Grain Size Hardening in Mg and Mg-Zn Solid Solutions
- PDF / 863,478 Bytes
- 10 Pages / 593.972 x 792 pts Page_size
- 43 Downloads / 250 Views
TRODUCTION
THE Hall–Petch (H-P) equation
[1–4]
ry ¼ ro þ kd1=2
takes the form ½1
where d is the grain size and ry is the yield strength. The term ro is usually interpreted as a friction stress suitably modified by the Taylor factor and k as a stress intensity factor related to the difficulty in transferring slip from grain to grain and to the nucleation of multiple slip within grains.[5,6] Experiments show that Eq. [1] describes the relationship between the strength and grain size in both cubic and hexagonal metals.[7–11] However, in the case of Mg and its alloys, the literature[11–15] shows large discrepancies in the values of both ro and k. There are several reasons that may account for the apparently contradictory observations from different laboratories in pure Mg[7,9] and its alloys.[16–18] First, the strong dependence of both parameters on orientation texture affects ro and k in opposite directions.[9,19] Second, both parameters depend on the deformation mode (slip or twinning) and, therefore, on the proportions of each,[20–23] this being especially important in pure Mg.[24–26] Third, the strain at which the yield strength is determined[10,27,28] can depend on the individual investigator. Fourth, solid C.H. CACERES, Reader in Casting Technology, is with the ARC Centre of Excellence for Design in Light Metals, Materials Engineering, School of Engineering, The University of Queensland, Brisbane, QLD 4072, Australia. Contact e-mail: [email protected] GEMMA E. MANN, formerly Postgraduate Student, CAST Co-operative Research Centre, Materials Engineering, School of Engineering, The University of Queensland, is now Lecturer with Central Queensland University, Rockhampton, QLD 4701, Australia. J.R. GRIFFITHS, Post-Retirement Fellow, is with CSIRO Process Science and Engineering, Kenmore, QLD 4069, Australia. Manuscript submitted May 14, 2010. Article published online January 11, 2011 1950—VOLUME 42A, JULY 2011
solution effects on both ro and k are important and are yet to be quantified, as explained in more detail below. Solid solution effects on the strength of Mg are anisotropic;[29] in dilute Mg-Zn and Mg-Al alloys, solute atoms harden the basal plane,[30–32] whereas they cause solid solution softening of the prism and pyramidal slip systems.[33–35] In concentrated Mg-Zn alloys (c = 0.5 to 2.6 at. pct, where c is the solute concentration), Zn causes extensive hardening, which has been ascribed to short-range order.[32] Extension {10-12} twins are preferentially activated at low strains in compression[25,29] creating a tension/ compression (t/c) asymmetry in the yielding behavior, which may be quite pronounced in the presence of intense basal texture[13,36] although less so in random polycrystals.[29] The twinning stress in Mg and its alloys appears to have a stronger dependence on grain size than does yield by dislocation slip,[13,21–23] as it does in other face-centered cubic, body-centered cubic, and hexagonal metals.[20,37–39] A larger k value should, therefore, be expected whenever twinning dominates, in parti
Data Loading...
