Effect of Mo addition on microstructure and mechanical properties of (Ti,W)C solid solution based cermets

(Ti,W)C solid solution was synthesized by milling a mixture of C and oxides and then reducing it at 1350 °C for 2 h. The microstructure and mechanical properties of (Ti,W)C solid solution based cermets with various Mo additions were systemically studied. The dark core–gray rim carbide grains and/or gray carbide grains embedded in black Ni based binder phase were observed. The grain size of the cermets decreased with increasing Mo addition, while excessive Mo addition would result in agglomeration and inhomogeneity of the grains. The dark core was transformed from (Ti,W)C into (Ti,W,Mo)C and Mo concentration in the dark core increased with increasing Mo content. The fracture toughness of the cermets decreased with the increase of Mo content, while the hardness and TRS reached a peak value at 10 wt.% and 15 wt.% Mo additions respectively, and declined with the further increase of Mo content.     

The fabrication of multi-core structure cermets based on (Ti,W,Ta)CN and TiCN solid-solution powders

A novel multi-core structure cermets consisted of both black-core/rim structure and grey-core/rim structure were obtained by partially replacing TiCN powder with (Ti,20W,15Ta)CN powder via low-pressure sintering process. The toughness and strength of TiCN-based cermets were optimized and its feature of high hardness was maintained simultaneously. Systematically, it was investigated that the influences of various weight ratios of both (Ti,20W,15Ta)CN/TiCN and Co/Ni on the microstructure and mechanical properties of the multi-core cermets. The results showed that the addition of (Ti,20W,15Ta)CN powder could cause the refinement of the core size and the occurrence of the secondary phase (W,Mo,Ti)_3 + x(Co,Ni)_3 − xC (0 < x ≤ 1), both of which are responsible for the significant improvement of the mechanical properties. The appearance of the secondary phase was found under two circumstances, one was when the weight ratio of (Ti,20W,15Ta)CN/TiCN was 6:4 while that of Co/Ni was 5:5(cermet M60) and the other was when that of (Ti,20W,15Ta)CN/TiCN was 5:5 with pure Co binder (cermet C50). And there is a monotonous escalation of the fracture toughness (K_IC) of the cermets while increasing the (Ti,20W,15Ta)CN content. The optimal comprehensive mechanical performance was found in cermet M60 with transverse rupture strength (TRS) of 1903.32 MPa, Vickers hardness (HV₃₀) of 16.33 GPa and fracture toughness of 12.19 MPa·m^1/2.     

Effect of (Ti,W, Mo,V)(C,N) powder size on microstructure and properties of (Ti,W, Mo,V)(C,N)-based cermets

Three kinds of (Ti, 15 W, 5Mo, 0.2 V)(C, N) powders with different particle size were prepared from a mixture of oxides and carbon powders by carbothermal reduction-nitridation method. (Ti, W, Mo, V)(C, N)-based cermets were obtained by mixing Co, Ni, WC, MoC₂ and (Ti, 15 W, 5Mo, 0.2 V)(C, N) powders, and then processed via a conventional P/M technique. The influence of (Ti, 15 W, 5Mo, 0.2 V)(C, N) particle size on the microstructure and mechanical properties of (Ti, 15 W, 5Mo, 0.2 V)(C, N)-5WC-10MoC₂-7Co-8Ni cermets has been studied. When the particle size of (Ti, W, Mo, V)(C, N) is 0.2–0.5 μm, (Ti, 15 W, 5Mo, 0.2 V)(C, N)-based cermets can be characterized by weak core/rim structure. With the particle size of (Ti, W, Mo, V)(C, N) increasing to 1–1.5 μm, the microstructure of (Ti, 15 W, 5Mo, 0.2 V)(C, N)-based cermets develops into composite structure that consists of typical core/rim and no core/rim. Accordingly, typical core/rim structure is obtained in the case of the particle size of 3–5 μm. With the coarsening of raw (Ti, 15 W, 5Mo, 0.2 V)(C, N) powders, the fracture toughness of (Ti, 15 W, 5Mo, 0.2 V)(C, N) cermets is greatly improved, but the hardness continues to decline. (Ti, 15 W, 5Mo, 0.2 V)(C, N) cermets with composite structure have higher bend strength of 2165 MPa.     

Study on the formation of core–rim structure in Ti(CN)-based cermets

Ti(CN)-based cermets were synthesized from Ti(CN)WCMo₂CTaCNiCo composite powders by vacuum-low pressure sintering. The phase evolution and the formation of core–rim structure in Ti(CN)-based cermets were systemically investigated during difference reaction stages at 950–1450 °C. The results show that the secondary carbides such as Mo₂C and TaC are begun to dissolve at 950 °C, finished at 1150 °C, and the solution temperature of WC phase is range from 1150 to 1300 °C, which are result in increase of the cermets lattice constant. At the same time, the inner rim is also formed, and Ti(CN)-based cermets are composed of (Ti, W, Mo, Ta)(CN) and Ni/Co solid solution phase. While at 1350 °C, it was found that the outer rim began to precipitate from the liquid phase with the metal binder. With increase of sintering temperature, mechanical properties of cermets improved obviously were related intimately to the increase of outer rim thickness.     

Effects of sintering processes on the mechanical properties and microstructure of Ti(C,N)-based cermet cutting tool materials

In this study, two types of Ti(C_0.7,N_0.3)-based cermet cutting tool materials (Ti(C,N)–Mo–Ni–Co, named as TMNC, and Ti(C,N)–WC–Mo–Ni–Co–TaC–HfC, named as TWMNCTH) were fabricated by the hot pressed sintering process at different temperatures (from 1380 °C to 1500 °C) for different holding times (from 30 min to 60 min) in a vacuum atmosphere and at a compressive stress of 32 MPa. The polished surface and the fracture surface of the two types of cermets were observed by a scanning electron microscope (BSE/SEM) and energy dispersive spectrometry (EDS), and the relationships among sintering processes, mechanical properties and microstructure were discussed. The experimental results showed that the sintering temperature and holding time both had a great influence on the flexural strength and a small effect on the hardness and the fracture toughness of the two types of cermets. The two cermets both had the optimal comprehensive mechanical properties when they were sintered at 1400 °C for 30 min. The sintering temperature and holding time also had a great influence on the microstructure of the two cermets, and the grain sizes increased when the sintering temperature varied from 1400 °C to 1500 °C and the holding time varied from 30 min to 60 min. The properties and microstructure of the two cermets were also compared. The results indicated that the cermet TWMNCTH had a lower flexural strength, a similar value of fracture toughness, a higher hardness and a thicker rim in the microstructure.     

Absence of the core–rim microstructure in TixTa1 − xCyN1 − y-based cermets developed from a pre-sintered carbonitride master alloy

(Ti,Ta)(C,N) solid solution-based cermets with cobalt as the binder phase were synthesised by a two-step milling process. The titanium–tantalum carbonitride solid solution (the ceramic phase) was obtained via a mechanically induced self-sustaining reaction (MSR) process from stoichiometric elemental Ti, Ta, and graphite powder blends in a nitrogen atmosphere. Elemental Co (the binder phase) was added to the ceramic phase, and the mixture was homogenised by mechanical milling (MM). The powdered cermet was then sintered in a tubular furnace at temperatures ranging from 1400 °C to 1600 °C in an inert atmosphere. The chemical composition and microstructure of the sintered cermets were characterised as ceramic particles grown via a coalescence process and embedded in a complex (Ti,Ta)–Co intermetallic matrix. The absence of the typical core–rim microstructure was confirmed.     

Effects of Cr3C2 addition on the corrosion behavior of Ti(C,N)-based cermets

The Ti(C, N)-based cermets with different Cr₃C₂ addition were prepared and the effects of Cr₃C₂ addition on the microstructure and properties of cermets were discussed. The corrosion behavior of the cermets with different Cr₃C₂ addition was investigated emphatically in 2 mol/L HNO₃ solution. The results indicate that there is no obvious effect of Cr₃C₂ addition on the densification of the cermets, and all cermets are almost fully densified during sintering. The thickness of rim phase is reduced and the core size is increased remarkably in the cermets with 1 wt.% and 3 wt.% Cr₃C₂ addition; the grains are refined significantly in the cermets with the increase of Cr₃C₂ addition to 5 wt.%. The hardness and transverse rapture strength of the cermets are improved with Cr₃C₂ added properly. In HNO₃ solution, the corrosion resistance of cermets is improved remarkably by Cr₃C₂ addition. The corrosion of binder phase is predominant in the cermets in which the Ni binder phase without Cr has lower corrosion resistance than the rim phase; whereas the corrosion resistance of binder phase with high Cr content is better compared to the rim phase, so that the degradation of rim phase is predominant and a reticulate binder phase forms. With the increase of Cr₃C₂ addition, the Mo content in rim increases, and it is bad for the corrosion resistance of rim phase. Additionally, the inner rim phase has lower corrosion resistance than the outer rim phase owing to the higher Mo content.     

Site preference of early transition metal elements in C15 NbCr2

The site preference of early 3d (Ti, V), 4d (Zr, Mo) and 5d (Hf, Ta, W) transition metal elements in C15 NbCr₂ Laves phase was studied using first-principles calculations. According to the present calculations, at T = 0 K, Zr, Hf and Ta consistently have a preference for the Nb sites in Nb-rich, Cr-rich and stoichiometric NbCr₂, while the site preference of Ti, V, Mo and W varies strongly with alloy composition. Using a statistical–mechanical Wagner–Schottky model based on the canonical ensemble, the finite temperature site occupancy behavior of those transition metal elements in NbCr₂ was further predicted. It was found that the site preference of Ti, V, Mo and W also depends strongly on temperature. The calculated results compare favorably with the experimental measurements using ALCHEMI and synchrotron X-ray diffraction techniques.     

Shape memory behavior of Ti–Ta and its potential as a high-temperature shape memory alloy

In this study, the effect of Ta content on shape memory behavior of Ti–Ta alloys was investigated. The shape memory effect was confirmed in Ti–(30–40)Ta alloys. The martensitic transformation start temperature (M_s) decreased by 30 K per 1 at.% Ta. The amount of ω phase formed during aging decreased with increasing Ta. A stable high-temperature shape memory effect was confirmed for Ti–32Ta (M_s = 440 K) during thermal cycling between 173 and 513 K. On the other hand, the high-temperature shape memory effect of Ti–22Nb, which has a similar M_s to Ti–32Ta, exhibited poor stability due to the large amount of ω phase formed during thermal cycling. It is suggested that Ti–Ta is an attractive candidate for the development of novel high-temperature shape memory alloys.     

A study of the microstructures and oxidation of Nb–Si–Cr–Al–Mo in situ composites alloyed with Ti,Hf and Sn

The effects of alloying on the microstructures, solidification path, phase stability and oxidation kinetics of Nb_ss/Nb₅Si₃ base in situ composites of the Nb–Ti–Si–Al–Cr–Mo–Hf–Sn system have been investigated in this study. All the studied alloys are classified as hyper-eutectic Nb silicide base in situ composites and have lower densities compared to nickel-based superalloys. The Nb₃Si silicide formed in the Hf-free alloys and transformed to Nb_ss and αNb₅Si₃ during heat treatment at 1500 °C. This transformation was enhanced by the addition of Ti. The Nb_ss and Nb₅Si₃ were the equilibrium phases in the microstructures of the Hf-free alloys. In the presence of Ti, the βNb₅Si₃ only partially transformed to αNb₅Si₃, suggesting that Ti stabilises the βNb₅Si₃ to lower temperatures (at least to 1300 °C). Furthermore, alloying with Hf stabilised the hexagonal γNb₅Si₃ (Mn₅Si₃-type) silicide in the Hf-containing alloys. The addition of Sn promoted the formation of the Si-rich C14 Laves phase and stabilised it at 1300 °C. This is attributed to the Sn addition decreasing the solubility of Cr in the Nb_ss of the Nb–Ti–Si–Al–Cr–Mo–Hf–Sn system whilst increasing the Si solubility. The Si solubility in the C14 Laves phase was in the range ∼6.6 to 10.5 at%. The lattice parameter of the Nb_ss in each alloy increased after heat treatment signifying the redistribution of solutes between the Nb_ss and the intermetallic phases. The oxidation resistance of the alloys at 800 °C and 1200 °C increased significantly by alloying with Ti and Sn. Pest oxidation behaviour was exhibited by the Nb–18Si–5Al–5Cr–5Mo (as cast), Nb–24Ti–18Si–5Al–5Cr–5Mo (as cast), Nb–24Ti–18Si–5Al–5Cr–2Mo (heat treated) and Nb–24Ti–18Si–5Al–5Cr–2Mo–5Hf (heat treated) alloys at 800 °C. Pesting was eliminated in the alloy Nb–24Ti–18Si–5Al–5Cr–2Mo–5Hf–5Sn at 800 °C, indicating that the addition of Sn plays an important role in controlling the pest oxidation behaviour at intermediate temperatures. The oxidation behaviour of all the alloys at 800 °C and 1200 °C was controlled by the oxidation of the Nb_ss and was sensitive to the area fraction of Nb_ss in the alloy.     

Erosion-corrosion behavior of Ti(C,N)-based cermets containing different secondary carbides

The present work investigated the effects of secondary carbides (Mo₂C\WC\TaC\NbC) on the erosion-corrosion behavior of Ti(C,N)-based cermets. The results indicate that the erosion-corrosion resistance of Ti(C,N)-based cermets is enhanced in the order of NbC, TaC, WC and Mo2C addition. The contribution of erosion to the erosion-corrosion of Ti(C,N)-based cermets is much more significant than that of corrosion, and it increases with the decreased mechanical properties. The synergistic effect plays a dominant role in the degradation of Ti(C,N)-based cermets in erosion-corrosion conditions. There are two modes to ceramic phase degradation in erosion conditions: large ceramic grains are prone to deterioration through crack initiation and propagation → grain fracture → fragment removal; finer ceramic grains trend to be pulled out after the deterioration of binder and interface. The binder loss is determined by the corrosion resistance of binder, the erosion resistance of binder and the erosion resistance of ceramic phase.     

Effect of the Cr content on the sintering behaviour of TiCN–WC–Ni–Cr3C2 powder mixtures

In TiCN–W–Cr–Ni cermets produced by liquid phase sintering melting occurs at lower temperatures as their Cr content increases. For low Cr additions (up to 4 wt.%) eutectic temperatures are close to those found in the TiC–WC–Ni system. For 8 wt.% Cr and above, temperatures are similar to those found in the Cr–Ni–C system. The precipitation of M₇C₃ carbides is observed to start at 8 wt.% Cr in samples sintered at 1425 °C for 1 h. This sets a limit for the Cr solubility in the binder phase of these cermets around 18 wt.%. The dissolution of WC and Cr₃C₂ particles starts at temperatures as low as 1150 °C, but that the homogenization of the binder phase is only achieved after melting. The carbonitride phase exhibits the typical precipitation of inner and outer rims onto Ti(C,N) cores. However, a fine precipitation of Ni-rich particles is found inside Ti(C,N) cores, likely related to coalescence phenomena.     

Microstructure characterization of large TiC-Mo-Ni cermet tiles

A detailed characterization of large (150 mm × 150 mm), 6 to 12 mm thick, commercially produced tiles of a TiC-Mo-Ni cermet with ~ 13 vol% Ni binder and a microstructure consistent with processing via self-propagating high temperature synthesis (SHS) has been conducted. Many mechanical property defining attributes of the materials were highly reproducible, including the composition, phase content, TiC particle size distribution (with an average particle diameter of 8–10 μm), and density of 5.52 × 10³ kgm^− 3. However, sufficient variability in the distribution of metal elements within the carbide particles, interparticle contiguity, the distribution of porosity, and residual stress were discovered that the mechanical behavior is expected to exhibit significant variability. The spheroidal shaped TiC particles had a multilayered (onion ring like) composition with rings of locally higher Mo concentration, rather than the more usual Ti-rich core and a single Mo-rich rim that enhances wetting with the Ni-binder. The TiC particles also had a high contiguity factor of 0.30–0.47. Recent assessments of liquid phase sintered cermets indicate significant loss of fracture resistance as the contiguity increases above 0.25. Hot isostatic pressing (HIP) at 1250 °C and 100 MPa was unable to reduce the porosity, which remained as large pockets of insufficient metal binder material (a form of shrinkage porosity) between the spheroidal carbide particles. X-ray diffraction measurements indicated the presence of significant residual stress in the as-received and the HIP condition materials. A stress relief heat treatment at 900 °C succeeded in eliminating this residual stress consistent with its origination from thermal gradients associated with rapid cooling.     

Three-dimensional atom probe microscopy study of interphase precipitation and nanoclusters in thermomechanically treated titanium–molybdenum steels

Atom probe microscopy was used to generate tomographic analyses of solute clustering and precipitation reactions in a Ti–Mo added microalloyed steel under simulated strip-rolling conditions. It was observed that the interphase row spacing of precipitates was reduced with the application of a pre-strain. The atom probe data also revealed the coexistence of nanoclusters and precipitate particles, even after isothermal holding for 3600 s. These microstructural features occurred both within 3-D interphase precipitate sheets, and in randomly selected fields of view. A bimodal distribution of larger (～8–10 nm) precipitates coexisted with smaller nanoclusters (～3 nm) within the interphase sheets/rows. Both the nanoclusters and the precipitates possessed a disc morphology, although nanoclusters with less than ～30 atoms were more irregular in shape. The size of the nanoclusters and the precipitates was expressed as a Guinier radius, and this varied between 0.5 and 8 nm for both strain conditions, with the average size ～1.8 nm. The composition of the nanoclusters varied over a wide range, yet was mostly rich in C. All of the nanoclusters and precipitates consisted of a mixture of Ti, Mo and C and the average precipitate composition was close to that of MC carbide stoichiometry, where M represents a mixture of Ti and Mo. In the majority of cases, the Ti/Mo ratio in the MC carbides was > 1. As the Guinier radius increased above 2.5 nm, the composition range became narrower, towards the MC carbide stoichiometry, with a small amount of Fe (～3–12 at.%).     

Titanium diboride composite with improved sintering characteristics

Titanium diboride (TiB₂) and its ceramic composites were prepared by hot pressing process. The sintering process, phase evolution, microstructure and mechanical properties of TiB₂ ceramics prepared by using different milling media materials: tungsten carbide (WC/Co) or SiAlON was studied. It was found that the inclusion of WC/Co significantly improved the sinterability of the TiB₂ ceramics. A core/rim structure with pure TiB₂ as the core and W-rich TiB₂, i.e. (Ti,W)B₂ as the rim was identified. Microstructure analysis revealed that this core/rim structure was formed through a dissolution and re-precipitation process. In addition, silicon carbide (SiC) was also introduced to form TiB₂–SiC composites. The addition of SiC as the secondary phase not only improved the sinterability but also led to greatly enhanced fracture toughness. The optimum mechanical properties with Vickers hardness ~ 22 GPa, and fracture toughness ~ 6 MPa m^1/2 were obtained on TiB₂–SiC composites milled with WC/Co.     

Effect of nitrogen and secondary carbide on the microstructure and properties of (Ti0.93W0.07)C–Ni cermets

Solid-solution powders of (Ti_0.93W_0.07)C and (Ti_0.93W_0.07)(C_0.7N_0.3) were synthesized via high-energy ball milling and carbothermal reduction processes. After blending powders with Ni and other carbides, cermets were prepared by a blending and sintering process at 1510 °C for 1 h. We observed a typical core/rim structure consisting of solid solution phases. We also found that secondary carbide and nitrogen have a remarkable influence on the cermet microstructures. Further, with an increase in the Mo₂C content, the mechanical properties of these cermets were enhanced significantly; the hardness of carbide and carbonitride cermets increased from 9.3 and 12.7 to 12.9 and 13.9 GPa, respectively. All results are discussed in terms of the thermodynamic stability and dissolution behavior of the constituent carbides and carbonitride.     

Microstructure instability in TiC-316L stainless steel cermets

Titanium carbide (TiC) based cermets are commonly used in wear and corrosion resistance applications. The microstructural evolution, and related compositional instability, of TiC-based cermets prepared with a 316-L stainless steel binder is described in the present work. Samples were fabricated using a simple vacuum melt-infiltration procedure, with 5 to 30 vol.% binder. Infiltration temperatures ranged from 1475 °C to 1550 °C, held for up to 240 min, typically resulting in sintered samples with densities in excess of 99% of theoretical. It is demonstrated that irregularly shaped grains (concave/hollow) can arise after sintering, especially at 1475 °C, which is discussed in terms of the ‘instability of the solid-liquid interface’ theory. It is demonstrated that a complex, multi-layer core-rim structure arose for the cermets, with accommodation of selected steel constituents into the rim of the TiC grains. In particular, it is shown that the Mo in the original 316-L stainless steel is essentially fully depleted from the metallic binder phase, forming a Mo-rich inner-rim layer on the TiC grain cores.     

On the use of TiO2 in Ti(C,N)-WC/Mo2C-(Ta,Nb)C-Co/Ni cermets

In this study, Ti(C,N)-WC/Mo₂C-(Ta,Nb)C-Co/Ni cermets with varying [Mo]/([W] + [Mo]) mole ratios and constant [W] + [Mo] content were prepared by using TiO₂ and carbon as substitutes for a part of Ti(C,N). In situ carbothermal reaction took place with subsequent liquid phase sintering. The thermal behavior and outgassing behavior were conducted using differential thermal analysis/thermal gravity analysis (DTA-TG) and mass-spectrometric evolving gas analysis (MS-EGA). The microstructure was investigated by scanning electron microscopy (SEM). After being sintered in N₂ atmosphere, cermets without TiO₂ additions generally showed rimless microstructures with black grains independently distributed from other phases, whereas the cermets with TiO₂ addition showed complete core-rim microstructures. The average grain size generally decreased with increasing Mo content, and increased with increasing TiO₂ substitution. Moreover, the average grain size increased with decreased N₂ pressure from 50 mbar to 10 mbar at any [Mo]/([Mo] + [W]) ratio. The most interesting result was for the Mo-free cermets with 5 mol% TiO₂ and 10 mol% TiO₂ addition sintered in 50 mbar N₂: both the hardness and fracture toughness increased compared with the cermets without TiO₂ addition. The HV-K_IC behavior of the majority of the cermets was superior to the industrial grades.     

Effect of graphite size on the tribological behavior of Ti (C,N)-based cermets self-mated wear pairs

In the present work, graphite of different particle size ranging from 2 μm to 32 μm is added into the system to lubricate the Ti (C, N) cermets, but the addition amount is fixed at 1 wt%. The sliding wear tests were carried out using block-on-ring equipment. The distribution morphology of graphite, mechanical properties and wear behavior were studied in this paper. The results indicate that when the graphite is added into the cermet, graphite particles join together and are distributed to clusters of different size which are like bird nests. The range of the “bird-nest” clusters increase with the improvement of graphite size when the graphite particle size is smaller than 12 μm but decrease when larger than 12 μm. Furthermore when the graphite grain size increase to 32 μm, most of the particles exist in the cermets individually instead of distributing like bird nest The mechanical properties decrease with the improvement of graphite size, and when the particles size of graphite increase to 32 μm the mechanical properties get worse sharply. The wear mechanism of the cermets with graphite, adhesion and plastic deformation are dominated. The extruded graphite forms a tribofilm, and protects the surface from the hard abrasive particles ploughing. When the particles size of graphite added is 22 μm, cermets which have coarser graphite particles and more but moderate-in-size graphite clusters possess the best effect of lubrication