Materials Science Forum Vols. 654-656

Abstract: The thermodynamic stability ’- Co3(Al,W) phase (L12) in the Co-Al-W ternary system at 900 °C was investigated through microstructure and EPMA analysis on a heat-treated bulk alloy. To promote microstructural evolution, the bulk alloy was cold rolled before heat treatment. By heating at 900 °C, the ’ phase was formed discontinuously in contact with the -Co (A1) phase. With increasing heat treatment time, however, the fraction of ’ phase decreased while that of , CoAl (B2) and Co3W (D019) phases increased. These results are consistent with our previous work with a diffusion-couple method, indicating that the ’ phase is metastable, and the three phases of, CoAl and Co3W are thermodynamically stable at 900 °C.

Abstract: The microstructures and hardness property of dual two-phase intermetallic alloys that arecomposed of various kind of volume fractions of geometrically closed packed (GCP) Ni3Al(L12) and Ni3V(D022) phases were studied. The hardness of dual two-phase intermetallic alloys basically was explained by mixture rule in hardness between primary Ni3Al precipitates and eutectoid region.Nb and Ti addition raised the hardness of dual two-phase intermetallic alloys by solid solute hardening in the constituent phases.The additional hardening arising from interfacial area between primary Ni3Al precipitates and eutectoid region was also found. As temperature increases, theadditional hardening decreased for the base and Nb added alloys but decreased little for the Ti added alloys.

Abstract: Intermetallic TiAl alloys with a significant volume fraction of the body-centered cubic β-phase at elevated temperatures have proven to exhibit good processing characteristics during hot-working. Being a strong β stabilizer, Mo has gained importance as an alloying element for so-called β/γ-TiAl alloys. Unfortunately, the effect of Mo on the appearing phases and their temperature dependence is not well known. In this work, two sections of the Ti-Al-Mo ternary phase diagram derived from experimental data are shown. These diagrams are compared with the results of in-situ high-temperature diffraction experiments using high-energy synchrotron radiation.

Abstract: A near-α high temperature titanium alloy, Ti-6Al-2.5Sn-4Zr-0.7Mo-0.30Si, was produced with various Y contents from 0 to 0.7wt% by arc-melting technique to study the influence of Y on its microstructure and mechanical properties. It was found that small amount of Y obviously refines the grain size of Ti-6Al-2.5Sn-4Zr-0.7Mo-0.30Si alloy. SEM and TEM observations revealed that Y-containing phase precipitated at the β grain boundary in the form of Y2O3 particles. Hardness and compression tests performed at room temperature revealed the strengthening effect of Y.

Abstract: The effects of electropulsing treatment on the precipitation behaviour of grain boundary carbides in GH3044 alloy were investigated. The results showed that the initial temperature of precipitation of M23C6 type carbides on grain boundary could be decreased by electropulsing treatment under the condition of high current density of 10.0kA/mm2 with a frequency of 5Hz and pulse width of 15s. The volume percentage of M23C6 type carbide was greatly increased to 274.60% comparing with that of the aging treatment at the same temperature. However, the precipitation of M23C6 type carbide was inhibited by electropulsing treatment under the condition of high frequency of 45Hz with the current density of 2kA/mm2 and pulse width of 15s. The volume percentage of M23C6 type carbide was decreased to 18.81% comparing with that of the aging treatment at the same temperature. It has been found that the diffusion of solute atom in the alloy can be promoted by the electric effect with the electropulsing. As a result, the thermodynamic condition and kinetics of the precipitation of M23C6 type carbide were changed, and both the initial and peak temperatures of precipitation were decreased.

Abstract: Multi-component Nb-(11,15)Si-5Hf-30Ti-4Al-4W-2B-(8,16)Cr alloys have been proposed, attempting to obtain the Nb-Si based alloys with a comprehensive property. The results show that three phases of NbSS, Nb5Si3 and Laves Cr2Nb exist in the alloys with a Cr content of 16 at%. With increasing Si and Cr contents the fracture toughness KQ decreases, while the oxidation resistance at 1250°C and strength at 1250°C and 1350°C exhibit an increasing tendency. The 15Si-16Cr alloy shows the highest strength and oxidation resistance, and the lowest toughness; they are 385MPa at 1350°C, 215mg/cm2 at 1250°C for 100 h, and 5.45 MPa•m1/2 at room temperature, respectively. The 11Si-8Cr alloy with an NbSS/Nb5Si3 microstructure only has the highest toughness of 11.87 MPa•m1/2, its strength and oxidation resistance are the lowest.

Abstract: Microstructure, mechanical properties and cold workability of quaternary Ni3(Si,Ti) intermetallic alloys with L12 structure, which were alloyed with two atomic percent of a refractory element X (X: Hf, Ta and W), were investigated. The Ta-added Ni3(Si,Ti) alloy showed an L12 single phase microstructure, while the microstructure of the Hf-added alloy was comprised of Ni5Hf and/or Ni3Hf intermetallic dispersions in the L12 matrix, and that of the W-added alloy consisted of fcc Ni solid solution phase within the L12 grain. In the homogenized condition, hardness increased in the order of the Hf-added, the Ta-added and the W-added alloys. The hardenings of the Hf-added and the Ta-added alloys were attributed to second-phase dispersion hardening and distinctive solid solution hardening, respectively. Among these alloys, only the W-added alloy was successfully cold-rolled to thin sheet with a thickness of 200 m. It was found that both room-temperature and high-temperature tensile strength of the W-added alloy sheet was enhanced compared with that of the unalloyed Ni3(Si,Ti) sheet. Also, high-temperature tensile ductility was significantly improved in the W-added alloy sheet, by suppressing the propensity of brittle intergranular fracture.

Abstract: The precipitation of carbide particles and high temperature strength in a Fe3Al base alloy with C, Cr and Mo were investigated. The -Fe3AlC (E21) and M2C (B81) phases precipitated in the temperature range between 1000 °C and 600 °C in periods up to 10 h. The -Fe3AlC phase tended to precipitate in a film-like morphology on grain boundaries by annealing at relatively high temperatures. The M2C phase was dispersed in a fine needle shape within grains by annealing at 700 °C. The film-like  precipitates covering grain boundaries as well as finely dispersed M2C particles was suggested to contribute to strengthening at 600 °C.

Abstract: The effect of Al addition on microstructures and tensile properties of cold-rolled Ni3(Si,Ti) intermetallic alloys with L12 ordered structure, which were fabricated through thermomechanical processing from arc-melted ingots, were investigated. Addition of 4 and 8 at.% Al to Ni3(Si,Ti) was conducted in two ways that Al substituted for Ti site and both for Ni and Ti sites, respectively. The alloys made by the former way showed a two-phase microstructure consisting of disordered fcc Ni solid solution dispersions in the L12 matrix, irrespective of Al contents, while the 4 at.% Al alloy made by the latter way exhibited an L12 single-phase microstructure. These alloys were successfully cold-rolled to thin sheets with a thickness of 200 μm except the 8 at.% Al alloy made by the latter way. For the thermomechanically processed 4 at.% Al alloys, high-temperature yield stress was higher in the alloy made by the latter way than in the alloy made by the former one, suggesting that the single-phase microstructure consisting of whole L12 ordered structure is favorable for high-temperature tensile property.

Abstract: Inconel 718C is a nickel-base alloy that is difficult to machine. This paper presents a study of the influence of grain size in as-cast workpieces on cutting forces of Inconel 718C. The end milling tests were performed to understand the effect of various grain structures on machinability of nickel-base superalloy under wet condition using carbide insert. The collected data of cutting forces were analyzed using polynomial regression methods. The results show that grain refining of Inconel 718C can effectively decrease cutting force and improve the machinability of nickel-base superalloy.