Influence Of Pre-stretching And Aging On The Microstructure And Mechanical Properties Of Mg-5Zn Alloy

As a light-weight and environmental friendly structural material, magnesium and its alloys have attracted great research attention, and a bright future is there for magnesium and its alloy in automotive, aerospace industries. However, the low absolute strength, poor creep resistance, limited plasticity at room temperature, poor formability and mechanical processability, restrict their application on industrial scale, while the strengthen and processability are the main goal-checking for practical application, and ground breaking innovations are acquired to substitute aluminum alloys by magnesium alloys. Conclusively, there is climacteric need of developing high-performance magnesium alloys. The precipitate particles, especially strengthen precipitate particles in alloys, have great impact on strength, formability, and creep resistance, and different precipitate types may lead to different mechanical response. Thus, magnesium alloys properties can be improved by controlling the precipitate particle types.In this work, Mg-Zn binary alloys were extruded for microstructural and mechanical testing. A strategy, combing pre-stretching and annealing was used to control the precipitation conditions. Experimental techniques such as OM and Transmission electron microscope(TEM) were used to characterize precipitate particle’s locations. TEM was used to measure the size and aspect ratio of the precipitates, and high-resolution TEM(HRTEM) was used to confirm the orientation relationships. XRD and high-resolution TEM(HRTEM) were used for phase analysis, at the same way, XRD data was used to calculate the c/a ratio of different samples. Finally, re-stretching and compression tests were carried out on different amount of pre-stretched samples with different aging time to investigate the relationship between microstructure and mechanical properties. The main conclusions are as follows:① The extruded Mg-5Zn rods received a relative weak fiber basal texture after solution treatment at 400oC for one hour, and lots of dislocations and {101-2} tension twins were obtained after pre-stretching. It was noticed that the twin number increases with the increasing amount of pre-stretching.② The 1′ phase in pre-stretched Mg-Zn alloys preferentially nucleated at the grain boundaries, twin boundaries or within the twins during ageing process. the twins provide sites for heterogeneous nucleation. And pre-stretching promoted the nucleation and increased the growing rate of 1′. For peakage samples, increase in pre-stretching amount lead to decrease in size and aspect ratio of precipitates, however their densities were increased with more uniform distribution.③ The nature of 1′ did not changed after pre-stretching and aging. It is Mg Zn2 with a Laves structure, and the orientation relationship between 1′ and the matrix are [0001] a//[21-1-0] 1′ ,(21-1-0) a/ /(0001)1′ .④ The tensile and compressive properties are greatly influenced by pre-dislocations and pre-twins resulted from pre-stretching than the particles precipitated during aging. Small pre-stretching level and aging technique can improve the yield strength, but decrease the ductility, while severe pre-stretching(10% pre-stretching) and over aging technique can reduce the asymmetry of tension and compression.