Evolution Of Precipitate-free Zones And Precipitates During Creep Of Mg-Y Alloy

Magnesium alloys are widely used in aerospace,vehicles and electronic products at present.However,the poor elevated temperature performance of magnesium alloys has always restricted the application of magnesium alloys.The applicable temperature of common heat-resistant magnesium alloys systems is basically below 200?.If magnesium alloys are to be applied in other fields,such as aircraft or automotive power systems,their service temperature needs to be raised to 250? or even 300?.Creep-resistance is one of the most important factors to improve the high-temperature properties of magnesium alloys.Mg-Y alloy is an important system in heat-resistant Mg-RE alloys.It is very important for the design of heat-resistant alloys to study the evolution of grain boundaries structure and precipitates in Mg-Y alloys during creep at 250-300?.In this paper,the creep tests of Mg-3.3Y-0.1Zr(at.%)alloy at 250?,80-120 MPa and 300?,30-70 MPa have been completed.Microstructure of the grain boundary and grain in crept samples and corresponding formation mechanism were also carried out.Through characterization of precipitate-free zones in crept Mg-3.3Y-0.1 Zr alloy and comparision experiment with crept Mg-2.5Gd-0.1Zr alloy,the features of A-type PFZ and B-type PFZ are summarized.(a)A-type PFZ has one array of grain boundry precipitates which divides PFZ into two parts.The widths of these two parts can be different.The grain boundary is the same as the location of precipitation phase at grain boundary.In the Al-type PFZ with different widths,the precipitation phase at grain boundary is close to the grain with small Schmid factor.The probability of A-type PFZ in crept samples is high.(b)B-type PFZ has two arrays of grain boundry precipitates which divide PFZ into three parts.The grain boundary can be the same as one side of the two grain boundary precipitates,or it can shuffles between the two arrays of particles.The probability of B-type PFZ in crept samples is low.Quasi in-situ creep characterization indicates that there are other types PFZ,like X0-type PFZ and XAB-type PFZ.Different types PFZ have transformation relationship,which is controlled by coordination of diffusion and dislocation.The crystal structure,morphology and orientation of precipitates in crept Mg-3.3Y-0.1Zr alloy were analyzed.Unlike to Mg-Gd alloy in the literature,only one variant ?' phase in Mg-Y alloy is observed after creep tests.This selected ?' variant with the angle between[100]p-and the applied stress being?45° is proposed to grow preferentially under the role of applied stress.?'precipitates stack in a terrace-like style in the linear precipitate chains,which are approximately perpendicular to the stress like Mg-Gd alloy.Nano-scale plate-like ?' precipitates from dynamic precipitation whose morphology is obviously different from ?' precipitates in peak-aged samples are also observed in crept samples.According to phase field simulations,the interfacial energy is the dominant factor in morphology and distribution of dynamic precipitation of these nano-scale ?' precipitates.