Masaharu Tokizane

Recrystallization and formation of austenite in deformed lath martensitic structure of low carbon steels

The effect of prior deformation on the processes of tempering and austenitizing of lath martensite was studied by using low carbon steels. The recrystallization of as-quenched lath martensite was not observed on tempering while the deformed lath martensite easily recrystallized. The behavior of austenite formation in deformed specimens was different from that in as-quenched specimens because of the recrystallization of deformed lath martensite. The austenitizing behavior (and thus the austenite grain size) in deformed specimens was controlled by the competition of austenite formation with the recrystallization of lath martensite. In the case of as-quenched (non-deformed) lath martensite, the austenite particles were formed preferentially at prior austenite grain boundaries and then formed within the austenite grains mainly along the packet, block, and lath boundaries. On the other hand, in the case of lightly deformed (30 to 50 pct) lath martensite, the recrystallization of the matrix rapidly progressed prior to the formation of austenite, and the austenite particles were formed mainly at the boundaries of fairly fine recrystallized ferrite grains. When the lath martensite was heavily deformed (75 to 84 pct), the austenite formation proceeded almost simultaneously with the recrystallization of lath martensite. In such a situation, very fine austenite grain structure was obtained most effectively.

Superplastic behaviour of Ti-48at.%Al two-phase titanium aluminide compacts made from mechanically alloyed powder

Abstract An HIP compact of MA-processed powder having a nominal composition of Ti-48at.% Al was produced. The compact consisted of a large amount of TiAl(λ) and a small amount of Ti 3 Al ( α 2 ), in a completely ultra-fine equiaxed grain structure. This two-phase compact showed typical superplastic deformation behaviour. A maximum elongation of 550% was obtained. A strain exponent, n = 2, and grain size exponent, p = 2, were determined from the results of a strain-rate-change test and a creep test at constant initial stress using samples having various grain sizes, respectively. The activation energy for creep, Q c at constant stress was calculated to be 350 kJ/mole. It is concluded that the superplastic deformation mechanism of the material under study is grain boundary sliding controlled by lattice diffusion in the TiAl phase.

Stress-strain behavior and shape memory effect in powder metallurgy TiNi alloys

The shape memory properties of the TiNi alloy produced by a powder metallurgical method have been evaluated from tensile stress-strain curves. The contamination of the powders during atomization can be suppressed by applying the Plasma Rotating Electrode Process (P-REP), so that the compact made by Hot Isostatic Pressing (HIP) is expected to exhibit the shape memory effect identical to the typical alloy grown from melt. The fracture behavior of the P/M alloy is also studied, and the improvement of fracture strength of the P/M alloy is attempted.

Ultra-fine austenite grain steel produced by thermomechanical processing

Summary We have tried to make fine austenite grains by applying thermomechanical processing to steel with C, Mn and Mo. The thermomechanical processing consists of tempering and subsequent cold rolling of martensite, followed by austenitization. The stabilization of the substructure of tempered martensite and acceleration of the nucleation of austenite lead to the refinement of austenite grains. These are due to alloying with Mn and Mo. We can produce fine austenite grains, 0.9μm in diameter, by applying the thermomechanical processing to steel with 0.4% C, 3% Mn and 1% Mo.

Shape Memory and Mechanical Properties in Powder Metallurgy TiNi alloys

ABSTRACT Using prealloyed powders made by the Plasma rotating Electrode Process (P-REP) method, a TiNi shape memory alloy was produced by the Hot Isostatic Pressing (HIP). The obtained compact has a relative density of 99.6% and chemical analysis showed no detectable increase in impurity contents. The results of electrical resistivity measurements and tensile test showed that the P/M alloy produced by this method exhibits shape memory effect (SME) and superelasticity due to thermally and/or stress-induced martensitic transformation. An R-phase transformation also occurs, and SME and superelasticity due to this transformation were observed. It was found that the transformation and mechanical properties in the P/M alloy prepared are almost identical to those of the typical alloy grown from a melt.