Qingyou Han

Nanoparticulate reinforced metal matrix nanocomposites – a review

Particulate reinforced metal matrix composites (MMCs) are widely used in many industries. With the development of nanotechnologies, the particulates scales down to the nano-level to enhance MMCs. MMCs with nano-sized reinforcements are termed as metal matrix nanocomposite (MMNCs). MMNCs have shown promising properties. Many researchers are using different methods to fabricate MMNCs. In this paper, some commonly used methods used to fabricate MMNCs are introduced including: powder metallurgy, casting, rapid solidification and in-situ fabrication. The enhancing mechanism of MMNCs is discussed. The properties obtained by different methods are illustrated. Some unsolved problems are also listed.

Particle pushing : critical flow rate required to put particles into motion

Particles can be rejected by the growing solid as a result of fluid flow. It has been observed that without fluid flow, particles are engulfed by the growing solid at very low growth rates. When the flow rate is higher than a critical value, a particle can be put into motion and thus prevented from being engulfed. The critical local flow rate required to put a particle into motion has been measured and modelled. This critical flow rate depends on the size and density of the particle, the roughness of the interface and the growth rate of the solid. For small particles, a very small sideways motion of the liquid is enough to put a particle into motion.

Design perspectives for creep-resistant magnesium die-casting alloys

The microstructure of die-cast magnesium alloys is highly non-uniform, which leads to a non-uniform distribution of the solidus/homologous temperature in the α(Mg) phase and a non-uniform distribution of deformation stresses and strains in the specimen during creep testing. Experimental observations suggest that significant creep deformation occurs in the α(Mg) phase in and adjacent to the eutectic regions while deformation in the primary α(Mg) dendrites is less pronounced. This article addresses the effect of the non-uniform as-cast microstructure on the creep resistance of die-cast magnesium alloys. Computational thermodynamic simulations were carried out to determine solute segregation, solidus temperature, and the corresponding homologous temperature distribution in the α(Mg) phase. Transmission electron microscopy studies provided evidence of non-uniform creep deformation in the creep-tested specimens. The results suggest that the creep resistance of magnesium alloys is determined by the weakest aggregate and/or phase in the alloy, viz., the α(Mg) phase in and adjacent to the eutectic regions. Microstructural design efforts that increase the homologous temperature or reinforce the eutectic α(Mg) phase hold significant promise for increasing the creep resistance of magnesium alloys.

Fabrication of nylon-6/carbon nanotube composites

A new technique to fabricate nylon-6/carbon nanotube (PA6/CNT) composites is presented. The method involves a pretreatment of carbon nanotubes synthesized by catalytic pyrolysis of hydrocarbon and an improved in-situ process for mixing nanotubes with the nylon 6 matrix. A good bond between carbon nanotubes and the nylon-6 matrix is obtained. Mechanical property measurements indicate that the tensile strength of PA6/CNT composites is improved significantly while the toughness and elongation are somewhat compromised. Scanning electron microscopy (SEM) analysis of the fractured tensile specimens reveals cracking initiated at the wrapping of the CNTs PA6 layer/PA6 matrix interface rather than at the PA6/CNT interface.

Different growth regimes during directional dendritic growth

Abstract In a recent paper using a numerical treatment of array dendritic growth Hunt and Lu [1] predicted that two growth regimes exist during the array growth of dendrites. The present paper describes experimental work which clearly shows that the two regimes exist and that the model predicts the transition with remarkable accuracy. The work also showed experimentally that the form of non-dimensionalisation successfully describes cellular and dendritic growth. The importance of the transition to conventional casting processes is discussed.

Numerical and experimental investigation of upsetting with ultrasonic vibration of pure copper cone tip

This paper is the first time to create a new model for the cone tip upsetting with the ultrasonic vibration, namely, during the tip upsetting process, the ultrasonic wave transmits in the specimen at the same time. Firstly, the experiments of the tip upsetting with ultrasonic vibration were conducted and the deformed samples were obtained. However, a detailed analysis and understanding of the forming mechanism is not possible on the basis of the conventional experimental observations because the ultrasonic vibration processing phenomenon occurs at high speed. Therefore, the finite element method was applied to understand the processing mechanism. Abaqus/Explicit was used for the finite element analysis in this study. Based on a valid model, the forming process, stress and strain distributions, and effect of the ultrasonic vibration strength on forming process were revealed for the tip upsetting with ultrasonic vibration.

Ultrasonic shot peening

This paper gives a brief overview of published results pertinent to ultrasonic shot peening, a relatively new method to introduce severe plastic deformations in the surface of materials and obtain nanosized grains. Among reported benefits are increased surface hardness and fatigue resistance, and possibility to perform nitriding at lower temperatures and in shorter times. Being a new technology, there is a lack in available literature on ultrasonic shot peening. For that reason, some of the results published for conventional shot peening are also presented. We believe, however, that the results selected here are transferrable to ultrasonic shot peening, or at least can serve as a reference point for this process.

Particle pushing: the attachment of particles on the solid-liquid interface during fluid flow

We have measured the number of insoluble particles that attach to artificial interfaces under different flow conditions. In downward laminar flow, very few particles are attached even if there is a high concentration of particles near the interface and the number of particles attached is independent of the concentration of particles. In downward turbulent flow, the number of particles attached to an interface depends on the roughness of the surface, the direction of fluid flow, the size and density of particles, and the orientation of the interface. In horizontal flow, the dominant factor is sedimentation. An increase in fluid flow tends to reduce the number of particles attached to the interface, and thus the number of particles trapped. A crude criterion for the attachment of particles to a rough interface is proposed.

Particle pushing: the distribution of particles in liquid near a solid surface

Abstract The distribution of insoluble particles in liquid within a few millimetres of a solid surface has been measured. The results show that the concentration of particles near a solid interface depends on the direction and velocity of fluid flow, on the density and size of the particles, and on the orientation of the surface. Under certain conditions there is a particle-deficient layer near the wall, which, in the case of a solidyfing material, means that fewer particles will be trapped by the growing front.

Fundamental understanding of Na-induced high temperature embrittlement in Al–Mg alloys

Sodium is an undesirable impurity in aluminium–magnesium alloys. In trace amounts it leads to high temperature embrittlement (HTE), due to intergranular fracture, which results in edge cracking during hot rolling. In the present work, the results of a thermodynamic investigation to elucidate the mechanism are presented. Correlations between HTE, phase formation, temperature and composition in Al–Mg alloys were determined. It is suggested that: (i) HTE is related to the formation of an intergranular Na-rich liquid phase, which significantly weakens the strength of grain boundaries; (ii) for a given Mg content, there exists a maximum Na content above which HTE cannot be avoided; and (iii) for a given alloy, a proper hot-rolling temperature should be chosen with respect to Na and Mg contents to suppress HTE. The HTE sensitive zone and a hot-rolling safe zone of Al–Mg–Na alloys are defined as functions of processing temperature and alloy composition. The tendency of HTE formation was evaluated based on thermodynamic simulations of phase fraction of the intergranular Na-rich liquid phase.