In-Sn合金のマルテンサイト変態の結晶学的研究

Abstract

The fct(β,c⁄a<1)\ ightleftarrowsfct(α′,c⁄a>1) phase transformation in indium-rich tin solid solutions has been studied by means of X-ray and electron diffraction, with supplement of metallographic observations. The β\ ightleftarrowsα′ phase transformation is of the diffusionless (martensitic) kind, and takes place with a mechanism involving cooperative atomic movements. The atomic movements of the phase transformation can be explained by considering a coupling of two predominant TA1 phonon modes with low frequencies, as in the cases of the In-Tl, In-Cd and In-Pb alloys. After the reverse transformation (α′→β) upon heating, the orientations of the reverted β phase crystal do not always agree with the original ones of the initial β phase (matrix) crystal, i.e. the β\ ightleftarrowsα′ phase transformation is not always crystallographically reversible. It was also found that the indium-tin alloys are slightly decomposed during heating for the reverse transformation; the decomposition process depends strongly on the grain size. The behavior of forward and reverse transformations is quite similar to that of the burst type in Fe-Ni alloys, although for the latter a decomposition process does not interfere with the transformation. It is concluded that the partial shape recovery is mainly associated with an imperfect crystallographical reversibility and also that a gradual decrease in shape memory effect, which was observed after keeping at room temperature for a long period of time, is related with the intermediate stage of α′→(α+β) decomposition. The imperfect crystallographical reversibility may result from the large atomic displacements of the phase transformation.