Microstructure Stability and Creep Strength in a Die-Cast AX52 Magnesium Alloy

Abstract

Superior creep strength of a heat resistant AX52 magnesium alloy is ascribed to the grain boundary eutectic Al 2 Ca phase covering the primary -Mg grains. The eutectic phase is stable in morphology at temperatures below 473 K, while it collapses during long term exposure at temperatures higher than 473 K. The microstructural change of the alloy during high temperature exposure is characterized by the decrease in the grain boundary coverage by the eutectic phase. The creep strength of the alloy decreases with the decrease in the grain boundary coverage, and the correlation between the creep strength and the grain boundary coverage is discussed.

Precipitation Strengthening of a Mg-Al-Ca–Based AXJ530 Die-cast Alloy

“…[23,24] Studies on creep properties and deformation mechanisms of die-cast Mg-Al-Cabased alloys postulate that the eutectic structure along grain boundaries suppresses local deformation or sliding of the a-Mg grains in the vicinity of the grain boundaries. [5,[25][26][27][28] In a die-cast AX52 (Mg-5Al-2Ca, wt pct) alloy, dislocation climb in the a-Mg grains is reported as the rate-controlling process at stress levels lower than the yield stress. [25] Recently, the creep properties and microstructure of Mg-Al-Ca-based AXJ530 and AX44 (Mg-4Al-4Ca, wt pct) alloys processed by permanent mold casting were investigated.…”

Effect of Prior Deformation on Creep Behavior of a Die-Cast Mg-Al-Ca Alloy

“…8,14,15) The eutectic structure consists of the phase and a ternary (Mg,Al) 2 Ca intermetallic compound with a C36 dihexagonal structure, 16,17) and the ternary compound would transform to C15-Al 2 Ca equilibrium phase during high temperature exposure. 16,18,19) It has been demonstrated that the eutectic intermetallic phase covering the primary -Mg grains detected in the alloys is stable in morphology below 473 K. 20) Dislocations are introduced within the primary -Mg grains for the Mg-AlCa alloys in the die-casting process, 12,21) which may be due to the fast cooling under geometric constraint that was experienced by the die-cast specimen. [22][23][24] The dislocation density in the -Mg grains for the as die-cast Mg-Al-Ca alloy specimens tends to be rather high in the order of 10 13 m À2 .…”

“…6. The creep rate at 10 À2 h is higher than the minimum creep rate by a factor of $50, when the aging time at 423 K is 0.01 h. The creep rate slightly increases with aging time in the range from 0.01 to 10 h. It is supposed that the basal segments of dislocations, which bow out and glide on the basal planes during the prior deformation at 423 K/80 MPa/160 h, 21) revert back to a large extent within 0.01 h (36 s) during the aging at 423 K. The eutectic intermetallic phase covering the primary -Mg grains is stable in morphology below 473 K, 20) then, the increase in creep rate at 10 À2 h with the aging time at 423 K shown in Fig. 6 would be ascribed to the reversion of the basal segments of dislocations rather than the collapse of the eutectic intermetallic phase.…”

Creep and Corrosion Properties of the Extruded Magnesium Alloy Containing Rare Earth

“…Therefore, to improve heat resistance in Mg-Al based alloy, it seems an effective way to develop an alloy containing stable phases in thermal condition, which suppress the grain boundary sliding. Although some magnesium alloys containing Ca element that exhibit favorable mechanical properties, even at moderate temperature, developed in the past few years [11][12][13][14][15] but the castability is inferior to Mg-Al-Zn alloy. The addition of rare earth (RE) elements effectively works to improve the mechanical properties of magnesium alloys at elevated temperatures as well as the corrosion resistance.…”