Tensile strength and work hardening of ultrafine‐grained high‐purity copper

Sputter‐deposited high‐purity copper specimens with grain sizes from 8.4 to 0.056 μm were tested in tension to investigate the influence of grain size on yield strength for small grain sizes. The smallest grain size in the sputter‐deposited copper was a factor of 40 smaller than the smallest grain size previously available for yield‐strength–grain‐size studies of high‐purity copper. The 0.2% offset yield strengths varied from 73.4 MPa for the coarsest‐grained copper to 481 MPa for the finest‐grained copper. The hardening was related to grain‐ and twin‐boundary spacings. The yield strength obeyed the Hall‐Petch relation even for the finest grain size of 0.056 μm, giving σy=6.39+3.74(l)−1/2 MPa, where σy is the 0.2% offset yield strength and l is the mean intercept length (mm) between boundaries. The data agreed with the Ashby model, σ∼ (e/l)1/2, only at very low strains (e<0.001) for the specimens with very fine grain sizes (l⩽0.077 μm).