Designing a magnesium alloy with high strength and high formability

Although magnesium alloys, as the lightest structural alloys, offer significant potential for automotive applications, their applications remain limited due to their poor formability at room temperature. Since the strategies used for improving formability usually result in a degradation of strength, there are no high strength magnesium alloys showing good formability. Here we report an alloy design concept that can simultaneously provide high strength and good formability. Such designed alloy when subjected to an appropriate processing technique shows a combination of strength and formability that surpasses those of the existing magnesium alloys reported so far. The alloy design concept used in the present study is based on the utilization of alloying elements that can induce precipitation, as well as maximize the segregation of other texture-controlling alloying elements. Such developed alloy is expected to broaden the application of Mg alloy sheets, which are now starting to gain acceptance by automotive industries.

Magnesium alloys, as the lightest structural alloys having a density about one-fourth that of steels, offer significant potential for improving energy efficiency of various transportation systems such as automobiles^1,2. Numerous R&D efforts in the last decade have led to the development of various types of magnesium alloys having good combination of strength and ductility^3,4 and excellent corrosion resistance⁵. Although there are many variants of magnesium alloys showing large ductility, however, they usually show poor formability at room temperature³, hindering their widespread applications in automobiles. Poor formability of magnesium alloys arises from several factors; development of strong basal texture during rolling (thermomechanical treatment) and limited number of available deformation modes due to magnesium having hexagonal close-packed (hcp) structure⁶. Although Mg alloys are readily formable at warm or high temperatures despite their poor formability at room temperature, forming at warm or high temperatures is quite energy intensive and inefficient. Therefore, to make Mg alloys attractive for applications in automobiles, their formability at room temperature should be improved. There have been numerous attempts to improve formability of magnesium alloys and it has been shown that the formability of Mg alloys can be improved by texture randomization/weakening from the typical strong basal texture of conventional Mg alloys, which is usually achieved by the modification of thermomechanical processing^7,8 and the addition of rare earth elements (REEs)^9,10,11. The randomized/weakened basal texture of REE-containing alloys is certainly beneficial for formability, but also results in low strength. Such inverse relationship between formability and strength is also applicable to other non-REE containing Mg alloys³. Figure 1 shows the Index Erichsen (IE) value (which is an indication of stretch formability; see Methods) as a function of yield strength of various Mg alloys^{3,8,12,13,14,15,16}. As shown in Fig. 1, yield strength of Mg alloys having the IE values larger than 8 mm is lower than ~160 MPa. However, as yield strength of Mg alloys increases to 200 MPa, their IE values become lower than 6 mm, making them non-formable at room temperature. It is apparent that the conventional approaches that improve formability have adverse effects on strength or vice versa. Therefore, new alloy design and processing concepts should be utilized to develop high strength Mg alloys with good formability at room temperature.

Fig. 1

Yield strength and stretch formability represented by the Index Erichsen (IE) value at room temperature of various Mg alloys sheets. Higher IE values mean that the alloys exhibit better formability. Overall, Mg alloys display an inverse relationship between IE value and yield strength, as illustrated by the area between two dash lines. The developed TRC-AZMX3110 alloy shows a much better combination of strength and formability than others. Different Mg alloy systems are marked by different colors and symbols and the corresponding references are cited

In this study, we report an alloy design concept that can simultaneously provide high strength and good formability at room temperature. The newly developed Mg-3Al-1Zn-1Mn-0.5Ca (wt.%) (AZMX3110) alloy using the alloy design concept shows a much improved combination of formability (IE value) and yield strength, namely IE value of 8 mm with yield strength of 219 MPa, as compared to commercial as well as experimental Mg alloys, as presented in Fig. 1. Such improvement in the combination of formability and strength has been realized by a judicious control of alloy chemistry and a selection of appropriate process for such alloy chemistry. The composition of AZMX3110 alloy is quite similar to that of the most familiar and commonly used Mg-3Al-1Zn (AZ31) alloy. It is expected that such composition of the developed alloy would lead to an early acceptance by the users due to its similarity to AZ31 alloy.

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