Microstructures And Properties Of Mg-Y-MM-Zr Alloys

Magnesium alloys with rare earth elements not only have the inherent advantages as normal magnesium alloys, but also have corrosion-resistant and heat resistant characteristics. Therefore, it has been the research focus at home and aboard. The rare earth metals are divided into two main subgroups. One of them is cerium subgroup which has small solid solubility and can form steady phases. And the other one is yttrium subgroup which has large solid solubility and can form precipitation phases during aging treatment. In addition, because a large quantity of rare earth is used in function materials and the government controls the mining of rare earth resource for its irrecoverable ability, the price of pure rare earth metals is increasing year by year and the production cost of rare earth magnesium alloy rise. However, the associated rare earth metal has low utilization efficient. According to the background mentioned above, we investigate the effect of mish metal on the microstructures and properties of magnesium alloy with yttrium element by using OM, SEM, TEM, DSC and mechanical test to provide theoretical basis and experimental data for developing heat-resistant magnesium alloys and balance use of rare earth resource.The yttrium has the strongest reducing power of seize the oxygen atom at MgO, and the lanthanum is the weakest. The cerium has the less possibility of burning loss during the reaction between the rare earths and covering flux. And the yttrium and lanthanum have a greater possibility. The eutectic structures and the DSC curves areas of low melting point phases are increasing with the mish metal addition. When the content of mish metal is no more than 3%, the Mg-MM phase solidify at first as one of the second phases and then it is covered by the eutectic structures formed by Mg-Y and Mg, when the content of MM is 5%,the Mg-MM phase is not covered anymore and the Mg-MM phases distribute in grain boundaries independently. The layered eutectic structure is rich in yttrium element.The ultimate tensile strength (UTS) of the alloys is increasing, the elongation is decreasing and the yield ratio is rising with the mish metal addition and the alloy containing 3% mish metal has the highest strength with UTS of 240MPa. The brattle fracture characteristic of alloy is more and more obvious. The mechanism of twinning coordinating deformation disappears when the alloy contains 5% mish metal and its crack propagation is along the eutectic structures.The homogenization regulation of miniature ingot is decided by observing microstructures, hardness testing and DSC curve, where is 535℃for 18h. There are two typical phases remained after homogenization, one is Mg12MM phase and the other one is Mg4.26Y95.74 phase, which has the same crystal structure as Mg24Y5. The mechanical properties are improved after homogenization but the fracture characteristic does not change, while the crack is born intra-granular and Mg-MM phases. A two-step homogenization treatment is settled as 480℃×6h+535℃×16h, in order to prevent over burning and industrial production.The hot deformation behavour of WE91 alloy has been investigated. The results show that the alloy is sensitive to strain rate. The flow stress of WE91 magnesium alloy during high temperature deformation can be represented by Zener-Hollomon parameter in the hyperbolic Arrhenius-type equation. A mathematical model which is expressed by a quintic polynomia with strain is established to predict the stress-strain curves of this alloy during the deformation. Two hundred calculated samples are selected for comparing with the experimental samples and the fitting degree is 5.62% which means the model modify the need of engineering calculation.The nucleation mechanism of dynamic recrystallization of WE91 alloy can be summarized into four types:nucleating in deformation zone, nucleating close to original grain boundary, nucleating intra-grain and the particle stimulated nucleation.There is precipitation during the deformation which can be reduced by rising up the deformation temperature or strain rate. The processing maps have been calculated and analyzed according to the dynamic materials model.The processing map at strain of 0.916 exhibits three domains with peak efficiency of 49%,44% and 42%, respectively. It is found that the alloy can be extruded at 693K. The forms of distortion failure of WE91 alloy are stress concentration in deformation zone, mixed grain structure caused by adiabatic shearing and interface cracking between the second phase and the matrix. The extrusion condition is determined according to the processing map.The WE91 alloy after extrusion has obvious age-hardening response according to the age-hardening curves which have their own peak values and the mechanical properties are quite different. The UTS of extrusion WE91 alloy is 315MPa and it improve to 390MPa after aging 225℃for 28h which is as the T5 heat treatment system. Theβ′precipitated in peak aging alloys are the main strengthening phases and these phases are equiangular, which is nearly 40nm in length and 10nm in depth. The relationships between the matrix andβ′are<001>β′‖< 0001>αand{100}β′‖{1120}α.The strength of WE91 ally after T5 treatment decreases a little while rising up the test temperature and it decreases remarkably when the temperature is over 250℃. The alloy has more transgranular fracture at room temperature and the crack is born in Mg-MM phase particles.The precipitate free zone promotes the crack propagation. The transgranular fracture decreases while rising up the test temperature, the second particle breakage and intergranular fracture are the main crack sources. The mish metal addition transforms the way to fracture and improve the stability.The creep activation energy Q and strain coefficient n of WE91-T5 alloy are 116.2kJ/mol and 2.58 by linear regression calculation, respectively, which indicates the creep mechanism is boundary diffusion at 200℃.The creep-resistant mechanism is that intragranularβ′precipitation phases hinder the dislocation movement. Theβ′phases become coarser and transform in situβ1 phase and it also can hinder the dislocation movement after testing at 250℃.The T5 heat treatment and grain size can improve the alloy creep-resistant behavior.The extrusion WE91 alloy presents intergranular creep fracture at 250℃and it presents ductile fracture characteristics at 300℃, the original grains are elongated which is parallel to the force direction. At both temperatures mention above, the Mg-MM phases are good enough to stay stable. The boundaries which have more Mg-MM phases have less cracks and the failure during the creep test is maily caused by Mg-Y coarsing at high temperature.