| Bulk metallic glasses (BMGs) have attracted great attention from both the researchers of fundamental science and practical application due to a series of excellent properties, such as high strength, high hardness, good corrosion resistance and high wear resistance, et al. However, the lack of significant plastic deformation ability at room temperature is the main obstacles for the BMGs used as structural materials. Thus, understanding the possible factors which affect the mechanical properties, especially the plastic strain becomes a focus in order to put the materials into use as soon as possible.In this paper, the effects of minor Nb addition on thermal stability and mechanical properties in Zr47-xCu47Al6Nbx(x=0,1,2) and Zr50.5-xAl9Ni4.05Cu36.45Nbx(x=0,1,2,4) alloys were firstly investigated. On the basis, the effect of brittle phase on fracture surface and mechanical properties was investigated in Zr46Cu47Al6Nbi alloy. In addition, effects of casting diameter and cryogenic treatment on (Ti0.361Zro.332Ni0.058Be0.249)91Cu9 alloy and Ti45.7Zr33Ni3Cu5.8Be12.5 alloy were investigated. The main experimental results are as following:For the alloy of Zr47-xCu47Al6Nbx(x=0,1,2) with 2mm in casting diameter, the largest supercooled liquid region (ΔTx=69.0K) and fracture strength (σf=1830MPa) can be reached while x=2. For Zr50.5-xAl9Ni4.05Cu36.45Nbx(x=0,1,2,4) alloy with the same casting diameter, the largest supercooled liquid region (ΔTx=68.1K) and fracture strength (af=2032MPa) can be reached while x=4. The above results show that the minor Nb additions is a practical method of improving supercooled liquid region and fracture strength of ZrCu based bulk metallic glasses.The effects of brittle phase precipitated in the amorphous matrix for Zr46Cu47Al6Nb1 alloy with diameters of 3 and 5mm on mechanical properties were studied. The volume fraction of amorphous phase in the matrix and the fracture strength decrease as the sample size increases compared to the amorphous alloy with 2mm in casting diameter. The crack initiation and rapid propagation between brittle phase and amorphous matrix also decrease the strength of alloy. The plasticity of alloy is unable to be improved because the split between brittle phase and amorphous matrix can not hinder the proliferation of shear band and induce the formation of multiple shear bands. Therefore the brittle phase can not be regarded as the one that can toughen amorphous phase.For (Ti0.360Zr0.332Ni0.058Be0.249)91Cu9 BMG with 2,3 and 4mm in diameters, the plastic strains are 1.21,0.75 and 0.33%, respectively. The plastic strain decreases while the value of fracture strength varies in the range of 1970±20MPa, which indicate that the plastic strain should be dependent on the casting size. Furthermore, the size dependent of plasticity and density of vein pattern on the fracture surface were discussed on the basis of free volume.The investigations of morphology and mechanical properties in Ti-based Ti45.7Zr33Ni3Cu5.8Be12.5 BMGC have shown that the volume fraction of P-Ti(Zr, Cu) dendrite-like phase increase as the sample size increases. For specimen with 2,3 and 4mm in diameters, the plastic strains are 1.33%,1.81% and 1.52% while the values of fracture strength vary in the range of 1909±3MPa, which indicate that the casting size has little effect on strength but the maximun plastic strain corresponding to an optimal size of dendrite-like phase.The amorphous nature in (Ti0.361Zr0.332Ni0.058Beo.249)91Cu9 BMG is not changed, but the plastic stains increase from 0.75% to 1.92%,3.75% and 0.79% while the fracture strengths change from 1968MPa to 2015MPa,2078MPa and 1958MPa after cryongenic treatment for 12, 24 and 48h, respectively. In other words, the largest value of strength and plastic strain can be reached after cryogenic treatment for 24h. The observed propagation of shear band on later surface and discontinuous slide of crack along the shear direction indicate that the transmission rate of shear band should be slow down, which consequently improved the plasticity. The morphology of precipitated phase and compressive strain are not obviously changed after CT maybe associated with the stable structure in Ti45.7Zr33Ni3Cu5.8Be12.5 BMGC. |
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