Research On Mg-Gd-Cu Alloys With Long Period Stacking Ordered Structures

Magnesium alloys with rear earth have become an important part ofcommercialized magnesium alloys due to their high strength, high toughness,good ability of corrosion and heat resistant. Moreover, the discover of longperiod stacking ordered(LPSO) structure in magnesium alloys would not onlyimprove the obdurability and high temperature performance of alloys, but alsohave significance on bring down the cost of materials by reducing theconsumption of rear earth metals.In this study, Mg-Gd-Zn-Cu, Mg-Gd-Cu and Mg-Gd-Cu-Zr alloys withLPSO structures were prepared by conventional permanent mold casting andsubsequent heat treatment. The microstructures of these alloys were investigatedby OM, EDS, SEM, XRD and TEM methods, the mechanical properties werealso tested. The main research contents in this paper include: the influence ofelemental Cu to the formation of LPSO structures in Mg-Gd alloy system.Besides, the effect of alloying elements addition and cooling rate after heattreatment on the microstructures and mechanical properties were also studied.The main conclusions are shown as follows: with the increase of Cucontent in Mg96Gd3Zn1-xCuxalloys, the area fractions of eutectic structure weredecreased and LPSO structures phases were increased. The addition of Cu into Mg-Gd-Zn alloys promoted the formation of Mg72.5Gd15.7Zn11.8LPSO structuresphases. Meanwhile, some Zn atoms were replaced by Cu atoms to form newMg86.5Gd5.4Cu5.3Zn2.7LPSO phases. The Brinell Hardness of as-cast alloys wereincreased with increasing Cu contents; The T4state Mg96Gd3Zn0.6Cu0.4alloywith different morphologies were gained after solid solution treatment, theeutectic structures dissolved and massive lamellar LPSO phases precipitated.According to the microstructures of T4state alloys with different soaking timeand cooling rate, it is believed that long soaking time and low cooling speedwere both beneficial for the formation of lamella LPSO structures.14H-LPSO phases were observed in as-cast Mg-Gd-Cu alloy. And theseLPSO phases were stable on heat treated, there is no obvious changes of theirmorphologies and compositions between as-cast and T4state alloys. The Mg5Gdphases in as-cast alloy dissolved and some lamellar Mg92Gd6Cu2LPSO phasesprecipitated at the corresponding positions after heat treatment. The T6stateMg96Gd3Cu1alloy exhibited superior mechanical properties (UTS=232MPa,δ=3.4%, Brinell Hardness=108HB) due to the strengthen of LPSO phases andage-hardening.The grains of as-cast Mg96Gd3Cu1alloy were refined and homogenizedobviously with Zr addition. When Zr content increased from0.1at.%to0.5at.%,the refining effect on grains increasing obvious, but higher Zr addition cannotbring about stronger grain refining effect. Thus, the critical value of Zr additionis0.5at.%, and it exhibits the best combined mechanical properties at ambienttemperature. No changes were found on the composition of as-castMg96-xGd3Cu1Zrxalloy by Zr addition, it is composed of-Mg, Mg5Gd andLPSO phases. The combined contribution of lamellar and block LPSO phaseslead to the excellent mechanical properties of T6-treated Mg95.5Gd3Cu1Zr0.5alloy(UTS=298MPa,=5.6%, Brinell hardness=122HB).