Materials Science Forum Vols. 838-839

Abstract: The liquid hot isostatic pressure in superplastic conditions was applied to eliminate macroporosity in a die cast Al-12%Si eutectic alloy. The removing of macroporosity provides the yield stress increase of 56%, the ultimate tensile strength increase of 46%, the total elongation increase of 21%, the fracture toughness increase of 340% and the fatigue strength increase of 58%. Remarkable improvement of mechanical properties is caused by suppression of crack initiation on internal coarse pores.

Abstract: Currently liners of high-pressure hydrogen storage container for fuel cell vehicles are manufactured from the 6061 aluminum alloy pipes through spinning at elevated temperatures. Since the surroundings of the containers are reinforced with a large amount of high-cost CFRP, the use of 6066 or 6069 aluminum alloy with higher strength than 6061 is demanded to lower the cost of the container. However, the formability of these aluminum alloys at elevated temperatures has not been elucidated yet. In this study, tensile deformation characteristics of 6066 and 6069 aluminum alloys at temperatures ranging from 25 to 550°C were investigated. The total elongation of 6066 aluminum alloy was higher than that of 6069 aluminum alloy at 450°C. This may be caused by the lower volume fraction of constituent particles. The flow stresses of the two alloys were almost the same, and were decreased with increasing testing temperature. The increase in elongation and decrease in strength observed in the two alloys were attributable to dynamic recovery.

Abstract: Tungsten has higher melting point than any other metals. Therefore, tungsten is applied to the electrode for resistance welding. The electrodes for resistance welding of tungsten are mainly used for fusing joining because they have high heat resistance and are hard to be alloyed with work metal pieces. However, the cracks on electrode surface occur during cyclic welding processes. In this study, to reveal the relationship between the elevated temperature deformation characteristics and fracture mechanism of recrystallized and unrecrystallized tungsten, tensile tests were carried out with varying strain rate and testing temperature. At 400°C, the total elongation at a strain rate of 1.4×10-1 s-1 in the recrystallized tungsten decreased more than the total elongation at the other strain rate. At 600°C, total elongation increased by approximately 10%, if the strain rate was increased from 1.4×10-3 s-1 to 1.4×10-1 s-1. Then, the recrystallized tungsten at the strain rate of 1.4×10-1 s-1 had more necking than that of 1.4×10-3 s-1. The may be caused by an environmental embrittlement due to oxygen in the high temperature atmosphere.

Abstract: A commercial AA2519 alloy with a chemical composition of Al-5.64Cu-0.33Mn-0.23Mg-0.15Zr (in wt. %) was subjected to two-step thermomechanical processing (TMP) providing the formation of fully recrystallized structure with an average grain size of ~7 mm in 3 mm thin sheets. Superplastic tensile tests were performed in the temperature interval 450-535°C and initial strain rates ranging from ~2.8 x 10^-4 to ~6.0 x 10^-1 s^-1. The highest elongation-to-failure of ~750% appears at a temperature of ~525°C and an initial strain rate of ~1.4 × 10^-4 s^-1 with the corresponding strain rate sensitivity coefficient of ~0.46.

Abstract: Grain refinement is an important prerequisite for advent of superplasticity. In particular, as the grain size is smaller, the superplasticity appears at higher strain rates and lower temperatures. Severe plastic deformation (SPD) is a useful process for achieving significant grain refinement. This presentation shows that applicability of the SPD process is enhanced when it is operated under high pressure through high-pressure torsion (HPT) and high-pressure sliding (HPS). It is demonstrated that commercially available conventional alloys but less ductile alloys such as Mg alloys, age-hardenable high-strength Al alloys (A2024, A7075) and Ti alloys become superplastic after processing by HPT or HPS.

Abstract: Some features ofstructure, way of production and mechanical properties of titanium andtwo-phase titanium alloys with ultrafine-grained (UFG) microstructure werediscussed. Various methods of UFG structure formation was considered andoptimal conditions for attaining of a homogeneous microstructure with thesmallest grain size were determined. The kinetics of microstructure evolutionin titanium and two-phase titanium alloys during large (severe) plasticdeformation in a wide temperature interval and the mechanisms of ultrafinegrain formation were analyzed. The influence of grain size on static mechanicalproperties was determined. Significantly reduced temperatures of superplasticforming of the UFG conditions were shown. Some examples of practical applicationsof the UFG two-phase titanium alloys were discussed.

Abstract: Characteristics of mechanical behavior during superplastic flow and associated microstructural evolution in the wrought AlCoCrCuFeNi high-entropy alloy were studied. The alloy had complex microstructure with fine grain/particle size of ≈2.1 μm. 4 different phases with volume fractions from 7% to 46% and different deformation characteristics were found in the alloy. Very high tensile elongations of up to 1240% were observed during deformation at temperatures of 800°C–1000°C and at strain rates of 10^-4 s^-1–10^-1 s^-1 despite presence of pronounced softening stage followed by steady state flow stage. Microstructure of the alloy after tensile testing was studied in detail. Phase transformations were analyzed employing thermodynamic modeling and their role in strain accommodation is discussed.

Abstract: The development of submicrocrystalline structure in a Cu-0.3wt.%Cr-0.5wt.%Zr during multidirectional forging (MDF) and equal channel angular pressing (ECAP) was investigated in comparison. A large number of strain-induced subboundaries with low-angle misorientations appeared at early deformation. The subsequent straining led to an increase in the misorientations of these subboundaries, resulting in the formation of submicrocrystalline structure at sufficiently large strains. The process of microstructural evolution can be considered as continuous dynamic recrystallization. MDF provided faster kinetics of new ultrafine grain formation as compared to ECAP. The fraction of ultrafine grains with a size below 2 μm comprised 0.59 or 0.23 after MDF or ECAP to a total strain of 4, respectively. The grain refinement kinetics could be accelerated by the presence of second phase precipitates. The fraction of ultrafine grains after MDF to a strain of 4 achieved 0.36 or 0.59 in the solution treated or aged samples, respectively.

Abstract: The recent studies on grain refinement in austenitic stainless steels during large strain deformations are critically reviewed. The paper is focused on the mechanism of structural changes that is responsible for the development of submicrocrystalline structures that can be interpreted as continuous dynamic recrystallization developing under conditions of warm working. The final grain size that is attainable by large strain warm working can be expressed by a power law function of temperature compensated strain rate with an exponent of about -0.15. The development of submicrocrystalline structures is assisted by the deformation microbanding and dynamic recovery, which are characterized by opposite temperature dependencies. The grain refinement kinetics, therefore, are characterized by a weak temperature dependence for a wide range of warm working conditions.

Abstract: Zn–22Al alloy was processed using a well-designed two-step equal channel angular extrusion/pressing (ECAE/P), and ultrafine-grained (UFG) microstructure with 200 nm grain size was achieved. UFG Zn-22Al was subjected to long-term (up to 60 days) aging at room temperature (RT) and it was seen that natural aging caused limited grain growth in the microstructure. Grain sizes of about 300 nm, 350 nm and 350 nm were measured after 15, 30 and 60 days aging, which mean that UFG Zn-22Al alloy has a good microstructural stability at RT up to 60 days. ECAPed Zn-22Al alloy showed a maximum elongation of about 400% at a high strain rate of 5x10^-2 s^-1 and maximum elongation decreased with increasing grain size. Elongation to failures of ~375% and ~350% were obtained with the samples having 300 nm and 350 nm grain sizes, respectively. In addition, natural aging slightly decreased the strain rate at which superplastic region formed. While the maximum elongation occurred at the strain rate of 5x10^-2 s^-1 in ECAPed UFG alloy, it took place at lower strain rate of 1x10^-2 s^-1 after aging for all time periods. Also, flow stress of the alloy increased with increasing grain size during natural aging.