Materials Science Forum Vols. 561-565

Abstract: Crack propagation tests on a bulk metallic glass, Zr55Cu30Ni5Al10, were conducted either in aqueous sodium chloride (NaCl) solutions or deionized water. Crack growth experiments were conducted under cyclic loading at a stress ratio of 0.1 or 0.5 under a loading frequency of 20 or 1.0 Hz. The experiments were also conducted under a sustained load. Although the crack growth rate in deionized water was almost identical to that in air, the rate in NaCl solution was much higher than that in air even in a very low concentration of NaCl such as 0.01%. In 3.5% NaCl solution, the time-based crack propagation rate during cyclic loading, da/dt, was determined by the maximum stress intensity factor, Kmax, but was independent of the loading frequency and the stress ratio, and the rate was almost identical to that of environment-assisted cracking under a sustained load.

Abstract: The kinetics of structural relaxation in a Pd40Ni40P20 bulk metallic glass was investigated by the volume relaxation due to density experiments and the enthalpy relaxation due to specific heat experiments. A two-step relaxation process was found in the volume relaxation, while the enthalpy relaxation seemed to be one-step relaxation process with a spectrum of relaxation times. First-step volume relaxation only in as-quenched glass was the process with a spread of relaxation times at lower relaxing temperature, while a Debye-type relaxation behavior was observed at higher temperature near Tg and in pre-annealed glass. The comparison of the kinetics of volume and enthalpy relaxations at the same temperature showed a disagreement with the prediction of free volume theory.

Abstract: Using ultrasonics, acoustic characteristics of Pd40Cu30P20, Zr55Cu30Al10Ni5, Zr65Pd12.5Ni10Al7.5 Cu5, Cu55Zr30Ti10Pd5, Cu45Zr20Hf25Ag10 and Ti41.5Cu42.5Ni7.5Hf5Zr2.5Sn1 glassy alloys were examined in terms of complex elasticity. The order of bulk modulus (K), Lamè parameter (λ ), Young (E ) and shear (G ) moduli of the glassy alloys is monopoltical character of glassy alloys, except for polymers and rubbers. For metals and alloys, ceramics, polymers and glassy alloys, Poison’s ratio correlates well to ratio G /K. Complex elasticity indicates that viscoelasticity of the glassy alloys is predominated by volumetric motion.

Abstract: Spark plasma sintering (SPS), as a developed rapid sintering technique, has a great potential for producing larger metallic glassy alloy specimens in a variety of shapes than those fabricated by casting methods, and can readily produce composites by dispersing crystalline particles in the glassy matrix. In this study, the Ni52.5Nb10Zr15Ti15Pt7.5 bulk metallic glassy matrix composites dispersed homogeneously with ceramics ZrO2 particulates were fabricated by the SPS process. The plastic ductility of the Ni52.5Nb10Zr15Ti15Pt7.5 glassy matrix composites was improved by adding ZrO2 particulates into the glassy alloy. The matrix of the fabricated composites maintained a glassy phase after the SPS process.

Abstract: High-strength nanocrystalline Ni-W alloys containing 16.9 at. % W with average grain size of about 6 nm in diameter has been obtained by electrodeposition. At room temperature, the nominal tensile strength of the alloy was attained to about 1600 MPa, while the plastic strain before fracture was a very low value of 0.05 %. In this case, highly localized shear bands were observed near the fractured surface of the tensile test specimen. When the samples were annealed at 300 °C under a static tensile stress of 327 MPa, the plastic strain was largely increased at the initial period of annealing and then tended to saturate, i.e., 0.54 % for 2 h, respectively. Grain size of the Ni-W alloys was almost saturated to 10 ~ 15 nm after annealing at 300°C for 2 h. It may be expected that the high tensile stress during grain growth might be effective to obtain the large uniform plastic deformation of nanocrystalline Ni-W alloys.

Abstract: The fatigue behavior of metals is strongly governed by the grain size variation. As the tensile strength, the fatigue limit increases with decreasing grain size in the microcrystalline regime. A different trend in mechanical properties has been demonstrated in many papers for metals with ultrafine (< 1 m) and nanocrystalline (< 100 nm) grain size in particular in the yield stress and fatigue crack initiation and growth. The fatigue behavior of electrodeposited nanocrystalline Ni (20 and 40 nm mean grain size) and nanocrystalline Co (20 nm) has been analyzed in the present paper by means of stress controlled tests. The monothonic mechanical properties of the materials were obtained from tensile tests by employing an Instron 5800 machine by measuring the strain with an extensometer up to 2.5% maximum strain. The strain gage specimen dimensions measured 20 mm length and 5 mm width, all the specimens were produced by electro-discharge machining. The low cycle fatigue tests were performed with specimens of the same geometry of the tensile ones in tension-tension with load ratio R=0.25. The fatigue crack propagation experiments were carried out by employing single edge notched specimens measuring 39 mm in length, 9.9 mm in width and with an electro-discharge machined edge-notch of 1 mm. All the endurance fatigue and crack propagation tests were performed at 10 Hz.

Abstract: In order to examine the correlation between a mechanical property and an excess free volume for metallic glass Pd44Cu31Ni8P17, longitudinal and transverse ultrasonic velocities were measured to estimate longitudinal and transverse elastic constants, c11 and c44. An as-quenched sample, an annealed one at a temperature just below a glass transition temperature Tg and a crystalline one were prepared. The as-quenched sample contains the excess free volume depending on the preparing process. The Young’s modulus E, the Poisson’s ratio, a bulk modulus and c12 are estimated using c11 and c44. The values of c11, c 44 , c12 and E of the as-quenched one are smaller than those of the annealed sample losing excess free volume by about 0.2 % and much smaller than those of the crystalline one. The Poisson’s ratio for the as-quenched sample and the annealed one are 0.389 and 0.387, respectively, which are much larger than that for the crystalline one with 0.349.

Abstract: Resistance microwelding is an important joining process used in the fabrication of miniature instruments. The small-scale resistance spot weldability of Zr based glassy thin sheet alloys was investigated. Metallic glass sheets were successfully welded and amorphous structure was maintained in both weld nugget and heat affected zone. Joint strength that is evaluated by shear breaking force increased as the welding current increased, resulting from nugget growth. Fractured surface showed typical vein pattern.

Abstract: A Zr65Al7.5Ni10Cu17.5-xPdx (x=0-17.5 at. %) alloy system is found to exhibit a different dependence of glass-forming ability (GFA) on atmosphere-pressure during mold casting process. High-Pd alloys (x=7.5-17.5) show a remarkable increase of critical size for glass formation (dc) as casting-atmosphere (Ar) pressure varies from vacuum to ambient. No significant change of dc, however, is observed in low-Pd alloys (x=0-5) despite the variation in atmosphere pressure. The origin of the phenomenon is unveiled by correlating the cooling characteristic during mold casting and on-cooling phase transformation characteristic of the alloy system.

Abstract: Different reactivity of ions has been implanted into Zr-Cu metallic glass to obtain nano-structured surface with controlled elasticity. The penetration of glass forming element of Ni+ into crystalline Zr-Cu stabilizes glassy phase to induce crystalline-amorphous (c-a) transition during implantation process. In the meanwhile, penetration of N+ into glassy matrix induces precipitation of (Zr, Cu)N at the mean penetration depth of N. Critical N concentration for nitride formation is estimated to be (Zr,Cu)-20at%N, which also suggests existing of N solid solution of glassy phase. Inert element of Ar+ yields dispersion of nano-voids among glassy matrix. Nano-indentation tests reveal that Young’s modulus of ion implanted glassy film dramatically changes with respect to the induced nano-structure, to decrease 0.4 times for Ar+, to increase 1.3 times for N+ as comparison with that for as-deposited state.