The Effects Of Multiple RE Addition And Pre-Cold Rolling On The Age Hardening Response Of Mg-Gd Based Alloys

In order to improve the age hardening response of Mg-Gd based alloys and develop high performance Mg alloys,the designed Mg-Gd-Y-Nd-Ho-Er-Zn-Zr alloys prepared by semi-continuous casting method were used as the experimental material.Combined with simple solid solution + ageing and solid solution + pre-cold rolling + ageing processes,the effects of multiple rare earth(RE)and pre-cold rolling on the age hardening response of the studied alloys were investigated.The microstructure evolution of alloys during heat treatment and pre-cold rolling was characterized by optical microscope(OM),scanning electron microscope(SEM),X-ray diffraction(XRD),electron backscatter diffraction(EBSD)and transmission electron microscope(TEM).The microstructure of peak-aged alloy was the focus of research.This study revealed the promoting effect of multiple RE addition and pre-cold rolling on the age hardening response of Mg-RE alloys.The main results can be summarized as follows.The microstructure of Mg-2.1Gd-2.0Y-1.9Nd-0.9Ho-1.0Er-0.5Zn-0.4Zr(wt%)cast alloy is mainly composed of ?-Mg matrix and(Mg,Zn)3RE phase,and the(Mg,Zn)3RE phase dissolved into ?-Mg matrix after solution treatment at 520 °C for 16 h.It is known that the age hardening response of Mg-Gd binary alloy is obvious only when the content of Gd element is pretty high(above 15 wt%).However,this research shows that the studied alloy with 8 wt% RE content has superior age hardening response(?HV = 35.7)when aged at 200 °C,indicating that multiple RE(Gd,Y,Nd,Ho,Er)alloying instead of single Gd can effectively promote the age hardening response.The peakaged T6 alloy has excellent comprehensive mechanical properties(?b = 306 MPa,?s = 215 MPa,? = 5.7%)and moderate thermal conductivity(55.0 W/(m?K)),and the improved strength and thermal conductivity is mainly attributed to the formation of ???/?? phases during ageing treatment.The Mg-2.1Gd-1.7Ho-1.4Y-1.3Nd-0.9Er-0.5Zn-0.5Zr(wt%)solution-treated alloy has good formability after 12 % reduction by pre-cold rolling at room temperature.The high density dislocations,{101 2} tensile twins,basal stacking faults(SFs)and stronger basal texture are introduced by pre-cold rolling.Dislocations can accelerate the atoms diffuse and provide precipitate nucleation sites during ageing treatment.Therefore,T3 alloy can reach to the peak-aged T8 condition(99.2 HV)aged at 200 °C for just 9 h while T4 alloy get the peak-aged T6 condition aged at 200 °C for 36 h.The relative higher strength of T8 alloy is attributed to the denser and finer ??? precipitates,dislocations and the special orientation relationship between prismatic ??? precipitates and basal SFs.All the strengthening factors in the above T8 alloy are introduced by the simple pre-cold rolling at room temperature and ageing treatment.