Papers by Author: Hiromi Miura

Abstract: Dynamic recrystallization (DRX) behavior in a coarse columnar-grained Cu-0.65Sn-0.025P and Cu-0.025P (mass%) alloys were systematically investigated by compression tests at temperatures between 1073 K and 1253 K and at true strain rates from 2 x 10^-4 s^-1 to 2 x 10^-1 s^-1 in vacuum. As a model sample, an orientation-controlled bicrystal having [0 1 twist 18^o boundary was prepared and also hot deformed. Appearance of the peaks stress, where DRX onsets, was much delayed in Cu-Sn-P alloy compared with that in Cu-0.025P alloy. The onset of DRX was, therefore, obviously impeded by the small addition of Sn to Cu-P. While nucleation of new grains took place almost at random in Cu-Sn-P when strain rate was high enough, it tended to appear more preferentially at grain boundary with decreasing strain rate and with increasing temperature. The most of new grains were annealing twins formed behind the migrating grain boundary. Because grain boundary migration took place more extensively with increasing temperature and with decreasing strain rate, the preferential nucleation at grain boundary became more significant.

Abstract: The evolution process of ultrafine grains during hot severe plastic deformation (SPD) was studied in several aluminum alloys. The structural changes can be characterized by the evolution of deformation bands such as microshear bands (MSBs) at moderate strains. The process of strain-induced grain formation can be categorized into the three stages irrespective of deformation mode and temperature: i.e. i) an incubation period for new grain evolution in low strain; ii) a grain fragmentation by frequent development of MSBs and subsequently new grains in medium strain, and iii) a full development of fine grains in large strain. Temperature effect on the new grain formation in aluminum alloys is also analysed in detail and the mechanism operating is discussed.

Abstract: A commercial AZ61 magnesium (Mg) alloy composed of coarse initial grains was multi-directionally forged (MDFed) under decreasing temperature conditions from 673 K to 463 K up to a cumulative strain of ΣΔε = 6.4 at a true strain rate of 3 × 10^-3 s^-1. A pass strain of Δε = 0.8 was employed. The average grain size decreased gradually with an increase in the cumulative strain. After straining to ΣΔε = 6.4 (i.e., after 8 passes of MDF), equiaxed ultrafine grains (UFGs) with an average size of approximately 0.8 μm were uniformly produced. These grains were relatively coarse as compared with MDFed Mg alloys having initially finer grains. The hardness of the AZ61 Mg alloy increased monotonically up to 910 MPa with decreasing grain size. The Hall-Petch relationship held within this experimental condition.

Abstract: Dynamic recrystallization (DRX) behaviour in a newly developed Cu-Sn-P alloy for heat exchangers and tubes was systematically investigated. For this purpose, Cu-Sn-P alloys with different content of Sn were deformed in compression at temperatures between 1073 K and 1213 K and at various strain rates from 2 x 10-4 s-1 to 2 x 10-1 s-1. The onset of DRX was more advanced with increasing temperature and with decreasing strain rate. Full DRX was not achieved at the testing conditions of lower temperature and higher strain rate even after straining to ε = 1.0. This tendency was more significant in the alloy with higher Sn content. With increasing Sn content, the flow stress and the obtained grains size became higher and finer, respectively. These experimental results indicate the important role of Sn for strengthening and microstructual control.

Abstract: Dynamic recrystallization (DRX) behavior in a Cu-0.65Sn-0.025P (mass%) alloy (Cu-Sn-P), which had been newly developed for high strength copper tubes, was systematically investigated. For this purpose, an orientation-controlled bicrystal ( =28o); a model samples of the as-casted billet having coarse columnar grains, was hot deformed in compression at 1073 K at true strain rates from 2 x 10-3 s-1 to 2 x 10-1 s-1 in vacuum. Appearance of peak stress, where DRX sets in, was much delayed in Cu-Sn-P alloy compared with that in Cu and the other copper alloys. While nucleation of new grains preferentially took place at grain boundary, this tendency became more significant with decreasing strain rate. Almost all the new grains were annealing twins (3) formed behind the migrating grain boundary. The more preferential nucleation at grain boundary with decreasing strain rate could be, therefore, reasonably understood by easier and more extensive occurrence of grain boundary migration at lower strain rate.

Abstract: New thermo-mechanical processes (TMPs) to produce ultrafine-grained copper alloys utilizing continuous recrystallization (cRX) were proposed. These methods stand on our hypothesis that the evolution of ultrafine grains can be evolved by a mechanism of cRX even during severe plastic deformation at ambient temperature. A TMP of warm compression of 10 to 20 % of Cu-1.7mass%Fe alloy followed by annealing was cyclically repeated. The slight reduction, low-temperature annealing and pinning of grain boundaries by precipitates efficiently impeded occurrence of discontinuous recrystallization (dRX). The evolved substructures with nodes of the Fe precipitates gradually changed to new grains surrounded by low- and high-angle boundaries with increasing number of the repeated processes. Ultrafine grains with average size of 0.7 m were successfully evolved. However, the onset of dRX triggered extended grain coarsening accompanied by grain-boundary migration under conditions of insufficient annealing temperature and large pass stain. Another TMP cycles of cold rolling and annealing also induced fine-grained structure of about 0.6 m. The above results improved that ultrafine grain refinement can be realized simply by a mechanism of cRX even in the metallic materials with low stacking fault energy.

Abstract: The evolution mechanisms of ultrafine grains processed by severe plastic deformation are studied in ferritic steel, copper and aluminum alloys. The structural changes are characterized by the evolution of deformation bands such as microshear bands (MSBs) at moderate strains. The process of strain-induced grain formation can be subdivided in the following three stages irrespective of deformation temperature: i.e. an incubation period for new grain evolution in low strain; grain fragmentation by frequent development of MSBs in medium strain, and a full development of new grains in large strain. A mechanism of new grain formation during SPD, i.e. the MSB-based model, is proposed and discussed comparing with the subgrain-based model.

Abstract: A bulky SUS316 austenitic stainless steel (SUS316) was multi-directionally forged (MDFed) at 77 K and 300 K up to a cumulative strain of = 6 at maximum. With increasing cumulative strain, the grains were subdivided by mechanical twinning and martensitic transformation. Especially, mechanical twins accelerated grain fragmentation by subdivision of the initial grains and by intersection of the previously formed twins during MDF. The intersection of twins caused finally evolution of packet grains, which were composed of lamellar structured twins. The packet size and the lamellar twin spacing decreased down to 35 nm and 15 nm by MDF to  = 6 at 77 K. The average grain size achieved was estimated to be about 10 nm. Twinning appeared more frequently and uniformly at 77 K than at 300 K. Tensile test at 300 K revealed ultimate tensile strength of 2.1 GPa and fracture strain of about 0.2. The fracture strain, however, appeared to be constant over ∑Δε = 2.4 independent of cumulative strain. The observed excellent balance of strength and ductility of the nano-grained SUS316 is discussed in relation with the effects of twins on grain fragmentation and mechanical properties.

Abstract: The static recrystallization (SRX) behavior of nano grained (NGed) Cu-30mass%Zn alloy processed by Multi-directional forging (MDF) was investigated. The NGed Cu-Zn alloy showed characteristic annealing behavior. The SRX nucleation and its grain growth occurred more readily in the samples deformed to higher cumulative strain at lower temperature. The frequency of new grain formation in the samples MDFed at 77 K was much higher than that at 300 K. The new grains were composed of fine annealing twins with thickness from 10 to 200 nm. The average grain size fully recrystallized was less than 300 nm.