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.

Regularity of strain distribution in short-fiber/whisker reinforced composites

Based on studies on the strain distribution in short-fiber/whisker reinforced metal matrix composites, a deformation characteristic parameter, lambda is defined as a ratio of root-mean-square strain of the reinforcers identically oriented to the macro-linear strain along the same direction. Quantitative relation between lambda and microstructure parameters of composites is obtained. By using lambda, the stiffness moduli of composites with arbitrary reinforcer orientation density function and under arbitrary loading condition are derived. The upper-bound and lower-bound of the present prediction are the same as those from the equal-strain theory and equal-stress theory, respectively. The present theory provides a physical explanation and theoretical base for the present commonly-used empirical formulae. Compared with the microscopic mechanical theories, the present theory is competent for stiffness modulus prediction of practical engineering composites in accuracy and simplicity.

Effect of Fe content on the thermal stability and dynamic mechanical behavior of NdAlNiCu bulk metallic glasses

The dependence of microstructure and thermal stability on Fe content of bulk Nd60Al10Ni10Cu20-xFex (0 less than or equal to x less than or equal to 20) metallic glasses is investigated by means of differential scanning calorimetry (DSC), X-ray diffraction (XRD) and high-resolution transmission electron micrograph (HRTEM). All samples exhibit typical amorphous feature under the detect limit of XRD, however, HRTEM results show that the microstructure of Nd60Al10Ni10Cu20-xFex alloys changes from a homogeneous amorphous phase to a composite structure consisting of clusters dispersed in amorphous matrix by increasing Fe content. Dynamic mechanical properties of these alloys with controllable microstructure are studied, expressed via storage modulus, the loss modulus and the mechanical damping. The results reveal that the storage modulus of the alloy without Fe added shows a distinct decrease due to the main a relaxation. This decrease weakens and begins at a higher temperature with increasing Fe content. The mechanism of the effect of Fe addition on the microstructure and thermal stability in this system is discussed in terms of thermodynamics viewpoints. (C) 2004 Elsevier B.V. All rights reserved.

Microstructure and mechanical properties of Zr-Cu-Al bulk metallic glasses

Zr49Cu46Al5 and Zr48.5Cu46.5Al5 bulk metallic glasses(BMGs) with diameter of 5 mm were prepared through water-cooled copper mold casting. The phase structures of the two alloys were identified by X-ray diffractometry(XRD). The thermal stability was examined by differential scanning calorimetry(DSC). Zr49Cu46Al5 alloy shows a glass transition temperature, T, of about 689 K, an crystallization temperature, T-x, of about 736 K. The Zr48.5Cu46.5Al5 alloy shows no obvious exothermic peak. The microstructure of the as-cast alloys was analyzed by transmission electron microscopy(TEM). The aggregations of CuZr and CuZr2 nanocrystals with grain size of about 20 nm are observed in Zr49Cu46Al5 nanocrystalline composite, while the Zr48.5Cu46.5Al5 alloy containing many CuZr martensite plates is crystallized seriously. Mechanical properties of bulk Zr49Cu46Al5 nanocrystalline composite and Zr48.5Cu46.5Al5 alloy measured by compression tests at room temperature show that the work hardening ability of Zr48.5Cu46.5Al5 alloy is larger than that of Zr48.5Cu46.5Al5 alloy.

Critical serrated flow features during nanoindentation in La-based bulk metallic glasses

La57.6Al17.5(Cu,Ni)(24.9) and La64Al14(Cu,Ni)(22) bulk metallic glasses (BMGs) were prepared by copper-mould casting method. Plastic deformation behavior of the two BMGs at various loading rates was studied by nanoindentation. The results showed that the La57.6Al17.5(Cu,Ni)(24.9) BMG with a glass transition temperature of 423 K exhibited prominent serrated flow at low loading rates, whereas less pronounced serrated flow at high rates during nanoindentation. In contrast, the La64Al14(Cu,Ni)(22) BMG with a glass transition temperature of 401 K exhibited prominent serrated flow at high loading rates. The different rate dependency of serrated flow in the two La-based BMGs is related to the different glass transition temperature, and consequently the degree of viscous flow during indentation at room temperature. A smoother flow occurs in the alloy with relatively lower glass transition temperature, due to the relaxation of stress concentration.

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.

The annular crack in a nonhomogeneous matrix surrounding a fiber under torsional loading .2. The crack going through the bimaterial interface into the fiber

The axisymmetric problem of an elastic fiber perfectly bonded to a nonhomogeneous elastic matrix which contains an annular crack going through the interface into the fiber under axially symmetric shear stress is considered. The nature of the stress singularity is studied. It is shown that at the irregular point on the interface, whether the shear modulus is continuous or discontinuous the stresses are bounded. The problem is formulated in terms of a singular integral equation and can be solved by a regular method. The stress intensity factors and crack surface displacement are given.

Tensile and compressive deformation of a short-glass-fiber-reinforced liquid-crystalline polymer

Results of tensile and compression tests on a short-glass-fiber-reinforced thermotropic liquid crystalline polymer are presented. The effect of strain rate on the compression stress-strain characteristics has been investigated over a wide range of strain rates epsilon between 10(-4) and 350 s-1. The low-strain-rate tests were conducted using a screw-driven universal tensile tester, while the high-strain-rate tests were carried out using the split Hopkinson pressure bar technique. The compression modulus was shown to vary with log10 (epsilon) in a bilinear manner. The compression modulus is insensitive to strain rate in the low-strain-rate regime (epsilon = 10(-4) - 10(-2) s-1), but it increases more rapidly with epsilon at higher epsilon. The compression strength changes linearly with log10 (epsilon) over the entire strain-rate range. The fracture surfaces were examined by scanning electron microscopy.

The interlaminar fracture-behavior and toughening mechanisms of new carbon-fiber-reinforced bismaleimide composites

The interlaminar fracture behaviour of carbon fibre-reinforced bismaleimide (BMI) composites prepared by using a new modified BMI matrix has been investigated by various methods. Laminates of three typical stacking sequences were evaluated. Double cantilever beam, end-notch flexure and edge-delamination tension tests were conducted under conventional conditions and in a scanning electron microscope. The strain energy release rates in Mode I and Mode III G(lc) and G(llc), as well as the total strain energy release rate, G(mc), have been determined and found to be higher than those for laminates with an epoxy matrix. Dynamic delamination propagation was also studied. The toughening mechanisms are discussed.

Fatigue fracture-behavior of carbon-fiber-reinforced modified bismaleimide composites

The fracture toughness and fatigue fracture behaviour of carbon-fiber-reinforced modified bismaleimide (BMI) composites have been studied. These composites were found to have higher fracture toughnes, better damage tolerance and longer fatigue life than carbon-fiber composites with epoxy matrices. Delamination is the major mode of failure in fatigue and it is controlled by the properties of the matrix and interface. The improved performance is dire to the presence of thermoplastic particles in the modified BMI matrix which gives rise to enhanced fiber/matrix adhesion and more extensive plastic deformation. The fatigue behaviour also depends on the stacking sequence, with the multidirectional [45/90/-45/0] fiber-reinforced modified BMI composite having a lower crack propagation rate and longer fatigue life than the unidirectional laminate. This arises because of the constraint on the damage processes due to the different fiber orientation in the plies.

Theoretical and experimental researches of post-micro-buckling for fiber-reinforced composites

Post-microbuckling is a fundamental feature of compressive failure process for the unidirectional-fiber-reinforced composites and laminated composites. The post-microbuckling behavior of composites under compression in the light of the Kevlar49-reinforced 648/BF3.400 (brittle epoxy) and EP (flexible epoxy) is studied, theoretically and experimentally. Analytical results of compressive strength are in good agreement with experimental results, qualitatively and quantitatively. By the experimental research, the post-microbuckling feature of the advancing kink band model is clearly displayed.

Creep characterization of a fiber reinforced plastic material

Creep behavior of [±45°]_s composite material is characterized by using uniaxial creep and recovery tests. The well-known Schapery nonlinear viscoelastic consti tutive relation was modified to make it suitable for characterizing the creep behavior of this material. Then, using this modified Schapery constitutive equation, by which the vis coplastic and creep damage can be taken into consideration, the creep behavior of [±45°]. glass fiber reinforced epoxy laminate was studied. The constitutive parameters of the material were determined experimentally, and the procedure and method of determination of the material parameters are proved to be valid.

Experimental relaxation behavior of chopped-strand mat glass-fiber reinforced polyester beams in 3 point bending

A dimensionless relation of the form for collating fatigue crack starting growth data is proposed in which Δkth represents the stress intensity factor range at the threshold. Based on experimental results, this relation attains the value of 0.6 for a fatigue crack to start growth in the Austenitic stainless steel investigated in this work. Metallurgical examinations were also carried out to show a transgranular shear mode of cyclic cleavage and plastic shear.

Static and fatigue failure behavior of carbon-fiber reinforced epoxy composite-materials with edge notch

A study of carbon fiber reinforced epoxy composite material with 0° ply or ±45°ply(unnotched or with edge notch) was carried out under static tensile and tension-tensioncyclic loading testing. Static and fatigue behaviour and damage failure modes in unnotched/notched specimens plied in different manners were analysed and compared with each other.A variety of techniques (acoustic emission, two types of strain extensometer, high speed pho-tography, optical microscopy, scanning electron microscope, etc.) were used to examine thedamage of the laminates. Experimental results show that when these carbon/epoxy laminateswith edge notch normal to the direction of the load are axially loaded in static or fatiguetension, the crack does not propagate along the length of notch but is in the interface (fiberdirection). The notch has no substantial effect on the stresses at the unnotched portion. Thedamage failure mechanism is discussed.