Morphological and structural studies of thermoelastic martensites in a Ag-38 At. Pct Zn alloy

Abstract

Thermoelastic martensites formed upon subzero cooling a Ag-38 at. pct Zn alloy have been studied by means of optical and electron microscopy, electron diffraction, and electrical resistivityvs temperature measurements. Two different morphologies, slender banded and spear shaped, are observed in cooled bulk specimens, both being thermoelastic. The slender banded martensite is retained at room temperature in thin foils after subzero cooling and possesses the 9R type long period stacking order structure with internal stacking faults on the basal plane. This 9R martensite is “normal” with an orthorhombic lattice:a=4.81,b=2.82 andc=20.5Å (a:b:c=1.71:1:7.27). The spear shaped martensite is also retained at room temperature in subzero cooled thin foils and in shape memory cycled ribbons as well. This morphology exhibits a monoclinic 3R type stacking order structure with internal twins and stacking faults. Lattice parameters of the 3R martensite area=4.83,b=2.87,c=6.94Å and β=87.4 deg (a:b:c=1.68:1:2.41). This structure is equivalent to that of AuCuI (L1₀, fct) with lattice parametersa=4.07,c=3.89, andc/a=0.96. The twin and stacking fault planes in the 3R martensite are the (201) and (001) planes respectively. The basal planes of the 9R and 3R martensites are common, being generated from the {110} plane of the CsCl type parent phase, and the two martensites differ only in the shuffling mode on the {110} plane upon transformation. The thermelastic nature of the 3R martensite is slightly different from that of the 9R one.