Plastic deformation in Ce-based bulk metallic glasses during depth-sensing indentation

The deformation behavior and the effect of the loading rate on the plastic deformation features in (numbers indicate at.%) Ce60Al15Cu10Ni15, Ce65Al10Cu10Ni10Nb5, Ce68Al10Cu20Nb2, and Ce70Al10Cu20 bulk metallic glasses (BMGs) were investigated through nanoindentation. The load-displacement (P-h) curves of Ce65Al10Cu10Ni10Nb5, Ce68Al10Cu2, and Ce70Al10Cu20 BMGs exhibited a continuous plastic deformation at all studied loading rate. Whereas, the P-h curves of Ce60Al15Cu10Ni15 BMG showed a quite unique feature, i.e. homogeneous plastic deformation at low loading rates, and a distinct serrated flow at high strain rates. Moreover, a creep deformation during the load holding segment was observed for the four Ce-based BMGs at room temperature. The mechanism for the appearance of the "anomalous" plastic deformation behavior in the Ce-based BMGs was discussed. (c) 2006 Elsevier B.V. All rights reserved.

Crack propagation in the power-law nonlinear viscoelastic material

An analysis on crack creep propagation problem of power-law nonlinear viscoelastic materials is presented. The creep incompressilility assumption is used. To simulate fracture behavior of craze region, it is assumed that in the fracture process zone near the crack tip, the cohesive stress sigma(f) acts upon the crack surfaces and resists crack opening. Through a perturbation method, i. e., by superposing the Mode-I applied force onto a referential uniform stress state, which has a trivial solution and gives no effect on the solution of the original problem, the nonlinear viscoelastic problem is reduced to linear problem. For weak nonlinear materials, for which the power-law index n similar or equal to 1, the expressions of stress and crack surface displacement are derived. Then, the fracture process zone local energy criterion is proposed and based on which the formulas of cracking incubation time t

Microstructure, deformation and failure of polymer bonded explosives

Polymer bonded explosives (PBXs) are highly particle filled composite materials comprised of explosive crystals and a polymeric binder (ca. 5-10% by weight). The microstructure and mechanical properties of two pressed PBXs with different binder systems were studied in this paper. The initial microstructure of the pressed PBXs and its evolution under different mechanical aggressions were studied, including quasi-static tension and compression, ultrasonic wave stressing and long-pulse low-velocity impact. Real-time microscopic observation of the PBXs under tension was conducted by using a scanning electron microscope equipped with a loading stage. The mechanical properties under tensile creep, quasi-static tension and compression were studied. The Brazilian test, or diametrical compression, was used to study the tensile properties. The influences of pressing pressures and temperatures, and strain rates on the mechanical properties of PBXs were analyzed. The mesoscale damage modes in initial pressed samples and the samples insulted by different mechanical aggressions, and the corresponding failure mechanisms of the PBXs under different loading conditions were analyzed.

Simple phenomenological determination of contact stiffness and elastic modulus of Ce-based bulk metallic glasses through nanoindentation

The viscoelastic deformation of Ce-based bulk metallic glasses (BMGs) with low glass transition temperature is investigated at room temperature. Contact stiffness and elastic modulus of Ce-based BMGs cannot be derived using the conventional Oliver-Pharr method [W. C. Oliver and G. M. Pharr, J. Mater. Res. 7, 1564 (1992)]. The present work shows that the time dependent displacement of unloading segments can be described well by a generalized Kelvin model. Thus, a modified Oliver-Pharr method is proposed to evaluate the contact stiffness and elastic modulus, which does, in fact, reproduce the values obtained via uniaxial compression tests. (c) 2007 American Institute of Physics.

Study of three elastic-plastic constitutive models by non-proportional finite deformations of OFHC copper

Experimental stress-strain data of OFHC copper first under torsion to 13% and then under torsion-tension to about 10% are used to study the characteristics of three elastic-plastic constitutive models: Chaboche's super-positional nonlinear model, Dafalias and Popov's two surface model and Watanabe and Atluri's version of the endochronic model. The three models, originally oriented for infinitesimal deformation, have been extended for finite deformation. The results show (a) the Mises-type yield surface used in the three models brings about significant departure of the predictions from the experimental data; (b) Chaboche's and Dafalias' models are easier than Watanabe and Atluri's model in determining the material parameters in them, and (c) Chaboche's and Watanabe & Atluri's models produce almost the same prediction to the data, while Dafalias' model cannot accurately predict the plastic deformations when a loading path changes in its direction. Copyright (C) 1996 Elsevier Science Ltd

Study on fatigue crack nucleation of electrodeposited nanocrystalline nickel

The mechanism of fatigue crack nucleation for nanocrystalline (nc) nickel was experimentally investigated in this paper. The samples of electrodeposited ne nickel were loaded cyclically by using a three point bending instrument at first. Then, atomic force microscopy (AFM) was used to scanning the sample surface after fatigue testing. The results indicated that, after fatigue testing, there are vortex-like cells with an average size of 108nm appeared along the crack on nc nickel sample. And, the roughness of sample surface increased with the maximum stress at the surface.

盐岩地层套管蠕变荷载试验及其数值分析

中原油田在钻遇盐岩层的油水井中,某些区块套管的损坏率高达65%,盐岩层蠕变是套管损坏的一个重要原因.通过对6块盐岩岩心进行地下条件的蠕变模拟试验,得出了盐岩层的蠕变规律和特征参数,建立了套管承受外载的有限元计算模型,并编制了计算程序.利用所编制的软件对中原油田某口井的套管蠕变载荷进行了数值模拟计算.结果表明,作用在套管上的径向载荷与相位角和蠕变时间有关,并且蠕变条件下套管的最大载荷超过原地应力10%左右.

Factors Resulting In Micron Indentation Hardness Descending In Indentation Tests

Some factors that affect the experimental results in nanoindentation tests such as the contact depth, contact area, load and loading duration are analyzed in this article. Combining with the results of finite element numerical simulation, we find that the creep property of the tested material is one of the important factors causing the micron indentation hardness descending with the increase of indentation depth. The analysis of experimental results with different indentation depths demonstrates that the hardness decrease can be bated if the continuous stiffness measurement technique is not adopted; this indicates that the test method itself may also be one of the factors causing the hardness being descended.

Effects of convection and solid wall on the diffusion in microscale convection flows

The diffusive transport properties in microscale convection flows are studied by using the direct simulation Monte Carlo method. The effective diffusion coefficient D is computed from the mean square displacements of simulated molecules based on the Einstein diffusion equation D = x2 t /2t. Two typical convection flows, namely, thermal creep convection and Rayleigh– Bénard convection, are investigated. The thermal creep convection in our simulation is in the noncontinuum regime, with the characteristic scale of the vortex varying from 1 to 100 molecular mean free paths. The diffusion is shown to be enhanced only when the vortex scale exceeds a certain critical value, while the diffusion is reduced when the vortex scale is less than the critical value. The reason for phenomenon of diffusion reduction in the noncontinuum regime is that the reduction effect due to solid wall is dominant while the enhancement effect due to convection is negligible. A molecule will lose its memory of macroscopic velocity when it collides with the walls, and thus molecules are hard to diffuse away if they are confined between very close walls. The Rayleigh– Bénard convection in our simulation is in the continuum regime, with the characteristic length of 1000 molecular mean free paths. Under such condition, the effect of solid wall on diffusion is negligible. The diffusion enhancement due to convection is shown to scale as the square root of the Péclet number in the steady convection regime, which is in agreement with previous theoretical and experimental results. In the oscillation convection regime, the diffusion is more strongly enhanced because the molecules can easily advect from one roll to its neighbor due to an oscillation mechanism. © 2010 American Institute of Physics. doi:10.1063/1.3528310

Investigation on the microstructure and mechanical properties of a cast Mg–6Zn–5Al–4RE alloy

Mg-6Zn-5Al-4RE (RE = Mischmetal, mass%) alloy was prepared by metal mould casting method. The microstructure and mechanical properties of the as-cast and heat-treated alloys were investigated

Effects of rare earth on the structure and properties of Mg–6Zn–5Al–4Gd–1RE (RE = Ce or Y) alloys

The microstructures and mechanical properties of Mg-6Zn-5Al-4Gd-1RE (RE = Ce or Y) alloys were investigated. The addition of Ce or Y obviously refines the grain size for the Mg-6Zn-5Al-4Gd-based alloy, while the Y element has a better refining effect. The Ce and Y show different grain-refining mechanisms: Ce addition mostly promotes the growth of secondary dendrite, while Y addition mainly increases the heterogeneous nucleation sites.

The influence of mischmetal and tin on the microstructure and mechanical properties of Mg-6Zn-5Al-based alloys

The influence of the addition of mischmetal (MM) and tin (Sn) (total content of mischmetal and tin = 4 wt.%) on the microstructure, aging behavior and mechanical properties of Mg-6Zn-5Al-based alloys has been investigated. The microstructure of the as-cast alloys consists of alpha-Mg. Mg-32(Al,Zn)(49), Al2Mg5Zn2, Mg2Sn and Al2MMZn2 phases, and the morphology of these intermetallic phases varies with different MM and Sri additions.

Microstructures and mechanical properties of as-cast Mg–5Al–0.4Mn–xNd (x=0, 1, 2 and 4) alloys

Mg-5Al-0.4Mn-xNd (x=0, 1, 2 and 4wt.%) alloys were prepared by metal mould casting method. The microstructures and mechanical properties were investigated. The results demonstrated that Al11Nd3 phase was formed and mainly aggregated along the grain boundaries with the addition of Nd. Meanwhile, the grain sizes were greatly reduced with the increasing Nd content.

Effect of Nd on the microstructure, mechanical properties and corrosion behavior of die-cast Mg-4Al-based alloy

Die-cast Mg-4Al-0.4Mn-xNd(x = 0, 1, 2, 4 and 6 wt.%) magnesium alloys were prepared successfully and influences of Nd on the microstructure, mechanical properties and corrosion behavior of the Mg-4Al-0.4Mn alloy have been investigated. The results showed that with the addition of Nd binary Al2Nd phase and Al11Nd3 phase. which mainly aggregated along the grain boundaries, were formed, and the relative ratio of above two phases was in correlation with the Nd content in the alloy. Meanwhile, the grain sizes were greatly reduced with the increasing Nd content. It was found that due to the addition of Nd both the tensile properties and corrosion resistance were improved substantially.

Effect of Y for enhanced age hardening response and mechanical properties of Mg-Ho-Y-Zr alloys

In this study, compositional dependence of age hardening characteristics and tensile properties were investigated for Mg-4Ho-xY-0.6Zr alloys (x = 0, 3 5, and 7 wt%). The result showed that with increasing Y content, the hardness of the alloys increased in the as-quenched and aged-peak conditions. Considerable age hardening response was recognized for the alloys. When the alloy containing 7% Y showed the most remarkable age hardening response at aging temperature of 250 degrees C.