Zhirui Wang

Effect of grain size on mechanical properties of nanocrystalline materials

Abstract The possibility of a dislocation mechanism in the deformation process of nanocrystalline materials is reviewed and analyzed. The present theoretical calculation, by taking the anisotropic characteristic of crystallographic symmetry and different choices of critical shear strength into account, results in a reasonable limit in grain size for applying dislocation pile-up theory to nanocrystalline materials. The deviation from the Hall—Petch relationship is rationalized in terms of a small number dislocation pile-up mechanism. A composite model is proposed to evaluate the strength of nanocrystalline materials. It is shown that this model can be used for interpreting the various cases observed in Hall—Petch studies. An analytical expression for assessing the creep rate of nanocrystalline materials by a diffusion mechanism, including triple line diffusion, is derived. It is predicted that the creep rate due to triple line diffusion will exhibit a stronger grain size dependence than that due to grain boundary diffusion.

Room temperature creep behavior of nanocrystalline nickel produced by an electrodeposition technique

Abstract Deformation processes of nanocrystalline (6–40 nm) nickel produced by an electrodeposition technique were studied. First, the results of unidirectional tensile tests were discussed with respect to the deviation from the Hall-Petch relationship. It was suggested that such a mechanical behavior exhibited by nanocrystalline materials could be described by a composite model proposed previously. Further experimental work on static and dynamic creep tests under the load control condition showed that nanocrystalline nickel electrodeposits exhibited a significant room temperature creep behavior. It appeared that grain boundary sliding and diffusive matter transport within the intercrystalline region played an important role in terms of deformation mechanisms of nanocrystalline materials. The contributions of dynamic creep to stress-strain behavior and, in turn, to the assessment of the Hall-Petch relationship for nanocrystalline materials are discussed.

Isokinetic analysis of nanocrystalline nickel electrodeposits upon annealing

Abstract The grain growth kinetics of nanocrystalline nickel electrodeposits was studied by transmission electron microscopy and differential scanning calorimetry at different heating rates. It was found that, upon annealing, the nanocrystals in the nickel electrodeposits appeared to grow abnormally and released about 415.7 ± 3.5 J/mol of heat. The mechanism of the abnormal grain growth was attributed to the subgrain coalescence. The method for determination of the grain growth activation energy as well as all the other kinetic parameters in the Johnson-Mehl-Avrami equation was proposed based on an isokinetic analysis. This method is applicable to general types of transformation process governed by a single activation energy under the isokinetic condition. The activation energy for the grain growth of nanocrystalline nickel electrodeposits was found to be about 131.5 kJ/mol using this method. The difference between the present method and the Kissinger and Ozawa method was addressed in terms of their physical backgrounds.

Stress distribution in particulate-reinforced metal-matrix composites subjected to external load

Finite element calculations were carried out to study the stress distribution in particulate SiC-reinforced Al metal-matrix composites (MMCs) subjected to external load. The results showed that, in addition to the effect of the aspect ratio of the reinforcement, particle distribution and grouping behavior-clustering, resulting from the casting process in the present materials, also contribute considerably to the nonuniform stress distribution in the material. The triaxial stress state around a particle or inside a particle cluster may change the von Mises stress and result in plastic deformation features, such as strain localization. Furthermore, the relative orientation of the particle distribution to the external load was found to be important to the stress distribution. As the applied stress increases, the state of triaxial stress inside a cluster appears to promote the early particle cracking, interface debonding, and void formation in the ductile matrix. Experimental observations of slip band distribution and other features in compression tests are consistent with the calculated results.

Formation of magnesium aluminate (spinel) in cast SiC particulate-reinforced Al(A356) metal matrix composites

Transmission (TEM) and scanning electron microscopy (SEM) are employed to study the SiC/Al-alloy interface in a cast SiCp/Al(A356) metal matrix composite (MMC). Magnesium aluminate (spinel), MgAl2O4, was found at the interface as a reaction product after material processing. Comparisons of the crystal structure, structure factor, and interface reaction ther-modynamics between MgAl2O4 and MgO have been carried out. The results from these com-parisons confirm the experimental observation;i.e., the favored interface phase is magnesium aluminate (spinel). Based on the thermodynamic analysis, the presence of oxygen in various forms in the system during processing, such as SiO2, A12O3, and MgO, is believed to be the source which supplies the oxygen for the formation of MgAl2O4.

Mechanical behavior of cast particulate SiC/AI (A356) metal matrix composites

Mechanical tests were carried out to study the deformation behavior of particulate SiC-reinforced Al (A356) matrix composites produced through direct casting using the molten metal mixing method. The matrix alloy-Al (A356) was also tested as a control material for comparison. The elastic constant and yield strength of the composite material were found higher than those of the control alloy, but the ultimate tensile strength (UTS) and the ductility were lower. The Tsai-Halpin equation was found applicable for calculating the elastic constant if an average particle aspect ratio could be determined. The strain-hardening behavior of the tested composite material appeared very different from that of the control alloy. The high strain-hardening rate in the early stage of plastic deformation of the composite was rationalized by the interaction between the hard particles and the ductile metal matrix; on the other hand, the low hardening rate recorded from intermediate strain amplitude to fracture was attributed to the early coalescence of voids and other microdamages. Particle-matrix interface debonding, particle cracking, and void for-mation in the metal matrix were considered to be responsible for the low ductility. Deformation asymmetry of the composites was noticed, not only through the Bauschinger effect, but also through the difference in virgin specimens’ yield stresses in tension and compression.

Cyclic deformation behavior and dislocation structures of [001] copper single crystals-I Cyclic stress-strain response and surface feature

Abstract Cyclic deformation behavior of [001] copper single crystals was investigated in symmetrical pull-push fatigue tests at the ambient temperature and with constant plastic shear strain amplitudes ( γ pl ) in the range of 1.0 × 10 −4 to 3.0 × 10 −3 . The formation and development of surface slip bands (SBs) were examined in situ by a light microscope. A rapid initial hardening stage followed by a pronounced overshooting or significant softening was found with the crystals tested at γ pl ≥ 4.8 × 10 −4 . The cyclic stress-strain curve of [001] crystals does not show any plateau behavior, instead it follows, if corrected with the Taylor factor, the power law function for copper polycrystals. The fatigue limit, defined as the critical strain amplitude below which SBs do not form, was found to be approximately 1.7 × 10 −4 , a value much higher than that for single slip oriented crystals, but very close to that for polycrystals modified by the Taylor factor. In situ observation revealed that the primary SBs appeared at the very beginning of cyclic deformation and frequently they were not persistent. The secondary (critical) SBs usually formed at the overshooting stage after rapid hardening. Deformation bands with the characteristics of kink bands were detected in the final stage of the cyclic deformation. Analysis indicated that the rapid initial cyclic hardening is caused by the formation of Lomer-Cottrell locks, while the overshooting/softening behavior is related to the operation of secondary slip or the formation of dislocation labyrinth structure. The preferred combination of primary and critical slip during cyclic deformation is also discussed in terms of dislocation reactions.

Microscopic characteristics of fatigue crack propagation in aluminum alloy based particulate reinforced metal matrix composites

Abstract Microscopic characteristics of fatigue crack propagation in two aluminum alloy (A356 and 6061) based particulate reinforced metal matrix composites (MMCs) were investigated by carrying out three point bending fatigue tests. The impedance offered by the reinforcing particles against fatigue crack propagation has been studied by plotting the nominal and actual crack lengths vs number of cycles. Surface observation shows that fatigue cracks tend to develop along the particle-matrix interface. In the case of Al (A356) MMCs, stronger interaction of fatigue crack with Si particles, as compared to SiC particles, was evident. In both MMC materials, particle debonding was more prominent as compared to particle cracking. The attempted application of Davidsons model to calculate ΔK th indicated that for cast MMCs the matrix grain including the surrounding reinforcing particles has to be considered as a large “hard particle”, and the grain boundary particles themselves behave like an hard “egg-shell” to strengthen the material.

Cyclic deformation response of planar-slip materials and a new criterion for the wavy-to-planar-slip transition

This paper will start with the review of mechanical response and dislocation structure evolution of single crystals of planar-slip alloys during cyclic deformation. Experimental results with typical planar-slip materials have demonstrated that, unlike typical wavy-slip crystals, planar-slip materials do not exhibit ‘real’ cyclic saturation behaviour, nor is there any evidence for the formation of persistent slip bands and dislocation ladder structures. Comparisons of the following three aspects of cyclic deformation, namely the mechanical response, the surface morphology and the dislocation structures, between wavy-slip and planar-slip materials will then be presented. Although it is a recognized fact that, on many occasions, the value of stacking-fault energy (SFE) can be employed as the criterion for distinguishing planar slip from wavy slip, detailed comparisons have shown that one cannot always obtain a satisfactory result if such a criterion for the transition is employed alone. Other considerations, such as the criterion based on the reciprocal width of the stacking fault and the approach based on a short-range-ordered structure, are then discussed. Further study has indicated that a more indicative factor for the transition of wavy-slip mode to planar-slip mode, at least in many Cu-based alloys, can be deduced in terms of the free-electron-to-atom ratio e/a of the alloy. Still further analysis and comparison show that the transition actually occurs when the SFE no longer decreases rapidly with the increase in the e/a ratio, that is in the solute concentration. In fact, a critical value or a critical range of e/a ratios for such a transition in these alloy systems could indeed be determined. To recognize the range of application of the e/a ratio approach, other alloy systems have also been examined and will be discussed in this paper.This paper will start with the review of mechanical response and dislocation structure evolution of single crystals of planar-slip alloys during cyclic deformation. Experimental results with typical planar-slip materials have demonstrated that, unlike typical wavy-slip crystals, planar-slip materials do not exhibit ‘real’ cyclic saturation behaviour, nor is there any evidence for the formation of persistent slip bands and dislocation ladder structures. Comparisons of the following three aspects of cyclic deformation, namely the mechanical response, the surface morphology and the dislocation structures, between wavy-slip and planar-slip materials will then be presented. Although it is a recognized fact that, on many occasions, the value of stacking-fault energy (SFE) can be employed as the criterion for distinguishing planar slip from wavy slip, detailed comparisons have shown that one cannot always obtain a satisfactory result if such a criterion for the transition is employed alone. Other considerations,...

Finite element simulation to investigate interaction between crack and particulate reinforcements in metal-matrix composites

Abstract Elastic-plastic finite element simulations have been carried out to determine the effect of the presence of single particles and particle clusters ahead of the crack tip on the stress distribution at the crack tip. The resulting knowledge about stress distribution has been used to provide insight regarding the predominant factors that influence crack propagation. This study indicates that the presence of particle(s) relatively far away from the crack tip increases the crack opening stress at the crack tip. The crack opening stress is, however, subdued as the crack approaches the particle(s). Particle clustering influences the stress distribution by suppressing plastic deformation inside the cluster and hence plastic deformation occurs predominantly outside the cluster. Experimental observation of slip bands and crack propagation characteristics in SiC-reinforced aluminum A356 confirms this fact. It is also found that particle clustering creates higher interface shear stress and von-Mises stress regions in the particle away from crack tip, while the highest interface normal stress region exists at the interface of the particle along the crack line. Therefore, depending upon the stress that predominates, the crack can either deviate and go around the cluster region or go straight through the cluster region.