Materials Science Forum Vols. 561-565

Abstract: The pseudoelastic behavior of Fe3Al single crystals doped with an extra element (e.g. Ti, V, Cr, Mn, Co, Ni, Si, Ga, Ge) was investigated. In binary Fe-23.0at.%Al crystals with the D03 structure, 1/4[111] superpartial dislocations moved independently dragging the nearest-neighbor anti-phase boundaries (NNAPB) during loading. During unloading, the NNAPB pulled back the superpartials decreasing its energy resulting in a giant pseudoelasticity of which the recoverable strain is about 5 %. Addition of a third element significantly affected the pseudoelastic behavior of Fe3Al single crystals. Mn- or Ga-doped crystal demonstrated a giant pseudoelasticity. In particular, Ga-doping was found to be effective in the enhancement of the pseudoelasticity. On the other hand, the amount of strain recovery decreased upon doping of the other elements. The frictional stress of the superpartials, the back stress of the NNAPB and ordered domain structure in the crystals changed upon doping, which was closely related to the pseudoelastic behavior.

Abstract: Zr and/or Nb added Fe3Al based intermetallic alloys (i.e., Fe3Al-Zr, Fe3Al-Nb and Fe3Al-Zr-Nb) were arc-melted, homogenized, hot-rolled and then annealed to evaluate microstructure and tensile property at room temperature as well as at a high temperature (873K). After annealing, the rolled alloys exhibited a recrystallized microstructure containing coarse second phase particles, except for the Nb-added alloy with a minor content of Nb. Relatively high tensile elongation as well as high tensile strength was observed at room temperature in the Zr-added alloys with a minor amount of Zr. Also, these alloys showed relatively high tensile strength and elongation at high temperature (873K). The results suggest that tensile ductility as well as strength of Fe3Al-based alloys can be improved by introduction of the second phase dispersions.

Abstract: Mechanical properties of thermomechanically fabricated D03 Fe-33Al and B2 Fe-38Al intermetallic alloys containing Zr were investigated by means of tensile test and microhardness measurement. The Zr-added ternary alloys showed fine-grained microstructure containing large (Fe,Al)12Zr τ1 phase particles, while the binary alloy showed a single-phase microstructure consisted of coarse recrystallized grains. By introducing the large τ1 phase particles to Fe-Al matrix, tensile strength at room temperature as well as at high temperature (873K) was enhanced but tensile ductility at both temperatures decreased. On the other hand, it was found that vacancy hardening which was significant in the alloys with high contents of Al (i.e., Fe-38Al) was reduced by the large τ1 phase particles.

Abstract: Superalloy powders, which have a small particle size and spherical shape, were prepared by a spark plasma discharge process. The electrical parameter, spark voltage, current and pulse duration have very important effects on the size distribution of produced superalloy powders. When current or pulse duration changed the mean size significantly changed.

Abstract: Plastic deformation behavior of Ni3V with D022 structure was examined using the single crystals containing two dominant variants of three. At [557] loading orientation, {111}1/6<112] twinning is dominantly operative at low temperatures, but {111}1/2<112] slip is activated at high temperatures accompanied by a rapid drop of yield stress.

Abstract: Nb and/or Cr added Ni3(Si,Ti) as well as unalloyed Ni3(Si,Ti) intermetallic thin foils (i.e., Ni3(Si,Ti), Ni3(Si,Ti)+Nb, Ni3(Si,Ti)+Cr and Ni3(Si,Ti)+Nb,Cr) were fabricated from arc-melted polycrystalline ingots by thermomechanical process and subsequent heavy cold-rolling. Tensile property at room temperature as well as at high temperature and oxidization behavior of the cold-rolled foils with a thickness of ~200μm were investigated. The Ni3(Si,Ti) and Ni3(Si,Ti)+Nb alloys showed a single-phase microstructure consisting of L12 phase, while the Ni3(Si,Ti)+Cr and Ni3(Si,Ti)+Nb,Cr alloys exhibited a two-phase microstructure with A1 (fcc) Ni solid solution phase within the L12 grains. All the cold-rolled foils showed high tensile strength (over 2GPa) at room temperature although no plastic elongation was observed. The addition of Nb and/or Cr slightly enhanced the room-temperature tensile strength of the Ni3(Si,Ti) alloy. On the other hand, the addition of Nb and/or Cr prominently enhanced high-temperature tensile strength as well as oxidization resistance, while the addition of Cr improved high-temperature elongation.

Abstract: Sound billets were produced at BIAM with a twin-scanning spray forming facility. Microstructure and mechanical properties of In718 alloy produced with this technology were examined. Both tensile strength and stress rupture properties are excellent with the rupture life even doubled after received a direct aging heat treatment.

Abstract: The results of atmosphere-controlled tensile test in various conditions (with different strain rate at different temperature under vacuum, air, or water vapor atmosphere) revealed that the addition of boron and chromium would improve the elongation as well as ultimate tensile strength (UTS) of the Ni-19Si-3Nb based alloys over a wide range of temperature under air and water vapor atmosphere. The UTS and elongation can reach to 1270 MPa and 14%, respectively at 873K in each kind of atmosphere. On contrary, the alloy without boron addition only presents ductile mechanical behavior in vacuum. This is evident that boron and Cr elements present positive effect on suppressing the environmental embrittlement in air and water vapor atmosphere from room temperature to 1073 K for the Ni-19Si-3Nb base alloy. In addition, both of UTS and elongation present quite insensitive on the strain rate when test at the temperature below 973 K. However, the UTS exhibits very dependent on the strain rate when test temperature above 973 K, decreasing the ultimate tensile strength with decreasing strain rate.

Abstract: Nb-Si based in-situ composites have great potential for the application of high temperature structure components. In this paper, the influence of microstructure on the compression behavior of Nb-Si in-situ composite forged at high temperature was studied. The alloy with nominal composition of Nb-12Si-24Ti-4Cr-4Al-2Hf was consumable arc-melted, and then isothermal forged at 960°C. Scanning electron microscopy (SEM), energy-dispersive spectroscopy (EDS) and X-ray diffraction (XRD) were used to characterize the microstructure, composition and phases. The compressive behaviors at 1250°C were carried out by Gleeble thermo-mechanical simulator. The results showed that the microstructures were inhomogenous in the different sections of the ingot, and no evident directional texture formed, and the main phases were niobium solid solution, Nb5Si3 and Ti5Si3. However, no macro-elements segregation in the ingot was observed. The compression strength was in the range of 140-360MPa. BSE observation showed that irregular-shaped Nb5Si3 and Ti5Si3 phases distributed in Nb solid solution and the size of Nb5Si3 in three tested samples was 10μm. Large size of eutectoid texture existed in the sample with strength of ~140MPa. On the contrary, in the sample with higher strength of 360MPa, eutectoid structures were hardly detected. The results suggested that the strength decreased gradually with size increase of eutectoid structure.

Abstract: In this study, the high temperature oxidation behavior of polycrystalline MoSi2 in a low-pressure atmosphere was investigated. Polycrystalline MoSi2 was produced by the spark plasma sintering process. Oxidation tests were carried out at 1500°C at either 10Torr or 760Torr in an Ar-20%O2 atmosphere. For both conditions, the weight change peaked at the initial oxidation stage, and then their weights gradually increased with increasing oxidation time. The sample weight became heavier in the ambient pressure than in the low-pressure, but the evaporation oxidation was not significant in the low-pressure condition. After the low-pressure oxidation tests, the formation of Mo5Si3 in the MoSi2 substrate was identified. The oxidation resistance of MoSi2 at 1500°C is discussed based on the obtained results.