X-Ray investigation of one-dimensional disorder in supersaturated Co-Cu Alloys

Abstract

Single crystalline grains of coarse polycrystals of Co alloyed with 8 to 10 wt\% Cu were studied by X-ray diffractometry. A martensite phase with one-dimensionally disordered h.c.p. and long-period polytype structures is observed. Up to four martensite variants (domains) are formed in one crystal, where the h.c.p. and the polytypic phases are stacked along any one of the four 111c directions. Stacking disorder in these martensite structures was analysed by Monte Carlo simulation. The polytype structures are formed both after quenching and slow cooling from 1100 °C. The long-period polytype in quenched samples has a disordered 7T1 structure which can be described in terms of a non-random distribution of h.c.p. lamellae within the f.c.c. matrix. The high-temperature f.c.c. phase has a perfect structure before and after transformation. It is suggested that the formation of polytypes is related to small variations of chemical composition in different grains of the material.