Rong Shi Chen

Microstructure and strengthening mechanisms of a cast Mg-1.48Gd-1.13Y-0.16Zr (at.%) alloy

The microstructure and mechanical properties of a Mg–1.48Gd–1.13Y–0.16Zr (at.%) alloy in the as-cast, solution-treated, peak-aged and over-aged conditions have been investigated by a combination of thermodynamic calculations and experimental approaches. It is shown that both the Mg24(Gd,Y)5 and cuboid-shaped Mg5(Gd,Y) phases exist in the as-cast sample, which is in good agreement with the Scheil solidification model. The former is dissolved during solution treatment, while the latter persists and coarsens. Subsequent artificial ageing results in the formation of metastable β′ precipitates within the α-Mg matrix and along the grain boundaries. The peak-aged alloy exhibits maximum ultimate tensile strength and tensile yield strength of 370 and 277xa0MPa, respectively, at room temperature. Moreover, the strengths decrease gently from room temperature to 250xa0°C with a gradual increase of elongation. The strengthening contributions to the yield strength are quantitatively evaluated from individual strengthening mechanisms by using measured microstructural parameters. The modelled yield strengths are compared with the experimental results and a reasonable agreement is reached.

Semisolid Microstructural Evolution of Equal Channel Angular Extruded Mg-Al Alloy During Partial Remelting

Using equal channel angular extrusion (ECAE) process, which combine the advantages of grain refinement and induced strain, for preparing semisolid billets is a relatively new Strain Induced Melt Activation(SIMA)method. This paper investigates the remelting and semisolid isothermal holding behavior of Mg-9Al alloy after ECAE processing at 350°C. It has been found that 2 passes ECAE-ed billets are qualified for obtaining spherical grains after remelting and isothermal holding treatment. Increasing the ECAE process from 2 passes to 8 passes has no further significant effects on the size and roundness of the solid particles at the semisolid state. The effects of temperature and isothermal holding time on microstructural evolution have also been investigated. In addition, the solid solution treatment before the ECAE processing affected greatly on microstructural evolution of the alloy during ECAE processing and thus the following remelting and isothermal holding behavior. The Mg17Al12 precipitates were uniformly decomposed from the saturated solid solution in the solutionized samples during ECAE processing. The coalescence of grains together with the self-blocking effect generated more entrapped liquid in the solutionized sample at semisolid state. Moreover, the solid particles of solutionized samples have bigger size and grow faster, which are detrimental to SSM processability.

Development of a High Strength and High Ductility Magnesium Alloy

A new steel mold gravity casting magnesium alloy of low-cost, high strength, and high ductility has been developed and studied. This new magnesium alloy, which is designated as IMR-41, exhibits high strength (Yield Tensile Strength≈145 MPa, Ultimate Tensile Strength≈280 MPa) and high ductility (Elongation≈8%) at room temperature. The alloying elements are inexpensive ones and the cost of IMR-41 is similar to AZ91 series. The influence of small X element addition and heat treatment on the microstructures and mechanical properties are discussed. The IMR-41 combines the virtues of AZ91 series and AM60 series to some extend and shows great potential application on wheels of lightweight vehicles or motorcycles, etc. which require high strength and high ductility simultaneously.

Microstructure Evolution of Mg-Y-Nd-Zr Alloy during High Temperature Deformation

The variation of the flow stress and the microstructural evolution during hot deformation at different test temperatures and strain-rates are studied. During hot deformation, dynamical recrystallization did not occur completely; after annealing at 523K, microstructural changes due to static recrystallization and small grain sizes of several micrometers and fine nanometer-precipitates can be attained. The stress exponent (n) and the activation energy (Q) for high temperature deformation have been evaluated and the deformation mechanisms at different temperature have also been discussed.

Preparation of Semi-Solid Billet of ZW61 Alloy by Equal Channel Angular Extrusion

Strain induced and melt activated (SIMA) is used for the preparation of semi-solid billet through equal channel angular extrusion (ECAE) as strain induced step in SIMA and completing melt activated step by semi-solid isothermal treatment, by which semi-solid billet with fine spherical grains can be prepared. As comparison, extruded ZW61 alloy is also remelted to the semi-solid state. Furthermore, both of the semi-solid isothermal treatment microstructures in as-cast and extruded ZW61 alloy are discussed. ECAE process is found more potential for semi-solid thixoforming for as-cast ZW61 alloy.

Microstructure and Mechanical Properties of Extruded Mg-Al-Ca Based Alloys

Mechanical properties and microstructure of extruded AZ91(-Ca) alloys have been studied in this paper. The results showed that Ca has no significant effect on reducing grain size of the extruded AZ91 alloy. The ambient temperature tensile tests showed that the ultimate and yield strength of extruded AZ91 alloy decreased by addition of Ca. At elevated temperature, Ca addition improves the yield strength of both AZ91 alloy. The variations in microstructure and mechanical properties of the AZ91 alloy are also discussed in terms of the effects of Ca on grain refinement and formation of constituent phases.

Characterization of Mushy Zone of ZWx2 (x=2, 4, 6) Magnesium Alloys

The potentials of Mg-Al system alloys for semi-solid forming have been extensively evaluated these years. In the present studies, mushy zone properties of the Mg-Zn system alloys, i.e. ZWx2 (x=2, 4, 6), are characterized and analyzed by using cooling curve thermal analysis method and continuous torque measurement technique. The results show that Tn1 (first characteristic temperature of primary crystal nucleation), Tn2 (second characteristic temperature of primary crystal nucleation) and Tch (temperature of dendritic coherency) decrease with the increasing of zinc content, and the temperature difference of (Tn1-Tn2) almost remains constant, while the temperature difference of (Tn2-Tch) changes dramatically when Zn content increases to 6wt.%; The results also show that the fs-cc (solid fraction of dendritic coherency) of ZWx2 alloys ranges from 0.4 to 0.51 while ZW22 and ZW62 alloys having higher fs-ch and ZW42 alloy having the lowest fs-ch; also, strength of alloys in mushy zone does not develop until a certain fraction solid( corresponding to the dendrite coherency point) is reached. The results obtained by cooling curve thermal analysis method and continuous torque measurement techniques are comparable to each other.

Structural Evolution and Deformation Behavior of Extruded AZ91 with Ca Addition in the Semi-Solid State

The microstructural evolution and mechanical response in compression in the semi-solid state of previously extruded AZ91 alloys containing two levels of Ca additions (1mass%Ca (AZC911) and 2mass%Ca (AZC912)) have been studied. Ca additions have a significant effect on microstructural evolution and compression behavior of the AZ91 alloy. At 515°C, the liquid fraction in AZC911 is larger than that in AZC912, so that the compression stress of the AZC912 alloy was found to be much larger than that of the AZC911 alloy. This behavior is explained through DSC analysis which suggests that some solid Al2Ca phase remains in AZC912 alloy at this temperature. Increasing the remelting temperature for this alloy leads to more liquid and coarsening of the solid particles occurs with increasing holding time.

Microstructural Evolution and Deformation Mechanisms for Superplasticity of NiAl Intermetallic

The microstructural evolution during superplastic deformation of the extruded stoichiometric NiAl polycrystals were systemically investigated in various conditions of temperature, strain rate and strain by means of optical microscopy (OM) and transmission electron microscopy (TEM). Consequently, The deformation microstructures corresponding to the large tensile elongation consisted of subgrains, low angle grains as well as high angle grains, which indicated that continuous dynamic recrystallization (CDRX) process was operating during superplastic deformation.