Synthesis And Properties Of Cu-based Cu-Zr-Al Bulk Glassy Alloys

Bulk glassy alloys (BGAs) have been expected to be used in wide application fields, because they exhibit many unique properties such as high strength, large elastic strain, high hardness, good soft magnetic properties, excellent corrosion resistance and viscous flow workability in the supercooled liquid region. For practical applications as the function materials, BGAs are requested to simultaneously possess glass-forming ability (GFA), high strength and good toughness. Recently, much attention has been paid to Cu-based BGAs because they have not only lower cost but also higher strength and higher thermal stability against crystallization as compared to other BGA systems. However, the lower GFA and plasticity are limited their use in industry. Therefore, in the present thesis, the thermal properties, GFA and mechanical properties of Cu-based glassy alloys were systematically investigated. Series of Cu-Zr-Al-based alloys exhibiting outstanding mechanical properties together with critical diameter for glass-formation (dc) over one centimeter were developed.We first examined thermal stability of Cu-Zr-Al alloy system by ribbon samples. The ternary glassy alloys possess large supper cooled liquid regionΔTX over 70 K in the composition range of Cu100-x-yZrxAly (x=41-48 at.%; y=4-10 at.%). The GFA of ternary Cu-Zr-Al glassy alloys were investigated by copper mold tilt casting. It revealed that the ternary Cu-based BGAs have large dc of centimeter order. The glassy alloys Cu47Zr45Al8 and Cu4yZr46Al7 exhibit highest critical diameter of 15 mm. The Cu-based BGAs with high GFA over 10 mm exhibit high compressive fracture strength of over 2200 MPa and the Young's modulus of 99-120 GPa, but low plasticity.For developing new Cu-based BGAs with higher GFA and plasticity, we systematically investicated the efferts of element substituting and microalloying technique on GFA and properties of Cu-Zr-Al glassy alloys. The results indicated that the Ag and heavy rare earth elements can inprove the thermal stability, GFA and plasticity.The 3-4 at.% of Ag adding into Cu-Zr-Al alloys have led to appreciable decrease of the liquidus temperature (Tl) and improved the stability of liquid. The quarternary Cu-based BGAs exhibit large GFA of 18 mm in diameter. The plasticity was also improved by the addition of Ag. The Cu45Zr43Al4Ag8 exhibit good mechanical properties under a compressive deformation mode, i.e., high yield strength of 1863 MPa, Young's modulus of 94 GPa and distinct plastic strain of 0.57%.It was found that the addition of minor Y, Dy and Lu greatly improved the stabilization of supercooled liquid as well as the GFA for Cu47Zr45Al8 alloy. Fully glassy alloy rods with diameters exceeding 20 mm could be successfully fabricated using a copper mold casting method. The 25 mm rods with full glassy structure were obtained for the (Cu47Zr45Al8)99Y1 and (Cu47Zr45Al8)96Y4 alloys. These alloys exhibit highest GFA of Cu-based BGAs in present. The heavy rare earth elements with limit range can increase the free volume of the ternary Cu-Zr-Al alloys. The (Cu47Zr45Al8)97Lu3 bulk glassy alloy exhibits good plasticity of 4.65% under compressive applied load at ambient temperature together with high fracture strength of about 1840 MPa and Young's modulus of 92 GPa. The sensitivity of elastic properties to small compositional variation has enabled the accurate determination of a well-defined transition from plasticity to brittleness as Poisson's ratio falls below the critical values.The high Cu-bearing alloys developed by rapid solidification have a well-developed fibrous structure of fcc-Cu and tetragonal Cu5Zr phases. The microstructure and the well-developed fibrous structure contribute to the high conpressive strength. The Cu90Zr9Al1 alloy exhibits high yield strength over 1500 MPa and hardness of 400 Hv. The yield strength, Young's modulus and hardness decreased with Al content increasing.