EFFECT OF GAMMA[--]EPSILON CYCLIC TRANSFORMATIONS ON THE SHAPE-MEMORYCHARACTERISTICS OF AN FE-MN-SI-CR-NI ALLOY

Effects of gamma <-- --> epsilon cyclic transformations on the shape m emory characteristics have been studied in an Fe-14Mn-6Si-9Cr-5Ni (wt. %) alloy. First, the effects of two cyclic transformations (one is the gamma --> epsilon transformation by cooling and its reverse epsilon - -> gamma transformation by heating, and the other is the gamma --> eps ilon; stress-induced transformation and its reverse transformation by heating) on the transformation temperatures were investigated by diffe rential scanning calorimetry (DSC). M(s) and M(f) temperatures slightl y decrease with increasing number of the gamma <-- --> epsilon (epsilo n is formed on cooling) cyclic transformations. The effect of the gamm a <-- --> epsilon cyclic transformation (epsilon is stress-induced) on the A(f) temperature is fairly large, and it is remarkably increased with increasing number of the gamma <-- --> epsilon cyclic transformat ions. The amount of martensite remarkably increases with increasing nu mber of the gamma <-- --> epsilon cyclic transformations (epsilon is f ormed on cooling). Next, the effect of gamma <-- --> epsilon cyclic tr ansformation (gamma --> epsilon stress-induced transformation and its reverse transformation by heating under various stresses) on the recov ery stress was investigated. When the gamma <-- --> epsilon cyclic tra nsformation is repeated, the recovery stress first increases remarkabl y and then decreases monotonically irrespective of the stress applied during the reverse transformation. This increase of recovery stress is attributed to not only the increase of elongation when the specimen i s deformed but also to the increase of stacking faults, which act as n ucleation sites for martensite. The monotonic decrease of recovery str ess is attributed to the decrease of elongation.