Study On Strengthening Of ZA84+(x)Y Magnesium Alloy

Magnesium alloy, which has promising application prospect in the fields of automobile, aero-space and 3C products, is praised as"new green engineering material in 21st century". Because of low cost and good high-temperature performance, Mg-Zn-Al alloy have been used in automobile industry for engine producing. AS one of important Mg-Zn-Al alloys with excellent capability , ZA84 magnesium alloy have high application value and development potential. But the problem that high temperature performance and room temperature performance is difficult to compatible needs the further research to increase their application value.By means of as-cast forming technique and test facilities such as optical microscope, scanning electron microscope(SEM), X-ray diffraction (XRD),electronic universal testing machine and et cetera, the effects of Y addition on the microstructures and mechanical properties of the Mg-8Zn-4Al alloys have been investigated. Meanwhile,effects of heat treatment on the microstructure and mechanical properties of the Mg-8Zn-4Al alloys have been studied. The strengthening mechanism of Y on the testing alloy have been in-depth explored。(1) The matrix phase of Mg-8Zn-4Al cast alloys areα-Mg,Ï„-Mg₃₂(Al,Zn)₄₉ phase andφ-Al₂Mg₅Zn₂ phase which exist as dispersive island-like.Ï„-Mg₃₂(Al,Zn)₄₉ phase which exist as intermittent strip-like, semi-continuous and continuous net-like.(2) With the content of rare earth Y increasing, the microstructure of alloy and grain were first refined and then thicken. When the content of Y less than 0.5%, theÏ„-Mg32(Al, Zn)49 phase changes semi-continuous network into granular fracture, and as the content of Y is 0.5%, the alloy gets best refinement. When the containing Y more than 0.5%, the network organizations re-emergence, and gradually becomes coarser, while the new phase with Y comes out, its molecular formula closes to Mg₅₀Al₁₀Zn₃Y₃₇.(3) With solution treatment to ZA84 magnesium alloy at 350℃at different times for 4 20h, mesh netted Mg32(Al, Zn)49 phase gradually fracture into broken granular with the growth of solution time. And the phase will slowly dissolve into the matrix. When the solution time is 16h, the coarse second phase almost disappeared in the alloy, only leaving a small number of small phases, while the solution time continue extend the burning phenomenon occurred in the alloy. The ZA84 magnesium alloy after solid solution at 350℃and 16h gets the aging treatment at 180℃at different times for 0 24h. With the extending of the aging time, the number of precipitated particles in the alloy increased, and the particles were dispersed. When the time is 16h, the number of precipitates in the organization reached maximum. When the time is too long, the aging phenomenon appeared in the organization, leading to the coarsening of tissue. (4) With the content of Y increasing, the brinell-hardness value of alloy first increased and then decreased, and reaches maximum as the Y content is 1.0%. The impact toughness of the alloy with Y content increasing linearly increased. With the addition of 1.5% Y, the experimental values of impact toughness of the alloy is about double of that of alloy without addition of rare earth. With the content of Y increasing, tensile properties increased and then decreased under room temperature. The changes of tensile strength under high temperature (150℃) were similar with the state of room temperature, but while the content of Y surpasses 1.0% the tensile strength is increasing slightly. Moreover while the content of Y is 0.5%, the best mechanical properties are got. The tensile strength and yield strength were 208.4MPa and 154.6MPa at the room temperature. Compared to the matrix alloy they are increased by 15.6% and 27.5%; At 150℃, the tensile strength and yield intensity was 182.7MPa and 146MPa.Compared to the matrix alloy, they are improved 14.5% and 26.7%.(5) With solution and aging treatment, the value of the alloy hardness significantly increased. With aging time extended, the hardness value first increased and then decreased, as well as the aging time is 6h, the hardness reached maximum. Above all, the hardness value of the ZA84 magnesium alloy with 1.0% Y addition increased to 79.2HB, which is increased by 35.2% compared with matrix alloy.