Rim Phase Research Articles

TiC-(Ti,M)C core-rim structures in solid-state manufactured steel-based MMCs

Core-rim structures in TiC-based metal matrix composites are known to be an important microstructural feature influencing the mechanical properties of the material. While this is a widely investigated topic for metal matrix composites consolidated by techniques utilizing a liquid phase, little research is available for core-rim structures in solid-state manufactured TiC-steel composites. In this work we study core-rim structures of 10 vol% TiC particles in a heat-treatable steel matrix, solid-state consolidated at 1140 °C by hot isostatic pressing. Before consolidation, TiC and steel powders were mixed by either tumble blending or attritor milling, which allows characterizing the influence of TiC particle size on the core-rim structure. SEM and STEM/EDS microstructural investigations show that in both materials a rim phase of the type (Ti,M)C (M = V, Mo, W, Cr) forms during solid-state consolidation and heat treatment. It is thereby shown that TiC in steel-based composites can be thermodynamically instable and form a core-rim structure even in the absence of a liquid phase. While significantly more (Ti,M)C forms in the attritor milled sample, it is less enriched in matrix elements compared to the tumble blended sample. Comparing thermodynamic calculations with the amount and composition of (Ti,M)C formed, it is shown that the attritor milled composite is much closer to thermodynamic equilibrium of matrix steel + 10 vol% TiC than its blended counterpart. Moreover, it is found by TEM and XRD that the rim phase has a NaCl crystal structure and grows epitaxially on the surface of original TiC particles.

Effect of NbC addition on the microstructure, mechanical properties and thermal shock resistance of Ti(C,N)-based cermets

In this paper, Ti(C,N)-WC-TaC-xNbC--Mo-Ni cermets were prepared by vacuum sintering. The microstructure, mechanical properties and thermal shock resistance of Ti(C,N)-based cermets with different NbC contents was explored. The results showed that with the increase of NbC content from 0 to 3 wt%, the rim phases around black core became complete, which refined the grain size of the cermets. However, white core appeared with the further increase of NbC content to 9 wt%, and the volume proportion of rim phases increased. Subsequently, the thickness of rim phases around white core increased and the black cores without rim phases appeared. The mechanical properties and thermal shock resistance of the cermets increased first and then decreased with the increase of NbC content from 0 to 9 wt%. When the NbC content was 3 wt%, cermets showed the best mechanical properties with transverse rapture strength of 2387 MPa, hardness of 92.1 HRA and fracture toughness of 8.88 MPa·m1/2. Meanwhile, when the NbC content was 3 wt%, cermets showed the best thermal shock resistance with critical temperature difference of 360 °C, due to its higher transverse rupture strength and lower thermal expansion coefficient.

Improvement in densification process and properties of Ti(C,N)-based cermets with vanadium carbide addition

This study investigates effects of vanadium carbide addition on densification process, microstructure, and properties of Ti(C,N)-based cermets prepared by powder metallurgy method. Results show that vanadium carbide in cermets promotes the diffusion of tungsten into rim phase of microstructure and enhances densification process of cermets. Incorporation of vanadium carbide can improve the hardness and transverse rupture strength of Ti(C,N)-based cermets due to solid-solution strengthening mechanism. Cermets with 2.0 wt% vanadium carbide exhibit an excellent hardness of 16.13 GPa along with transverse rupture strength of 2510 MPa. Furthermore, the addition of vanadium carbide in cermets can also enhance abrasion resistance of Ti(C,N)-based cermets.

Effects of carbon addition on the microstructure, mechanical properties, corrosion resistance and tribological behaviour of Ti(C, N)-based cermets

In this work, the microstructure and properties of Ti(C, N)-based cermets with different carbon additions that were prepared by a powder metallurgy method were systematically studied through SEM, TEM, mechanical properties tests, electrochemical tests and friction tests. The results showed that the micromorphology of the core-shell structure in the cermets was strongly affected by carbon content. The increase in carbon addition can lead to a finer hard core phase and thicker rim phase in the cermets. In addition, both the fracture toughness and transverse rupture strength of cermets tended to increase first and then decrease with increasing carbon content, while hardness slowly decreased. The cermet with 0.3 wt% added carbon showed the highest fracture toughness (12.94 MPa · m1/2) and transverse rupture strength (1961 MPa). Furthermore, the corrosion resistance of the cermets decreased with increasing carbon addition in 0.2 mol L−1 H2SO4 solution during electrochemical tests on account of the alloying elements precipitating from the binder phase. Additionally, after room-temperature friction tests, the wear patterns of the cermets changed from abrasive wear to adhesive wear with increasing carbon addition due to the softening of the binder phase. However, plastic deformation and oxidation were the main wear behaviours of cermets in the high-temperature friction process.

The influence of NaCl-H2O fluids on reactions between olivine and plagioclase: An experimental study at 0.8 GPa and 800–900 °C

The lower to middle continental and oceanic crust is dominated by mafic gabbroic rocks, which are variably metamorphosed at amphibolite-granulite facies conditions forming reaction rims or veins containing secondary pyroxenes, amphibole and phlogopite. Metamorphic fluids in the mid-crust are typically NaCl-rich brines. In this study, olivine and plagioclase were reacted with varying amounts of fluid (0–2 wt% added) that had compositions ranging from pure H2O to NaCl solutions (0–2 mol) in a series of experiments using the piston cylinder apparatus at 0.8 GPa and temperatures from 800 to 900 °C. Coronas of clinopyroxene, clinopyroxene + amphibole, and clinopyroxene + phlogopite were formed. The thicknesses of reaction rims, i.e. the relative amount of reaction rim products, increase with the relative amount of fluid present in the system, and are therefore linked to the level of fluid saturation along grain boundaries and interphase boundaries. Samples with no added H2O, show little to no signs of reaction, indicating the necessity for fluids in initiating a reaction between olivine and plagioclase. The concentration of NaCl in the fluid also increases the rates of rim growth. At 900 °C and 2 wt% added fluid, Cl-rich hydrous dacite-trachyte melt compositions formed and are similar to low degree partial melts of oceanic crust. The Cl contents of secondary amphiboles are positively correlated with the FeO and K2O contents of the amphibole. Consumption of fluid during the formation of hydrous rim phases may lead to high Cl within transient fluid filled pores along grain boundaries, which may lead to locally Cl-rich amphiboles.

Fabrication and wear mechanism of Ti(C,N)-based cermets tools with designed microstructures used for machining aluminum alloy

In this work, microstructure of Ti(C,N)-based cermets can be designed and controlled with extra carbon additions. The influences of microstructure on the mechanical properties and wear mechanism of cermets tools are investigated systematically by scanning electron microscope, X-ray diffraction and machining test of 7075 aluminum alloys. The increasing carbon additions in cermets can react with alloy elements dissolved in binder phase of cermets and promote the formation of rim phase, which may induce the disappearance of Co3W3(C,N) phase (η phase), reduction of core phase content and increase of rim phase content. With increasing carbon additions, the hardness of cermets decreases slightly, while transverse rupture strength of cermets can increase firstly and then decrease. Moreover, increasing carbon additions can augment the friction coefficient of Ti(C,N)-based cermets, which may induce improvement of cutting temperature and aggravation of abrasion at a relatively low cutting speed. However, with increasing cutting speeds, the existence of Co3W3(C,N) phase in cermets tools can accelerate wear process of tools.

Influence of ZrC addition on the microstructure, mechanical properties and oxidation resistance of Ti(C,N)-based cermets

In this study, Ti(C,N)-WC-NbC-ZrC-Co-Ni cermets were prepared by sintering-hip at 1450 °C. The effect of ZrC addition on the microstructure, mechanical properties, oxidation resistance and wear resistance of Ti(C,N)-WC-NbC-Co-Ni cermets were explored in detail. The results show that ZrC addition plays the role of inhibitor in the dissolution–reprecipitation process, which can increase the wear-resistant carbide phases and inhibit the precipitation of brittle (Ti,W,Nb)(C,N) rim phase. Therefore, the core-rim structures are refined and the Nb content in binder increases, which enhance mechanical properties and oxidation resistance of cermets. With the increasing ZrC content, the oxidation resistance of cermets can be improved constantly, while the transverse rupture strength, fracture toughness and wear resistance of these cermets increase first and then decrease. The cermet with 1 wt% ZrC exhibits the transverse rupture strength of 2549 MPa and highest fracture toughness of 13.0 MPa m1/2. The oxidation weight gain of cermets containing 5 wt% ZrC after holding 100 h at 750 °C in air is 2.8 × 10−6 g mm−2, which is only 22% of that in the cermets without ZrC addition.

Optical System Design of Telescope Based on Mixed Modulation Diffractive Lens

Using of diffractive optical elements (DOE) in the Schupmann system offer a new way for the construction of ultra-large aperture telescope. The Fresnel Corrector, one of the DOEs, is the key device in the Schupmann system, which is used to correct the chromatic aberration introduced by the diffractive primary lens called Magnifying Glass. Generally, in a large aperture telescope (>20 m), the Fresnel Corrector is a diffractive lens with a large aperture and a small f-number, which is difficult to process. In this article, an improved device with a small F number but a large rim feature size, called amplitude and phase hybrid modulation Fresnel diffractive optical element (APHMFDOE), is used here as the Fresnel corrector. First, APHMFDOE with appropriate parameters is designed to match the dispersion of the Magnifying Glass so that the system meets the achromatic condition. Second, the optical characteristics of this improved system are simulated and compared with those of the general system based on the conventional Fresnel corrector. Our approach introduces a new dispersion correction device, which not only can eliminate the chromatic aberration caused by Magnifying Glass, but also can reduce the processing difficulty of Fresnel Corrector.

Comparison of Ti(C, N)-based cermets by vacuum and gas-pressure sintering: Microstructure and mechanical properties

Ti(C, N)-based cermets with different carbon content were prepared by vacuum sintering and gas pressure sintering (Ar, 20MPa). Effects of carbon content and sintering method on the microstructure, composition and mechanical properties of cermets were studied. Results showed average grain size and thickness of rim phase to increase with higher carbon or by gas pressure sintering, which was caused by the accelerated dissolution-precipitation process. More uniform morphology with broadened grain size distribution was obtained by gas pressure sintering, leading to improved density and properties. Cermets with 0.8wt%C showed the best mechanical properties, i.e. 2105MPa (TRS), 10.55MPam1/2 (KIC), and 1581 (HV10), although containing residue Ni2(W, Ti)4C. Further increasing the carbon addition leads to thick rims and high amount of white core-grey rim phases, which can deteriorate the properties. Adequate control of size and composition of the core-rim structure plays a key role in ensuring satisfactory mechanical properties.

Effect of Mo on Microstructures and Wear Properties of In Situ Synthesized Ti(C,N)/Ni-Based Composite Coatings by Laser Cladding.

Using Ni60 alloy, C, TiN and Mo mixed powders as the precursor materials, in situ synthesized Ti(C,N) particles reinforcing Ni-based composite coatings are produced on Ti6Al4V alloys by laser cladding. Phase constituents, microstructures and wear properties of the composite coatings with 0 wt % Mo, 4 wt % Mo and 8 wt % Mo additions are studied comparatively. Results indicate that Ti(C,N) is formed by the in situ metallurgical reaction, the (Ti,Mo)(C,N) rim phase surrounding the Ti(C,N) ceramic particle is synthesized with the addition of Mo, and the increase of Mo content is beneficial to improve the wear properties of the cladding coatings. Because of the effect of Mo, the grains are remarkably refined and a unique core-rim structure that is uniformly dispersed in the matrix appears; meanwhile, the composite coatings with Mo addition exhibit high hardness and excellent wear resistance due to the comprehensive action of dispersion strengthening, fine grain strengthening and solid solution strengthening.

Effects of HfB₂ and HfN Additions on the Microstructures and Mechanical Properties of TiB₂-Based Ceramic Tool Materials.

The effects of HfB2 and HfN additions on the microstructures and mechanical properties of TiB2-based ceramic tool materials were investigated. The results showed that the HfB2 additive not only can inhibit the TiB2 grain growth but can also change the morphology of some TiB2 grains from bigger polygons to smaller polygons or longer ovals that are advantageous for forming a relatively fine microstructure, and that the HfN additive had a tendency toward agglomeration. The improvement of flexural strength and Vickers hardness of the TiB2-HfB2 ceramics was due to the relatively fine microstructure; the decrease of fracture toughness was ascribed to the formation of a weaker grain boundary strength due to the brittle rim phase and the poor wettability between HfB2 and Ni. The decrease of the flexural strength and Vickers hardness of the TiB2-HfN ceramics was due to the increase of defects such as TiB2 coarse grains and HfN agglomeration; the enhancement of fracture toughness was mainly attributed to the decrease of the pore number and the increase of the rim phase and TiB2 coarse grains. The toughening mechanisms of TiB2-HfB2 ceramics mainly included crack bridging and transgranular fracture, while the toughening mechanisms of TiB2-HfN ceramics mainly included crack deflection, crack bridging, transgranular fracture, and the core-rim structure.

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-(5–25wt%)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 (Ti0.76, W0.13, Mo0.11)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.

Fabrication, microstructure and mechanical properties of self-diffusion gradient cermet composite tool materials

Self-diffusion gradient cermet composite tool materials were fabricated by vacuum hot pressing process at different temperatures (from 1450°C to 1575°C) for different holding times (from 20min to 50min) under 32MPa. A new subsurface layer enriched with metal binder was formed due to self-diffusion driving force after sintering. The surface layer had a higher hardness, while the substrate had a high flexural strength, as well as the subsurface layer had a good interface bonding strength. The relationships among sintering processes, mechanical properties and microstructure were discussed. The experimental results showed that both the sintering temperature and holding time had a great influence on the flexural strength and fracture toughness, while a relatively small effect on the hardness. For the sintering parameters considered, it was revealed that the sintering temperature of 1500°C and the holding time of 40min were in the vicinity of optimum to yield the desired mechanical properties and fine microstructure. Both grain sizes and the rim phase of the Ti(C,N) increased and the Ni content in the substrate decreased when sintering temperature varied from 1450°C to 1575°C and holding time varied from 20min to 50min. Compared with Ti(C,N)-based cermet, the self-diffusion gradient cermet exhibited better comprehensive mechanical performance.

Enhanced Mechanical Properties of Ti(C,B)-Based Cermets with Multi-Component AlCoCrFeNi High-Entropy Alloys Binder

In this study, Ti (C,N)-based cermets with multi-component AlCoCrFeNi high-entropy alloys binder were successfully fabricated by mechanical alloying and low pressure sintering. The results indicate that the introduction of AlCoCrFeNi HEAs binder extended the formation process of (W,M)C rim phase by WC diffusion-solution mechanism, and repressed the precipitation process of the outer rim phase. On the other hand, the TEM micro-area analysis revealed a lath modulated structure, which massive nanoparticles are precipitated in the spinodal plate zone, 3-20nm in diameter. The nanocrystalline dispersion would provide an effective precipitation strengthening effect. The results of mechanical properties revealed that the cermets with AlCoCrFeNi HEAs binder achieved the co-enhancement of hardness and toughness. The novel material exhibits a high Vickers hardness of 1787 MPa, along with a good fracture toughness value of 11.4 MPa m1/2.

Effect of the Ratio of Co to Ni+Co on the Microstructures and Mechanical Properties of Ti(C, N)-Based Cermets

The microstructures of the prepared Ti(C, N)-based cermets with various ratios of Co to Ni+Co were studied using X-ray diffractometry (XRD) and scanning electron microscopy (SEM). Mechanical properties such as transverse rupture strength (TRS), fracture toughness (K1C) and hardness (HRA) were also measured. The results showed that when Ni was partly replaced by Co, the core size of hard particle and the thickness of rim phase changed. With the increasing of the ratio of Co to Ni+Co, the porosity of the cermets increased gradually, the fracture toughness of the cermets decreased gradually, the transverse rupture strength increased firstly and then decreased, the hardness changed slightly。When the ratio of Co to Ni+Co was 0.2, the cermets had better transverse rupture strength (TRS), which was characterized by fine grains and the moderate thickness of rim phase in the binder.

Effects of secondary carbides on the microstructure, mechanical properties and erosive wear of Ti(C,N)-based cermets

The present work investigated the effects of secondary carbides (Mo2C/WC/TaC/NbC) on the microstructure, mechanical properties and erosion behavior of Ti(C,N)-based cermets. The results indicate that Ti(C,N)-based cermets with different secondary carbides have the typical core/rim microstructures which include the Ti(C,N) core phases and rim phases of (Ti,X)(C,N) solid solution (where X: Mo/W/Ta/Nb). Mo2C addition significantly refines the ceramic grain and enhances the hardness, indentation fracture toughness and transverse rupture strength (TRS) of cermets. The volume fraction of rim phase increases while the core size decreases in the cermets containing TaC and NbC. Furthermore, the considerable solubility of carbides (WC and Mo2C) in metal Ni strengthens the binder phase and decreases the thickness of brittle rim phase. The combined effects result in an increasing trend of mechanical properties in order of NbC, TaC, WC and Mo2C containing cermets. Strengthened binder, moderate thickness of rim and refined grain jointly enhances the mechanical properties of Ti(C,N)-based cermets, resulting in the improvement of erosion resistance.

Effects of heating rate and metal binder on the microstructure and mechanical properties of self-diffusion gradient cermet composite tool materials

Self-diffusion gradient cermet composite tool materials with different metal binder contents and Ni/(Co + Ni) mass ratios were fabricated to improve the mechanical properties. The effects of heating rate and metal binder on the microstructure and mechanical properties of self-diffusion gradient composite were investigated. For the heating rates considered, it was revealed that the heating rate of 30 °C/min was in the vicinity of optimum to yield the desired mechanical properties and fine microstructure. It was found that the flexural strength increased and the hardness decreased with increasing Ni binder content, while the addition of Co binder led to an excessively thick rim phase which was detrimental to the flexural strength. Applying a proper amount of Ni binder appeared to be beneficial for fabricating gradient composites with desired self-diffusion process during sintering. For the experimental conditions considered in this study, the best mechanical properties can be obtained when 6 mass% Ni in the surface and 12 mass% Ni in the substrate were used, which gave flexural strength, substrate hardness and surface hardness at 1488 ± 190 MPa, 20.96 ± 0.30 GPa and 25.12 ± 0.48 GPa, respectively.

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

Short carbon fibre (Cf) reinforced TiCN-based cermets (Cf/TiCN composites) were produced by powder metallurgy method with pressureless sintering technology. The phase evolution, microstructure and fracture morphology of Cf/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 Cf did not change the ‘core–rim’ microstructure but improved the mechanical properties of TiCN-based cermets. The Cf/TiCN composite containing 3 wt-% Cf 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 Cf. Toughening and strengthening mechanisms of Cf/TiCN composite were concluded as crack deflection and branch, as well as the pull-out, fracture and bridging of carbon fibres.

Effect of carbon addition on the densification behavior, microstructure evolution and mechanical properties of Ti(C, N)-based cermets

Four cermets with composition of TiC-10TiN-32Ni-16Mo-(9−x)WC-xC (x=0, 0.5, 1.0 and 1.5wt%) were prepared by vacuum sintering. The effect of carbon addition on the densification behavior, phase and microstructure development, hardness (HRA) and transverse rupture strength (TRS) of the prepared cermets was studied. The full densification temperature was almost identical for cermets with carbon addition in the range of 0–1wt%, but it obviously decreased from 1340°C to 1280°C for cermets with excess carbon addition (up to1.5wt%). The carbon addition had no obvious effect on the phase transformation, in all four cermets existed only Ti(C, N) and Ni phases when the sintering temperature was or exceeded 1340°C. However, the carbon addition had a significant effect on the final microstructure of cermets sintered at 1400°C, grey rim phase gradually became thin with increasing carbon addition, which was attributed to the fact that higher carbon addition inhibited the dissolution process of carbides in Ni phase. Accordingly, the content of alloy elements in Ni-based binder phase reduced gradually with increasing carbon addition, resulting in the decrease of Ni lattice parameter. Typically, 1wt% carbon-doped Ti(C, N)-based cermets sintered at 1400°C exhibited good mechanical properties, which was characterized by homogeneous microstructure, moderate thickness of rim phase and a relatively low solution hardened binder phase.

Effects of metal binder on the microstructure and mechanical properties of Ti(C,N)-based cermets

To optimize the mechanical properties of Ti(C,N)-based cermets used as tool materials, the cermets with different Ni–Co binder contents and Co/(Ni+Co) weight ratios were prepared. The effects of metal binder content and Co/(Ni+Co) ratio on the microstructure and mechanical properties of Ti(C,N)-based cermets were investigated by scanning electron microscope (SEM) with energy dispersive spectroscopy (EDS), X-ray diffraction (XRD), and measuring the transverse rupture strength (TRS), Vickers hardness (HV) and fracture toughness (KIC). The experimental results reveal that increasing Ni–Co binder content can increase the thickness of rim phases by improving the solid solution reaction and the wetting of hard phases. The cermets with 25wt.% binder addition present good comprehensive mechanical properties, which is attributed to the moderate rim phases and uniformly distributed Ti(C,N) grains. The Co/(Ni+Co) weight ratios in binder have a great influence on the grain sizes and microstructure features of Ti(C,N)-based cermets, in virtue of the synergic effects between the wettability of Co and the solubilizing capacity of Ni on hard phases. The cermets with pure Co as binder exhibit optimal mechanical properties with a TRS of 1767±81MPa, a hardness of 12.26±0.10GPa and a KIC of 8.40±0.47MPam1/2, which meet the requirements for tool materials. And the cermets with a Co/(Ni+Co) ratio of 0.2 have the second best mechanical properties with a TRS of 1848±201MPa, a hardness of 11.12±0.40GPa and a KIC of 9.43±0.54MPam1/2, in which the lower hardness can be further improved by optimizing the compositions and sintering process.