Development of new creep-resistant magnesium alloys with low cost

Abstract

As the lightest metallic structural materials, magnesium (Mg) and its alloys have attracted increasing interest in automotive and aerospace industries because of the benefits to energy conservation and improved fuel efficiency through significant weight reduction. However, the insufficient high-temperature mechanical and creep properties of magnesium alloys have restricted the wider applications in industries, such as the powertrain components. So far, rare earth elements (RE)-containing Mg alloys are known to exhibit outstanding elevated-temperature creep resistance that is unachievable by other Mg systems, such as Mg-Al, Mg-Zn, etc. However, previous strategies for the development of creep-resistant Mg-RE based alloys mainly relied on the precipitation hardening effect produced by high content of RE, which led to extremely limited applications due to the high cost. For the purpose of achieving more applications, it is of commercial interest to develop new creep-resistant Mg alloys with low RE content through new strategies. In addition to precipitation hardening, the solid solution strengthening of RE solutes in Mg is also outstanding even in the case of dilute RE concentration. But the actual mechanism of the superior solid solution strengthening of RE to other common-used solutes in Mg is still unclear because the classic elastic interactions model has failed in explanation. Based on unaddressed issues above, the present work aims to (i) select alternative alloying addition to reduce the usage of the typical RE, Gd, and therefore develop new creep-resistant Mg alloys with low cost; (ii) understand the mechanism behind different solute strengthening effect in Mg.Compared with Gd, Ca is a cheaper alloying addition and also known for the improvement in creep resistance of Mg. But the high-temperature mechanical and creep properties based on Mg-Gd-Ca system have never been investigated. Therefore, Mg-3Gd-2Ca (wt.%) based alloys were newly developed in present work based on a published alloy, Mg-6Gd-2Zn-0.6Zr, through the partial replacement of Gd with Ca. The as-solid solution treated Mg-3Gd-2Ca alloy was found to exhibit compatible compressive strengths and better stress relaxation resistance at 180  when compared with the as-solid solution treated Mg-6Gd-2Zn-0.6Zr alloy. Further addition of other solutes, including Al, Zn, Si, Sn and Zr, showed marginal or even detrimental effect on the high-temperature performance of the Mg-3Gd-2Ca alloy. The results highlighted the effect of Ca that can retain the loss of high-temperature strengths and creep resistance caused by the reduction in Gd content, due to the comparable solid solution strengthening effect.To understand the mechanism contributing to higher solid solution strengthening of RE and Ca than other elements, the solute distribution inside the Mg-3Gd-2Ca alloy was characterized by atom probe tomography (APT) and high angle annular dark field-scanning transmission electron microscopy (HAADF-STEM). Gd and Ca solutes in the α-Mg solid solution were verified having strong preference to distribute as co-clusters in several atoms scale. Those co-clusters frequently formed in a triangular shape and/or other structures extended from this triangular shape when viewed from [0001]Mg direction, which were generally referred as short-range order structure. In comparison, atom probe tomography analysis on a Mg-3.68Al (at.%) specimen indicated that Al solute atoms distributed within the Mg solid solution in an apparently random manner. Therefore, an extra local (short-range) ordering strengthening mechanism in addition to the atomic and modulus mismatch hardening, were proposed to contribute to the high solid solution strengthening effect in the Mg-Gd-Ca based alloy. This local order strengthening in solid solution was expected to provide an athermal forest hardening effect and extend the athermal strength regime in a range of temperatures, and hence improved the thermal stability of the alloy at high temperatures. On the contrary, the absence of short-range order structure was responsible for the weaker solid solution strengthening of Al solute in Mg alloys at elevated temperature.As Mg-RE based alloys usually reached optimal mechanical properties at peak-aging condition, the effect of heat treatment on the Mg-3Gd-2Ca alloy has been investigated. Peak-aging at 180  for 32 hours led to a significant increase in room temperature strengths of the alloy by about 50 MPa due to the precipitation hardening by three types of precipitates, block-shape Mg2Ca, rod-like Mg5Gd on prismatic planes and a novel Mg-Gd-Ca basal plate, however the as-solid solution treated alloy exhibited better performances at elevated temperatures, including compressive strengths, stress relaxation resistance and creep resistance. Microstructural characterization illustrated that the higher strengths and stress relaxation resistance of the as-solution treated alloy at 180  were totally attributed to the solid solution strengthening effect, and the better creep resistance was due to the synergistical effects of solid solution strengthening (particularly at early stage of creep) and dynamic precipitation hardening formed during long-term creep. Therefore, owing to the presence of local order effect, solid solution strengthening was suggested to be an important mechanism ultimately determining the mechanical and creep properties of this low-RE containing Mg alloy at elevated temperatures.In addition, the creep resistance of the newly developed Mg-3Gd-2Ca alloy could be further improved by the minor addition of Nd and Mn. The modified Mg-2Gd-1Nd-2Ca-0.5Mn alloy exhibited one order of magnitude lower creep rate and three times lower creep strain than the as-solid solution treated Mg-3Gd-2Ca alloy when tested at 210 ℃  and 100 MPa. Nd solutes in Mg solid solution was verified by atom probe tomography (APT) that exhibited stronger co-clustering preference with other solute atoms than that between Gd and Ca solutes, leading to a higher local order strengthening effect on resisting creep. The addition of Mn was observed to present as polygonal-shape α-Mn precipitates and served as heterogeneous nucleants for dynamic precipitates, refining their size and increasing the number densities by two order of magnitudes. As a result, the remarkably improved creep resistance of the Mg-2Gd-1Nd-2Ca-0.5Mn alloy was attributed to a combination of improved solid solution strengthening by Nd and increased precipitation hardening by Mn addition.At last, the potential of newly-developed Mg-Gd-Ca based alloys for industry applications has been evaluated through comparison with commercialized creep-resistant Mg-RE based alloys, WE43, EV31 (Elektron 21) and QE22, in terms of stress relaxation and creep behaviours, and raw material cost. Commercialized Mg-RE based alloys exhibited optimal stress relaxation and creep resistance at peak-aged condition, whereas new Mg-Gd-Ca based alloys (Mg-3Gd-2Ca, Mg-2Gd-1Nd-2Ca and Mg-2Gd-1Nd-2Ca-0.5Mn) were seen to have better performance at as-solid solution treated condition even though noticeable aging responses were obtained. Nevertheless, the compared results indicated that Mg-Gd-Ca based alloys exhibited stress relaxation and creep properties better or at least comparable to the EV31 and QE22 alloys, but slightly inferior to the WE43 alloy. However, their raw material costs were estimated to be at least 24% cheaper than the three commercial Mg-RE based alloys.In summary, the present PhD thesis has advanced understanding on the solid solution strengthening mechanisms in Mg alloys, and developed a number of new Mg alloys based on Mg-Gd-Ca system. Based on experimentally characterization, a local order strengthening mechanism in addition to the classic elastic strain interaction was proposed to determine the solid solution strengthening effect of Mg alloys at elevated temperatures. Owing to the local ordering effect, the new Mg-3Gd-2Ca alloy exhibited good mechanical and creep properties after solid solution treatment. With further Nd and Mn additions, the Mg-2Gd-1Nd-2Ca-0.5Mn alloy exhibited comparable creep resistance to the currently commercialized Mg-RE based alloys while it had a much lower raw material cost. Therefore, it is reasonable to conclude that the newly developed Mg-Gd-Ca alloys in present thesis are cost-effective and highly creep-resistant with great potential for future industry applications.