Nano-spaced stacking faults create stronger, lightweight magnesium alloys; potential for structural applications in automotive and aerospace
|Summary of room temperature tensile yield strength and uniform elongation of earlier efforts and the new work. Numbers adjacent to data points are references cited in Jian et al. Source: Jian et al. Click to enlarge.|
Researchers from North Carolina State University have developed a new technique for creating ultra-strong magnesium alloys that maintain good ductility. By introducing stacking faults with nanoscale spacing into a magnesium alloy using conventional hot rolling, they produced a yield strength of 575 MPa, an ultimate strength of 600 MPa, and moderate ductility (uniform elongation of 5.2%).
The nano-spaced stacking faults are essentially a series of parallel fault-lines in the crystalline structure of the alloy that isolate any defects in that structure. The process can lead to Mg alloys with superior mechanical properties with potential structural applications in the automobile and aerospace industries, the researchers suggested in an open access paper in the journal Materials Research Letters.
Magnesium and its alloys have attracted extensive attention in the recent years due to their abundance, low-density, good castability and recyclability. However, the application of Mg alloys has been substantially hindered by their relatively low strengths (tensile yield strength: ∼100–250 MPa for commercial casting Mg alloys) and limited ductility (elongation: 2– 8%) at room temperature.
Outside of traditional precipitation control, Mg-alloy strengthening typically relies on two general approaches: non-traditional, esoteric processing and grain refinement. An example of non-traditional processing for high strength is rapid solidification/powder metallurgy, which was used to obtain a yield strength of ∼600 MPa in a Mg–Zn–Y alloy with uniform distribution of long-period ordered structures. While the resultant properties are remarkable, such unconventional processing technologies limit potential industrial application. In addition, the ultra-high strength is usually accompanied by marked losses in ductility.
The second general approach of grain refinement for strengthening has been used to obtain ultrafine (<1 μm) grains in Mg alloys. The high concentration of grain boundaries (GBs) in the ultrafine-grained microstructure provides barriers to motion of dislocations and consequently promotes the strength improvement. Nevertheless GB strengthening mechanism alone provided limited contribution to macroscopic yield strengths, which, in these reports, are typically less than 400 MPa. More importantly, refining the grain size is reported to suppress the propensity of deformation twinning which, in addition to dislocation slip is an important mechanism for enhancing strength and ductility. Ultrafine-grained microstructures also suffer from strength reduction via grain growth at a relatively low temperature (0.32 Tm ) , thus limiting the potential for further shaping or processing.—Jian et al.
The researchers selected an alloy of magnesium, gadolinium, yttrium, silver and zirconium for the study because they thought they could introduce the faults to that specific alloy using hot rolling.
The team suggested that their introduction of a high density of stacking faults with nanoscale spacing provides for a high density of barriers to block and pin dislocations and retention of work hardening for enhanced ductility.
While the resulting Mg alloy material is not as strong as steel, it is so much lighter that its specific strength—a material’s strength divided by its density—is actually much higher, said Dr. Suveen Mathaudhu, a co-author of a paper on the research and an adjunct assistant professor of materials science and engineering at NC State under the US Army Research Office’s Staff Research Program.
In theory, you could use twice as much of the magnesium alloy and still be half the weight of steel. This has real potential for replacing steel or other materials in some applications, particularly in the transportation industry—such as the framework or panels of vehicles.—Suveen Mathaudhu
W. W. Jian, G. M. Cheng, W. Z. Xu, H. Yuan, M. H. Tsai, Q. D. Wang, C. C. Koch, Y. T. Zhu, S. N. Mathaudhu (2013) Ultra-strong Mg Alloy via Nano-spaced Stacking Faults. Materials Research Letters doi: 10.1080/21663831.2013.765927