Study On The Microstructure And Mechanical Properties Of Mg-Y-Sm-Zr Alloys

The WE series alloys, based on the Mg-Y-Nd-Zr system, identified as WE54 (5wt.%Y, 3.3wt%RE, 0.5wt.%Zr) and WE43 (4wt.%Y, 3.3wt.%RE, 0.5wt.%Zr) become more and more attractive for aerospace and automotive industries because of their high room temperature mechanical properties, excellent high temperature creep resistance and good corrosion resistance. These alloys are strengthened essentially by precipitation hardening. Samarium belongs to the same subgroup as neodymium, and the maximum solubility of samarium in solid magnesium is higher than neodymium. Therefore it is reasonable to assume that the precipitation hardening effect of Mg-Y-Sm-Zr alloys is higher than that of WE series alloys based on Mg-Y-Nd-Zr system, which may result in a higher strength. But there is little research on strengthening effects and strengthening mechanisms of Mg-Y-Sm-Zr alloys. It is the purpose of present study to investigate the effects of Sm on microstructure and mechanical properties of Mg-Y-Sm-Zr alloys. Then the heat treatments and plastic deformation of Mg-4Y-4Sm-0.5Zr alloy are investigated. And the precipitation sequence and the precipitation strengthening mechanism in Mg-4Y-4Sm-0.5Zr alloy are emphasized. Finally, the high temperature compression and compression creep behavior of extruded Mg-4Y-4Sm-0.5Zr alloy are contrastively studied.Thermo-mechanical treatments containing solution, artificial ageing and extrusion are carried out on Mg-4Y-xSm-0.5Zr (x=1, 4, 8) alloys. Effects of samarium on microstructure and mechanical properties of Mg-Y-Sm-Zr alloys during thermo-mechanical treatments are investigated. The microstructure of as-cast alloys involves Mg solid solution + eutectic compounds. The eutectic phase has the same fcc crystal structure (a=2.246nm) and similar composition to Mg5Sm. But some Y elements are dissolved in it. And there are also some small RE-enriched quadrate phases which have the fcc crystal structure (a=0.5581nm) in the as-cast Mg-Y-Sm-Zr alloys. The eutectic phases in Mg-4Y-1Sm-0.5Zr and Mg-4Y-4Sm-0.5Zr alloys are solutionized into the Mg-matrix after solutionized at 525℃for 8h. There is still large amount of the second phases remained at grain boundaries in the solutionized Mg-4Y-8Sm-0.5Zr alloy for the incomplete dissolution of the samarium. The fcc crystal structure Mg5(Sm,Y) eutectic phase is transformed to the body-centred tetragonal crystal structure Mg41Sm5 phase with little Y element dissolved in it. The strength and elongation of Mg-4Y-8Sm-0.5Zr alloy are low because of the large amount undissolved fragile compounds; the precipitation strengthening effect of Mg-4Y-1Sm-0.5Zr alloy is not evident because of the low Sm content; as a result, the Mg-4Y-4Sm-0.5Zr alloy has the best mechanical properties.The eutectic phases in Mg-4Y-4Sm-0.5Zr alloy dissolve into the matrix and the alloying elements Y and Sm homogenously distributes through out the grains after solutionized at 525℃for 8h. Elongation (EL) greatly increase for the dissolution of eutectic phase and homogenous distribution of alloying elements, and ultimate tensile strength (UTS) increases a little for the solution strengthening effect of Y and Sm. The alloy peak-aged at 225℃has the highest EL. But the UTS and yield strength (YS) are relatively low. The alloy peak-aged at 175℃has the highest hardness, but the ageing time is long, and the UTS, YS and EL are not the highest. And the highest UTS and YS are reached when the alloy peak-aged at 200℃. Very fine scale precipitates form inside the grains and along the grain boundaries when under-aged at 200℃for 16h, resulting of the highest UTS and EL. With increasing the ageing time, The EL of alloy greatly decreases for increasing amount and coalescence of the precipitates along the grain boundaries, and the UTS decreases respectively. The optimal ageing parameter 200℃for 16 hours is chosen for Mg-4Y-4Sm-0.5Zr alloy. The precipitation strengthening of the alloying elements Y and Sm is the main strengthening mechanism in this Mg-4Y-4Sm-0.5Zr alloy. The mechanical properties of the alloy greatly increase after the solution-plus-ageing heat treatment.The UTS of 348MPa, YS of 217MPa and EL of 6.9% are attained after this solution-plus-ageing heat treatment.The precipitation sequence containingβ′,β1 andβis determined in the Mg-4Y-4Sm-0.5Zr alloy during ageing at different temperatures. Theβ′precipitates are formed within grains when the alloy is aged at 200℃for up to 3000h. This intermediate phase is thermally stable at 200℃, and there is no phase transformation occurred at this temperature. The globular shapeβ′precipitates are formed in the under aged state, which is corresponding to the highest strength of the alloy. The precipitates coalesce, and the plate shapeβ′precipitates are formed lying in the {2110 } habit planes with increasing ageing time. Theβ1 precipitates are formed within the grains when the alloy is aged at 250℃for 48h. These plate shapeβ1 precipitates with fcc crystal structure (a=0.74nm) lie in the {1010 } habit planes and in contact withβ′precipitates. Large numbers of plate shape equilibriumβphases with fcc crystal structure (a=2.223nm) precipitate along the {1010 } habit planes when the alloy is aged at 300℃for 13h. These equilibriumβprecipitates are transformed in situ from theβ1 precipitates.The grains of Mg-4Y-4Sm-0.5Zr alloy are evidently refined for the occurrence of DRX (dynamic re-crystallization) during high temperature (350℃and above) extrusion process. The mechanical properties of the alloy greatly increase due to the grain refining strengthening effect. The ageing heat treatment can directly carry out on the extruded alloy, and the precipitation strengthening effect in extruded alloy is a little increased with increasing extrusion temperatures. The UTS of 400MPa, YS of more than 300MPa and EL of 7% are attained after thermo-mechanical treatments.The high temperature compression and compression creep behavior of extruded Mg-4Y-4Sm-0.5Zr alloy are investigated. The strain rate sensitivity m is 0.4 and the activation energy Q is 92.1KJmol-1 when the alloy compressed at a strain rate of 2.5x10-4s-1 and at a temperature of 450℃, indicating that grain boundary sliding (GBS) is the main compression mechanism. The extruded Mg-4Y-4Sm-0.5Zr alloy has good creep resistance at temperature 200℃and at stress 180MPa. The creep activation energy Q is 156.7KJ/mol and the stress exponent n is 4.1 indicating that dislocation climb is the main creep rate controlling mechanism.