Multi-scale Examination of the Microstructure Evolution of AZ31 During Multi-pass Cross-Rolling
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INTRODUCTION
MAGNESIUM alloys are attracting increasing attention for automotive applications due to their high strength-to-weight ratio. The main limitation to widespread utilization of Mg alloys continues to be their poor formability at room temperature.[1] The origin of this limitation is often discussed in terms of the low number of operating independent slip systems in Mg at room temperature. Analysis of the critical resolved shear stress (CRSS)
for slip in Mg reveals that basal[2]slip In {0001} 11 20 is the most favored slip system.
addition, it is possible to have slip along the 1120 directions on the prismatic and pyramidal planes. None of these slip systems offers the possibility of accommodating deformation in the c-direction. Deformation along the c-axis could be accommodated by deforma[3,4] and possibly by slip on tion twinning
[5] 11 22 11 23 , which is often referred to as hc + ai slip. The extent to which hc + ai slip occurs has
S. LIANG, X. WANG, H.S. ZUROB, and N. BASSIM are with the Department of Materials Science and Engineering, McMaster University, 1280 Main St. W., Hamilton, ON L8S 4L7, Canada. Contact e-mail: [email protected] Manuscript submitted September 18, 2017.
METALLURGICAL AND MATERIALS TRANSACTIONS A
attracted a lot of attention because of the high CRSS for this type of slip.[2] As for deformation twinning, the twins groups: are generally divided into
three 12 1011 20 10 extension twins ( 11 86.3 deg),
12 contraction twins ( 1120 1011 10 56 deg), and 1011 to 1012 double-twins ( 1120 37.5 deg).[5–10] The first of these has a very low CRSS and is readily activated at low strains. As for contraction twinning, the CRSS is high and comparable to the CRSS for non-basal slip.[11] Recent studies have examined the microstructure evolution of AZ31 sheets during compression and cold rolling at room temperature.[12–16] These results show a complex interaction of basal slip, twinning, and shear banding. The above studies, however, were limited to low strains because the onset of shear banding and subsequent cracking precluded observations at high strains.[14,17–20] Higher deformation strains could be achieved through the use of cross-rolling. During cross-rolling, the specimen is rotated 90 deg after each deformation path. Several authors have argued that cross-rolling leads to a weaker basal texture compared to unidirectional rolling[21,22] and thus leads to higher deformation strains.[23] Of particular interest is the work of Couling et al.[24] who reported a high total cold-rolling reduction when they employed a cross-rolling procedure with a 2 pct reduction per pass.[25] The present work will attempt to shed more light on the microstructure evolution of alloy AZ31 during cross-rolling. Special emphasis will be placed on the high strains that have rarely been examined in the literature.
II.
EXPERIMENTAL
A. Sample Preparation The experiments were carried on 2-mm-thick sheets of alloy AZ31. The sheets were homogenized at 400 C for 24 hour
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