Rapid Grain Growth Research Articles

Influence of annealing temperature on microstructure, tensile properties and tensile deformation mechanism of metastable austenitic stainless steel repetitively cold-rolled and annealed

A metastable austenitic stainless steel was repetitively cold-rolled and reversely annealed. The effect of the final annealing temperature on microstructure, tensile properties and tensile deformation mechanism was investigated by electron backscattering diffraction, X-ray diffraction, and transmission electron microscopy. The results revealed that austenite grains were significantly refined and the strength was greatly enhanced by the repetitive cold-rolling and reverse annealing treatment. The reversed austenite grains gradually grew as the final annealing temperature increased from 800 °C to 950 °C, whereas rapid growth of austenite grains has been observed in 1000 °C-annealed samples, which can be ascribed to the gradual dissolution of M23C6 carbide particles with increasing annealing temperature and almost complete dissolution at the annealing temperature of 1000 °C. The yield strength and ultimate tensile strength showed an inverse relationship with annealing temperature due to grain growth, whereas the elongation exhibited a direct relationship due to the austenite stability caused by carbides dissolution. The microstructural examination of the annealed samples, with different tensile strains, indicated that the formation of strain-induced α′-martensite has been retarded, whereas the formation of deformation twins has been promoted by increasing the final annealing temperature. The tensile deformation mechanisms were dependent on the annealing temperature, including a transformation from strain-induced α′-martensite to a hybrid of strain-induced α′-martensite transformation and deformation twinning, and to deformation twinning as the annealing temperature increased from 800 °C to 1000 °C.

Carbonation curing of industrial solid waste‐based aerated concretes

AbstractMineral carbonation for concrete curing is a promising route to large‐scale CO2 sequestration and the economic production of building materials. In this work, the carbonation curing of a kind of lightweight concrete, aerated concrete, was comprehensively studied. Three typical industrial wastes – fly ash, blast furnace slag, and red mud – were employed to replace part of the cement and to reduce the carbon footprint of products. The effects of carbonation curing time, pressure, and water‐to‐solids ratio on the CO2 uptake and compressive strength were investigated. Aerated concrete with red mud showed the highest CO2 uptake capacity, followed by concrete with fly ash, and concrete with blast furnace slag. The main crystalline carbonation products, needle‐like calcite and rod‐like aragonite, were observed to be embedded in the surface of fly ash and blast‐furnace‐slag‐based specimens. The orderly structures of carbonated crystals, as well as the reduction of mesoscale porosity (< 50 nm), resulted in an enhancement of mechanical properties. For red‐mud‐based specimens, calcite crystals with the appearance of grains (smaller than 1μm) were formed due to deeper carbonation. The rapid growth of calcite grains may lead to cracks from inside red‐mud‐based specimens and cause a negative effect on mechanical properties. In terms of mechanical performance, the blast furnace slag specimens, after carbonation curing, exhibited the largest compressive strength of 7.3 MPa, which was over 197% higher than that of natural curing for the same duration. © 2019 Society of Chemical Industry and John Wiley & Sons, Ltd.

Valorization of Ferronickel Slag into Refractory Materials: Effect of Sintering Temperature

Preparation of refractory materials from ferronickel slag by sintering over a broad temperature range (1200°C to 1500°C) with addition of sintered magnesia was explored. The thermodynamic calculations indicated that the amounts of newly generated high-melting-point forsterite and spinel phases increase with increasing temperature. The experimental analysis demonstrated that elevating the sintering temperature promoted conversion and crystallization of forsterite and spinel phases from the original phase of the slag, with simultaneous reduction of low-melting-point enstatite. There was also rapid growth of spinel grains from about 0.5 μm to 5 μm, which should be controlled by selecting an appropriate temperature. The results showed that, by sintering the slag at 1400°C for 3 h with addition of 20 wt.% sintered magnesia, a high-quality refractory material with refractoriness of 1680°C, bulk density of 2.93 g/cm3, apparent porosity of 1.81%, and compressive strength of 166.62 MPa was obtained.

Annealing behavior of severely-deformed titanium Grade 4

The static-annealing behavior and evolution of the microstructure-strength relationship of severely-deformed commercial-purity titanium Grade 4 over the temperature range of 50–850 °C (0.16–0.57 Tm, where Tm is the melting point) were established. The severely-deformed material was obtained via equal-channel angular pressing (ECAP) using the Conform (ECAP-C) technique at 200 °C to an effective accumulated true strain of 8.4. The resulting ultrafine structure was stable to 400 °C. The excellent thermal stability was concluded to be associated with a strain-aging effect, i.e., the enhanced diffusion of solutes within this temperature interval resulting in the formation of solute atmospheres at/near dislocations. At 450–500 °C, rapid growth of strain-free grains occurred, which eliminated the severely-deformed microstructure and promoted softening. This process was deduced to be controlled primarily by grain-boundary energy and therefore was interpreted primarily in terms of grain growth rather than discontinuous recrystallization expected in this temperature range. A further increase in annealing temperature to 600 °C led to normal grain growth. Analysis of the microstructure-strength relationship suggested a significant influence of mechanical twinning on yield strength of the fully-annealed material. At 600 °C and higher temperatures, dissolution of constituent iron-rich particles was observed. This promoted a partial α → β transformation at the temperatures noticeably below the typical beta-transus of pure titanium (~880 °C). This phenomenon resulted in the precipitation of nanoscale β particles which imparted substantial strengthening. Water quenching of the material annealed at 850 °C gave rise to a β → α′ martensitic transformation. The latter process was governed by exceptionally strong variant selection and thereby provided a nearly-ideal restoration of crystallographic orientations of parent α-grains.

Understanding structural evolution of nanostructured Cu-Al2O3 composite powders during thermomechanical processing

A better understanding of structural coarsening mechanisms is of significance to effectively tackle structural instabilities in nanostructured materials. In this work, the microstructural evolution of nanostructured metallic powder particles during thermomechanical consolidation has been investigated to gain new insights into the evolution of reinforcing particles and matrix grains of a mechanically alloyed nanostructured Cu-5 vol.% Al2O3 composite powder extruded at either 300 or 900 °C, respectively. Unusual coarsening of Al2O3 particles and rapid growth of Cu grains occurred during extrusion at a low temperature of 300 °C; meanwhile the formation of elongated micron Cu grains was observed at a high extrusion temperature of 900 °C. Both the applied shear stress/strain and the sizes and distribution of second-phase nanoparticles were found to determine the grain structure of the Cu matrix after extrusion. Specifically, the shear stress-induced dynamic and particle-stimulated recrystallization and the movement of the Al2O3 particles are respectively responsible for the low-temperature rapid growth of the Cu grains and the coarsening of the Al2O3 particles. The development of the elongated micron Cu grains during extrusion at 900 °C is associated with anisotropic grain boundary migration and particle re-distribution.

The Effect of Precipitate Evolution on Austenite Grain Growth in RAFM Steel.

To study the effects of various types of precipitates and precipitate evolution behavior on austenite (size and phase fraction) in reduced activation ferritic/martensitic (RAFM) steel, RAFM steel was heated to various austenitizing temperatures. The microstructures of specimens were observed using optical microscopy (OM) and transmission electron microscopy (TEM). The results indicate that the M23C6 and MX precipitates gradually coarsen and dissolve into the matrix as the austenitizing temperatures increase. The M23C6 precipitates dissolve completely at 1100 °C, while the MX precipitates dissolve completely at 1200 °C. The evolution of two types of precipitate has a significant effect on the size of austenite. Based on the Zener pinning model, the effect of precipitate evolution on austenite grain size is quantified. It was found that the coarsening and dissolution of M23C6 and MX precipitates leads to a decrease in pinning pressure on grain boundaries, facilitating the rapid growth of austenite grains. The austenite phase fraction is also affected by the coarsening and dissolution of precipitates.

Jet electrodeposition of nanocrystalline nickel assisted by controllable friction

Abstract A controllable friction auxiliary method (CFAM) was developed to improve the quality of a coating surface prepared through flexible friction-aided jet electrodeposition (FFAJED) at a high current density. A rolling nozzle was also designed. This nozzle can provide controlled pressure constantly during deposition so that the proposed method can forcibly restrain the rapid growth of the coarse grains. The influence of different pressures and cathode scanning speeds on coating surface quality was discussed, and the optimum parameters of the method were determined. With the optimum parameters, the effect of different current densities on coating surface quality was studied. Results show that when the rolling pressure is 5 N and the scanning speed of the cathode is 860 mm/min, the surface quality of the coating is excellent. No obvious coarse grains, defects (e.g., pinhole or nodules), and apparent traces of friction were observed. The magnetic properties of Ni coating were improved too. The upper limit of current density in the preparation of Ni coating by CFAM reached 200 A/dm2, which is 1.54 times that of traditional jet electrodeposition (130 A/dm2) and 1.43 times that of FFAJED.

Imaging the water snow-line during a protostellar outburst.

A snow-line is the region of a protoplanetary disk at which a major volatile, such as water or carbon monoxide, reaches its condensation temperature. Snow-lines play a crucial role in disk evolution by promoting the rapid growth of ice-covered grains. Signatures of the carbon monoxide snow-line (at temperatures of around 20 kelvin) have recently been imaged in the disks surrounding the pre-main-sequence stars TW Hydra and HD163296 (refs 3, 10), at distances of about 30 astronomical units (au) from the star. But the water snow-line of a protoplanetary disk (at temperatures of more than 100 kelvin) has not hitherto been seen, as it generally lies very close to the star (less than 5 au away for solar-type stars). Water-ice is important because it regulates the efficiency of dust and planetesimal coagulation, and the formation of comets, ice giants and the cores of gas giants. Here we report images at 0.03-arcsec resolution (12 au) of the protoplanetary disk around V883 Ori, a protostar of 1.3 solar masses that is undergoing an outburst in luminosity arising from a temporary increase in the accretion rate. We find an intensity break corresponding to an abrupt change in the optical depth at about 42 au, where the elevated disk temperature approaches the condensation point of water, from which we conclude that the outburst has moved the water snow-line. The spectral behaviour across the snow-line confirms recent model predictions: dust fragmentation and the inhibition of grain growth at higher temperatures results in soaring grain number densities and optical depths. As most planetary systems are expected to experience outbursts caused by accretion during their formation, our results imply that highly dynamical water snow-lines must be considered when developing models of disk evolution and planet formation.

Modeling of Austenite Grain Growth During Austenitization in a Low Alloy Steel

The main purpose of this work is to develop a pragmatic model to predict austenite grain growth in a nuclear reactor pressure vessel steel. Austenite grain growth kinetics has been investigated under different heating conditions, involving heating temperature, holding time, as well as heating rate. Based on the experimental results, the mathematical model was established by regression analysis. The model predictions present a good agreement with the experimental data. Meanwhile, grain boundary precipitates and pinning effects on grain growth were studied by transmission electron microscopy. It is found that with the increasing of the temperature, the second-phase particles tend to be dissolved and the pinning effects become smaller, which results in a rapid growth of certain large grains with favorable orientation. The results from this study provide the basis for the establishment of large-sized ingot heating specification for SA508-III steel.

A New Method for Controlling Billet Temperature During Isothermal Die Forging of a Complex Superalloy Casing

Isothermal die forging is one of near net-shape metal-forming technologies. Strict control of billet temperature during isothermal die forging is a guarantee for the excellent properties of final product. In this study, a new method is proposed to accurately control the billet temperature of complex superalloy casing, based on the finite element simulation and response surface methodology (RSM). The proposed method is accomplished by the following two steps. Firstly, the thermal compensation process is designed and optimized to overcome the inevitable heat loss of dies during hot forging. i.e., the layout and opening time of heaters assembled on die sleeves are optimized. Then, the effects of forging speed (the pressing velocity of hydraulic machine) and its changing time on the maximum billet temperature are discussed. Furthermore, the optimized forging speed and its changing time are obtained by RSM. Comparisons between the optimized and conventional die forging processes indicate that the proposed method can effectively control the billet temperature within the optimal forming temperature range. So, the optimized die forging processes can guarantee the high volume fraction of dynamic recrystallization, and restrict the rapid growth of grains in the forged superalloy casing.

Correlation among composition, microstructure and mechanical properties of ZrO<SUB align="right">2 (Y<SUB align="right">2O<SUB align="right">3)/Al<SUB align="right">2O<SUB align="right">3 composite ceramics

The aim of this work is to correlate the microstructure of alumina toughened zirconia (ATZ) and zirconia toughened alumina (ZTA) composites with the Vickers hardness, flexural strength and fracture toughness. The monoclinic zirconia (m-ZrO2) phase has completely been transformed into tetragonal zirconia (t-ZrO2) phase for Al2O3 content. The presence of 3 mol% yttria-stabilised zirconia (3YSZ) particles on the grain boundaries effectively limits a rapid grain growth of the Al2O3 grains. The average grain sizes of pure Al2O3 and 3YSZ as well as Al2O3 and t-ZrO2 in the 3YSZ + 40 wt% Al2O3 composite were 3.78 µm, 1.82 µm, 0.89 µm and 0.83 µm, respectively. The 3YSZ + 40 wt% Al2O3 composite showed maximum flexural strength of 395 MPa, fracture toughness of 5.6 MPa.m1/2 and intermediate Vickers hardness of 15.01 GPa with the highly dense microstructure consisting of higher t-ZrO2 grains of the critical size, which make it suitable for dental applications.

Preparation, microstructure and mechanical properties of multicomponent Ni3Al-bonded cermets

Ni3Al intermetallic compound was synthesized by mechanical milling mixture of elemental Ni and Al powder after 20h milling; mechanical alloying (MA) first generated a Ni(Al) solid solution that transformed to Ni3Al intermetallic compound on further milling. Ti(C,N)–xNi3Al cermets were prepared through conventional powder metallurgy route at different sintering temperatures. The microstructure and mechanical properties of Ni3Al-bonded cermets sintered at different temperatures with different contents of Ni3Al were systemically studied. The dark core-gray rim carbide grains and/or dark core-white inner rim-gray outer rim carbide grains embedded in light-gray Ni3Al based binder phase were observed. Abnormal grains growth happened in all cermets and these grains tend to be lath like grains in this research. Moreover, with the rise of sintering temperature, the phenomenon of abnormal grains growth becomes more obvious, resulting in rapid growth of individual grains. Generally, transverse rupture strength (TRS) of cermets increased with the ascent of Ni3Al content and sintering temperature, however, it reached a peak value 1131±80.3MPa, when sintering temperature was 1450°C. HRA of cermets ascended firstly and then dropped down with the increase of binder content and sintering temperature. Fracture toughness (K1C) increased with Ni3Al content and got a peak value 16.1±0.55MPam1/2.

Densification behavior, mechanical properties and thermal shock resistance of tungsten alloys fabricated at low temperature

The low-temperature shrinkage of tungsten was greatly accelerated by the addition of trace Nb and Ni, and the addition of trace Nb and Ni also significantly promoted the final sintering density. The 99.1% of theory density for W–0.1wt.%Nb–0.1wt.%Ni material sintered at 1600°C was obviously greater than 93.7% of theory density for W material sintered at 2000°C. Ball milling treatment played an important role in promoting the sintering densification of W–0.1wt.%Nb–0.1wt.%Ni powder, and the powder milled for 10h (W10) could be sintered to near full density (99.4% of theory density) at 1600°C. The ball milling for 15h has no effect in improving the sintering density, but it induced rapid growth of tungsten grains. The microhardness and tensile strength of the sintered tungsten alloys were highly dependent on its sintering density and grain size. Improving the sintering density while controlling the grain growth could effectively promote the microhardness and tensile strength. Furthermore, the improvement of thermal shock resistance of the W10 alloy was due to good microstructure and the increase in the tensile strength.

Direct synthesis of barium zirconate titanate (BZT) nanoparticles at room temperature and sintering of their ceramics at low temperature

Using tetrabutyl titanate, zirconium nitrate and barium octahydrate as the raw materials, BZT nanoparticles with a grain size of ~10nm were directly synthesized at room temperature. With low energy consumption and without any contamination produce, the synthesis process is green, environmental-friendly, convenient and efficient. The graingrowth of the as-prepared nanoparticles annealed at different temperatures was checked, and a rapid graingrowth starting at 600°C was observed. The sintering characteristics were also studied and it was found that an adapted sintering aid was very important for the sintering of the BZT nanoparticles at low temperature. Here, adding the useful active liquid of Bi2O3–Li2O as the sintering aid results in obtaining the dense BZT ceramics with the relative density of 96% even sintering at the temperature as low as 900°C for 2h.

KINETICS OF GRAIN NUCLEATION IN SUPERCOOLED QUASI-TWO-DIMENSIONAL Cu

Grain nucleation kinetics of nanocrystalline Cu in a quasi-two-dimensional system was investigated using molecular dynamics. Nanoscale grains nucleate rapidly from supercooled Cu under isothermal annealing conditions. An Avrami exponent from the JMAK equation implies that the kinetics follows the continuous nucleation model. Observation of grain growth during the nucleation stage shows that the kinetics follows the classical grain growth model with a low growth exponent. The lower value of the exponent illustrates the rapid growth of grains during this stage. Using the growth exponent, the activation energy for nucleation in Cu was obtained and compared with the values obtained in earlier studies.

Nanocrystalline diamond microstructures from Ar/H2/CH4-plasma chemical vapour deposition

The incorporation of H2 into Ar plasma was observed to markedly alter the microstructure of diamond films. The addition of a small percentage of H2 (<1.5%) into the Ar plasma leads to the presence of stacking faults in plate-like diamond grains, the incorporation of 75% H2 induces the formation of the diamond polymorph (8H). Optical emission spectroscopy indicated that addition of H2 into the Ar/CH4 plasma decreased the CH/C2 ratio and increased the proportion of atomic hydrogen. The small proportion of atomic hydrogen in 1.5%H2–Ar plasma can only induce the formation of (111) stacking faults, resulting in scarcely distributed plate-like diamond grains. The large proportion of atomic hydrogen in 75%H2–Ar plasma causes the rapid growth of diamond grains, leading to the formation of polymorphs of diamond lattices. The tuning on the microstructure of the UNCD films by incorporating either small or large amounts of H2 in Ar-plasma can be attributed to the interaction of H-species with the grain boundary hydrocarbons. Such a capability opens up the potential for applications of UNCD films. Despite the complication in granular structure resulted from the CH4/(Ar–H2) plasma chemical vapour deposition, the formation of microstructures can be explained by the same pathway, the competition of the processes (i) formation of a hydrocarbon passivation layer and the re-activation of the hydrocarbon layer and (ii) secondary nucleation and the enlargement of diamond grains.

Structure and microstructure of EB-PVD yttria thin films grown on Si (111) substrate

Structure and microstructure of yttria thin films grown by electron beam physical vapour deposition on a stationary Si (111) substrate at room temperature (RT), 500° and 700 °C, were investigated by the grazing-incidence X-ray diffraction and scanning electron microscopy, respectively. X-ray photoelectron spectroscopy provided information on the surface contamination from the atmosphere and the Y oxidation state. A strong effect of the deposition temperature and the vapour flux incidence angle was found. The film deposited at RT is polycrystalline with very fine grains of the body-centered cubic (bcc) crystallographic symmetry. An increase of deposition temperature results in a rapid growth of bcc grains with an improved crystalline structure. Moreover, the based-centered monoclinic phase appears for the deposition temperature of 700 °C. Preferred grain orientation (texture) with two main components, (400) and (622), was observed in the films deposited at 500 °C whereas no texture was found for 700 °C. The microstructure exhibits the columnar feather-like structure of different degrees of perfection which can be explained by the shadowing effects caused by an oblique vapour flux incidence angle. Surface morphology of the films is governed by a combination of the triangular and four-sided (square) columns. All films were found to be dense with a little porosity between the columns.

Dense Nanostructured Hydroxyapatite Coating on Titanium by Aerosol Deposition

In order to improve biocompatibility of Ti metal substrates, 1‐μm‐thick nanostructured hydroxyapatite (HAp) coatings were deposited on the substrates through aerosol deposition, which sprays HAp powder with an average particle size of 3.2 μm at room temperature in vacuum. The original HAp particles were fractured into nanoscale fragments to form highly dense coating during the deposition process. Density of the HAp coating was 98.5% theoretical density (TD). Transmission electron microscopy observation revealed that the as‐deposited coating consisted of HAp crystallites with average grain size of 16.2 nm and amorphous phase. Tensile adhesion strength between the coating and the substrate was 30.5±1.2 MPa. Annealing up to 500°C in air increased crystallinity and grain size in the coating without any delamination or crack according to X‐ray diffraction analysis and electron microscopy. MTS assay and alkaline phosphatase activity measurements with MC3T3‐E1 preosteoblast cell revealed that the biocompatibility was greatly improved by postdeposition heat treatment at 400°C in air due to well‐crystallized HAp with average grain size of 29.3 nm. However, further heat treatment at 500°C deteriorated biocompatibility due to rapid growth of HAp grains to average size of 99 nm. Cross section of the coating on a commercially available Ti dental implant revealed full coverage of the surface with HAp.

Early-Stage Oxidation Behavior of Pt-Modified γ′-Ni3Al-Based Alloys with and without Hf Addition

The early-stage oxidation behavior in air of Pt-modified γ′-Ni3Al-based alloys of composition (in at.%) Ni–22Al–30Pt with and without 0.5Hf was investigated in terms of oxidation kinetics, scale evolution and Al2O3 phase transformation. Oxidation exposures included heating to and short-term holds at 1,150 °C. Hafnium addition did not appear to affect microstructural evolution and growth rate of the oxide scales during heating to 1,150 °C; however, it was found that Hf delayed the metastable-to-α-Al2O3 phase transformation, thus allowing continued fast growth of oxide scale. After the transient oxidation stage of up to about 10 min (including heating time), Ni-rich metallic particles precipitated in the lower part of the metastable Al2O3 layer, due to a decrease in the oxygen potential resulting from scale evolution. The present results indicated that the period of oxide phase transformation was followed by the establishment of steady-state oxidation kinetics. However, the steady-state kinetics were different for the two alloy systems. Specifically, after complete phase transformation to α-Al2O3, rapid growth of oxide grains occurred on the Hf-free alloy; whereas, the oxide grain size remained small for the Hf-containing alloy. Such a difference of transformation and subsequent grain-growth behavior greatly affected oxide thickening kinetics.

Introduction of preferential Zener drag effect in Monte Carlo simulation of abnormal Goss grain growth in the Fe–3%Si magnetic alloys

The selective growth of the Goss grains in textured materials such as Fe–3%Si is linked to the preferential weak Zener drag effect on their boundaries. The presence of AlN and MnS inhibitors permits the development of Goss texture by sudden and rapid growth of small grains possessing a {1 1 0}〈0 0 1〉 orientation. In Monte Carlo simulation of grain growth, particles are randomly distributed on such a theoretical microstructure. This assumption is not representative of real materials like Fe–3%Si sheets, which are used to the manufacture of transformers. In the present study, a model for classical Monte Carlo simulation is developed where preferential pinning effect of particles on grain boundaries is introduced through a critical radius Rn. The values of Rn are inversely proportional to the pinning force. The simulation procedure has been implemented using as starting state a real microstructure of grain oriented Fe–3%Si sheet characterised by orientation imaging microscopy (OIM™).