Effects of dynamic recrystallization on g grain refinement and improvement in micro segregation of elements such as Ni and Mn of as cast austenite in 9 % Ni steel were investigated by two kinds of experimental methods. The first one was a hot compression test using the specimens prepared from the strand cast slab and the hot rolled plate of 9 % Ni steel, and the other was a hot tensile straining test in the austenitic region after levitation melting and solidification. In the hot compression test, variations in onset strain and flow stress of steady state flow of dynamic recrystallization with hot deformation conditions were investigated. The onset strain was found to decrease below 0.25 at the temperatures above 1 523 K and the strain rate below 1ϫ10 Ϫ2 s Ϫ1. The activation energy value obtained from steady state flow stress was 421 kJ/mol. Dynamically recrystallized g grain size in as cast austenite of this steel was controlled simply by Z value with no dependence on the initial g grain size. This beneficial feature of dynamic recrystallization was confirmed by the experiment of tensile straining in austenite formed after levitation melting and solidification, where extremely coarse initial g grain size of around 1.9 mm was markedly refined down to 140 mm by straining such a small strain as 0.40. Micro segregation ratio of Ni and Mn in the strand cast 9 % Ni steel examined by EPMA analysis was 1.20 and 1.36, respectively. These values were found to decrease continuously with reduction in strain rate in hot deformation of austenite. That is, dynamic recrystallization in austenite taken place at lower strain rate deformation is much more effective for homogenization of segregated elements compared with high strain rate deformation.

The effect of small amounts of yttrium addition on refinement of b grain size and grain growth behavior in SP-700 alloy with Ti-4.5%Al-3%V-2%Fe-2%Mo was investigated in the temperature region from 1 223 to 1 573 K. Yttrium and oxygen contents in yttrium-added alloys were varied from 0.007 to 0.070 % and 0.017 to 0.086 %, respectively. Such a small amount of yttrium addition as 0.007 % resulted in marked b grain refinement. Variation of b grain size with the heating temperature exhibited rapid grain growth at the particular temperature, which became higher with the increase of yttrium or oxygen content in alloys. It was confirmed that b grain refinement and grain growth behavior with the heating temperature in yttrium-added alloys were brought about by fine precipitates of yttria and dissolution of these precipitates into the b matrix at the higher temperature. TEM and SEM observations of yttrium-added alloys revealed the cubic shape of yttria precipitates with the size of 0.1 to 0.2 mm, and this range of the precipitate's size was well fitted to results analyzed by modified Zener' model for pinning of grain boundary due to a fine particle. Based on assumption that the onset temperature of rapid grain growth corresponds to the re-dissolution temperature of yttria into the b phase, solubility product of Y 2 O 3 in reaction of 2Yϩ3O→ ← Y 2 O 3 was obtained in use of data of variation of the onset temperature of rapid grain growth with yttrium and oxygen contents in alloys. It was also confirmed that yttria powder could be used as an alloying additive of yttrium in melting of yttriumadded alloys.KEY WORDS: b grain refinement; yttrium; oxygen; SP-700 alloy; yttria; rapid grain growth rate; solubility product.ened through grain refinement and precipitation hardening in the transformed microstructure.8-11) One of the major difference between steel and titanium is that such microalloying elements forming carbide or nitride can not be utilized in titanium because of their high solubility in both b and a phases. However, if such finely dispersed compounds to enable to dissolve and re-precipitate into the b phase of titanium materials could be found instead of carbide or nitride, b processing or b heat treatment can be used as the final manufacturing processing, and moreover, new thermo-mechanical processing utilizing microalloying elements can be developed, realizing marked improvement of mechanical properties through microstructure control in titanium materials.Based on these backgrounds and understanding of the present status of titanium manufacturing technology, the authors paid attention to titanium boride of TiB and oxide of Y 2 O 3 as a grain refiner instead of carbide or nitride. Titanium alloys containing titanium boride or yttria added up to 2 % volume fraction were prepared by two routes of powder metallurgy and ingot making, and b grain size variation with the heating temperature was investigated.12,13) It was found in both cases of alloy preparation that b grain size was markedly refined with the increase of t...

Dynamic and static restoration behaviors of pure lead and tin were investigated by compression tests, and the deformation temperature and strain rate were varied in the range from 223 to 348 K and from 2 Â 10 À3 to 1 s À1 , respectively. Lead and tin used had two purity levels of 99.999% (5N) and 99.9% (3N), and 5N and 4N, respectively. The hot working simulator was reformed so as to enable compression tests at low temperatures ranged from 223 to 273 K. S-S curves observed in lead and tin were those in dynamic recrystallization and dynamic recovery types of metals, respectively, of which activation energies were 92 to 119 kJ/mol in lead and 49 to 52 kJ/mol in tin. Steady state flow stress in lead with 5N purity was lower than that of tin with 5N purity. A reduction of purity level from 5N to 3N in lead significantly increased flow stress, but difference in purity level of 5N and 4N in tin exerted tiny influence on flow stress at strain rate below 1 Â 10 À1 s À1 . Static recrystallization in lead with 5N purity completed in the holding time of less than 600 s even at 273 K, while tin with the same purity showed a slightly retarded recrystallization progress. A reduction of purity level in both metals extended the time period for completion of recrystallization by more than 2 orders.

New α+β type titanium alloy with Ti-4.5Al-6Nb-2Mo-2Fe was developed on the basis of using biocompatible elements and eliminating the cytotoxic ones such as Vanadium, while achieving the desirable mechanical properties such as appropriate strength, cold workability and low superplastic forming (SPF) temperature. The present study was conducted to investigate the effect of yttrium addition of less than 0.05% into this alloy on static and under superplastic deformation grain growth behavior. The new alloy bar manufactured by α+β processing and annealed at 1073K yielded extremely fine two-phase microstructure with α grain size around 2μm. Specimens were heated at temperatures of 1048, 1073 and 1098K and kept for times between 3.6 to 172.8KS. Yttrium forms in-situ Y2O3 particles, and the presence of these particles yield finer two phase microstructure due to their retardation effect on β phase grain growth. Grain growth behavior during hot deformation was investigated by hot compression test in use of a hot working simulator of THERMEC-Master Z. Strain rate was varied from 2×10-2 to 2×10-4S-1 and strain was 0.69. Grain size of both α and β phases increased with a reduction of strain rate, and Y2O3 particle was also effective to retard grain growth under hot deformation. It was confirmed from comparison of grain growth during isothermal heating with and without hot deformation that grain growth was much accelerated by deformation. All of these results were discussed based on grain growth mechanism or model for two-phase microstructures as well as superplastic deformation mechanism.