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Martensitic transformations in Ag-Cd and Ag-Zn alloys

University of British Columbia

A study has been made of the martensitic transformations occurring in β-phase silver-cadmium and silver-zinc alloys. In silver-cadmium alloys the M(s) temperature was found to change from -44°C to -137°C as the cadmium content changed from 44.2 at. % Cd to 47.0 at. % Cd. Alloys of silver-zinc did not show any martensitic transformation; even on cooling to liquid helium temperature. The thermal martensite in Ag-45 at. % Cd alloy was found to have an orthorhombic structure of the 2H type. This was confirmed by X-ray diffraction and electron microscopy. A spontaneous martensite with a face centred cubic structure was found to occur along the thin edges of perforated specimens used for electron microscopy, because of the relaxation of volume constraints during thinning. In both Ag-45 at. % Cd and Ag-41 at.% Zn alloys a <111> slip direction was found. Also it was shown that Ag-45 at. % Cd alloy was elastically anisotropic, a characteristic very common to β-phase alloys. In both Ag-Cd and Ag-Zn alloys a face centred tetragonal structure could be formed on deforming the specimens by rolling or by tensile deformation. The structure changed to close-packed on severe deformation e.g. by filing. At lower Cd and Zn contents this close-packed structure was face centred cubic, whilst at higher alloy concentrations, this structure was close-packed hexagonal. Pseudo-elasticity was found to occur by stress-induced martensitic transformation. Maximum pseudo-elasticity occurred at temperatures just above A(f) and the actual amount of pseudo-elasticity was found to be dependent on the orientation of the tensile axis. The strain memory effect was studied by deforming specimens below A(f) and then heating. At temperatures below M(f), deformation of the martensite takes place and it is suggested that there is a change in the martensite structure, involving a change from thermal martensite to stress-induced martensite. The experimentally determined habit planes for thermal, stress-induced and deformation martensites were found to agree well with the values obtained using phenomenological theory assuming a {110}<11Ī0> microscopic shear. The 'elastic' elongations accompanying the transformation could be accounted for using the theory. A mechanism suggesting the course of the transformation was developed.

Text

2011-04-29

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http://hdl.handle.net/2429/34172

Metals and Materials Engineering

University of British Columbia

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