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1 Prediction of shear-band thickness in metallic glasses M.Q. Jianga, W.H. Wangb and L.H. Daia, aState Key Laboratory of Nonlinear Mechanics, Institute of Mechanics, Chinese Academy of Sciences, Beijing 100190, People’s Republic of China bInstitute of Physics, Chinese Academy of Sciences, Beijing 100190, People’s Republic of China Received 20 November 2008; revised 31 January 2009; accepted 20 February 2009. Available online 27 February 2009. We derive an explicit expression for predicting the thicknesses of shear bands in metallic glasses. The model demonstrates that the shear-band thickness is mainly dominated by the activation size of the shear transformation zone (STZ) and its activation free volume concentration. The predicted thicknesses agree well with the results of measurements and simulations. The underlying physics is attributed to the local topological instability of the activated STZ. The result is of significance in understanding the origin of inhomogeneous flow in metallic glasses. 2 Elasticity, thermal expansion and compressive behavior of Mg65Cu25Tb10 bulk metallic glass G. Lia, b, Y.C. Lic, Z.K. Jiangb, T. Xua, L. Huanga, J. Liuc, T. Zhangb and R.P. Liua aState Key Laboratory of Metastable Materials Science and Technology, Yanshan University, Hebei Street 438#, Qinhuangdao 066004, People’s Republic of China bSchool of Materials Science and Engineering, Beijing University of Aeronautics and Astronautics, Beijing 100083, People’s Republic of China cBSRF, Institute of High Energy Physics, Chinese Academy of Sciences, Beijing 100039, People’s Republic of China Received 17 December 2008; revised 16 February 2009. Available online 25 March 2009. Abstract The existence of special covalently bonded short-range ordering structures in a Mg65Cu25Tb10 bulk metallic glass (BMG) is confirmed by thermal expansion and compression behavior. Under ambient conditions the linear thermal expansion coefficient obtained is almost constant in the glassy state with a value of 4.0 × 10−5 K−1. By fitting the static equation of state at room temperature under ambient conditions we find the value for bulk modulus B of 48.7 GPa, which is in excellent agreement with the experimental study by pulse-echo techniques of 44.7 GPa. Unlike many bulk metallic glasses, such as Zr- and Pd-based, which bulk modulus is much larger than 100 GPa, the value B of Mg65Cu25Tb10 BMG falls into the range of SiO2 and fluorozirconate glass ZBLAN. Moreover, the elastic constant of the Mg65Cu25Tb10 BMG is almost the same as those of ZBLAN. No evidence for the high-pressure phase transitions of the Mg65Cu25Tb10 BMG has been found up to 31.19 GPa at room temperature. 3 Ductile-to-brittle transition in a Ti-based bulk metallic glass X.J. Gua, , , S.J. Poonb, G.J. Shifletc and J.J. Lewandowskia aDepartment of Materials Science and Engineering, Case Western Reserve University, Cleveland, OH 44106-7204, USA bDepartment of Physics, University of Virginia, Charlottesville, VA 22904-4714, USA cDepartment of Materials Science and Engineering, University of Virginia, Charlottesville, VA 22904-4745, USA Received 6 February 2009; accepted 23 February 2009. Available online 27 February 2009. The effects of changes in cooling rate on the density, elastic constants and fracture energy have been determined for Ti40Zr25Cu12Ni3Be20 bulk metallic glass. It is shown that changes in cooling rate directly affect the measured density and elastic constants, while the fracture energy, ranging from 148.9 to only 0.2 kJ m−2, is also shown to correlate with these changes in cooling rate/density/elastic constants. The critical Poisson’s ratio for toughness in this alloy system is 0.35, somewhat higher than that obtained in other metallic glasses, although consistent with recent theoretical predictions. Further correlations with these changes in toughness on the fracture surface appearance are provided. 4 Influence of minor Si addition on the glass-forming ability and mechanical properties of Pd40Ni40P20 alloy N. Chena, D.V. Louzguine-Luzgina, G.Q. Xieb, T. Wadab and A. Inouea aWPI Advanced Institute for Materials Research, Tohoku University, Katahira 2-1-1, Aobaku, Sendai, Miyagiken 980-8577, Japan bInstitute for Materials Research, Tohoku University, Sendai 980-8577, Japan Received 7 January 2009; revised 23 February 2009; accepted 23 February 2009. Available online 25 March 2009. Abstract The influence of a minor Si addition on the glass-forming ability and mechanical properties of Pd40Ni40P20 alloy was investigated. It is suggested that the minor Si addition can adjust the composition to be closer to eutectic, which favors a large undercooling. Furthermore, Si addition led to an enhancement in the ductility of Pd–Ni–P bulk metallic glasses. Simultaneously it is of great interest that there are nanoscale and microscale wavy steps observed on the fracture surface of Pd40Ni40Si4P16 metallic glass. It is suggested that a quickly running crack dissipates excess energy in terms of stress wave, leaving such wavy traces on the fracture surface of the deformed Pd-based metallic glass. 5 Formation mechanism of lamellar chips during machining of bulk metallic glass M.Q. Jianga and L.H. Daia aState Key Laboratory of Nonlinear Mechanics, Institute of Mechanics, Chinese Academy of Sciences, Beijing 100190, China Received 27 November 2008; revised 19 February 2009; accepted 19 February 2009. Available online 28 March 2009. Abstract The unique lamellar chips formed in turning–machining of a Vit 1 bulk metallic glass (BMG) are found to be due to repeated shear-band formation in the primary shear zone (PSZ). A coupled thermomechanical orthogonal cutting model, taking into account force, free volume and energy balance in the PSZ, is developed to quantitatively characterize lamellar chip formation. Its onset criterion is revealed through a linear perturbation analysis. Lamellar chip formation is understood as a self-sustained limit-cycle phenomenon: there is autonomous feedback in stress, free volume and temperature in the PSZ. The underlying mechanism is the symmetry breaking of free volume flow and source, rather than thermal instability. These results are fundamentally useful for machining BMGs and even for understanding the physical nature of inhomogeneous flow in BMGs [ Last edited by hslining on 2009-9-21 at 23:24 ] |
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