Materials Science Forum Vols. 551-552

Abstract: The as-cast and hot worked microstructures of the newly developed β-solidifying ingot-metallurgy Ti-45Al-X (Nb,Mo,B) alloy and its superplastic properties in the hot worked condition have been studied. The obtained experimental findings were used for research of superplastic forming and diffusion bonding of sheet products, which were cut out of hot worked preform by spark cutting. It was shown that superplastic forming might be successfully applied to the obtained fine-grained sheet materials. Relatively low bonding temperatures and pressures were found to be sufficient to achieve sound joints in the sheet material.

Abstract: A novel approach to fabrication of globularized fine-grained structure in γ+α2 titanium aluminide alloys has been proposed. The approach included the use of a specially designed alloy Ti-43Al-X(Nb,Mo,B) and heat treatment. It was found that the ingot structure of the alloy might be partially globularized on a scale of bulk material using only globularization anneal excluding any hot working procedure. The microstructure and tensile mechanical properties of the alloy in the cast + heat treated condition were investigated. The tensile mechanical tests were performed in air in the temperature range of T=900-1130°C at an initial strain rate of ε′=1.7×10-4 s-1. High elongation (δ=160-230%) and low flow stresses (σ=36-100 MPa) typical of superplastic behavior were measured at T=1050-1130°C. It was demonstrated that the sheet material produced by spark cutting of the cast + heat treated alloy might be successfully hot formed.

Abstract: Ti2AlNb orthorhombic alloys exhibit great potential as advanced aerospace and structural materials serviced at elevated temperature. In this paper, pre-heat treatment of as-received hot rolling Ti-22Al-25Nb alloy was conducted. Fine, stable and equiaxed grain structure was obtained by the pre-heat treatment. The volume fraction of B2 increased when annealing at 980°C. The Ti-22Al-25Nb alloy showed characteristics of superplastic deformation when tested at 960°C. Maximum elongation of 280% has been obtained at strain rate of 1.0×10-4s-1.

Abstract: NiAl-based alloy is a promising material applied in the fields of aeronautic and astronautic instruments. In the paper the compression deformation behavior and microstructure evolution of NiAl-Cr(Mo)-Hf alloy at elevated temperature were studied. The results demonstrate that the alloy behaves good formability in the temperature ranging from 1320°C to 1360°C, in which the maximum initial strain rate is about 8.3×10-4s-1 and the maximum deformation resistance is lower than 40MPa. During compression at temperature between 1250°C and 1300°C the flow stress increased sharply with the increasing of the deformation degree. When compression deformation at 1320°C~1360°C, the flow stress decreased obviously and the flow stress decreased slightly after reached the maximum value. By analyzing the microstructure evolution during compression it can be concluded that as-casting microstructure was improved in deformation. The grains were refined and the brittle phases of lamellar Cr(Mo) existing at NiAl matrix were broken. The porosities in as-casting material were eliminated during compression and the density of the material increased. The fracture toughness of the alloy increased from 6.4MPa·m1/2 to 9.8MPa·m1/2 after compression.

Abstract: Superplastic behavior and microstructure evolution of an isothermally forged Ti-47Al-1Cr-1V-1.5Mo-1.5Nb alloy were investigated. The results showed that the strain rate sensitivity index, m, increased with strain during the superplastic deformation, and it kept as a constant when the strain reached a certain value. The maximum value of m was 0.53 at 900°C and strain rate of 5x10-4 s-1. During the superplastic deformation, the as received material with lamellae and subgrains were refined due to dynamic recrystallization, and small and equiaxed grains with high angle boundaries were formed, creating a better condition for superplastic deformation. Grain boundary sliding and boundary migration were the main superplastic deformation mechanisms and slip and twining were also very important during the superplastic deformation of the alloy.

Abstract: TiAl alloys are powerful candidates for light weight high temperature structural materials because of their excellent high temperature strength, low density and good oxidation resistance. Unfortunately, TiAl alloys are difficult to machine and hot working due to ordered structure, which impede large-scale application. Synthesis and superplastic deep drawing of a TiAl alloy were studied. Mechanically synthesized fine crystalline Ti/Al composite powders as precursor of TiAl alloy. The TiAl alloy with fine grain via reactive sintering was obtained. The deformation behavior under biaxial stress by means of deep drawing was achieved. The optimum temperature is 1100°C≤T≤1150°C. The microstructures corresponding to deep drawing in the part were studied.

Abstract: The effects of Zr ion irradiation on the mechanical properties of a typical superplastic ceramic, 3mol% yttria stabilized tetragonal zirconia polycrystal (3Y-TZP), were examined and discussed. The specimens were irradiated by Zr11+ ions with 130MeV at fluence level of 3.5×1012 and 2.1×1013 ions/cm2 in the TANDEM accelerator at Tokai Reasearch Establishment of JAEA. Microstructures after annealing and bending deformations to fracture of Zr ion irradiated 3Y-TZP were examined. It was found that the ratio of intergranular fracture to intragranular fracture was increased in the region that was affected by Zr ion irradiation. It seemed that grain boundary cohesion became relatively weak in the irradiated surface region. The influence of Zr ion irradiation on the mechanical properties almost disappeared when the irradiated 3Y-TZP was subsequently heated to 1173K.

Abstract: The 4mol%Sc2O3 doped ZrO2 (4ScSZ) showed a superplastic-like large elongation in a range of strain-rate at 1773K. The large elongation was caused by both of high strain-rate sensitivity and high strain hardening during the deformation. Ion conductivity of the 4ScSZ, deformed superplastically at a relatively high strain-rate region, was higher than that of non-deformed one, suggesting that superplastic deformation can improve its conductivity. These results are discussed with probable microstructural evolution.

Abstract: The Si3N4- Si2N2O composites are fabricated with amorphous nano-sized silicon nitride powders by the liquid phase sintering (LPS) method. XRD analysis shows sintered body consists of β-Si3N4 and Si2N2O. SEM experiment conforms that the average grain size of sintered body is less than 300nm. The complex-shape gears can be formed by a sinter-forging technology when the sintering temperature is 1600°C and the superplastic forging temperature is only 1550°C. Rod-shaped grains aligned along the perpendicular direction of pressure and the mechanical properties increase about 10% after the materials were forged.

Abstract: An average particle size of d ≈ 300 nm can be reduced to a nanocrystalline size of about 10 nm or less after 400 h HEBM process. Amorphous-like phase is also observed among the nanocrystalline particles. Using SPS, the nanocrystalline powder can successfully be consolidated into a dense nanocrystalline ZrO2-spinel composite of d ≈ 90 nm. As compared with the data for submicro-grain composite with d = 350 nm, nano-crystalization increased the strain rate by one order of magnitude or lowered the deforming temperature by about 100 K.