| Magnesium and its alloys have attracted extensive attention in 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 and limited ductility at room temperature. Outside of traditional precipitation control, Mg-alloy strengthening typically relies on two general approaches: non-traditional, esoteric processing and grain refinement. Here in this research, we processed a Mg-8.5Gd-2.3Y-1.8Ag-0.4Zr (wt.%) via conventional technique (hot-rolling at 450°C) with thickness reduction up to 88%. The main contents of this research, as well as the novelties, are discussed in the following ways.;Firstly, we reported a new mechanism for producing ultrahigh strengths (yield strength: ∼575 MPa, ultimate strength: ∼600 MPa) and maintaining moderate ductility (uniform elongation: ∼ 3% to 4%) in hot rolled Mg-alloy with relatively large grain sizes (13 µm). TEM studies show that a high density of nano-spaced SFs are the main defects inside of the grains and their density increased as rolling thickness reduction increased. The strength of the processed Mg alloy was found to increase as the mean spacing between adjacent SFs decreased. Nano-spaced SFs were found to be very effective in impeding the movement of dislocations and retaining strain hardening. Activation of non-basal dislocations during tensile testing accounts for the detected moderate ductility, in addition to the capability of retaining strain hardening.;Secondly, we predicted and calculated contributions from different strengthening mechanisms for the unltrahigh strength of hot rolled Mg alloy including solid solution, grain refinement, precipitation and texture evolution. The results showed that grain boundary strengthening, solid solution hardening, precipitates hardening and strong texture strengthening totally contribute 249.8 MPa to 278.5 MPa for the yield strength (∼575MPa) of 88% rolled Mg alloy. In other words, the nano-spaced SFs strengthening mechanism was the main strengthening factor, which solely contributed 326.2 MPa to 296.5 MPa, around 50% of the total yield strength.;Finally, the model of interactions between basal SFs and basal dislocations and non basal dislocations were established for the first time in Mg alloy. The model showed that the strengthening was proportional to the reciprocal of the SF spacing for both types of interactions between dislocations and SFs. Therefore, decreasing the SF spacing can increase interaction force which served as a barrier for a dislocation to move, and consequently improved the macroscopic strength of materials.;In summary, introducing nano-spaced SFs in Mg alloy can tremendously impede the movement of dislocations and retain strain hardening. It is expected that optimization of approaches that introduce a high density of nano-spaced SFs will enable other Mg-alloys with concurrent high strength and good ductility. |
