Effect of Cobalt Content on Microstructure and Strength in Severely Cold Drawn Ni-Co wires
The future upcoming technologies in aerospace, transport and nuclear industries demand materials with superior mechanical properties that are also economically viable. The quest to achieve theoretical strength of material by various techniques has been the aim of many material scientists across the globe. The strength of a material depends on the resistance experienced by dislocations during plastic deformation and in a polycrystalline material, it has been found that strength varies inversely with grain size (d) according to Hall-Petch relationship σ=σ0+KHP d−0.5 where σ is the yield stress at grain size d, σ0 is a lattice frictional stress and KHP is the Hall-Petch coefficient. For T/Tm < 0.5, grain boundaries are stronger than grains and they obstruct easy motion of dislocation, thereby contributing to strengthening. Extensive research on enhancing strength by grain size refinement led to development of various plastic deformation techniques such as high-pressure torsion, accumulative roll bonding and equi-channel angular pressing. Electro-deposition is a bottom up technique which is also used to produce bulk nanocrystalline materials. Studies carried on nanocrystalline (nc) materials showed that below a critical grain size ≤ 10 nm, grain boundary mediated deformation processes such as grain boundary sliding and grain rotation are activated, leading to a saturation in strength or weakening. The above processes cannot be used easily for industrial scale. Wire drawing is an to be an alternate method for producing ultrafine grained and nc materials. For alloys with weak solid solution strengthening, strengthening by grain refinement is the only way of strengthening and strain hardenability in these materials primarily depends on their stacking fault energy. For a given strain and lower the stacking fault energy, higher is the difficulty in cross slip of dislocations and higher is the hardening. The present study primarily focuses on the effect of stacking fault energy on strength and microstructural evolution in Ni-Co alloys during wire drawing strain. The Ni-Co alloys with 33, 50 and 60 wt. % Co were vacuum arc melted, suction cast into 3 mm diameter rods followed by homogenization at 1473 K for 24 h in vacuum of 10-5 mbar. These were drawn to 1900 μm diameter, homogenized again at 1473 K in vacuum of 10-5 mbar for 12 h. The metallographically polished samples revealed mean linear intercept grain sizes ranging from 20 to 70 μm from images electron back scattered diffraction (EBSD). Wire drawing was carried at room temperature on a Instron universal testing machine with a specially designed cage to mount the wire drawing dies and a three jaw chuck to grip the wire which was mounted at fixed end of the Instron. Polycrystalline diamond dies were used with 30% reduction in area for each die. Ni-Co alloys of 500, 200 and 100 μm diameters corresponding to a drawing true strain of 2.88, 4.05 and 5.88 were obtained and used for further study. Microscopic analysis of these wires showed formation of micro bands during deformation. Grains sizes in drawn wires correspond to the width of the grains. Ni-60 Co wires showed finer and more equiaxed grains and unlike the Ni-33Co and Ni-50Co which had wider and more elongated grains. Bulk texture and EBSD analysis on polished samples showed that increasing addition of Co leads to an increase in <100> fiber faction and a decrease of <111> fiber fraction. Taylor factor (M) values calculated from EBSD were found to be lower for Ni 60 Co alloy (M = 2.75 to 3.14 ) unlike the Ni 50 Co and Ni 33 Co alloys (M = 3.07 to 3.24). Tensile testing of these samples showed higher strength and higher ductility for Ni-60 Co alloy. The strength and ductility increased with increasing wire drawing strain in Ni-60 Co alloy . The strain rate sensitivity (m) calculated from strain rate jumps on monotonic tensile tests showed m to be in the range of 0.010 to 0.016, with an increase with higher Co content. The stress relaxation tests on drawn samples showed a decrease in activation volumes with increasing wire drawing strain. The activation volumes also decreased with an increase in Co content. It decreased with increase in wire drawing strain suggesting the dislocation intersecting mechanism. The Ni Co alloys of 100 μm diameter annealed at temperatures ranging from 973 K - 1123 K for varying times had grain sizes ranging from 2 to 10 μm. The Hall – Petch coefficient (KHP) and lattice frictional stress (σ0) calculated from tensile tests showed no effect of SFE on them and these values were in agreement with pure Ni data available in literature. The KHP for drawn wires were found to be higher than annealed ones. X-ray line profile analysis carried out on drawn wires to measure dislocation densities by using Williamson Hall method were in order of 1015 m-2 suggesting nearing saturation. A semi-empirical model has been used to predict strength for drawn condition.