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INTRODUCTION

MAGNESIUM alloys are the lightest structural metallic materials. Their good specific mechanical properties drive their use in the transportation industry with the aim of replacing heavier components.[1] However, they also have limitations regarding the elastic modulus, low temperature ductility, corrosion resistance and low creep resistance at high temperatures, limiting its use in structural applications at temperatures > 150 C. Therefore, in the past 10 years a considerable effort has been devoted to developing new alloys with the aim of improving these properties. Primarily, these efforts have been concentrated on processing new magnesium alloys by rare earth addition. Currently, creep resistant magnesium alloys contain relatively high amounts of yttrium and/or rare earth elements[2–5] and transition elements.[6–10] Another way to increase the yield strength, is to strengthen the material by a dispersion of oxides/particles made by powder metallurgy or metal matrix composite techniques.[11–15] The improvement of

JORGE A. DEL VALLE and OSCAR A. RUANO are with the Department of Physical Metallurgy, CENIM-CSIC, Av. Gregorio del Amo 8, 28040 Madrid, Spain. Contact e-mail: [email protected] Manuscript submitted September 19, 2019. Article published online February 19, 2020 2344—VOLUME 51A, MAY 2020

creep properties against high temperature is usually attributed to the presence of finely dispersed precipitates/particles that prevent the movement of dislocations. Therefore, introducing a dispersion of fine particles is an appropriate way to improve the creep resistance of magnesium alloys. However, the presence of second-phase particles often carries an associated grain refinement effect that can have an adverse effect on the creep resistance. As is well known, the grain refinement encourages a grain boundary sliding (GBS) deformation mechanism at increasing strain rates. For example, GBS controls plastic deformation at strain rates of 104 s1 at high temperatures in cast or forged AZ31 magnesium alloy with grain sizes < 20 lm.[16] In this case, it is possible to sustain high tensile elongations because of the low stress exponent of the related power creep law, phenomena termed superplasticity. In PM alloys with a microstructure that combines a fine grain size and dispersion of second-phase particles or oxides, the experimental evidence shows,[11–15] on one hand, that the presence of particles helps maintain a fine grain size at high temperatures favoring superplastic creep with a stress exponent n = 2 at high strain rates and stresses. However, on the other hand, data also show a sharp drop in the strain rates for a narrow range of low stresses. That is, very high stress exponents and low ductility are obtained for low stresses. It is common in the literature to attribute the behavior of high stress exponents to the presence of a METALLURGICAL AND MATERIALS TRANSACTIONS A

threshold stress, which in most cases is reported to be grain size and temperature dependent.[11,12,17] From the technological point of vie