Developments in deformation and recrystallization textures were studied in cold-rolled (50–90% reduction) ultra low carbon (ULC) steel using X-ray texture measurements and orientation imaging microscopy (OIM). During deformation, γ-fibre (ND//〈111〉) increased between 0 and 50% reduction but then did not change significantly, while α-fibre (RD//〈110〉) increased progressively from 0 to 90% reduction. After complete recrystallization, however, a steady increase in γ and almost no changes in α were observed with increasing strain. Developments in recrystallization textures were attributed to two parameters: (1) spacings (λi, as measured along the normal direction, ND, where i can be a specific component of α/γ-fibres) of the α/γ deformed bands; and (2) their relative ability to form recrystallized grains. While λi was determined by the deformation texture and the thicknesses of the deformed grains/bands and naturally decreased with increasing strain, estimations of parameter (2) were obtained from the so-called nucleation factors (Ni, defined as the number of recrystallized i grains per i band—as measured/estimated along the ND). At higher strains, noticeable drops in the Nis of α-fibre were observed. Two plausible causes for such drops were increased stored energy advantages for γ bands and orientation pinning in some of the α regions.
A study has been made of the spontaneous growth of tin whiskers from tin electrodeposits on phosphor bronze sheet. The driving force for the evolution of tin whiskers is a biaxial compressive stress of about 8 MPa developed in tin deposits by the formation of an intermetallic compound of Cu6Sn5, especially in grain boundaries of tin films. The biaxial compressive stress gives rise to strains normal to the film plane, which are dependent on the tin grain orientations. The shear stresses due to differences between strains of different grains along the thickness direction make the tin surface oxide film sheared approximately along boundaries of grains with particular orientations which are different from the major texture of the film. Tin extrudes from the grains, whose surface oxide films are sheared. The extrusion takes place continuously by expansion of the dislocation loops by climb and their subsequent glide toward the surface in the slip direction to form whiskers. (C) 1998 Acta Metallurgica Inc.
A micromechanical model describing “quasi-ductile” Hertzian contacts in otherwise brittle ceramics is developed. The elemental basis of the model is a discrete “fault” along an internal weak interface, constrained at its ends by an elastic matrix and subject to frictional sliding, in the subsurface zone of high shear stress in the Hertzian field. By summation over a prescribed density of shear faults within the active plastic zone, the analysis leads to a constitutive identation stress-strain function, with special provision for the incorporation of microstructural variables. Experimental data from a series of mica-containing glass-ceramics with contiguous platelet microstructures are used to confirm the essential predictions of the model. It is demonstrated that plasticity increases with volume fraction and aspect ratio, but not size, of the platelets. Parametric evaluations by curve fitting the indentation stress-strain data allow for predictions of intrinsic stress-strain responses for the glass-ceramics in conventional uniform stressing states.
An isothermal kinetics of non-equilibrium grain-boundary segregation was developed both for segregation processes and for desegregation processes within a phenomenological theory. An effective time concept of a cooling process was discussed. On these bases, a simple and accurate method for evaluation of the levels of non-equilibrium grain-boundary segregation during cooling was proposed. According to the method, we have calculated the levels of non-equilibrium segregation to austenite or prior austenite grain boundaries for boron in Fe-30%Ni alloy, aluminium in Inconel 600, chromium in 2.25%Cr1%Mo steel and tin in 2.25%Cr1%Mo0.08%Sn steel in different experimental conditions respectively. Results calculated from the kinetic model in this paper are in satisfactory agreement with the observed data of experimental measurements for all the above samples.
The superplastic Zn-22% Al eutectoid alloy was tested over a range of strain rates at 503 K using specimens machined with the rolling direction perpendicular to the tensile axis. It is shown that the mechanical properties of the alloy, including the elongations to failure, are essentially identical to those obtained when the rolling direction is parallel to the stress axis. Inspection after failure showed that cavities are formed in stringers and, as in the same alloy tested with the rolling direction parallel to the tensile axis, these stringers are aligned parallel to the stress axis. The results demonstrate that the cavities are not nucleated primarily at impurity or oxide particles, nor do they grow from pre-existing microvoids which may be introduced during thermomechanical processing. The cavities are located preferentially at the α β interfaces and at the associated triple points, and it is probable that they were nucleated at triple points and grain boundary ledges during bursts of grain boundary sliding.
