Microstructure and Evolution of Mechanically-Induced Ultrafine Grain in Surface Layer of Al-Alloy Subjected to USSP

Experiments were conducted to investigate the ultrafine-grained (UFG) microstructures in the surface layer of an aluminum alloy 7075 heavily worked by ultrasonic shot peening. Conventional and high-resolution electron microscopy was performed at various depths of the deformed layer. Results showed that UFG structures were introdued into the surface layer of 62 μm thick. With increasing strain, the various microstructural features, e.g., the dislocation emission source, elongated microbands, dislocation cells, dislocation cell blocks, equiaxed submicro-, and nano-crystal grains etc., were successively produced. The grain subdivision into the subgrains was found to be the main mechanism responsible for grain refinement. The simultaneous evolution of high boundary misorientations was ascribed to the subgrain boundary rotation for accommodating further strains. Formed microstructures were highly nonequilibratory.  2002 Acta Materialia Inc. Published by Elsevier Science Ltd. All rights reserved.

A Study of Composite Beam with Shape Memory Alloy Arbitrarily Embedded under Thermal and Mechanical Loadings

The constitutive relations and kinematic assumptions on the composite beam with shape memory alloy (SMA) arbitrarily embedded are discussed and the results related to the different kinematic assumptions are compared. As the approach of mechanics of materials is to study the composite beam with the SMA layer embedded, the kinematic assumption is vital. In this paper, we systematically study the kinematic assumptions influence on the composite beam deflection and vibration characteristics. Based on the different kinematic assumptions, the equations of equilibrium/motion are different. Here three widely used kinematic assumptions are presented and the equations of equilibrium/motion are derived accordingly. As the three kinematic assumptions change from the simple to the complex one, the governing equations evolve from the linear to the nonlinear ones. For the nonlinear equations of equilibrium, the numerical solution is obtained by using Galerkin discretization method and Newton-Rhapson iteration method. The analysis on the numerical difficulty of using Galerkin method on the post-buckling analysis is presented. For the post-buckling analysis, finite element method is applied to avoid the difficulty due to the singularity occurred in Galerkin method. The natural frequencies of the composite beam with the nonlinear governing equation, which are obtained by directly linearizing the equations and locally linearizing the equations around each equilibrium, are compared. The influences of the SMA layer thickness and the shift from neutral axis on the deflection, buckling and post-buckling are also investigated. This paper presents a very general way to treat thermo-mechanical properties of the composite beam with SMA arbitrarily embedded. The governing equations for each kinematic assumption consist of a third order and a fourth order differential equation with a total of seven boundary conditions. Some previous studies on the SMA layer either ignore the thermal constraint effect or implicitly assume that the SMA is symmetrically embedded. The composite beam with the SMA layer asymmetrically embedded is studied here, in which symmetric embedding is a special case. Based on the different kinematic assumptions, the results are different depending on the deflection magnitude because of the nonlinear hardening effect due to the (large) deflection. And this difference is systematically compared for both the deflection and the natural frequencies. For simple kinematic assumption, the governing equations are linear and analytical solution is available. But as the deflection increases to the large magnitude, the simple kinematic assumption does not really reflect the structural deflection and the complex one must be used. During the systematic comparison of computational results due to the different kinematic assumptions, the application range of the simple kinematic assumption is also evaluated. Besides the equilibrium study of the composite laminate with SMA embedded, the buckling, post-buckling, free and forced vibrations of the composite beam with the different configurations are also studied and compared.

A Numerical Model of Rapid Solidification Processing of Ni-Al Alloy in Planar Flow Casting

A numerical model has been developed for simulating the rapid solidification processing (RSP) of Ni-Al alloy in order to predict the resultant phase composition semi-quantitatively during RSP. The present model couples the initial nucleation temperature evaluating method based on the time dependent nucleation theory, and solidified volume fraction calculation model based on the kinetics model of dendrite growth in undercooled melt. This model has been applied to predict the cooling curve and the volume fraction of solidified phases of Ni-Al alloy in planar flow casting. The numerical results agree with the experimental results semi-quantitatively.

Study on Mechanical Properties of Nanocrystal Surface Layer of an Aluminum Alloy

The nanocrystal surface layer of an aluminum alloy induced by High Speed Shot Peening (HSSP) was investigated in this paper. The results of nanoindentation experiment show that the elastic modulus and the hardness of nanocrystal surface layer increased,by 8% and 20%, respectively. The elastic modulus and the hardness appear to be independent of the distance from nanocrystalized surface and the process time.

Depth dependence of nanohardness in a CuAlNi single crystal shape memory alloy

Instrumented nanoindentation was employed to study the depth dependence of nanohardness in a CuAlNi single crystal shape memory alloy that exhibits shape memory effect (SME). A Berkovich indenter and a cube comer indenter were used in this study, and the

Grain Refinement at the Nanoscale Via Mechanical Twinning and Dislocation Interaction in a Nickel-Based Alloy

A nanostructured surface layer was formed on an Inconel 600 plate by subjecting it to surface mechanical attrition treatment at room temperature. Transmission electron microscopy and high-resolution transmission electron microscopy of the treated surface layer were carried out to reveal the underlying grain refinement mechanism. Experimental observations showed that the strain-induced nanocrystallization in the current sample occurred via formation of mechanical microtwins and subsequent interaction of the microtwins with dislocations in the surface layer. The development of high-density dislocation arrays inside the twin-matrix lamellae provides precursors for grain boundaries that subdivide the nanometer-thick lamellae into equiaxed, nanometer-sized grains with random orientations.

A Discussion on Modeling Shape Memory Alloy Embedded in A Composite Laminate as Axial Force and Elastic Foundation

In this paper, the possible error sources of the composite natural frequencies due to modeling the shape memory alloy (SMA) wire as an axial force or an elastic foundation and anisotropy are discussed. The great benefit of modeling the SMA wire as an axial force and an elastic foundation is that the complex constitutive relation of SMA can be avoided. But as the SMA wire and graphite-epoxy are rigidly bonded together, such constraint causes the re-distribution of the stress in the composite. This, together with anisotropy, which also reduces the structural stiffness can cause the relatively large error between the experimental data and theoretical results.

Magnetic properties and magnetic domain structure of bulk glass forming Nd60Al10Fe20Co10 alloy

The transition from hard to soft magnetic behaviour with increasing quenching rate is shown for Nd60WAl10Fe20Co10 melt-spun ribbons with different thickness. Microstructure and magnetic domain structure of ribbons were studied by magnetic force microscopy (MFM). Particle sizes < 5 nm decreasing gradually with increasing quenching rate were deduced from topographic images which differ from large-scale magnetic domains with a periodicity of about 350 nm in all ribbons irrespective the coercivity. This indicates that the magnetic properties of the alloy are governed by interaction of small magnetic particles. It is concluded that the presence of short-range-ordered structures with a local ordering similar to the Al metastable Nd-Fe binary phase is responsible for the hard magnetic properties in samples subjected to relatively low quenching rate.

Thermal mismatch stress in SiC whisker reinforced aluminium composite: new measurement method by X-ray diffraction

A new X-ray diffraction method for characterising thermal mismatch stress (TMS) in SiC_w–Al composite has been developed. The TMS and thermal mismatch strain (TMSN) in SiC whiskers are considered to be axis symmetrical, and can be calculated by measuring the lattice distortion of the whiskers. Not only the average TMS in whiskers and matrix can be obtained, but the TMS components along longitudinal and radial directions in the SiC whiskers can also be deduced. Experimental results indicate that the TMS in SiC whiskers is compressive, and tensile in the aluminium matrix. The TMS and TMSN components along the longitudinal direction in the SiC whiskers are greater than those along the radial direction for a SiC_w–Al composite quenched at 500°C.

Mechanism of nucleation and precipitation in Li containing Al-Zn-Mg-Cu alloys

Investigations on the aging hardening behavior of four Al-Li-Zn-Mg-Cu alloys were carried out using differential scanning calorimetry, transmission electron microscopy and hardness measurement. It is shown that the addition of Li inhibits the formation of Zn-rich G.P. zones in Al-Zn-Mg-Cu alloys. The dominant aging hardening precipitates is delta'(Al3Li) phase. Coarse T ((AlZn)(49)Mg-32) phase, instead of MgZn2, precipitates primarily on grain boundaries, and provides little strengthening. The multi-stop aging involving plastic deformation introduces in the matrix a high concentration of structural defects. These defects play different role on the nucleation of Zn-rich G.P. zones in different alloys. For the Li free alloy, structural defects act as vacancy sinks and tend to suppress the homogeneous precipitation of G.P. zones, while for the Li containing alloys, these defects promote the heterogeneous nucleation of G.P. zones and metastable MgZn2. A significant aging hardening effect is attained in deformed Li containing alloys due to the extra precipitation of fine MgZn2 in the matrix combined with deformation hardening.

Interfacial microstructure and mechanical behaviour in laser clad TiCp/Ni alloy coatings

Coatings of TiCp reinforced composite have been produced by laser cladding. Two kinds of coating with different TiCp origins were investigated, i.e. undissolved TiCp and in situ TiCp. For undissolved TiCp, epitaxial growth of TiC, precipitation of CrB, and a chemical reaction occur at phase interfaces, and nanoindentation loading curves show pop in marks caused by the plastic deformation associated with crack formation or debonding of TiCp from the matrix. As for in situ TiCp, no pop in mark appears. Meanwhile, in situ TiCp produces hardness and elastic modulus values that are higher than those produced by the coating that contains undissolved TiCp.

Ab initio investigation of the elasticity and stability of aluminium

On the basis of the pseudopotential plane-wave (PP-PW) method in combination with the local density functional theory (LDFT), complete stress-strain curves for the uniaxial loading and uniaxial deformation along the [001] and [111] directions, and the biaxial proportional extension along [010] and [001] for aluminium are obtained. During the uniaxial loading, certain general behaviours of the energy versus the stretch and the load versus the stretch are confirmed; in each case, there exist three special unstressed structures: f.c.c., b.c.c., and f.c.t. for [001]; f.c.c., s.c., and b.c.c. for [111]. Using stability criteria, we find that all of these states are unstable, and always occur together with shear instability, except the natural f.c.c. structure. A Pain transformation from the stable f.c.c. structure to the stable b.c.c. configuration cannot be obtained by uniaxial compression along any equivalent [001] and [111] direction. The tensile strengths are similar for the two directions. For the higher energy barrier of the [111] direction, the compressive strength is greater than that for the [001] direction. With increase in the ratio of the biaxial proportional extension, the stress and tensile strength increase; however, the critical strain does not change significantly. Our results add to the existing ab initio database for use in fitting and testing interatomic potentials.

Microstructure and Tribological Properties of Laser Clad gamma/Cr7C3/Tic Composite Coatings on gamma-TiA1 Intermetallic Alloy

In order to improve the wear resistance of the gamma-TiAl intermetallic alloy, microstructure, room- and high-temperature (600 degrees C) wear behaviors of laser clad gamma/Cr7C3/TiC composite coatings with different constitution of NiCr-Cr3C2 precursor-mixed powders have been investigated by optical microscopy (OM), scanning electron microscopy (SEM), X-ray diffraction (XRD), energy-dispersive spectrometer (EDS), block-on-ring (room-temperature) and pin-on-disk (high-temperature) wear tests. The responding wear mechanisms are discussed in detail. Results show that microstructures of the laser clad composite coatings have non-equilibrium solidified microstructures consisting of primary hard Cr7C3 and TiC carbides and the inter-primary gamma/Cr7C3 eutectic matrix, about three to five times higher average microhardness compared with the TiAl alloy substrate. Higher wear resistance than the original TiAl alloy is achieved in the clad composite coatings under dry sliding wear conditions, which is closely related to the formation of non-equilibrium solidified reinforced Cr7C3 and TiC carbides and the positive contribution of the relatively ductile and tough gamma/Cr7C3 eutectics matrix and their stability under high-temperature exposure.