Effect of ausaging on the morphology of martensite in an Fe-25%Ni-7.5%Si alloy

Abstract
The martensitic transformations in an Fe-25%Ni-7.5%Si (mass%) alloy ausaged under various conditions were investigated by means of optical and transmission electron microscopy, differential scanning calorimetry, electrical resistivity-temperature measurements and hardness-testing. It was clarified that nanoscale particles of γ'-(Ni,Fe)₃ with Ll₂ structure were formed in the austenite matrix by ausaging between 400 and 600°C, while no precipitation occurred due to ausaging at 700°C. The Ms temperature was depressed by ausaging at 400 and 500°C, but hardly changed when the ausaging temperature was 600°C. It was observed that the morphology of martensite which was lenticular in unaged specimen changed to a mixture of lenticular and thin plate by ausaging at below 600°C.