Change of Length during Transformation of Iron under Stress

Abstract

IT is well known¹ that various metallic materials are mechanically weak while being transformed into another allotropic modification. We measured the change of length of specimens of pure iron² while the metal, kept under tensile stress, was heated through the transformation temperature at 910° C. In Fig. 1 the change of length for fine-grained iron has been plotted for one α–γ transformation, one γ–α transformation and for a complete α–γ–α cycle as a function of stress. The resultant strain for the complete cycle gives a straight line through the origin. The fine-grained material for these experiments was obtained by loading the iron with a small amount (0.008 per cent by weight) of nitrogen³. All measurements were made in vacuo. An iron–carbon alloy containing 0.2 per cent by weight of carbon behaves in essentially the same way when given a temperature cycle through the two-phase field α + γ.