Microstructures and mechanical properties of tungsten wire/particle reinforced Zr57Nb5Al10Cu15.4Ni12.6 metallic glass matrix composites

Haein Choi-Yim, Jan Schroers, William L. Johnson
2002 Applied Physics Letters  
Tungsten wire or particle reinforced metallic glass matrix composites are produced by infiltrating liquid Zr 57 Nb 5 Al 10 Cu 15.4 Ni 12.6 ͑Vit106͒ into tungsten reinforcements at 1150 and at 1425 K. X-ray diffraction, differential scanning calorimetry, and scanning electron microscopy are carried out to characterize the composite. The matrix of the composite processed at 1150 K is mostly amorphous, with some embedded crystals. During processing, tungsten dissolves in the glass-forming melt and
more » ... ss-forming melt and upon quenching precipitates over a relatively narrow zone near the interface between the tungsten and matrix. In the composites processed at 1425 K, tungsten dissolves in the melt and diffuses through the liquid medium, and then reprecipitates upon quenching. The faster kinetics at this high temperature results in a uniform distribution of the crystals throughout the matrix. Mechanical properties of the differently processed composites containing wires and particles are compared and discussed. The composites exhibit a plastic strain failure of up to 16% without sacrificing the high-failure strength, which is comparable to monolithic Vit106. The limited plasticity of monolithic bulk metallic glasses ͑BMGs͒ has triggered research on metallic glass matrix composites. To improve the toughness two different approaches have been followed. One is to introduce foreign particles into the matrix. It was found that a variety of reinforcement materials such as SiC, WC, Ta, or W can be introduced into the metallic glass matrix without inducing crystallization. 1-6 The size of these particles ranges from 3 to 100 m. One crucial contribution to improve the ductility is the formation of a strong interface between the reinforcement material and BMG. 7 To guarantee a strong interface another approach has been followed where the composite forms in situ. This is done by partially crystallizing the sample upon cooling 8 or upon subsequent heating. 9 The size of these reinforcement crystals varies between several nm to 20 m. Volume fraction of the reinforcement crystals can be controlled by varying composition, processing time, and temperature, though not as directly as in the case of direct addition of particles to the glass-forming melt. The Zr 57 Nb 5 Al 10 Cu 15.4 Ni 12.6 ͑Vit106͒ glass-forming alloy is one of the best glass-forming Zr-based alloys that does not contain Be. 10 In addition, it is very robust against heterogeneous nucleation at surfaces or interfaces. The thermal stability of this alloy with respect to crystallization is not compromised by adding crystalline particles into its molten state. 11 The plastic deformation range under compression of the glass was improved by 300% when WC, W, or Ta particles with only 5%-10% volume fraction were added to it. 12 This is attributed to the fact that the material no longer fails along a single shear band that traverses the sample but forms multiple shear bands in the presence of particles. In this letter, we combine the techniques of adding foreign particles and in situ composite formation. Thermal and microstructure investigations are presented for tungsten me-tallic glass composites processed at 1150 and 1425 K. The high-processing temperature of 1425 K was chosen in order to increase the amount of tungsten that dissolves in the melt during processing the Vit106 liquid. Upon cooling, tungsten precipitates out and forms uniformly distributed crystals that result in a dense array of tungsten crystals in the Vit106 matrix. We will show that the composite sample processed at 1425 K contained more than 15 at. % of tungsten dissolved in the matrix prior to cooling. The effect of increasing tungsten dissolution into the matrix on the mechanical properties of these composites is presented and discussed. Ingots of Zr 57 Nb 5 Al 10 Cu 15.4 Ni 12.6 were made by arc melting a mixture of the elemental metals ͑purity better than 99.5%, metal basis͒. Tungsten wires, with a nominal diameter of 250 m, were straightened and cut to 5 cm lengths. In addition, tungsten powders with an average diameter of 80 m were used as reinforcement. Composite specimens were cast in a resistive furnace by melting the ingots in an evacuated 7-mm-inner-diam 304 stainless-steel tube packed with wire or particle reinforcement, followed by pressure infiltration of the molten Vit106 alloy. The reinforcement particles were placed in the sealed end of the tube, which was necked about 2 cm above the reinforcement. Ingots of the matrix materials were placed in the tube above the neck. Prior to heating, the tube was evacuated and then flushed with argon gas several times to remove residual oxygen. The sample tube was heated under vacuum in a resistive tube furnace with temperature feedback control to minimize trapped gas in the composite sample. The sample was then heated to 1250 K, well above the liquids temperature of 1093 K of the Vit106 alloy. This overheating of the molten Vit106 is found useful in dissolving residual oxides and other impurities that degrade the glass-forming ability of the alloy. 13 The sample was held at this temperature for 10 min. The temperature was then lowered to 1150 K and allowed to stabilize. When the furnace reached this target temperature, a pressure of 80 psi a͒ Electronic
doi:10.1063/1.1459766 fatcat:gym7irghavbxpci7yaqr6olscq