Effects Of Creep Aging On The Microstructure And Mechanical Properties Of Mg-9Gd-2Nd-0.5Zr Alloy

The Mg-9Gd-2Nd-0.5Zr alloy was prepared by gravity casting method in this paper.The alloy was treated by solution treatment and(creep)aging treatment,and the effects of aging parameters such as temperature,time,stress magnitude and stress loading mode on the precipitation behavior and mechanical properties of the alloy were systematically studied by microstructure observation(OM,SEM,TEM,EBSD,etc.)and mechanical properties test(Vickers hardness,tensile test at room temperature).Moreover,the formation mechanism of stress orientation effect and the broadening mechanism of precipitate free zone(PFZ)were investigated.The results are as follows:The microstructure of Mg-9Gd-2Nd-0.5Zr alloy is mainly composed of?-Mg matrix and precipitates(??phase and?₁phase)after conventional aging treatment at 200??275?.When the alloy is aged at different temperatures for different times,the overall trend of hardness value increases at first,reaches the peak value and then decreases rapidly.However,with the increase of temperature,the time for the alloy to reach the aging peak value is shorter.When the aging temperature is 250?,the aging peak time is only 4h,and the hardness value of the alloy has little change compared with that of low temperature aging.The aging peak time at275?is still 4h,but the hardness value is obviously lower than that of aging at 250?for 4h.Compared with 200?and 225?,aging at 250?has a stronger promoting effect on element diffusion in the alloy,accelerating the precipitation of the second phase,shortening the time to reach the peak of aging,reducing the width of precipitate free zone at grain boundary,obtaining a large number of nanoscale??phase,and obtaining the highest strength through Orowan strengthening mechanism.The hardening effect is weakened due to the increasing of size??phase when the Mg-9Gd-2Nd-0.5Zr alloy aging at 275?.In the process of unidirectional stress creep aging,the application of stress can increase the dislocation density in the alloy providing a large number of nucleation position for the precipitation of the second phase,and provide additional nucleation energy for the precipitation of the second phase,so as to promote the precipitation of the second phase in the alloy and shorten the time for the alloy to reach the aging peak.When the stress of loading reaches the maximum value(100MPa),the time to aging peak can be reached only by 1h.The introduction of unidirectional stress will lead to the preferential orientation of the second phase precipitation in the alloy,that is,the stress orientation effect,which is not conducive to the increase of the strength of the alloy.However,the application of unidirectional stress can lead to the evolution of the texture of the alloy,and promote the grain orientation in the alloy to rotate to the base plane.The larger the loading stress is,the grain orientation is more inclined to the base plane direction,which is conducive to the improvement of the plasticity of the alloy.When the stress is 100MPa,the alloy obtains the highest elongation(9.5%),which is twice of the traditional aging.During cyclic stress creep aging,the application of cyclic stress can avoid the stress orientation effect caused by unidirectional stress,affect the precipitation behavior of the alloy,and make the distribution of precipitation phase more conducive to hinder the movement of dislocation.After cyclic stress creep aging,the alloy can reach the peak aging in a short time,and obtain a large number of fine precipitates.Moreover,the cyclic stress creep aging can also reduce the width of precipitate free zone.Thus,the alloy obtain greater strength after cyclic stress creep aging,compared with that after conventional aging at the same temperature and time,and increases compared with that after uniaxial tensile stress creep aging.However,the strength of the alloy only slightly improved compared with that after conventional aging at the same temperature,and the maximum value of 326.3MPa is obtained after creep aging at250?×c75MPa×2h.The stress loading mode has a great influence on the broadening of precipitate free zone at grain boundary.During the same time,the width of precipitate free zone in the alloy is:conventional aging<cyclic stress creep aging<unidirectional stress creep aging.This is mainly because the tensile stress can accelerate the diffusion of atoms from the parallel direction to the stretching axis to the perpendicular direction in the alloy.The formation of second phase consumes the solute atoms near the grain boundary,leading to the solute atoms poor near the grain boundary,resulting in the wider PFZ formed at the grain boundary.