analyzing the rim compositions. Variations in the WC content had a much-lesser influence, as an additive, on the general microstructure than that of other carbides that have been used in previous studies. However, the nitrogen content in Ti(C_{1− X} N _X ) had a significant effect on its microstructure. The composition of the rim structure was determined by the ratio of solutes that were dissolved in the liquid binder, under the given processing conditions. The dissolution rate of WC was ∼2 and 5 times faster than that of Ti(C,N) in the system at temperatures of 1300° and 1510°C, respectively. The results have been interpreted in terms of phase stability and precipitation phenomena.     

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.     

Experimental and thermodynamic investigation of gradient zone formation for Ti(C,N)-based cermets sintered in nitrogen atmosphere

The influence of N₂ atmosphere on the microstructure of gradient zone in Ti(C,N)-Mo₂C-Ni cermet was systematically investigated by the coupling analysis of experimental characterization and thermodynamic calculation. Under the guidance of calculated carbon window, the composition of Ti(C,N)-Mo₂C-Ni cermet was designed, and the cermet was produced via liquid-phase sintering at 1450 °C for 2 h under N₂ pressure of 20, 200, 400 and 600 mbar. The microstructure and element distribution of cermet were analyzed by using Scanning Electron Microscopy (SEM) equipped with Energy Dispersive X-ray spectroscopy (EDX). A homogeneous microstructure was obtained for cermet sintered in 20-mbar nitrogen atmosphere, whereas the thickness of gradient layer increased with nitrogen pressure. EDX mapping demonstrate that Mo and Ti are enriched in gradient zone, while Ni is lacking and partially segregated near the surface. The diffusion of elements in cermet is caused by the different nitrogen activity between surface and interior. The carbonitride grains show typical black core and gray rim structure in the bulk of cermets, while it present light-gray core and gray rim in the surface gradient layer. In addition, the Vickers microhardness measurement was performed for the gradient zone of cermets, and the hardness increased for cermets sintered in higher nitrogen pressures, which exhibit slower grain growth phenomena.