Microstructure and texture evolution in wire drawing and its role on the tension and torsion response of nickel microwires
It is now well known that the strength of the metallic materials depends on microstructural features such as dislocation density, grain size, and precipitates. The migration of lattice defects such as dislocations is a common mechanism for plastic deformation in conventional metals. The hindrance to the motion of defects by internal microstructural barriers enhances the strength of metals, leading to an inverse power law relation between barrier spacing and strength. This can be considered an intrinsic size effect as it only depends on the length scales of the internal microstructural features. The recent increase in the usage of micro-electronics mechanical system (MEMS) devices has made it essential to understand the mechanical behavior of deformed materials at micron and sub-micron scales. Studies have shown that in addition to the internal microstructural features, the external dimensions of a sample can play a vital role in defining mechanical properties on a small scale. This dependence is an extrinsic size effect because it is not a bulk material property but depends on external sizes. Wire drawing is a straightforward deformation technique through which metals can be deformed to very large strains while simultaneously decreasing the wire diameter to few microns. This makes it feasible to use drawn microwires for studying the interplay of both internal and external size effects. It has been reported that the mechanical behavior of microwires changes with a reduction in diameter. However, the literature is not coherent in this field. Some studies have reported a significant size effect, whereas others do not. Most of the investigations have been done on the as-received drawn microwires where no information regarding the wires' thermo-mechanical treatment was available. The research pertaining to the torsion behavior of drawn microwires is extremely limited, as most torsion studies on microwires are limited to the annealed microstructures. This study was undertaken to develop a comprehensive understanding of the microstructural evolution and its effect on the tension and torsion response of pure nickel during wire drawing. Starting with an annealed condition, wires were drawn to various strain levels/diameters. Tensile tests were performed on different diameter microwires to examine the strength, ductility, and size effect. The evolution of grain size, dislocation density, and grain orientation with drawing strain and their effect on the mechanical behavior has been studied. Stress relaxation tests were also performed to characterize the dislocation activation volumes (V) and strain rate sensitivities (m). A lab-scale wire drawing setup was developed to produce drawn microwires from an initial annealed wire diameter of 1.7 mm down to 30 μm, corresponding to a total strain of 8.1. The microstructural features and their evolution as a function of drawing strain was studied using electron back-scattered diffraction (EBSD) and X-ray diffraction (XRD). The evolution was analyzed in the framework of grain subdivision, and the generation of new high angle boundaries called geometrically necessary boundaries (GNB). Wire drawing led to grains elongated along the drawing axis, and a progressive increase in strength with more wire drawing strains. There was a drastic reduction in the spacing between transverse high angle boundaries, approaching ~100 nm at large strains. The dislocation density increased from an initial value of ~ 3x1014 m-2 at a strain of ~0.3 to a stabilized value of ~ 2x1015 m-2 beyond a strain of ~2. Texture studies on the cross-section of drawn wires revealed that the core region had a very strong <111> fiber texture different from the shell region with <112> fiber texture. The role of this texture gradient on strength was also examined experimentally. A reduction in the wire size by electropolishing, which reduced the shell region, led to an increase in strength and reduced ductility. Corresponding on the basis of earlier in-situ synchrotron studies that showed load transfer from the plastic deformation initiation of the <111> grains to the other textures in the shell, leading to strain hardening and ductility. Viscoplastic self-consistent (VPSC) simulations were performed to simulate the texture evolution and showed good agreement with the experimentally measured crystallographic texture. Although the dislocation densities measured by the XRD method showed saturation at large strains (ɛd > 2), there was no such saturation in the strength. The tension results of drawn wires were compared with severe plastic deformation (SPD) techniques to provide a broad framework for analyzing microstructure-strength-ductility relationships. Analyses based on Hall-Petch behavior revealed that the Hall-Petch slope of the severely deformed wires remained similar to that of the annealed wires, but there was a drastic increase in the Hall Petch constant σ0 due to additional dislocations with a constant density introduced by wire drawing to large strains. It was shown that severely deformed Ni processed by rolling or accumulated roll bonding follows a similar behavior as that observed in wire drawing, but with lower strengths related possibly to lower accumulated dislocation densities. An experimental torsion testing setup was developed to perform torsion tests on the microwires with diameters 100 μm and below. The torsion tests showed an opposite trend compared to the tension results, with the least drawn wire showing maximum yield strength and vice-versa. This was attributed to the increase in the intensity of <111> fiber texture with drawing strain and a corresponding reduction in other textures in the shell. A new process of reversing the direction of wire drawing at each new stage led to a quicker development of a more homogeneous structure with <111> fiber texture and no core-shell microstructural architecture. Mechanical tests on such wires were consistent with the behavior and analysis of traditional wires with a core-shell microstructural architecture.