Features of the formation of tetragonal martensite in In–4.5%Cd alloy

Using the methods of metallography, radiography and transmission electron microscopy (TEM) it is shown that during cooling of In–4.5 mass% Cd alloy below the FCC-FCT martensite transformation temperature a lath structure consisting of colonies of tetragonal plates is formed. From dark-field and electron diffraction analyses it is concluded that the laths consist of parallel martensite plates with a habit of {101}FCС. The average width of martensite plates in the lath on a (001) plane is about 0.1 μm. Pairs of neighbor plates of a martensite differ by the directions of tetragonal axis <001>. During the inverse FCT-FCC transition, a dissolution of martensite plates starts at grain boundaries, and along with shortening of martensite crystals, their thinning is observed. After complete dissolution of martensite crystals, they are often replaced with dislocations which are parallel to the boundaries of former martensite plates. In In–4.5 %Cd alloy, in the pre-transition region of FCT-FCC transformation (pre-austenite state) tetragonal broadening of Bragg's reflections and increasing of diffuse scattering located around them are observed on electron diffraction patterns. The results can be interpreted as a consequence of softening of the alloy’s lattice and pre-transition structure changes. The alloy structure in the austenite state after the complete reverse martensite transformation has a typical “ripple”-like contrast. It is found that after a cycle of FCT-FCC-FCT transformations recrystallisation of the alloy occurs with a several fold decrease of the grain size as compared to the initial size, and lath sizes, the length and width of martensite plates in the laths correlate with the change of the grain size of the alloy.