Creep of magnesium strengthened with high volume fractions of yttria dispersoids

Abstract

Creep experiments were performed on dispersion-strengthened-cast magnesium (DSC-Mg), consisting of unalloyed magnesium with 1 mm grain size containing 30 vol.% of 0.33 mm yttria particles. Strain rates were measured for temperatures between 573 and 723 K at compressive stresses between 7 and 125 MPa. DSC-Mg exhibits outstanding creep strength as compared with other magnesium materials, but is less creep resistant than comparable DSC-Al and other dispersion-strengthened aluminum materials. Two separate creep regimes were observed in DSC-Mg, at low stresses (|B30 MPa), both the apparent stress exponent (n app :2) and the apparent activation energy (Q app :48 kJ mol − 1 ) are low, while at high stresses (|\34 MPa), these parameters are much higher (n app =9-15 and Q app =230-325 kJ mol − 1 ) and increase, respectively, with increasing temperature and stress. The low-stress regime can be explained by an existing model of grain-boundary sliding inhibited by dispersoids at grain-boundaries. The unexpectedly low activation energy (about half the activation energy of grain boundary diffusion in pure magnesium) is interpreted as interfacial diffusion at the Mg/Y 2 O 3 interface. The high-stress regime can be described by dislocation creep with dispersion-strengthening from the interaction of the submicron particles with matrix dislocations. The origin of the threshold stress is discussed in the light of existing dislocation climb, detachment and pile-up models.

Impression creep behavior of extruded Mg–SiCp composites

“…Stress exponents in the respective range of 3-7 and 4-7 have also been reported for AZ91-20% Al 2 O 3 [35] and AE42 Mg alloy reinforced with Saffil short fibers and SiC particulates [25]. Our results, however, are in contrast to a series of works on the creep of Mg-based composites that reported unusually high stress exponents, due to the existence of threshold creep behavior [15][16][17]29]. Almost all of these studies have obtained more reasonable stress exponents after incorporating a threshold stress into their analyses.…”

“…The creep studies of Mg alloys containing micron-submicron-and nano-sized reinforcements are all indicative of an overall enhanced creep resistance of the produced composites over the monolithic base alloy [15][16][17]. Ferkel and Mordike [18] studied the effect nano-sized SiC on the tensile creep resistance of pure Mg using a powder metallurgy route.…”

Properties of magnesium alloys reinforced with nanoparticles and carbon nanotubes: a review

“…As-cast billets were hot extruded with an extrusion ratio of 20 at a temperature of 400°C. Microstructural characterization and mechanical properties [31] as well as creep behaviour [32] was investigated. The density of extruded Mg/30Y 2 O 3 was 2.7154±0.0024 g/cm 3 and, assuming no porosity and the densities of magnesium and yttria to be 1.74 g/cm 3 and 5.03 g/cm 3 respectively, a volume fraction of 29.7 ± 0.001% was estimated.…”

“…In[32] the creep behaviour was determined at temperatures between 200°C and 450°C and stresses between 7 and 125 MPa. A compressive creep with constant load was performed and the true stress was calculated at the deformation where the minimum creep rate was achieved.…”

Effect of sintering techniques on the microstructure and tensile properties of nano-yttria particulates reinforced magnesium nanocomposites

“…Graces et al investigated the microstructure, tensile and compressive properties to correlate the mechanical anisotropy with texture of magnesium matrix reinforced with micron size yttria reinforcements of two different sizes up to 20 vol.% [6]. Han and Dunand reported microstructure, room temperature tensile and compressive properties and creep of dispersion-strengthenedcast magnesium (DSC-Mg) containing 30 vol.% of submicron size Y 2 O 3 particulates [7,8]. However, Y 2 O 3 particulates reinforcement in nano-size investigated has convincingly increased both the strength and the ductility of magnesium simultaneously.…”