Atomic size and local order effects on the high temperature strength of binary Mg alloys

Abstract

The solid solution strengthening introduced by Ca (0.6 and 0.9 at.%) and Sn (0.5 to 2.5 at.%) was studied through tensile, compression and stress relaxation tests at room temperature, 373K (100°C) and 453K (180°C) on solution heat-treated and quenched specimens and compared with existing data for binary alloys containing Ca, Sn, Y, Gd, Nd, Zn and Al as well as for AZ91 alloy. At room temperature the solution-hardening rate introduced by Ca and Sn was much higher than that of Al, matching those of Y, Gd and Zn. Calcium also reduced the tension/compression asymmetry. At high temperature Ca effectively prevented stress relaxation, nearly matching Y, Gd and Nd. Tin was less effective, but still outperformed Al and AZ91 at low stresses. The effects at room and high temperature introduced by Ca and Sn appeared consistent with the presence of short-range order, in line with those introduced by Y, Nd, Gd and Zn. The larger than Mg atom size of Ca, Nd, Gd and Y can be expected to intensify the local order by strengthening the atomic bonds through its effects on the local electron density, accounting for their greater strengthening at high temperature. For given difference in atomic size, the effects on the local order are expected to be lesser for smaller sized atoms like Sn and Zn, hence their more subdued effects.

Development of new creep-resistant magnesium alloys with low cost

“…and Gd were suggested to have the highest potency on developing SRO and therefore significantly increasing the creep resistance of Mg alloys, followed by Nd. Later, research from the same group[124,125] experimentally verified the above hypothesis in terms of the compression and stress relaxation behaviours using binary Mg-M (M denotes the solutes) alloys that were just solid solution treated. It was found that the stress relaxation resistance at 180 C was consistent with the order forming the SRO of solutes.…”

“…Hence, the SRO hypothesis needs further investigation. But, previous studies [124][125][126] indicated that solid solution strengthening of dilute Y, Gd and Nd solutes may play a more important role than precipitation strengthening in improvement of the creep resistance of Mg alloys.…”

“…Gao et al [161] reported that the peak-aged Mg-1Ca-1Zn-0.6Zr (wt.%) alloy exhibited the minimum creep rates at a range of stresses at 150 ℃ about two order of magnitude lower than that of the peak-aged AZ91 alloy, owing to the presence of thermally stable discs on the basal plane of the Mg. On the other hand, unlike the Mg-REs, Mg-Sn and Mg-Zn systems, the low solid solubility of Ca in Mg (0.82at.%) limits the precipitation hardening response in Ca-containing Mg alloys. Nevertheless, effective solid solution strengthening effect by dilute Ca has been suggested in several studies [125,126]. As stated in Section 3.2.2, the tendency of Ca solutes for developing SRO in solid solutions is comparable to those of REs.…”

“…The flow stress of alloys as a function of the temperature for applied compressive strain of 0.02 (c). The figure preceding each solute is its concentration in at.% (reproduced from[124,125]).…”