Showing papers on "Grain growth published in 2019"

Crystalline Liquid-like Behavior: Surface-Induced Secondary Grain Growth of Photovoltaic Perovskite Thin Film.

Abstract: Surface effects usually become negligible on the micrometer or sub-micrometer scale due to lower surface-to-bulk ratio compared to nanomaterials. In lead halide perovskites, however, their "soft" nature renders them highly responsive to the external field, allowing for extended depth scale affected by the surface. Herein, by taking advantage of this unique feature of perovskites we demonstrate a methodology for property manipulation of perovskite thin films based on secondary grain growth, where tuning of the surface induces the internal property evolution of the entire perovskite film. While in conventional microelectronic techniques secondary grain growth generally involves harsh conditions such as high temperature and straining, it is easily triggered in a perovskite thin film by a simple surface post-treatment, producing enlarged grain sizes of up to 4 μm. The resulting photovoltaic devices exhibit significantly enhanced power conversion efficiency and operational stability over a course of 1000 h and an ambient shelf stability of over 4000 h while maintaining over 90% of its original efficiency.

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.

Effect of second phase particles on mechanical properties and grain growth in a CoCrFeMnNi high entropy alloy

Abstract: Effect of annealing of a cold-worked CoCrFeMnNi alloy at temperatures of 500–900 °C for 1–50 h on the structure and mechanical properties was studied in the present work. Annealing for an hour resulted in: i) recrystallization of the face-centered cubic (fcc) matrix at 600–900 °C; ii) precipitation of a Cr-rich body-centered cubic (bcc) phase at 500–700 °C or a sigma phase particles at 600–800 °C. Moreover, an increase in the annealing time to 50 h at 600 °C resulted in a continuous growth of both the fcc grans and bcc/sigma particles and in an increase in the fraction of the sigma phase at the expense of the bcc phase particles. The fcc grains growth was found to be controlled by the pinning effect of the second phase particles. Soaking for an hour at 500–600 °C resulted in a substantial increase in strength of the alloy due to the second phases precipitation. Meanwhile annealing at the higher temperatures as well as an increase in the annealing time at 600 °C resulted in softening; however, even after 50 h annealing, the alloy demonstrated reasonably high strength. In the latter case fine fcc grains, preserved due to the pinning effect by the second phases particles, contributed to strength mainly.

Preparation and anisotropic properties of textured structural ceramics: A review

Abstract: Ceramics are usually composed of randomly oriented grains and intergranular phases, so their properties are the statistical average along each direction and show isotropy corresponding to the uniform microstructures. Some methods have been developed to achieve directional grain arrangement and preferred orientation growth during ceramic preparation, and then textured ceramics with anisotropic properties are obtained. Texture microstructures give particular properties to ceramics along specific directions, which can effectively expand their application fields. In this review, typical texturing techniques suitable for ceramic materials, such as hot working, magnetic alignment, and templated grain growth (TGG), are discussed. Several typical textured structural ceramics including α-Al2O3 and related nacre bioinspired ceramics, Si3N4 and SiAlON, h-BN, MB2 matrix ultra-high temperature ceramics, MAX phases and their anisotropic properties are presented.

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.

Dense and pure high-entropy metal diboride ceramics sintered from self-synthesized powders via boro/carbothermal reduction approach

Abstract: Equimolar quinary diboride powders, with nominal composition of (Ti0.2Hf0.2Zr0.2Nb0.2Ta0.2)B2, were synthesized by boro/carbothermal reduction (BCTR) of oxide mixtures (MO x , M = Ti, Hf, Zr, Nb and Ta) using B4C as source of B and C in vacuum. By adjusting the B4C/MO x ratios, diboride mixtures without detectable MO x were obtained at 1600°C, while high-entropy diboride (HEB) powders with particle size of 1 μm was obtained at 1800°C. The phase, morphology and solid solution evolution process of the HEB powders during the BCTR process were comprehensively investigated. Although X-ray diffraction pattern indicated the powders synthesized at 1800°C were in a single-phase AlB2 structure, elemental mappings showed that (Ta, Ti)-rich and (Zr, Nb)-rich solid solution coexisted in the HEB powders. The distribution of niobium and zirconium atoms in HEB was unable to reach uniform until the HEB powders were spark plasma sintered at 2000°C. (Ti0.2Hf0.2Zr0.2Nb0.2Ta0.2)B2 ceramics with a relative density of 97.9% were obtained after spark plasma sintering the HEB powders at 2050°C under 50 MPa. Rapid grain growth was found in this composition when the sintering temperature was increased from 2000 to 2050°C, and the averaged grain size increased from 6.67 to 41.2 μm. HEB ceramics sintered at 2000°C had a Vickers hardness of 22.44 ± 0.56 GPa (under a load of 1 kg), a Young’s modulus of ~500 GPa and a fracture toughness of 2.83 ± 0.15 MPa m1/2. This is the first report for obtaining high density HEB ceramics without residual oxide phase, benefiting from the high quality HEB powders obtained.

Static recrystallization and grain growth behaviour of Al0.3CoCrFeNi high entropy alloy

Abstract: Single phase Al0.3CoCrFeNi (Al0.3) FCC high entropy alloy (HEA) synthesized through arc melting was subjected to cold rolling and subsequent annealing to study its recrystallization and grain growth behaviour. For the as-cast alloy, only partial recrystallization was achieved after cold deformation followed by annealing at 800 and 900 °C, which was attributed to the pining effect of Al Ni rich precipitates formed during annealing. The activation energy of static recrystallization (549 kJ/mol) for the as-cast Al0.3 HEA was significantly higher than that of conventional alloys due to the distorted lattice, sluggish diffusion and most importantly the interaction of Al Ni rich precipitates with the recrystallization. Homogenization heat treatment on Al0.3 HEA increased the static recrystallization kinetics and was associated with the lower volume fraction of precipitates formed during annealing. An increased grain growth exponent (n ≈ 5) and activation energy (583 kJ/mol) compared with conventional alloys, particularly at low annealing temperatures, was observed for the Al0.3 HEA mainly due to the pinning effect of Al Ni rich precipitates. Finally, through investigating the mechanical performance of the alloy after annealing in the precipitation-free (1050 and 1000 °C) and precipitation-containing (900 °C) temperature ranges, the relative contributions for grain boundary and precipitation strengthening were estimated for the annealed Al0.3 HEA.

Pressureless sintering of ZrB2 ceramics codoped with TiC and graphite

Abstract: In this research, the pressureless sintering of ZrB2 composites were studied to obtain the highest densification and investigate the effects of TiC (2–10% wt) and graphite (1–2% wt) additives on the sinterability. The results showed that by adding TiC, the porosity and water absorption of the samples decreased, resulting in an increase in the density of the samples up to ~98% of the theoretical density. The addition of graphite and TiC reduced the surface oxide layers, improved the density and prevented the grain growth. The graphite additive reacted with B2O3 and ZrO2 and allowed the system to achieve a complete densification at 2000 °C. Both TiC and graphite additives were completely consumed during the sintering process and the in-situ formation of TiB2 and ZrC phases were proved.

Enhancing the energy storage properties of Ca0.5Sr0.5TiO3-based lead-free linear dielectric ceramics with excellent stability through regulating grain boundary defects

Abstract: Dielectric materials with high energy densities and efficiencies are greatly required in the field of power electronics to satisfy demand. This study presents a regulating strategy through Zr4+ doping and oxygen treatment for reliably enhancing the energy storage performances of Ca0.5Sr0.5TiO3 ceramics. The introduction of Zr4+ inhibits grain growth, and grain boundary barrier effects are enhanced via oxygen treatment; these outcomes are systemically analyzed using complex impedance data. Exceptional energy storage performance is shown; an energy density of 3.37 J cm−3 and 96% energy efficiency under a breakdown strength of 440 kV cm−1 are obtained simultaneously in Ca0.5Sr0.5Ti0.85Zr0.15O3 ceramics following oxygen treatment, which are better results than for other reported linear dielectric systems. This system also possesses excellent stability, with minimal fluctuations (<10%) over wide frequency (1–1000 Hz) and temperature (20–120 °C) ranges. Furthermore, pulsed charging and discharging tests are carried out, aimed at evaluating the practical performance of the ceramic materials. A prominent CD value of 293.3 A cm−2 and PD value of 17.6 MW cm−3 are achieved, and the stored energy is released sharply (∼20 ns); the ceramics also possess outstanding temperature stability (20–120 °C) and fatigue resistance (300 cycles). These properties verify that these lead-free linear dielectric ceramics have potential for practical energy storage applications, especially in high-temperature and high-pressure environments.

Utilization of brittle σ phase for strengthening and strain hardening in ductile VCrFeNi high-entropy alloy

Abstract: General design concept used in high-entropy alloys (HEAs) have deviated from forming an fcc single phase to utilizing hard intermetallic phases in ductile fcc matrix. Here, we effectively exploited strengthening effects of a brittle intermetallic sigma (σ) phase to improve cryogenic tensile properties of a non-equi-atomic ductile VCrFeNi four-component HEA. We preferentially selected vanadium as a candidate alloying element to efficiently produce the σ phase through computational thermodynamic approach. This σ phase has beneficial effects on grain refinement through retardation of grain growth due to grain-boundary pinning, thereby leading to yield strength of 0.79–0.93 GPa. The extensive strain hardening results in tensile strength of 1.33–1.49 GPa and ductility of 23–47% at cryogenic temperature, which are enabled by nano-sized dislocation substructures rather than deformation twinning. Our results demonstrate how the intermetallic σ phase, which has been avoided in typical HEAs because of ductility deterioration, could be used in high strength HEA design.

Reducing residual stress by selective large-area diode surface heating during laser powder bed fusion additive manufacturing

Abstract: High residual stresses are typical in additively manufactured metals and can reach levels as high as the yield strength, leading to distortions and even cracks. Here, an in situ method for controlling residual stress during laser powder bed fusion additive manufacturing was demonstrated. By illuminating the surface of a build with homogeneously intense, shaped light from a set of laser diodes, the thermal history was controlled thereby reducing the residual stress in as-built parts. 316L stainless steel bridge-shaped parts were built to characterize the effect of in situ annealing on the residual stress. A reduction in the overall residual stress value of up to 90% was realized without altering the as-built grain structure (no grain growth). Some annealing effects on the cellular-dendritic solidification structure (patterns of higher solute content) occurred in areas that experienced prolonged exposure to elevated temperature. A comparison of the in situ process to conventional post-build annealing demonstrated equivalent stress reduction compared to rule-of-thumb thermal treatments. Use of this method could reduce or remove the need for post processing to remove residual stresses.