Effects of Rare Earth Element Additions on the Impression Creep Behavior of AZ91 Magnesium Alloy

Abstract

The effects of 1, 2, and 3 wt pct rare earth (RE) element additions on the microstructure and creep behavior of cast AZ91 Mg alloy were investigated by impression tests. The tests were carried out under constant punching stress in the range 200 to 650 MPa at temperatures in the range 425 to 525 K. Analysis of the data showed that for all loads and temperatures, the AZ91-2RE alloy had the lowest creep rates and, thus, the highest creep resistance among all materials tested. This is attributed to the formation of Al 11 RE 3 with a branched morphology, reduction in the volume fraction of the eutectic b-Mg 17 Al 12 phase, and solid solution hardening effects of Al in the Mg matrix. The stress exponents and activation energies were the same for all alloy systems studied, 5.3 to 6.5 and 90 to 120 kJ mol À1 , respectively, with the exception that the activation energy for the AZ91-3RE system was 102 to 126 kJ mol À1 . An observed decreasing trend of creep-activation energy with stress suggests that two parallel mechanisms of lattice and pipe diffusion-controlled dislocation climb are competing. Dislocation climb controlled by dislocation pipe diffusion is controlling at high stresses, whereas climb of edge dislocations is the controlling mechanism at low stresses.

Effect of Ca and Rare Earth Elements on Impression Creep Properties of AZ91 Magnesium Alloy

“…[9] According to their results, with addition of 2 wt pct RE to AZ91 alloy, the impression rate decreases about 50 pct. On the other hand, AZRC91 alloy shows an impression rate about 3 to 5 times lower than that of AZ91 alloy.…”

“…[4][5][6] Therefore, there have been various attempts to improve the hightemperature properties of AZ91 alloy by adding Ca, Sr, Sb, Bi, and rare earth (RE) elements. [7][8][9][10][11][12][13][14][15][16][17] The Ca is a relatively inexpensive element and its addition to Mg-Al base alloys can improve the maximum increase of both ambient and creep strength for the equal weight unit additions compared to Sr and RE elements. [18] An improvement in creep properties of AZ91 alloy has been reported by Amberger et al [7] with the addition of Ca up to 5 wt pct.…”

“…[19][20][21][22] Similarly, results of many studies have shown that RE elements up to about 2 wt pct can improve high-temperature mechanical properties of AZ91 alloy, whereas they show declines in corrosion resistance and room-temperature mechanical properties at concentrations over 1 wt pct. [9][10][11][12][13] Wang et al [20] have reported that RE elements have little effect on hot tearing susceptibility of AZ91 alloy up to about 1 wt pct. Also, it has been claimed that the fluidity of AZ91 alloy is increased by adding RE elements.…”

Effects of Zr Additions on the Microstructure and Impression Creep Behavior of AZ91 Magnesium Alloy

“…The localized creep behavior of magnesium alloys has mostly been investigated by indentation tests using spherical indenters [20][21][22] and, in some more recent cases, by impression techniques. [23][24][25][26][27] The impression creep of Zr-containing Mg alloys has not been studied before, and thus, the aim of this study is to investigate the creep properties of this alloy using the impression testing technique.…”

Review of recent studies in Magnesium matrix composites

“…An observed decreasing trend of creep-activation energy with stress suggests that two parallel mechanisms of lattice and pipe diffusion-controlled dislocation climb are competing. Dislocation climb controlled by dislocation pipe diffusion is controlling at high stresses, whereas climb of edge dislocations is the controlling mechanism at low stresses [53].…”