Abnormal grain growth in Al of different purity

The transition from normal to abnormal grain growth has been studied in four Al alloys of various purity (2N, 3N, 4N and 5N). The temperature and time for the onset of abnormal grain growth depend strongly on the deformation and homogenization treatment. Generally, the formation of large grains before cold rolling makes easier the transition to abnormal grain growth during the subsequent annealing. The abnormal grain growth can take place only above a certain temperature which decreases with increasing alloy purity. The onset time of the abnormal grain growth decreases with increasing temperature. It can be qualitatively explained by the dissolution of submicron particles of a second phase.     

Abnormal grain growth in AISI 304L stainless steel

The microstructural evolution during abnormal grain growth (secondary recrystallization) in 304L stainless steel was studied in a wide range of annealing temperatures and times. At relatively low temperatures, the grain growth mode was identified as normal. However, at homologous temperatures between 0.65 (850 °C) and 0.7 (900 °C), the observed transition in grain growth mode from normal to abnormal, which was also evident from the bimodality in grain size distribution histograms, was detected to be caused by the dissolution/coarsening of carbides. The microstructural features such as dispersed carbides were characterized by optical metallography, X-ray diffraction, scanning electron microscopy, energy dispersive X-ray analysis, and microhardness. Continued annealing to a long time led to the completion of secondary recrystallization and the subsequent reappearance of normal growth mode. Another instance of abnormal grain growth was observed at homologous temperatures higher than 0.8, which may be attributed to the grain boundary faceting/defaceting phenomenon. It was also found that when the size of abnormal grains reached a critical value, their size will not change too much and the grain growth behavior becomes practically stagnant.     

Prediction model for the austenite grain growth in a hot rolled dual phase steel

The main purpose of this work is to develop a pragmatic model to predict isothermal austenite grain growth in a hot rolled dual phase steel. Austenite grain growth kinetics have been studied in different heating conditions, involving soaking time and soaking temperature as well as heating rate. The contribution of the initial grain size to the time exponent in Beck equation was analyzed mathematically. The time exponent and initial grain size were also quantitatively described by models proposed. It was found that the time exponent value which had a wide range increased when the temperature decreased. No meaningful activation energy can be obtained for the grain growth process when the time exponent varies. The initial grain size increases with the soaking temperature. The model predictions present a good agreement with experimental austenite grain growth data.     

Austenite grain growth modelling in weld heat affected zone of Nb/Ti microalloyed linepipe steel

Abstract

A model to predict the austenite grain size in an Nb/Ti microalloyed steel weld heat affected zone (HAZ) was developed. The present work investigates grain growth behaviour under the influence of pinning carbonitrides. The steel has been subjected to austenitising heat treatments to selected peak temperatures at various heating rates that are typical for thermal cycles in the HAZ. The effect of temperature and heating rate on the grain size is studied. A model is proposed for the dissolution of NbCN precipitates. This model has been coupled to an austenite grain growth model in the presence of pinning particles. This coupling leads to accurate prediction of the austenite grain size along the heating path simulating selected thermal profiles of the HAZ.     

Critical assessment 30: Grain growth – Unresolved issues

Grain growth is a broad field of study that covers several scientifically and technically interesting characteristics. Normal grain growth kinetics and its associated parabolic growth law and self-similar size distributions, grain boundary pinning, abnormal grain growth, the effect of triple junctions and quadruple points as well as grain growth in thin films are the most important current topics of grain growth research. In this critical assessment of grain growth in metals, we present and discuss some of the most important unresolved issues that have been a subject of research for many years.     

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

Abstract

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.     

Role of convective heat removal and electromagnetic field structure in the microwave heating of materials

The temperature distributions arising in a low-loss dielectric sample in the process of microwave heating have been studied by means of numerical simulation. The convective heat removal has been demonstrated to play the determining role in the energy balance of the sample. The obtained temperature distributions have been compared with the results of a purposely designed microwave heating experiment with multi-channel temperature measurement. The temperature on the surface of the sample agreed well to the simulation accounting for convective heat removal. The temperature measured inside the sample was higher than predicted by simulation. The electrodynamic calculations have shown that due to diffraction effects the electromagnetic field inside the sample is inhomogeneous even if the sample is irradiated by microwaves isotropically. Thus, it is concluded that in order to simulate the microwave heating process accurately, it is necessary to account for air convection and calculate the structure of electromagnetic field inside the sample.     

升温速率对7B04铝合金板材晶粒组织和超塑性的影响

采用形变热处理法制备7B04铝合金细晶板材,利用EBSD和高温拉伸等实验方法研究退火过程中升温速率对板材晶粒组织和超塑性的影响。结果表明:升温速率为5.0×10~(-3)K/s时,退火后板材的轧向和法向的平均晶粒尺寸分别为28.2μm和13.9μm,形核效率为1/1000。随着升温速率的提高,合金平均晶粒尺寸不断减小,形核效率不断提升。当升温速率提高至30.0K/s时,其轧向和法向的平均晶粒尺寸分别降低至9.9μm和5.1μm,形核效率提升至1/80。此外,板材的伸长率也随着升温速率的提高而增大,在773K/8×10~(-4)s~(-1)的变形条件下,试样的伸长率从100%提高至730%。     

A statistical investigation of normal and abnormal grain growth in iron

The evolution of the statistical distribution of linear grain sizes during isothermal grain growth in unstrained or slightly strained pure iron specimens was investigated. A log normal distribution was confirmed to fit the data well for normal growth, and was shown to be applicable in the initial and final stages of abnormal growth. Analysis of statistical parameters of the size distributions proves that the coefficient of variation /gs¯d significantly distinguishes between abnormal and normal growth processes.     

Austenitic grain size evolution and continuous cooling transformation diagrams in vanadium and titanium microalloyed steels

The evolution of the austenitic grain size in medium carbon steels microalloyed with vanadium and titanium was studied as a function of reheating temperature, heating rate, and titanium content. High resolution dilatometric techniques were used to determine the continuous cooling transformation (CCT) diagrams for two different austenitization temperatures. The microstructure and hardness were determined for different cooling rates. The results revealed a significant effect of titanium concentration on the austenitic grain growth control. The smallest grain size was found in the steel with a Ti concentration = 0.019 wt%. Low heating rates produced smaller grain sizes than high heating rates although an abnormal grain growth took place. In these steels, at temperatures above 1050 °C the influence of the reheating temperature on their hardness for cooling rates around 2 °C · s^–1 was negligible. The higher reheating temperatures caused a slight increase in their hardenability. Finally, it was found that the greater the titanium content, the greater the hardness of these steels, but only when the titanium percentages were higher than 0.020 wt%.     

Effect of reactive gas mass flow on the composition and structure of AIN films deposited by reactive sputtering

The mass flow of the reactive gas during sputtering of AIN has been studied in order to control film stoichiometry as well as film structure and morphology. It has been observed that the film composition and sputtering rate depend strongly on the deposition conditions and, in particular, on the mass flow and the sputtering pressure. It has been possible to deposit transparent, almost stoichiometric as well as non-transparent, metal-rich AIN films. The metal-rich films had a sputtering rate which was a factor of six higher than that for the stoichiometric films. The crystalline texture and the morphology have been studied as a function of the total sputtering pressure for both film types. For the transparent films, this novel deposition procedure may be applied in surface acoustic wave devices. The non-transparent AIN films exhibit some similarities to films obtained by ion vapour deposition or ion-beam-enhanced deposition methods, and the films exhibit interesting tribological properties.     

Graded compositions and microstructures by infiltration processing

Mullite/alumina particulate composites were fabricated by infiltrating porous alumina preforms with an SiO₂-containing sol followed by a heating step to cause mullite formation and densification. Electron microprobe analysis was performed to obtain concentration profiles across sections of the sintered composites. These analyses revealed the existence of concentration gradients, the mullite content decreasing with increasing distance from the surface of the bodies. Analyses with a scanning electron microscope also indicated a microstructural effect; the alumina grain size in composite bodies tended to increase with distance from the surface of the sample. These two effects (microstructural and compositional) have been related and it has been concluded that while the presence of mullite limits grain growth in alumina, the mullite content must be at least 5 wt % in order for grain growth to occur in a controlled fashion. The results point out the potential of the infiltration approach as a means for tailoring the composition and microstructure of ceramic bodies.     

The role of grain boundary energy in grain boundary complexion transitions

Recent findings about the role of the grain boundary energy in complexion transitions are reviewed. Grain boundary energy distributions are most commonly evaluated using measurements of grain boundary thermal grooves. The measurements demonstrate that when a stable high temperature complexion co-exists with a metastable low temperature complexion, the stable complexion has a lower energy. It has also been found that the changes in the grain boundary energy lead to changes in the grain boundary character distribution. Finally, recent experimental observations are consistent with the theoretical prediction that higher energy grain boundaries transform at lower temperatures than relatively lower energy grain boundaries. To better control microstructures developed through grain growth, it is necessary to learn more about the mechanism and kinetics of complexion transitions.     

Origination and oscillations of normal zone in superconductors

Dynamics of normal (N) zone regions (resistive domains) in uniform and nonuniform superconductors has been investigated both theoretically and experimentally. An analytical theory has been proposed to describe the resistive domains (RD) dynamics in nonuniform superconductors with the alternating current. The origination and localization of RD have been considered. The self-exited relaxation oscillations of the voltage were observed experimentally. The oscillations are due to the self-exited ones of the RD length when the inductance of the circuit is large enough. The superconducting sample in our experiment was made from the multifilament Nb-40%Ti cable, the characteristic values of the oscillations frequency f and the inductance L were of the order of 1 Hz and 20 μH respectively. The theory proposed is in a good agreement with the experiment.     

Role of magnesia and silica in alumina microstructure evolution

The effects of MgO and SiO₂ additive distributions on alumina grain morphology have been characterized using high-resolution imaging secondary ion mass spectrometry (HRI-SIMS). In alumina samples singly-doped with MgO, the concentration of Mg segregated to grain boundaries is independent of grain boundary length for a majority of grain boundaries studied. Mg segregant therefore redistributes from grain boundaries to microstructural sinks, such as pores and/or second phases, during grain coarsening. In samples singly-doped with SiO₂, abnormal grain growth develops and the concentration of Si at grain boundaries is also independent of grain boundary length. Redistribution of segregants is again necessary in this case to maintain constant grain boundary composition. Codoping with Mg/Si > 1 suppresses abnormal grain growth as a result of increased mutual solid solubility of both ions and an associated decrease in grain boundary segregation. Grain growth kinetics for doped aluminas are reconsidered in light of these observations.     

Grain refinement and orientation of AZ31B magnesium alloy in hot flow forming under different thickness reductions

An analysis of the hot flow forming of Mg-3.0Al-1.0Zn-0.3Mn (AZ31B) alloy was conducted by experiments and numerical simulations. The effects of different thickness reductions on the microstructure and mechanical properties were investigated at a temperature of 693 K, a spindle speed of 800 rev/min and a feed ratio of 0.1 mm/rev. Thickness reductions have great influence on the uniformity of microstructure along the radial direction (RD) and the grain sizes become refined and uniform when the thickness reduction reaches 45%. The c-axes of most grains are approximately parallel to the RD, with a slight inclination towards the axial direction (AD). The best mechanical properties with UTS of 280 MPa and YS of 175 MPa near the outer surface while 266 MPa and 153 MPa near the inner surface have been achieved due to grain refinement and texture. Moreover, the material flow behavior and stress/strain distributions for single-pass reductions were studied using the ABAQUS/Explicit software. The calculated results indicate that the materials mainly suffer from triaxial compressive stresses and undergo compressive plastic strain in RD and tensile strains in other directions. The higher stress and strain rate near the outer surface lead to more refined grains than that of other regions along the RD, whereas the orientation of the maximum principal compressive stress leads to a discrepancy of the grain orientations in RD.     

Mechanical behaviors of Al2O3 nanoparticles reinforced polyetheretherketone

The precipitation kinetics of Si in an Al–1.7 wt.%Si alloy after different thermal treatments has been studied by means of transmission electron microscopy (TEM), dilatometry and differential scanning calorimetry (DSC). The results obtained are explained by a model based on simple nucleation and growth/dissolution laws and are compared with measured precipitate size distributions. The evolution of precipitates in water-quenched samples during linear heating depicts the exothermic formation of platelets and globular Si precipitates (200–300 °C). The endothermal dissolution of Si platelets starts at lower temperatures than that of the globular precipitates. Coarsening and finally dissolution of globular precipitates is observed with increasing temperature. Samples slowly cooled from the solution treatment temperature present mostly globular precipitates, which are nucleated during cooling. Here, an exothermal effect related to the growth of Si precipitates increasing their volume fraction is observed at relatively high temperatures (350–460 °C) during linear heating. The formed precipitates are stable up to 460 °C, where the modelled critical radius becomes bigger than most of the Si precipitates formed so far.     

Effect of interface energy on abnormal grain growth in pyrite films prepared by sol-gel method

The pyrite films were prepared on quartz and glass substrates by the sol-gel dip coating process and sulfuration treatment. The film microstructure, chemical composition and morphology dependent on sulfuration temperature were investigated. The sulfuration treatment at 773-923 K for the films prepared by the sol-gel dip coating process can prepare pyrite films. Some grains show abnormal growth during sulfuration treatment. The grain size of abnormal growth increased to a limited value with increasing the sulfuration temperature. The grain size in pyrite films has bimodal distribution. The first peak is of Lognormal distribution and the second peak Gauss distribution. By determination of the abnormal grain growth activation energy, it can be concluded that the reduction in interface energy at the interface between the substrate and film should be responsible for the grain abnormal growth.     

Grain growth behaviour on reheating Al–Nb-containing steel in the homogenised condition

ABSTRACT

Grain size development during reheating is important to the mechanical properties of steels, and non-uniform grain growth including abnormal and bimodal grain growth are undesirable for steel manufacturing. In this paper, reheating treatments for Al–Nb-containing steel (0.057?wt-% Al and 0.019?wt-% Nb) were selected based on Thermo-Calc predictions for the dissolution temperatures of precipitates in the steel for a fully homogenised condition. Abnormal grain growth occurred in the homogenised Al–Nb sample during reheating, associated with random dissolution of the grain boundary-pinning AlN precipitates, while bimodal grain growth was not observed. So material in a homogenised condition would not be expected to provide a fully uniform grain structure during reheating treatment.     

Abnormal growth of austenite grain of low-carbon steel

Niobium is an important alloying element for the steel. To know further the effect of Nb in the steel, the contrast experiments on the austenite grain growth of the 0.015%Nb and free Nb steels were carried out using Gleeble 1500 thermomechanical simulator. The experimental results indicate that the austenite grain of 0.015%Nb steel is finer than that of Nb free steel at 1150–1230 °C. And when the heating temperature arrives the critical temperature 1240 °C, the austenite grain of Nb steel suddenly grows up, while the austenite grain of Nb free steel changes little. Finally, the austenite grain of Nb steel is obviously coarser than that of Nb free steel. By transmission electron microscopy (TEM) using a carbon extraction replica technique, the precipitates of Nb(C,N) were not observed in the 0.015%Nb steel. It is concluded that the grain-boundary internal adsorption of Nb atoms leads to the result.