Effect Of Cryogenic Rolling And Yb Addition On The Microstructure And Mechanical Properties Of Mg-14Li-1Al Alloy

Mg-Li alloy is widely used in military and civil fields because of low density,high specific strength and high specific stiffness.However,Mg-Li alloy is prone to dynamic recrystallization during plastic processing,which weakens the work hardening ability and limits the application of the alloy.In this thesis,Mg-14Li-1Al alloy is processed by means of cryogenic rolling and Yb alloying,so as to inhibit the softening and improve the strength of the alloy.Mg-14Li-1Al and Mg-14Li-1Al-0.1Yb alloys in the state of as-cast,homogenization annealing,rolling at room temperature(RTR),cryogenic rolling(CR)and annealing were investigated by OM,XRD,SEM and TEM respectively.The results show that the as cast Mg-14Li-1Al alloy contains?-Li phase with an average grain size of 423?m.After homogenization annealing,the grain size of the alloy has no obvious change and the microstructure is more uniform.The dynamic recrystallization grains with sub-micron size were observed in RTR sample and smaller grain size in CR sample,which indicates that the recrystallization grain growth is inhibited by CR.In addition,a large number of nano-twins appeared in the CR sample.After annealing,the average grain size of CR is smaller than that of RTR sample.The average grain size of as-cast Mg-14Li-1Al-0.1Yb alloy is 220?m,which shows that Yb plays an important role in refining grain.In the as-cast alloy,the size of Al₃Yb phase is about 500nm.After CR,Al₃Yb phase is broken with a size of about 20nm,which has a significant dispersion strengthening effect.Due to the introduction of Yb element,the stacking fault energy of the CR alloy reduces and a large number of stacking fault structures appear,which inhibit the nucleation and growth of dynamic recrystallization.However,no nano-twin is found in the CR alloy.The grain size of the CR Mg-14Li-1Al-0.1Yb alloy is smaller than that of the CR Mg-14Li-1Al alloy under the same annealing condition,which indicates that the addition of Yb element can inhibit the growth of recrystallization.The mechanical properties of the two alloys were tested by the tensile machine.The results show that the properties of Mg-14Li-1Al alloy can be improved by rolling.Among them,the CR alloy has the best performance,with tensile strength of 223MPa and elongation of 44.7%.And the strength and elongation decrease after annealing.Compared with Mg-14Li-1Al alloy,the strength and elongation of as-cast Mg-14Li-1Al-0.1Yb alloy are significantly improved.The strength of the CR Mg-14Li-1Al-0.1Yb alloy increases to 241MPa,but the elongation decreases to 31.1%.After annealing,the strength decreases obviously and the elongation increases.In addition,the electrical resistivity of two alloys was tested.The results show that the resistivity of the CR Mg-14Li-1Al alloy is higher than that of the RTR specimen,indicating that more dislocations can be accumulated during low temperature rolling.Compared with the CR Mg-14Li-1Al alloy,the resistivity of the CR Mg-14Li-1Al-0.1Yb alloy is further increased.It indicated that,Yb addition brings about more defects in the alloy.In conclusion,CR can significantly accumulate dislocations and improve the work hardening ability of the alloy.Nano-twin is found in the CR Mg-14Li-1Al alloy,which greatly improves the strength and elongation of the alloy.After Yb alloying,the grain of as-cast alloy is refined obviously and Al₃Yb phase is formed.After CR,Al₃Yb phase distributes in the microstructure with smaller size dispersion,which plays a significant dispersion strengthening effect.In addition,the stacking fault energy of the Mg-14Li-1Al-0.1Yb alloy is reduced,resulting in a large number of stacking faults,which plays a certain role in preventing dislocation movement.However,no nano-twin is found,thus the elongation of the Mg-14Li-1Al-0.1Yb alloy is relatively low.