The Two Way Memory Effect in relation to the nucleation and growth of martensite

Abstract

The shape memory properties observed in some alloys are due to a structural transformation of martensitic type which occurs quasi reversibly in a narrow temperature range. The high and low temperature phases will be called austenite and martensite respectively regardless of their structures which can change from one alloy to the other. The one way memory effect can be described as follow : a sample of a given shape in the austenitic state, cooled in the martensitic state does not change its macroscopic shape. At this temperature, it can be apparently plastically strained by an applied stress; a reheating in the austenite state regenerates the initial shape. This cycle can be repeated, the sample can be strained in martensite in the opposite direction, it will recover again its initial shape by heating. After some special thermomechanical treatment, on cooling. in the way austenite to martensite, a spontaneous macroscopic strain of the sample appears, which vanishes on heating in the autenitic state due to the simple memory. Therefore, without any external applied stress, a spontaneous change occurs between two different shapes respectively in the austenitic and the martensitic states. This phenomenon is called the Two Way Memory Effect (TWME). In the alloys which exhibit a thermoelasttc martensitic transformation, the one way memory effect is always effective, it is inherent to t h e martensitic transformation. The TWME must be induced by some thermomechanical treatment : it is an acquired property. This paper will first shortly describe the relation between the martensitic transformation and the one way memory effect, then the TWME. Afterwards the thermomechanical treatment inducing the TWME which is called "Training" will be detailed. Finally, the possible origins of the TWME will be reviewed.