Development of low-alloyed and rare-earth-free magnesium alloys having ultra-high strength

Abstract

Developing ultra-high strength in rare-earth-free Mg alloys using conventional extrusion process is a great challenge. What is even more difficult is to achieve such a goal at a lower processing cost. In this work, we report a novel low-alloyed Mg-2Sn-2Ca alloy (in wt. %) that exhibits tunable ultra-high tensile yield strength (360e440 MPa) depending on extrusion parameters. More importantly, there is little drop in mechanical properties of this alloy even when it is extruded at a speed several times higher than those used in the reported high strength Mg alloys. Examination of as-extruded microstructures of this Cacontaining Mg alloy reveals that the ultra-high strength is mainly associated with the presence of surprisingly submicron matrix grains (down to~0.32 mm). The results suggest that the Ca addition promotes accumulations of the pyramidal dislocations, which eventually transform into the low angular grain boundaries (LAGBs). The high number density of LAGBs separate the a-Mg matrix via either discontinuous dynamic recrystallization (DDRX) mechanism in the early stage or the continuous dynamic recrystallization (CDRX) mechanism in the later stage of extrusion, which effectively enhances the nucleation rates of the DRXed grains. More importantly, large amount of Ca segregation along LAGBs, accompanied with dynamically precipitated Mg 2 Ca nano-phases, are detected in the present nonseverely deformed samples. It is the combination of solute segregations and numerous Mg 2 Ca nanoprecipitates that contributes to the formation of the ultra-fine grains via pinning mechanism. The ultrafine grains size, Ca enrichment in most LAGBs, and residual Mg 2 Ca nano-precipitates would in turn contribute significantly to the enhancement of the yield strength of the as-extruded Mg-2Sn-2Ca (wt.%) alloy. The low content of alloying elements and the fast one-step extrusion process render the present alloys low-cost and thus have great potential in large-scale industry applications.

Enhancing the Mechanical Properties of AZ80 Alloy by Combining Extrusion and Three Pass Calibre Rolling

“…Surprisingly, the ultra-fine grain size of AZ80 alloy (~0.85 μm), which is effectively pined by nano-precipitates during caliber rolling, is even finer than that of 18 pass caliber rolled AZ31 (1.5 μm) [17]. Furthermore, the ultra-fine grain structure is even comparable with that obtained by double extruded Mg-Ca alloy (~0.7 μm) [11], and the ultra-high strength Mg-2Sn-2Ca (TX22) alloy (~0.65 μm) extruded at low temperature (200-240 °C) [28]. Accordingly, the submicron grain size, nano precipitates in present CRed AZ80 alloy can necessarily contribute to good mechanical performance.…”

“…After three pass caliber rolling, the strength of the CRed AZ80 alloy was greatly enhanced with TYS and UTS being 370 MPa and 446 MPa, respectively, both much higher than those of the corresponding as-extruded sample. The comparison of CRed AZ80 in this study with the other recently developed high strength Mg alloys [4,6,[8][9][10][11][12][13][14]17,19,21,[28][29][30][31] is shown in Figure 9. The UTS of the CRed AZ80 sample are much higher than those previously reported high strength Mg-8Al-4Sn-2Zn [21], Mg-10Sn-1Al-1Zn [29], Mg-3.5Al-3.3Ca-0.4Mn [30], and Mg-2Sn-2Ca [28] alloys.…”

“…The comparison of CRed AZ80 in this study with the other recently developed high strength Mg alloys [4,6,[8][9][10][11][12][13][14]17,19,21,[28][29][30][31] is shown in Figure 9. The UTS of the CRed AZ80 sample are much higher than those previously reported high strength Mg-8Al-4Sn-2Zn [21], Mg-10Sn-1Al-1Zn [29], Mg-3.5Al-3.3Ca-0.4Mn [30], and Mg-2Sn-2Ca [28] alloys. Besides, The ultra-high UTS of the CRed AZ80 fabricated here is even higher than that of LTSS-extruded AZ80 [10], 18 pass CRed AZ31 [17], HPRed AZ91 [9], HRDSRed AZ91 [12], ARBed AZ91 [13], 20 pass MDFed AZ61 [14], as well as the ECAPed Mg-8Gd-3Y-1Zn containing long-period stacking ordered phases [31].…”

Microstructure Evolution and Mechanical Properties of AZ80 Mg Alloy during Annular Channel Angular Extrusion Process and Heat Treatment

“…The eutectic β-phases were dissolved into the matrix and more equiaxed grains were developed by T4 treatment, which improved the elongation of the alloy. After T6 treatment, profuse lamellar DPs and a small amount of finer CPs precipitated along the grain boundaries and interiors, playing a vital role in the phase-hardening behavior by the Orowan mechanism [32,33], and thus significantly improving the mechanical properties of the extruded alloy. To further clarify the performance difference of extruded alloy in different states, the SEM microstructure near the fracture surface of extruded, T4, T5, and T6 specimens is given in Figure 10.…”

Microstructural Characteristics and Mechanical Properties of a Novel Extruded Dilute Mg–Sn–Mn–Ca Alloy

“…Moreover, the extruded Mg-Sn-based alloys with high Sn content commonly have sub-microscale and microscale Mg 2 Sn particles co-existing at the grain boundary in the form of network, which prevents further performance improvement (Cheng et al, 2010). Many previous reports have verified that decreasing the alloying element contents of Mg alloys could efficiently reduce the thermally stable phase contents and modify the morphologies of the strengthening precipitates (Hofstetter et al, 2015;Pan et al, 2018;Wang et al, 2018;Hu et al, 2019), thus improving the extrudability and ductility. Accordingly, studies have recently been concentrated on micro-alloyed systems with low content of Sn in order to reduce the cost of products (Chai et al, 2019a).…”