Microstructural evolution and strength variability in microwires

Tensile experiments on cold-drawn Ni microwires with diameters from similar to 115 to 50 gm revealed high strengths, with significant strength variability for finer wires with diameters less than similar to 50 gm. The wires showed pronounced necking at fracture. The coarser wires with diameters > 50 mu m exhibited conventional ductile cup-cone fracture, with dimples in the central zone and peripheral shear lips, whereas finer wires failed by shear with knife or chisel-edge fractures. Shear bands were observed in all samples. Further, through- section microscopy of selected fractured samples revealed that the shear bands did not go across the enitre specimen for the coarser wires. The shear bands led to grain fragmention, with a reduction in grain aspect ratio as well as rotations away from the initial < 111 > orientations. The strength data were analysed based on a Weibull approach. The data could be rationalized in terms of failure from volume defects in coarser wires, with a high Weibull modulus, and from surface defects in finer wires, with a low Weibull modulus and greater variability. (C) 2015 Elsevier B.V. All rights reserved.

Orientation Selection and Microstructural Evolution in Directionally Solidified Tb0.3Dy0.7Fe1.95

Tb0.3Dy0.7Fe1.95 alloy was directionally solidified by using a modified Bridgman technique at a wide range of growth rates of 5 to 100 cm/h. The directionally grown samples exhibited plane front solidification morphology up to a growth rate of 90 cm/h. Typical island banding feature was observed closer to the chilled end, which eventually gave rise to irregular peritectic coupled growth (PCG). The PCG gained prominence with an increase in the growth rate. The texture study revealed formation of strong aOE (c) 311 > texture in a lower growth rate regime, aOE (c) 110 > and ``rotated aOE (c) 110 > aEuroe in an intermediate growth regime, and aOE (c) 112 > in a higher growth rate regime. In-depth analysis of the atomic configuration of a solid-liquid interface revealed that the growth texture is influenced by the kinetics of atomic attachment to the solid-liquid interface, which is intimately related to a planar packing fraction and an atomic stacking sequence of the interfacial plane. The mechanism proposed in this article is novel and will be useful in addressing the orientation selection mechanism of topologically closed packed intermetallic systems. The samples grown at a higher growth rate exhibit larger magnetostriction (lambda) and d lambda/dH owing to the absence of pro-peritectic (Tb,Dy)Fe-3 and formation of aOE (c) 112 > texture, which lies closer to the easy magnetization direction (EMD).