The Formation of Macrotwins in NiAl Martensite

Abstract

Using high resolution electron microscopy, Boullay & Schryvers [6] observed and documented interesting macrotwin interfaces in bulk and splat-cooled Ni₆₅Al₃₅ polycrystals. This alloy undergoes a cubic (bcc) to tetragonal (bct) martensitic transformation. The macrotwin interfaces separate two different plates of martensite, each plate involving the same two martensitic variants but with microtwin interfaces that are nearly orthogonal. They measured the orientations both of the microtwin interfaces and of the rotations of the variants comprising the microtwins, in the vicinity of the macrotwin plane and at distances of up to several hundred nanometres from this plane. In addition, they described atomic scale details of the macrotwin region, in particular identifying two different ways in which the microtwins meet at the macrotwin plane, one in which they appear to cross, and the other in which they meet in a stepped configuration (see Fig. 1). In the bulk samples, which form the main focus of this paper, the grain-size is large (of the order of 1mm) compared to that of the macrotwins. In the splat-cooled samples it is smaller (of the order of 1μm).

Research supported by EC TMR Contract FMRX-CT98-0229 on ‘Phase Transitions in Crystalline Solids’.

Research supported by the above EC contract and the IUAP project of the Belgium Federal government on “Reduced Dimensionality Systems” (P4/10).