Modelling the transition from strengthening to softening due to grain boundaries

• Published in: Materials science & engineering. A, Structural materials : properties, microstructure and processing Vol. 483; pp. 95 - 98
• Main Authors: Zeng, X.H., Eisenlohr, P., Blum, W.
• Format: Journal Article Conference Proceeding
• Language: English
• Published: Amsterdam Elsevier B.V 15.06.2008; Elsevier
• Subjects: Applied sciences; Elasticity. Plasticity; Exact sciences and technology; Grain boundary; Mechanical properties and methods of testing. Rheology. Fracture mechanics. Tribology; Metals. Metallurgy; Spontaneous annihilation; Statistical dislocation model; Thermally activated annihilation; Transition; 66.30.Pa; Spontaneous annihilation; Thermally activated annihilation; Transition; Statistical dislocation model; 62.25.+g; 61.82.Rx; 61.43.Bn; 61.72.Bb; Grain boundary; Grain size; Strengthening; High strain; Plastic flow; 61.43.Bn; 61.72.Bb; 61.82.Rx; 62.25.+g; 66.30.Pa; Transition metal; Plastic deformation; Dislocation; Statistical model; Fine grain structure; Statistical dislocation model; Grain boundary; Spontaneous annihilation; Thermally activated annihilation; Transition; Copper; Activation energy; Hall Petch relationship; Flow stress
• ISSN: 0921-5093; 1873-4936
• DOI: 10.1016/j.msea.2006.09.130

Published data on pure Cu at or close to its saturation state of deformation suggests a Hall–Petch relation of flow stress at ambient temperature in the range of grain sizes 1  μ m  > d > 10  nm [W. Blum, Y.J. Li, J. Chen, X.H. Zeng, K. Lu, Int. J. Mater. Res. 97 (2006) 1661–1666] while for elevated temperatures ultrafine-grained Cu produced by severe plastic deformation exhibits softening relative to conventional grain sizes d > 10 μ m [Y.J. Li, X.H. Zeng, W. Blum, Acta Mater. 52 (2004) 5009–5018]. Presuming grain boundaries as impenetrable dislocation obstacles, these observations are rationalized by (i) a reduction of the mean slipped area and corresponding shift in the main location for dislocation storage from grain interior to boundaries with decreasing d; (ii) thermally activated dissolution of dislocation dipoles occurring faster at boundaries than within the grain interior owing to different coefficients of diffusion. A simple statistical dislocation model is derived from these ideas and compares to experimental results in semi-quantitative agreement.