Showing papers on "Grain size published in 2019"

Tailoring the microstructure and mechanical properties of AISI 316L austenitic stainless steel via cold rolling and reversion annealing

Abstract: Tensile properties of cold rolled AISI 316L stainless steel after full reversion of martensite to austenite, recrystallization of retained austenite, and grain growth were studied at 850, 950, and 1050 °C. At higher temperatures, it was found that the kinetics of the reversion and recrystallization processes enhance but coarser grain sizes will be obtained at the end of recrystallization. At 1050 °C, appreciable grain growth was observed after the completion of the recrystallization process, which was not the case for a low temperature of 850 °C. At the stage of full recrystallization, by decreasing the annealing temperature, the yield stress (YS) and the ultimate tensile strength (UTS) values increased and total elongation decreased, which was related to the grain size strengthening by the Hall-Petch law. However, the Hall-Petch slope for the UTS was found to be much smaller than that of YS, which reveals that YS has greater grain size dependency. The latter was ascribed to the improved work-hardening behavior and enhanced transformation-induced plasticity (TRIP) effect by coarsening of grain size. To obtain high-strength and ductile steel with tensile toughness higher than 300 MJ/m3 and yield ratio of ∼0.5, the average grain size of ∼3 μm was found to be desirable.

Reinforcement size dependence of mechanical properties and strengthening mechanisms in diamond reinforced titanium metal matrix composites

Abstract: Diamond particulates reinforced titanium matrix composites (TiMMCs) were fabricated by spark plasma sintering technique at different reinforcement sizes of 5, 100, 200 nm and 3 μm. The dependence of reinforcement size on the mechanical and tribological properties in the TiMMCs was studied, paying particular attention to the nanoscale effects. The enhancement in strength of the composites was elucidated on the basis of strengthening mechanisms characterized by load transfer, thermal mismatch, grain size, and Orowan strengthening. The strengthening mechanisms were quantitatively analyzed and evaluated as a function of particle size. The results revealed that the presence of 5 nm diamond particles enhance strength by interacting with dislocations, while simultaneously retarding grain growth. Although the micro-composite has a little higher strength than the nanodiamonds reinforced composite, the latter has a combination of high strength and high ductility as wells excellent tribological properties.

Can zinc alloys be strengthened by grain refinement? A critical evaluation of the processing of low-alloyed binary zinc alloys using ECAP

Abstract: In this work, the effect of equal channel angular pressing (ECAP) on the microstructure and mechanical properties of zinc and zinc alloys with Ag, Cu, and Mn additions (0.5 at%) was investigated. Four passes of ECAP Route BC was performed at room temperature for each material. Properties of investigated materials after ECAP were compared to their coarse grained counterparts obtained via indirect hot extrusion at 300 °C. Highest strengthening effect was observed for alloy containing the Mn addition. Grain refinement in materials after ECAP was obtained, mean grain diameter is equal to 20 µm in the case of pure zinc, and less than 3.2 µm for alloys. Strain rate dependent plasticity increase was observed for all fine grained materials, with maximum elongation of 510% measured for Zn-Cu alloy after ECAP. Grain refinement did not result in increased yield and ultimate tensile strength of alloys after ECAP. In all investigated materials tensile properties after ECAP were 20 ~ 60% lower than in hot extruded samples. Based on the tensile properties, microstructure and texture analysis, the changes in the main deformation mechanisms were considered. It was presented that the crystallographic texture and grain size are the main factors affecting twinning, slip and non-slip deformation mechanisms resulting in large differences in observed mechanical properties.

Mechanical Response of He-Implanted Amorphous SiOC/Crystalline Fe Nanolaminates.

Abstract: This study investigates the microstructural evolution and mechanical response of sputter-deposited amorphous silicon oxycarbide (SiOC)/crystalline Fe nanolaminates, a single layer SiOC film, and a single layer Fe film subjected to ion implantation at room temperature to obtain a maximum He concentration of 5 at. %. X-ray diffraction and transmission electron microscopy indicated no evidence of implantation-induced phase transformation or layer breakdown in the nanolaminates. Implantation resulted in the formation of He bubbles and an increase in the average size of the Fe grains in the individual Fe layers of the nanolaminates and the single layer Fe film, but the bubble density and grain size were found to be smaller in the former. By reducing the thicknesses of individual layers in the nanolaminates, bubble density and grain size were further decreased. No He bubbles were observed in the SiOC layers of the nanolaminates and the single layer SiOC film. Nanoindentation and scanning probe microscopy revealed an increase in the hardness of both single layer SiOC and Fe films after implantation. For the nanolaminates, changes in hardness were found to depend on the thicknesses of the individual layers, where reducing the layer thickness to 14 nm resulted in mitigation of implantation-induced hardening.

Controlling mechanical properties of additively manufactured hastelloy X by altering solidification pattern during laser powder-bed fusion

Abstract: Like other manufacturing processes, controlling the microstructure of additively manufactured parts is essential to reach the desirable mechanical properties. However, available reports on the control of as-build microstructure and mechanical properties of Ni-base superalloys during laser powder-bed fusion (LPBF) process are not comprehensive. This article aims at a systematic approach to study the effect of scanning strategies and build orientations on solidification patterns in the printed LPBF Hastelloy X parts. The as-built microstructure (grain size, texture) and mechanical responses (yield strength, ultimate tensile strength (UTS), and elongation) are also presented. Results reveal that the stripe unidirectional scan pattern leads to the largest grain size (>850 μm) with the lowest mechanical strength. These samples also exhibit the strongest crystallographic texture, resulting in a planar anisotropic mechanical response (~22 MPa difference in UTS). On the other hand, the stripe rotation scan strategy (67° rotation) leads to a randomly oriented and finer grain structure (~110 μm) with a higher UTS (~800 MPa) due to grain refinement observed in these samples. In addition, the aspect ratio of the columnar grain structure was observed to influence the mechanical response of these parts. UTS of horizontally printed parts were ~26% more than the vertical parts for the stripe scan strategy (67° rotation). However, changing the solidification pattern (stripe XY with 90° rotation) was observed to reduce this difference to ~18%. These findings can be used to tune the microstructure of as-built LPBF parts to obtain an optimal mechanical behaviour.

Three-dimensional grain growth during multi-layer printing of a nickel-based alloy Inconel 718

Abstract: Heterogeneous grain structure is a source of the inhomogeneity in structure and properties of the metallic components made by multi-layer additive manufacturing (AM). During AM, repeated heating and cooling during multi-layer deposition, local temperature gradient and solidification growth rate, deposit geometry, and molten pool shape and size govern the evolution of the grain structure. Here the effects of these causative factors on the heterogeneous grain growth during multi-layer laser deposition of Inconel 718 are examined by a Monte Carlo method based grain growth model. It is found that epitaxial columnar grain growth occurs from the substrate or previously deposited layer to the curved top surface of the deposit. The growth direction of these columnar grains is controlled by the molten pool shape and size. The grains in the previously deposited layers continue to grow because of the repeated heating and cooling during the deposition of the successive layers. Average longitudinal grain area decreases by approximately 80% when moving from the center to the edge of the deposit due to variable growth directions dependent on the local curvatures of the moving molten pool. The average horizontal grain area increases with the distance from the substrate, with a 20% increase in the horizontal grain area in a short distance from the third to the eighth layer, due to competitive solid-state grain growth causes increased grain size in previous layers.

A comparative study on single-laser and multi-laser selective laser melting AlSi10Mg: defects, microstructure and mechanical properties

Abstract: AlSi10Mg samples fabricated by using a multi-laser beams selective laser melting system have been studied in this paper. The relative density, defects, microstructure and mechanical properties of the isolated and overlap samples are investigated. It is found that the width and depth of the melt pools in overlap area are larger than that in isolated area, which leads to an obvious boundary between the overlap area and isolated area. Moreover, a few small pores were found in the melt pools along the overlap boundary. The isolated and overlap area in as-built samples have similar microstructure, microhardness and tensile strength. The morphology and distribution of Si change into particles from continuous dendrite after annealing treatment. However, it does not cause the grains to grow up. The microhardness and tensile strength have little difference due to almost the same grain size. The slight difference in strength and elongation of isolated and overlap samples are supposed to be caused by small pores. It can be concluded that the microstructure significantly influences the microhardness and strength of the AlSi10Mg samples, while the small pores have limited influence. There are a few smooth surfaces on the fractures of both isolated and overlap samples, which turn out to be the melt pool boundaries. This indicates that the cracks mainly propagate along the melt pool boundaries.

The effect of post annealing temperature on grain size of indium-tin-oxide for optical and electrical properties improvement

Abstract: Background (Problem) Indium tin oxide (ITO) is a transparent conductive oxide (TCO) thin film used as a transparent electrode. Given its high demand for the manufacture of transparent electrodes (high visible light transmittance, low resistance) in applications such as liquid crystal displays, touch screens, light emitting devices and solar cells, ITO thin films have attracted immense research interest. Objectives This study determines the effect of grain size on the optical and electrical properties of the ITO thin films under different annealing temperatures. Materials and methods ITO thin film was deposited at room temperature by a high frequency magnetron sputtering method using a target composed of In2O3 and SnO2. The structural, optical and electrical properties of the thin films annealed at 250 °C, 350 °C, 450 °C and 550 °C for 1 h were then analyzed. Results The research shows the grain size of indium-tin oxide thin films is strongly related to annealing conditions. The grain size was found to increase with increasing annealing temperature, although the crystal structure did not change for all the samples. It was observed that the lowest resistivity (500 × 10−4 Ω-cm) and highest optical transmittances (90–98%) of ITO films were obtained at annealing temperature of 450 °C. At low annealing temperatures, the measured resistivity is dependent on the effect of grain size, where it decreases with increasing grain size. Conclusion This work showed that the grain size of ITO thin films is strongly influenced by post annealing technique and conditions applied, thus providing a tool for enhancing the optical and electrical properties of the film.

Crystal Orientation and Grain Size: Do They Determine Optoelectronic Properties of MAPbI3 Perovskite?

Abstract: Growing large, oriented grains of perovskite often leads to efficient devices, but it is unclear if properties of the grains are responsible for the efficiency. Domains observed in SEM are commonly misidentified with crystallographic grains, but SEM images do not provide diffraction information. We study methylammoinium lead iodide (MAPbI3) films fabricated via flash infrared annealing (FIRA) and the conventional antisolvent (AS) method by measuring grain size and orientation using electron back-scattered diffraction (EBSD) and studying how these affect optoelectronic properties such as local photoluminescence (PL), charge carrier lifetimes, and mobilities. We observe a local enhancement and shift of the PL emission at different regions of the FIRA clusters, but we observe no effect of crystal orientation on the optoelectronic properties. Additionally, despite substantial differences in grain size between the two systems, we find similar optoelectronic properties. These findings show that optoelectronic quality is not necessarily related to the orientation and size of crystalline domains.

Microstructure and mechanical properties of friction stir welded AA6092/SiC metal matrix composite

Abstract: There is a need for improved understanding on the effects of friction stir welding (FSW) on the metallurgical and mechanical properties of aluminium matrix composite (AMC). In this study, AA6092/SiC/17.5p-T6 AMC joints were produced by using FSW with varying tool rotation and traverse speeds. The microstructural characterisation by scanning electron microscopy equipped with electron backscattered diffraction (EBSD) system revealed a substantial grain refinement and a homogeneous distribution of reinforcement particles in the nugget zone. The grain size of the nugget zone was greatly influenced by weld pitch, as a key indicator to control the amount of heat input, exposure time and cooling rate. Vickers microhardness profile across the welding zone revealed a significant difference in microhardness among the base metal, heat affected zone, thermo-mechanically affected zone and nugget zone. The tensile strength of the cross-weld specimens showed a high joint efficiency of about 75% of the base metal combined with relatively high ductility. Low-cycle fatigue properties were investigated in the axial total strain-amplitude control mode (from 0.3% to 0.5%) with R = e min / e max = − 1 . The results indicate that the fatigue life of the cross-welded joints varies with grain size in the nugget zone and it is lower than that of the base metal. A significant improvement of fatigue life is found to be related to the finer equiaxed grains dominated by high angle grain boundaries in the nugget zone.