Mechanisms of creep deformation in Mg-Sc-based alloys

Abstract

Binary Mg-Sc alloys show only a very weak age-hardening response due to the low diffusivity of Sc in Mg and exhibit inferior creep resistance compared to WE alloys. The addition of a small amount of Mn (Ͻ1.5 wt pct) improves their creep behavior markedly, decreasing the minimum creep rates by up to about two orders of magnitude at temperatures above 300 °C compared to WE alloys. This is due to the precipitation of fine Mn 2 Sc phase basal discs, which are very effective obstacles in controlling creep at temperatures at which cross-slip of basal dislocations and nonbasal slip are the rate controlling mechanisms. The addition of Ce improves the creep resistance even more due to the effect of the grain boundary eutectic. The effect of Mn 2 Sc discs can still be seen in alloys with a low Sc content (ϳ1 wt pct) and with the addition of rare earth (RE) elements (Gd, Y, Ce ϳ4 wt pct). Very thin hexagonal plates containing RE and Mn, which lie parallel to the basal plane of the Mg matrix, augment the effect of the Mn 2 Sc precipitates at elevated temperatures (ϳ250 °C). The triangular arrangement of prismatic plates of metastable or stable phases of Mg-RE systems controls effectively the motion of basal dislocations during the creep of these alloys at elevated or high temperatures. The combined control of basal slip, cross-slip of basal dislocations, and of nonbasal slip in low Sc content alloys ensures minimum creep rates of about one order of magnitude lower than those observed in WE alloys, both at elevated and high temperatures.

Creep and Fracture Behavior of Peak-Aged Mg-11Y-5Gd-2Zn-0.5Zr (wt pct)

“…[2,6] However, HZ32 is being phased out due to its radioactivity. [6,18] The peak-aged WGZ1152-T6 exhibited superior creep resistance compared to the WE54-T6 at T=523-573K (0.58-0.64T m ) and σ=50-80MPa, and was comparable to the HZ32-T5 at T=573K and σ=30MPa. At T=573K and σ=50MPa, the minimum creep rate of the WGZ1152-T6 was almost two orders of magnitude lower than that for WE54-T6, and the creep rupture life of WGZ1152-T6 (676h) was more than ten times longer than that for WE54-T6 (53h).…”

“…WE54 is the most commercially used structural Mg alloys for elevated-temperature applications up to 523K, [2,17,18] and HZ32 is the Mg alloy which has been used at the highest service temperatures (618≤T≤643K). [2,6] However, HZ32 is being phased out due to its radioactivity.…”

“…The minimum creep rate of WGZ1152-T6 was comparable to that for HZ32-T5 at T=573K and σ=30MPa. [18] From the slop, the creep stress exponent n can be determined as 4.5 for all the test temperatures. strain ε II was determined by the following equation [19] II r min…”

“…The measured Q avg value of 221±20kJ/mol was much higher than the activation energy for lattice self-diffusion, Q sd , of Mg (135kJ/mol). [50] High activation energy values for creep have also been found in pure Mg, [37] Mg-0.8%Al solid solution alloy, [38] Mg-5Al-1Sr, [41] Mg-Sc based alloys, [18] and Mg-Y based alloys [6,49] . Vagarali et al [37] reported the activation energy for creep in pure Mg to be 220± 10kJ/mol in the temperature range 600-750K, and there was evidence of extensive non-basal slip.…”

HRTEM Observation of Metastable Phases in a Mg–15%Gd–6.4%Sc Alloy

“…6,7) We concluded that the 00 and 0 phases coexisted on monolayers of Gd and Y on f1 1 100g Mg planes during the first stage of hardening, and the remarkable increase in hardness in the second stage was caused mainly by the presence of the 0 phase. Mordike et al 8) reported that binary Mg-Sc alloys (10-19 mass%), as a result of an almost complete absence of precipitation, show negligible age hardening and that their minimum creep rate decreases with increasing Sc content. The addition of Mn (1 mass%) to Mg-Sc alloys containing up to 15 mass% of Sc improves the creep resistance due to the formation of a densely dispersed fine particles of the Mn 2 Sc phase that is stable at temperatures up to 573 K. Stulikova et al 9) reported that in the Mg-Gd-Sc-Mn alloy with low Sc content (0.91 mass%), a complicated precipitation process involving precipitation of fine Mn 2 Sc particles, the Mnand Gd-containing phases, and the metastable 0 phase is responsible for the superior creep resistance and peak agehardening at elevated temperatures.…”