Discontinuous and continuous precipitation in magnesium–aluminium type alloys

Abstract

The microstructure investigations of the AZ91 alloy and binary Mg–9 wt.% Al alloy after different heat treatments were presented. Solution annealing at 693 K for 26 h (with water quenching), followed by ageing at 423, 473, 543 and 623 K was carried out. After ageing at 423 K only discontinuous precipitates were observed whereas at 623 K only continuous ones were revealed in the microstructure of both alloys. At intermediate ageing temperatures (473 and 543 K) both discontinuous and continuous precipitates occurred competitively. Additional analyses revealed that after cooling a solid solution from 693 K (without quenching) only discontinuous precipitates were formed. On the other hand, after heating a supersaturated solid solution from room temperature to 665 K both discontinuous and continuous precipitates were observed simultaneously. On the basis of the obtained results a model of precipitate types dependent on heat treatments for Mg–9 wt.% Al alloys was proposed.

Introduction

In lightweight magnesium alloys, aluminium constitutes the main alloying element, chiefly because of its low price, availability, low density and the advantageous effect on corrosion and strength properties [1], [2], [3], [4], [5]. The microstructure of as-cast Mg–Al alloys is generally characterized by: a solid solution of aluminium in magnesium (an α-Mg phase with a hexagonal close-packed structure) and an α + γ eutectic (fully or partially divorced depending on its solidification rate) [5], [6], [7], [8], [9], [10]. The γ-phase (called also β-phase [11], [12], [13], [14], [15]) is an intermetallic compound with a stoichiometric composition of Mg₁₇Al₁₂ (at 43.95 wt.% Al) and an α-Mn type cubic unit cell. In comparison with binary Mg–Al alloys, new phases do not appear in commercial ternary alloys with zinc (like AZ91) when the Al to Zn ratio is larger than 3:1 [13], [14]. In this case, zinc substitutes aluminium in the γ-Mg₁₇Al₁₂ phase, creating a ternary intermetallic compound Mg₁₇Al_11.5Zn_0.5 or Mg₁₇(Al,Zn)₁₂ type [4], [15]. On the other hand, the presence of a small amount of manganese in commercial magnesium–aluminium alloys additionally causes the formation of aluminium–manganese intermetallic compounds Al₈Mn₅ or Al₁₁Mn₄ [16].

Magnesium–aluminium alloys are susceptible to heat treatment due to the variable solubility of their alloying elements in a solid state with temperature [5]. The maximum solid solubility of aluminium in magnesium is reasonably high at 12.9 wt.% Al at an eutectic temperature of 710 K whereas the equilibrium concentration at 473 K is about 2.9 wt.% Al. During conventional heat treatment, involving solution annealing at about 690 K for a minimum of 24 h, followed by ageing at about 430 K for 16 h (T6 conditions), the precipitation process appears and the formation of γ-phase precipitates occurs. During γ-phase precipitation, neither Gunier-Preston zones nor other metastable phases are formed. However, precipitation occurring in supersaturated alloys can take place either continuously or discontinuously. Discontinuous precipitation (DP) is the cellular growth of alternating plates of the secondary phase and near-equilibrium matrix phase at high angle boundaries. This heterogeneous reaction leads to the formation of a lamellar structure behind a moving grain boundary. Continuous precipitation (CP) forms in all the remaining regions of the supersaturated matrix. In most alloys, nevertheless, these two types of precipitates can also occur simultaneously [5], [17].

Recently, investigations concerning precipitation processes in Mg–Al alloys have been reported [13], [14], [15], [16], [17], [18], [19], [20], [21], [22], [23], [24], [25], [26], [27], [28], [29], [30], [31], [32]. In most of them, however, discontinuous and continuous precipitation were analysed separately, whereas these two types of precipitates can occur simultaneously and compete in an intricate manner because they nucleate and grow at different rates and with different mechanisms. Duly et al. [17] proposed a possibility to superimpose a “morphology map” on the phase diagram but it has not found confirmation in different experiments. Bradai et al. [25] reported that only discontinuous precipitates were formed after ageing at 498 K for Mg–7% to 11% Al alloys, whereas according to Duly’s results, at this temperature there should also be continuous precipitation. Continuous precipitates were revealed in the Mg–6% Al alloy after ageing at 475 K [15], while to be agreement with ref. [17] there should only be discontinuous precipitation. Similarly, only discontinuous precipitates for Mg–10% Al after ageing at 495–500 K were disclosed [25], [26], [27], [29]. For the AZ91 alloy only continuously precipitates after ageing at 373–573 K were analysed in refs. [13], [30], whereas only discontinuous precipitates after ageing at 457 K were revealed in refs. [19], [15]. These results are not only in agreement with the Duly model but also inverse.

It should also be noted, that both discontinuous and continuous precipitates in Mg–Al type alloys have a plate-like morphology with an accurate orientation relationship (OR) with a matrix phase. For both precipitates the predominant orientation relationship is the Burgers OR, namely: (0 0 0 1)_α || ($0\overline{1}0$)_γ and [$2\overline{1}\overline{1}0$]_α || [1 1 1]_γ [13], [21], [22], [30], [31]. Additionally, other ORs were also reported in Mg–Al based alloys, i.e. the Porter OR [13], the Gjömmes-Östrmoe OR [30], [31], the Crawley OR [13], [30], [31] or the Potter OR [30], [32].

In the present work, the influence of the ageing temperature and cooling/heating rate on the precipitation mechanism in Mg–9 wt% Al and AZ91 were analysed. Investigations of the individual microstructure development were realized using scanning electron microscopy (SEM).