The effect of the cyclic heat-treatments on the martensitic transformation was examined with iron-nickel binary alloys containing 24.50 to 30.70 atomic per cent nickel from the observations of dilatation curves, microstructures and mechanical properties. Ms temperatures lowered with the increase in the number of cyclic heat-treatments so that the amount of martensite transformed also decreased. The optical and the electron microstructures of martensite after cyclic heat-treatments consisted of finer martensite plates with high dislocation density. The hardness and the tensile strength of austenite were remarkably increased by cyclic heat-treatments at an early state, but in the case of martensite they were not remarkably increased. It was suggested that these results would be explained by mechanisms based on the hardening of austenite and the partial decomposition of martensite due to the cyclic heat-treatments
