Effects of Stress and Deformation on the Martensite Transformation

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Abstract
The relation between elastic and plastic strains and the martensite transformation from face‐centered to body‐centered structure has been investigated in Fe‐Ni 71–29 alloy. Transformation begins at about −30°C and is of the martensite type except that it is not all completed instantaneously but continues at decreasing rate at constant temperature. It was found that (1) There is no anomalous decrease in elastic moduli as the Ms temperature is approached, nor is Ms affected by applied elastic stresses. (2) Plastic deformation in a temperature range above Ms causes partial transformation in amounts decreasing to zero at temperature Md≈Ms +75°C, but stabilizes against further transformation by cooling. (3) Plasticyield stress increases sharply from Md to Ms then drops suddenly to a low value just below Ms . (4) The Ms temperature depends on grain size; a factor of ten increase in grain size reduces Ms about 30°C. These results are contrary to expectations on the basis of the usual homogeneous shear model for the transformation mechanism. They indicate that the martensite phase becomes thermodynamically stable at Md but that transformation does not begin until Ms because of surface and strain energy considerations in growth of a martensite plate from a small nucleus.