Grain boundary strengthening in strongly textured magnesium produced by hot rolling
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I.
INTRODUCTION
I T is well known that at low temperatures, grain boundaries strengthen metals and the dependence of the flow stress, ~r, is related to the average grain diameter, l, through the HallPetch relation: 1 tr = tr0 + k1-1/2
[1]
where tr0 and k are constants representing the intercept and slope of the Hall-Petch line. In hexagonal close packed (hcp) metals, e.g., Zn, Cd, Mg, Ti, Zr, Be, both these constants are temperature dependent. 2-7 Further, Armstrong 8 has shown that tr0 is related to the critical resolved shear stress (CRSS) for the easy slip system operating within the grain volume, and k is related to the CRSS for the more difficult slip/twinning systems required to operate near the grain boundaries in order to maintain the continuity of strain. Typically for magnesium, tr0 is related to the CRSS (~'0) for {0001 } (1120) slip, and k is dependent on the CRSS (re) for {10i"0}(1150). Recently, it was observed in cadmium materials 9'~~ that the Hall-Petch constants are also texture dependent, inasmuch as the magnitude of the orientation factors relating them to the relevant single crystal resolved shear stresses is decided by the texture intensity. The influence of the grain size on plastic flow in magnesium was earlier investigated by Hauser et al,4 Wilson and Chapman," and Wilson ~2'13using hot extruded materials. These results clearly indicate that tro and k are texture dependent and considerable strengthening can be achieved by having an intense basal texture. The aim of the present investigation was to study the influence of texture on grain boundary strengthening in hot rolled magnesium. The hot rolling process produces ~4 textures of the ideal type in magnesium sheets (i.e., {0001} G. SAMBASIVA RAO, formerly a QIP Research Scholar in the Department of Metallurgy, Indian Institute of Science, Bangalore 560 012, India, is Lecturer, Department of Metallurgy, Regional Engineering College, Warangal 506 004, India. Y.V.R.K. PRASAD is Associate Professor, Department of Metallurgy, Indian Institute of Science, Bangalore 560 012, India. Manuscript submitted February 25, 1982.
METALLURGICALTRANSACTIONS A
(1120)), unlike in extruded material. H The ideal texture facilitates an accurate study of the mechanisms of strengthening in terms of the basic processes, viz., slip, twinning, and cracking. In general, the approach in this investigation was to study the influence of texture on the Hall-Petch constants obtained on longitudinal and transverse sheet specimens as well as on square-rod specimens.
II.
EXPERIMENTAL
A. Material and Specimen Preparation Magnesium metal of 99.87 pct purity was used in this investigation. The maximum contents (wt pct) of the impurities were as follows: Cu : 0.005; Mn : 0.1; Ni : 0.001; Pb : 0.005; Si : 0.01; Sn : 0.001; Fe : 0.004. The starting material for mechanical processing to produce sheets was a cast slab of dimensions 40 mrn x 40 rnm • 18 mm which was hot forged at 720 K to a final thickness of 11 mm. This was done in several stages, each involving a reduction o
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