Preparation, Microstructure, Mechanical Properties And Strengthening Mechanism Of Mg-Zn-Y Alloy

Magnesium alloy is well known for its good biological compatibility, light weight and large ratios of strength-to-weight and rigidity-to-weight, outstanding processing and cast ability, good corrosion resistance, good thermal conductivity, vibration and the electromagnetic shielding, abundance, and easy recycling etc, making it qualifying as the most promising structural materials for preparation electrical case, transport equipments and areospace parts. However, magnesium alloy has relatively low strength, ductility which are need to be further improved. It is known that rapid solidification (RS) technology can effectively refine the microstructure, improve micro-segregation, thus expected to substantially improve the mechanical properties of magnesium alloy. Reciprocating extrusion (RE) process is a severe plastic deformation technology which can effectively refine the microstructure and can recover the original shape of the deformed material after RE operation.In this paper, Mg-Zn-Y alloys have been prepared with conventional solidification (CT) and Chill-Block Melt Spinning rapid solidification (RS) and reciprocating extrusion (RE) processes. The microstructure, mechanical properties and the strengthening mechanism of, the prepared materials have been studied. The interfaces welding mechanism of the RS ribbons are analyzed and concluded. The main detailed results are as following.KND- II rapid solidification equipment was modified basing on the principle of the chill block planar flow melt-spinning process (PFMS). RS ribbons of Mg-Zn-Y alloy with grain size less than 5μm were prepared. The cooling rate of the RS-Mg-Zn-Y alloy ribbon prepared was in between 1.14×10⁶ and 4.12×10⁷ K/s. The microstructures of the ribbons were consisted of matrix of supersaturatedα-Mg solid solution and a small amount of intercrystalline Z phase. Slightly microsegregation of the matrix of the ribbon was revealed.REed materials with refined grains and strengthening particles were prepared from CT-Mg-Zn-Y alloy and RS-Mg-Zn-Y alloy ribbons by using developed reciprocating extrusion apparatus which amounted on a vertical press machine. It was revealed that grain refinement was achieved by dynamic recrystallization during RE processing. The recrystallization was found mostly occurred at grain boundaries and within deformation banded areas. Refined strengthening particles were resulted from the breaking of the grain boundary solidified mixtures or intermetallics. During RE, the refined strengthening particles were transferred within the matrix following the flow of the deformed materials, and were redistributed uniformly after RE process. The more RE passes the material was processed, the more effective of the grain refinement process. Therefore, RE was an effective method to get refined Mg-Zn-Y alloy from as cast materials and RS ribbons.During RE, the true strain rate of each pass was 0.1503s"1. Therefore, severer true strain of the RS ribbons was accumulated due to the large strain rate by each pass of RE. So, larger Zener-Hollomon parameter (Z*), which could promote atomic diffusion and nucleation at temperatures between 300℃and 350℃. It is found that RE is an effective method to prepare magnesium materials with height density and height mechanical properties, and is a preferred method to obtain sound welded materials from RS ribbons.The strengthening particles of the RE-n-EX-RS-Mg-Zn-Y alloy were consisted of three kinds: one was solidified during RS process which was redistributed uniformly within matrix during RE. This kind of particle, - 100 nm, could not be refined during RE. The second with irregular shapes and size of 0.5μm, were broken from grain boundary mixture or compounds during RE. The third about 70nm in size were resulted from precipitation during the RE process.The RE-n-EX-RS-Mg-Zn-Y alloys have high tensile-strength, large ratio of ultimate tensile strength-to-tensile yield strength and high ductility. For example, the ultimate tensile strength of the RE-n-EX-RS-B1 or RE-n-EX-RS-B2 alloy is larger than 400 MPa. The ratio of ultimate tensile strength-to-tensile yield strength is larger than 0.8 for RE-n-EX-RS-B1 or RE-n-EX-RS-B2 alloy, and is about 1 for RE-n-EX-RS66 alloy. The elongation of the RE-n-EX-RS66 alloy is larger than 20%. The strengthening mechanism of RE-n-EX-RS-Mg-Zn-Y magnesium alloy was due to the contributions of the grain refinement strengthening, solution strengthening, dislocation strengthening, precipitation strengthening, dispersion strengthening, etc. However, the grain refinement strengthening and Orowan strengthening were the mostly effective strengthening mechanism.The thermal (line) expansion coefficient of Mg-Zn-Y alloy is related to the prepared technology. The thermal (line) expansion coefficients of the materials prepared have the following order:α_tCT-MG-Zn-Y>α_{tRE-n-Ex-CT-Mg-Zn-Y}>α_{tRE-n-EX-RS-Mg-Zn-Y}. RE and RS could obtained the materials with low thermal (line) expansion coefficient.