Extent Of Grain Growth Research Articles

Effects of Sr additions on the microstructural stability and mechanical properties of a cast Mg–4Zn alloy

The effects of Sr additions (0.3, 0.6 and 0.9 wt%) on the microstructural evolution, thermal stability, and mechanical properties of a cast Mg–4Zn alloy were investigated. The extent of grain growth and stability of the intermetallic compounds were studied by optical and scanning electron microscopy (SEM), respectively. Mechanical properties of the studied alloys were evaluated by shear punch testing (SPT) method and also the hardness test. Hardness and shear strength of the base alloy were increased by Sr additions in the as-cast condition, due to the grain refinement effect of Sr and also presence of the Sr-containing Mg17Sr2 and Mg70Zn25Sr5 precipitates. However, the optimum amount of Sr was determined to be 0.3 wt%, above which no further improvement in the shear strength of the as-cast alloys was observed, due to the coarsening of the precipitates. The obtained microstructural results indicated that while both of the base and Sr-containing alloys have appropriate thermal stability at 330 °C, the grain size of the base alloy increased significantly and also the Mg4Zn7 particles were dissolved in the matrix after annealing treatment at 400 °C for 96 h, resulting in considerable decrease in the shear strength. However, grain growth was trivial in the Sr-containing alloys, due to the presence of thermally stable Mg17Sr2 particles. Accordingly, the Sr addition was found to be beneficial to improve the stability of the microstructure and mechanical properties of the Mg–4Zn alloy.

An Anomaly in Creep Property Dependence on Grain Size for Ultrafine Grain Nanocrystalline Nickel at Higher Creep Temperature

In this present study, molecular dynamics simulation of creep for ultrafine grain NC Ni specimens with different grain sizes have been carried out under a constant 1 GPa applied load for various creep temperatures to study the dependence of grain growth on creep temperature and grain size during creep process and its influence on creep properties. It is observed that the extent of grain growth in ultrafine grain NC Ni during creep deformation process is more if creep in creep temperature is higher. A noteworthy anomaly, that is NC Ni with smaller grain exhibits better creep property compared to NC Ni with larger grain, is observed in case of higher creep temperatures (i.e. around or greater than 1400K).

Grain growth during ultrasonic welding of nanocrystalline alloys

The effects of ultrasonic welding on the structure of nanocrystalline feedstock were investigated using nanostructured Ag-W as a model system. Transmission electron microscopy of welded Ag-W foils revealed extensive grain growth as well as second phase precipitation along the weld interface. These structural changes were related to the ultrasonic welding process variables by combining classical models of frictional heating and grain growth. Process diagrams showing the extent of grain growth under a given set of welding conditions were computed and used to identify combinations of weld pressure and weld time that give strong bonds while still preserving the structure of the feedstock.

Synergistic effect of carbon nanotube as sintering aid and toughening agent in spark plasma sintered molybdenum disilicide-hafnium carbide composite

Hafnium carbide (HfC) along with sintering aids was consolidated at a relatively lower temperature i.e. 1600°C (i.e. T=~0.41Tm) under a uniaxial load of 50MPa by spark plasma sintering. Two different sintering aids such as molybdenum disilicide (MoSi2) and carbon nanotube (CNT) were added to enhance the densification and lower the extent of grain growth in the sintered pellets. Density of the sintered pellet increased from 96.0±0.8% in HfC +5wt% MoSi2 (HM) to 99.0±0.5% with the addition of 2wt% CNT in HfC+5wt% MoSi2 (HMC) at sintering temperature of 1600°C. Further, the extent of grain growth drastically reduced from 204% in HM to 50% in HMC. Analysis of linear shrinkage during densification revealed that CNT addition increased densification rate and decreased the time required to reach the density of 99.0±0.5% at 1600°C. Increased densification and lower degree of grain growth could be due to the synergistic effect offered by the CNT, which are as follows: (i) Lubrication effect of CNT, (ii) Lower activation energy for grain boundary diffusion (iii) Reduction in liquid phase sintering temperature and (iv) Grain boundary pinning. Fracture toughness of the sintered HM and HMC composite was obtained using indentation technique. By the addition of 2wt% CNT in HM, drastic increase of 91% in fracture toughness was seen. This significant improvement in fracture toughness was due to the enhanced densification and relatively lower grain size of HMC. Also crack bridging, crack deflection, crack arrest, CNT and graphene sheet pull-out and swording played major role in toughening of HMC pellet.

Solution-processing of ultra-thin CdTe/ZnO nanocrystal solar cells

We have carried out a detailed study into how modifications of the physical, chemical and optical properties of solution-processed, nanocrystalline CdTe layers influence the photovoltaic performance of sintered CdTe/ZnO nanocrystal solar cells. Such solar cells are fabricated through layer-by-layer assembly, which is enabled through an inter layer chemical and thermal treatment cycle. In this manner we are able to fabricate working solar cells with sintered CdTe layers as low as 90nm, provided that grain size is precisely controlled. We show that the extent of grain growth achieved during the CdTe sintering process is strongly dependent on nanocrystal surface chemistry and chemical environment, with the removal of the organic capping ligands and the introduction of CdCl2 prior to annealing leading to greatly enhanced growth. Due to the air processing involved and the nanocrystalline nature of the CdTe, the overall performance of these solar cells is shown to be strongly dependent on both annealing temperature and time, with optimal results requiring a balance between crystal growth and degradation due to oxidation. Using this simple bi-layer device structure, optimized treatment conditions result in power conversion efficiencies of up to 7.7% and peak internal quantum efficiencies in excess of 95%.

Thermal stability of Ni/NiO multilayers

The effects of NiO multilayer additions to pulsed-laser deposited nanocrystalline Ni on the grain growth and texture evolution during annealing have been studied using a combination of in situ annealing in the transmission electron microscope and the recently developed transmission electron backscatter diffraction technique. Grain growth in pure Ni proceeds in an abnormal manner with a small number of grains growing rapidly at the expense of the surrounding nanocrystalline matrix. The addition of NiO layers in the Ni suppressed grain growth and twin formation upon annealing, with increasing oxide content resulting in decreasing extent of grain growth. In pure Ni, the post-annealing texture was found to be composed of a strong {001} texture coupled with the four {122} twin variants from the {001}. With the addition of NiO, annealing twinning, and hence the {122} orientation, was suppressed and instead the final texture was composed of the {001} and {111} orientations.

Effects of Free Surface and Heterogeneous Residual Internal Stress on Stress‐Driven Grain Growth in Nanocrystalline Metals

By reevaluating the experimental study of Zhang et al. (2005), here we demonstrate that the extent of grain growth, previously proposed to be solely driven by external stress, may have been significantly overestimated. A new physical mechanism, termed as free surface assisted stress‐driven grain growth (or self‐mechanical annealing), is proposed and discussed in detail. Representing the cooperative effect of free surface and heterogeneous residual internal stress, the proposed mechanism is considered more favorable than the traditional pure stress‐driven mechanism for interpreting the abnormal grain growth widely observed in deforming nanocrystalline metals at room temperature.

Synthesis of ZnO based nanopowders via a non-hydrolytic sol gel technique and their densification behaviour and varistor properties

Hexagonal nanocrystalline varistor grade ZnO particles with size 50 nm and the specific surface area of 28 m2/g have been prepared by non-aqueous gelation technique involving diethylene glycol and triethanolamine. The as-prepared varistor nanopowders were analyzed with the support of XRD, TG/DTA, FTIR, TMA, SEM and TEM. Varistor discs were fabricated by pressing and their densification was studied at 850, 950, 1050 and 1150?C. The evolution of varistor microstructures, extent of grain growth and the influence of microstructure on the I-V properties were explored and presented. .

Temperature Dependent Microstructure and Mechanical Behavior in AZ31 Alloy Processed by an Asymmetric Rolling

An asymmetrical rolling of AZ31 alloy sheets was carried out at 473 K with a high reduction ratio of 70% in thickness prior to tensile-tests. For the as-rolled sheet, tensile tests were performed in a temperature range of 473 ~ 673 K at a constant strain rate of 1 × 10-3 s-1. Tensile elongation increases with increasing reduction ratio in the as-rolled sheets. A large elongation of above 530% was obtained in the specimen deformed at 673 K for the sheet fabricated by 70% thickness reduction. Although the extent of grain growth was remarkable in a high temperature range, the tensile elongation increases with increasing the tensile temperature.

Interaction between precipitation, normal grain growth, and secondary recrystallisation in austenitic stainless steel containing particles

With the aid of various complementary methods of microstructural analysis, the precipitation, grain growth, and secondary recrystallisation behaviour of an 15Cr–15Ni–1·2Mo–Ti–B (wt-%) austenitic stainless steel were studied over prolonged periods of time in the temperature range 600–1300°C. The experimental results showed that several types of precipitates were present in the material, and that the dissolution temperatures of each of these correlated with the type and extent of grain growth which was observed. It was, therefore, concluded that in the present study secondary recrystallisation was caused directly by the interaction of precipitates with grain boundaries. Furthermore, secondary recrystallisation produced a strong, predominantly {122{ 〈012〉 texture which has not previously been reported.

High temperature phase transformation kinetics and their effects on diffusion bonding of Ti-48Al-2Mn-2Nb

Diffusion bonding of the intermetallic compound Ti-48Al-2Mn-2Nb has been carried out in order to determine what effect temperature, time and starting base microstructure have on the bond line microstructure, in particular the extent of grain growth that occurs at the interface. The results showed that the non-equilibrium lamellar and duplex microstructures gave more grain growth at the interface compared with the equilibrium fully γ microstructure for bonding upto 45 minutes in the temperature range 1200 o C-1350 o C. It was also observed that the base microstructure in the duplex and fully γ samples was little changed during bonding, whereas in the lamellar samples decomposition of the lamellar (α 2 +γ) regions to allotriomorphic γ occurred, particularly at 1250 o C

Grain Growth in Superplastically Deformed Zinc Sulfide/Diamond Composites

Grain growth in superplastically deformed ZnS/diamond composites was much greater than in undeformed specimens exposed to the same time and temperature. The extent of grain growth depended on the volume fraction of diamond particles. Grain growth was suppressed by the addition of 10 wt% diamond particles in superplastically deformed specimens. In static annealing, however, only 1 wt% diamond particles were enough to supress grain growth, which is a factor of 10 smaller than estimated by the Zener condition.

Mechanical properties and microstructural characterization of Al-0.5%Cu thin films

Using a wafer curvature technique we have studied the variation of stress with tem-perature in Al-0.5%Cu thin films deposited on oxidized silicon wafers. Concurrently, the microstructural changes in the films induced by the thermal cycling inherent to this technique were studied with in-situ transmission electron microscopy heating experi-ments. On heating an as-sputtered film a stress drop occurs, corresponding to the onset of grain growth. The in-situ TEM experiments indicate that the extent of grain growth is significantly altered by the presence of compressive stresses in the film. During cool-ing, dislocation loops nucleate on {111} planes inclined to the film surface, although the grain size plays an important role in determining the extent to which this mechanism can account for the deformation. A native oxide can influence the stress levels in the film by pinning one end of the dislocation loops. Upon cooling below 200° C a rapid increase in stress occurs. Although this increase has been attributed to hardening due to the precipitation of excess copper, no evidence of precipitate-dislocation interactions were observed.

The Effect of UO2Density on Fission Product Gas Release and Sheath Expansion

The effect of UO2 density on fission product gas release and sheath expansion has been determined in an irradiation experiment in which the performance of fuel elements with densities between 10.42 and 10.74 g/cm3 was compared at ∫λdθ values of 39 and 42 W/cm.Diametral sheath strain was less for the lower density elements, but fission product gas release and the extent of grain growth were greater as the density decreased. A correlation between the extent of grain growth in the UO2 and the fractional gas release was found to exist in this test and to apply to a large number of other irradiations; it is suggested that this relationship may be causal and that the fractional gas release is not solely a function of temperature.