Microstructural evolution and strength variability in microwires

Tensile experiments on cold-drawn Ni microwires with diameters from ~115 to 50 µm revealed high strengths, with significant strength variability for finer wires with diameters less than ~50 µm. The wires showed pronounced necking at fracture. The coarser wires with diameters >50 µm 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.