Unit cell determination in CuZr martensite by electron microscopy and X-ray diffraction

As several other binary alloy compounds, stoichiometric CuZr has a B2 phase with a CsCl type bcc based structure. In the present system this phase appears as a line compound between 715°C and 935°C (1,2). Rapid cooling to below 140°C transforms this phase into at least two monoclinic structures (1,3) which have been shown to have martensitic characteristics, not unusual for B2 phase alloys, including shape memory behaviour (3,4). Twinning is observed inside as well as between plates or variants (3). Considering the choice of constituents, the martensite microstructure and behaviour and the monoclinicity of the product phases, a possible close relation with the martensitic phase transformation in the NiTi system can be expected (5,6). Unit cell dimensions for both monoclinic phases, one about twice the size of the other, were previously suggested on the basis of powder X-ray diffractometry and limited electron microscopy results (3). Later on these values have been corrected and a first suggestion on the possible atomic positions was made (7,8). As the samples reveal multiple phases and peak broadening indicates internal stresses, possibly due to the extreme smallness of the product grains or variants, fitting of the X-ray profiles becomes rather difficult and some uncertainty on these first results remains. The aim of the current investigation was to confirm or correct these unit cells by extensive selected area electron diffraction (SAED), high resolution electron microscopy (HREM) and improved fitting procedures of the existing X-ray diffraction data. The crystallographic relation between parent and product phases will also be discussed.status: publishe