Papers by Author: Zenji Horita

Abstract: (α + γ) two phase stainless steel (Fe-21%Cr-4.8%Ni-1.5%Mo) powder was processed by high pressure torsion (HPT) and consolidation at room temperature. The received powder had fully α single phase due to the rapid cooling during gas atomizing process. Specimens after HPT process were heat treated at 1173K for 3.6ks. It was revealed that the decomposition of α phase to γ took place during the heat treatment. Detailed microstructure observation showed that an equiaxed (α + γ) micro-duplex structure was developed and its average grain size was approximately 3.2 micrometers. The same heat treatment given to the material without HPT resulted in a coarse two phase microstructure.Therefore, it is considered that an ultra fine grained microstructure was caused by increasing of nucleation sites for γ phase due to severe plastic deformation (SPD) of HPT process. Electron backscatter diffraction patterns (EBSD) analysis indicated that α phase has a {110}/ND strong texture, that is, the α phase seems to have single orientated coarse grain structure. The γ precipitates indicated a {111}/ND strong texture, and the crystallographic orientation relationship of Kurdjumov-Sachs was observed.

Abstract: In this paper, comprehensive studies on the age-hardening behavior and precipitate microstructures of severely deformed and then artificially aged aluminum alloys have been conducted to clarify whether or not concurrent strengthening by ultrafine-grained and precipitation hardenings can be achieved. From our graphically-illustrated equivalent strain dependence of both the attained hardness and increment/decrement in hardness during aging (i.e. age-hardenability), three strategies to maximize the combined processing of severe plastic deformation and age-hardening technique are proposed. (1) Lowering of aging temperature and (2) utilization of microalloying elements can improve not only the attained hardness but also the age-hardenability of high-pressure torsion (HPT) specimens of Al-Mg-Si (-Cu) alloy due to the increased volume fraction of transgranular precipitates. A further increase in hardness can be achieved by (3) taking advantage of spinodal decomposition for HPTed Al-Li-Cu alloy, in which nanoscale precipitates of δ’ phase are successfully formed within ultrafine grains, irrespective of the higher number density of grain boundaries. The attained hardness of >HV290 in the latter alloy is almost the highest among conventional wrought aluminum alloys, and therefore our proposed strategies will be useful for designing concurrently strengthened severely-deformed age-hardenable aluminum alloys.

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.