Continuous measurement of m-parameter for Hart analysis of superplastic behaviour of ultra-fine grained AZ31 Mg alloy

J. Stráský, J. Stráská, M. Janeček


The ultra-fine grained materials (UFG) often show superplastic behaviour at elevated temperatures due to small grain size. On the other hand, UFG microstructure is often not stable at increased temperatures and undergoes recovery and recrystallization. In this paper, we investigate the superplastic properties of UFG AZ31 Mg alloy prepared by equal channel angular pressing. The computer controlled tensile tests were performed in a temperature range of 175°C−250°C and at strain rate around 10-4 s-1. The superplastic behaviour and strain hardening is related to temperature, strain-rate and microstructure stability. Methodology of continuous measurement of m-parameter during the tensile test to achieve the dependence of m-parameter on true strain is presented. The measurement is based on alternating two slightly different true strain rates, keeping the overall true strain-rate constant. The m-parameter decreases with true strain for all studied conditions from approx. 0.5 to 0.3 and the decrease is fastest for the highest considered temperature (250°C), which is attributed to recovery and recrystallization. The sample deformed at 250°C strain hardens over significantly shorter interval of true strain which also suggests microstructure changes. Limits of plastic instability according to Hart analysis were computed for all conditions. Hart analysis of stability of plastic deformation seems not to be sufficient for description of elongation until fracture for studied material.

References (19)

Y. Umakoshia, W. Fujitania, T. Nakanoa, A. Inoueb, K. Ohterac, T. Mukaid, K. Higashie. Acta Materialia 46 (13), 4469 (1998).
R.B. Figueiredo, M. Kawasaki, C. Xu, T.G. Langdon. Materials Science and Engineering A 493 (1–2), 104 (2008).
R.B. Figueiredo, T.G. Langdon. Advanced Engineering Materials 10 (1‐2), 37-40 (2008).
H.K. Lin, J.C. Huang. Materials Transactions 43 (10), 2424 (2002).
H.K. Lin, J.C. Huang, T.G. Langdon. Materials Science and Engineering A 402, 250 (2005).
R.B. Figueiredo, T.G. Langdon. Materials Science and Engineering A 501, 105 (2009).
A. Mohan, W.Yuan, R.S. Mishra. Materials Science and Engineering A 562, 69 (2013).
J. Stráská, M. Janeček, J. Čížek, J. Stráský, B. Hadzima. Materials Characterization 94, 69 (2014).
E.W. Hart. Acta Metallurgica 15, 351 (1967).
A. Considère, Ann. Ponts Chauss. (Set. 6). 9, 574 (1885).
T.G. Langdon. Metallurgical Transactions A 13 (5), 689 (1982).
P. Haehner. Acta Metall, Mater. 43 (11), 4093 (1995).
P. Málek. Materials Science and Engineering A 137, 21 (1991).
J. Vrátná, M. Janeček, J. Čížek, D.J. Lee, E.Y. Yoon, H.S. Kim. Journal of Materials Science 48 (13), 4705 (2013).
M. Janeček, J. Čížek, J. Gubicza, J. Vrátná. Journal of Materials Science 47 (22), 7860 (2012).
H. J. Frost, M. F. Ashby. Deformation-mechanism maps: the plasticity and creep of metals and ceramics. Pergamon Press, New York, ISBN 0080293387 (1982).
M.F. Ashby, R.A. Verrall. Acta Metallurgica 21, 149 (1973).
J.P. Young, H. Askari, Y. Hovanski, M.J. Heiden, D.P. Field. Materials Characterization 101, 9 (2015).
A. Molinari, J. Méc. Théor. Appl. 4, 659 (1985).