Showing papers by "H.F. Zhang published in 2006"

The effect of primary crystallizing phases on mechanical properties of Cu46Zr47Al7 bulk metallic glass composites

Abstract: Cu46Zr47Al7 bulk metallic glass (BMG) and its composites in plate with different thicknesses up to 6 mm were prepared by copper mold basting. Primary crystallizing phases with different microstructures and volume fractions could be obtained under different cooling rates, forming some composites with different mechanical properties. Under compression tests, the 2-mm-thick monolithic BMG has a yield strength of 1894 MPa and a high fracture strength of up to 2250 MPa at plastic strain up to 6%, exhibiting apparent "work-hardening" behavior. The 4-mm-thick Cu46Zr47Al7 BMG composite containing martensite phase yields at 1733 MPa and finally fails at 1964 MPa with a plastic strain of 3.7%.

Elevated temperature compression behaviour of Mg–Al–Zn alloys

Abstract: Flow stress behaviour of Mg–Al–Zn alloys was studied by compression testing using a Gleeble 1500 machine at temperatures of 200–450°C and at strain rates ranging from 0·005 to 5 s−1. The experimental results show that initial grain size and a distinctive basal texture lead to the great difference between as extruded AZ31 alloy and other two groups of cast alloys AZ31 and AZ80, including the variation in the flow stress equations and dynamic recrystallisation behaviour. The increment of alloying element Al will decrease stacking fault energy and enhance the process of dislocation climb, and therefore reduce the tendency for dislocation pile-up to cross-slip. The presence of the second phase particles will hinder both the formation and migration of recrystallisation fronts. As a result, for cast AZ80 alloy, dynamic recrystallisation is delayed and the activation energy for the plastic deformation process sharply increases from 166·75 to 220 kJ mol−1 as compared with cast AZ31 alloy under the same de...

Effects of tungsten fiber on failure mode of Zr-based bulk metallic glassy composite

Abstract: The authors systematically investigated the effects of tungsten fiber on failure mode as well as deformation and fracture mechanisms in tungsten fiber-reinforced Zr41.25Ti13.75Ni10Cu12.5Be22.5 bulk metallic glassy composite under uniaxial compression at room and high temperatures. At room temperature, the failure mode of the composite changes from shear fracture to longitudinal splitting failure with increasing fiber volume fraction. Similar to the observations in monolithic metallic glasses, the shear fracture angle of the composite is approximately equal to 39∼40 deg, indicating that the Mohr-Coulomb criterion is suitable to give the critical shear fracture condition of the composite. When the compression tests were performed below the glass transition temperature of Zr41.25Ti13.75Ni10Cu12.5Be22.5 metallic glassT g, the deformation behavior of the composite strongly depends on the strain rates and the test temperature, which is quite similar to the deformation behavior of monolithic metallic glasses in the supercooled liquid region. The corresponding failure mode of the composite changes from shear or splitting fracture to bending failure with decreasing strain rate or increasing test temperature. The failure modes at the temperature nearT g are mainly controlled by the metallic glass matrix due to the decrease in its viscosity at high temperature. Based on these multiple failure modes, the effects of test temperature and tungsten fiber volume fraction on deformation and fracture mechanisms are summarized.

Deformation and fracture behavior of tungsten fiber-reinforced bulk metallic glass composite subjected to transverse loading

Abstract: Deformation and fracture behavior of Zr41.25Ti13.75Ni10Cu12.5Be22.5 bulk metallic glass and its composite containing transverse tungsten fibers in compression were investigated. The monolithic metallic glass and the tungsten fiber composite specimens with aspect ratios of 2 and 1 are shown to have essentially the same ultimate strength under compression. The damage processes in the bulk metallic glass composite consisted of fiber cracking, followed by initiation of shear band in the glassy matrix mainly from the impingement of the fiber crack on the fiber/matrix interface. The site of the shear band initiation in the matrix is consistent with the prediction of finite element modeling. Evidence is present that the tungsten fiber can resist the propagation of the shear band in the glassy matrix. However, the compressive strain to failure substantially decreased in the present composite compared with the composites containing longitudinal tungsten fibers. Finally, the two composite specimens fractured in a shear mode and almost all the tungsten fibers contained cracks.

Fatigue damage and fracture behavior of tungsten fiber reinforced Zr-based metallic glassy composite

Abstract: The fatigue life, damage and fracture behavior of tungsten fiber reinforced metallic glass Zr41.25Ti13.75Ni10Cu12.5Be22.5 composites are investigated under cyclic push-pull loading. It is found that the fatigue life of the composite increases with increasing the volume fraction of tungsten fibers. Similar to crystalline metals, the regions of crack initiation, propagation and overload fracture can be discerned on the fracture surface of the specimen. Fatigue crack normally initiates in the metallic glass matrix at the outer surface of the composite specimen and propagates predominantly in the matrix. Different crack front profile around the tungsten fibers and fiber pullout demonstrate that fatigue crack may propagate around the fiber, leading to bridging of the crack faces by the unbroken fiber and hence improved fatigue crack-growth resistance. Locally decreased effective stiffness in the region where fiber distribution is sparse may provide preferential crack path in the composite. A proposed model was exercised to elucidate different tungsten fiber fracture morphologies in the fatigue crack propagation and overload fracture regions in the light of Poisson's ratio effect during fatigue loading. (c) 2005 Published by Elsevier B.V.

Synthesis and characterization of bulk amorphous Zr65Cu18Ni9Al8 and [Zr0.65Cu0.18Ni0.09Al0.08]98Er2 alloys

Abstract: Bulk metallic glasses (BMGs), especially Zr-based BMGs, have attracted lot of attention of materials scientists because of their very attractive physical, thermal and mechanical properties and a few unique applications. In the present study, Zr65CU18Ni9Al8 alloy was designed according to the criterion of conduction electron/atom (e/a ratio) similar to 1.395 and average atomic size of alloy (R-a) similar to 0.1498 nm. Addition of 2 at.% Er was carried out in the base alloy to investigate its effect on thermal and mechanical properties. Characterization of alloys was performed using the techniques of XRD, DSC, and SEM/EDS. Mechanical properties like Vicker's microhardness, nanohardness, elastic modulus, density and fracture strength were measured. Average shear angle was found to be similar to 35 +/- 1 degrees for base alloy and about 31 +/- 1 degrees for alloy containing 2 at.% Er. Wide supercooled liquid regions of 129 K and 119 K were found for the base alloy and the alloy containing 2 at.% Er. (c) 2006 Elsevier B.V. All rights reserved.

Specific heat characteristics near Tc and low temperature in MTG Y0.8Ca0.2Ba2Cu3Oy crystals

Abstract: The specific heat of a melt-textured-growth Y0.8Ca0.2Ba2Cu3Oy crystal has been measured from 100 to 70 K in an applied magnetic field up to 9 T and from 30 to 2 K in a magnetic field up to 4 T, respectively. The abnormal electronic specific heat near Tc can be simulated correctly by the London model, indicating that there is a coexistence of the quasiparticles and the Cooper pairs in the mixed state. The field-induced specific heat CDOS at low temperature (T<5 K) behaves as αT2 in zero field and AT(μ0H)1/2 in nonzero fields, which is well-known evidence for d-wave paring. The values of the Sommerfeld constant γn and the upper critical field μ0Hc2(0) deduced from the specific heat near Tc are consistent with that at low temperature (T<5 K).