熱膨脹曲線に現われたマルテンサイト変態の二重性とAl含有量の関係について

抄録

In our 1st report, we stated that there were two types of β′ transformed from β by martensitic way, the one occurring with large expansion by rapid cooling (Vc>500°C/min), and the other occurring with contraction at the rate of 500°C/min>Vc>15°C/min. In this report, we will give the results of our study on the effect of Al content on the duplicate property of the β-β′ transformation. We have obtained some interesting results, as follows: (1) A maximum quantity of expansion and a minimum critical cooling velocity for formation of the expanded β′ were recognized in 12.52%Al specimen. (2) The expansion increased as the quenching temperature rose in 12.18% and 12.64%Al specimens. (3) The γ′ martensite which occurred in the hyper-eutectoid alloys, for ex., 13.59% and 13.94%Al specimens, did not show such a duplicate property of the β′ martensite. (4) Abnormal thermal contraction and lowering of electrical resistance took place at 200∼300°C following tempering in 10.2% and 10.58%Al specimens. (5) The Ms temperature of contracted β′ transformed by both air-cooling and furnace-cooling lay within the temperature range of 250∼290°C. (6) Upon microscopic observation, we found the characteristic features of β′ transformed by water cooling, air-cooling and furnace-cooling, respectively. The propable nature of the transformation from β to martensite was also discussed.