Austenitic Model Alloy Research Articles

Refinement of Microstructure in Steels through Thermomechanical Processing

The refinement of microstructure is the most generally accepted approach to simultaneously improve the strength and toughness in steels. In the current study, the role of dynamic/static phase transformation on the ferrite grain refinement was investigated using different thermomechanical processing routes. A Ni-30Fe austenitic model alloy was also used to investigate the substructure character formed during deformation. It was revealed that the microstructure of steel could further be refined to the nanoscale through both the control of processing route and steel composition design.

Texture and substructure characteristics of dynamic recrystallization in a Ni–30%Fe austenitic model alloy

The substructure and texture development during dynamic recrystallization (DRX) of an austenitic Ni–30%Fe model alloy was investigated using hot torsion testing. The current results revealed that the DRX texture was dominated by grains with a low Taylor factor component. This was related to the preferred nucleation and lower consumption rates of these grains during DRX. The substructure of DRX grains was “random” in character and displayed complex subgrain/cell arrangements that largely depended on grain orientation.

Modeling the Flow Behavior, Recrystallization, and Crystallographic Texture in Hot-Deformed Fe-30 Wt Pct Ni Austenite

The present work describes a hybrid modeling approach developed for predicting the flow behavior, recrystallization characteristics, and crystallographic texture evolution in a Fe-30 wt pct Ni austenitic model alloy subjected to hot plane strain compression. A series of compression tests were performed at temperatures between 850 °C and 1050 °C and strain rates between 0.1 and 10 s−1. The evolution of grain structure, crystallographic texture, and dislocation substructure was characterized in detail for a deformation temperature of 950 °C and strain rates of 0.1 and 10 s−1, using electron backscatter diffraction and transmission electron microscopy. The hybrid modeling method utilizes a combination of empirical, physically-based, and neuro-fuzzy models. The flow stress is described as a function of the applied variables of strain rate and temperature using an empirical model. The recrystallization behavior is predicted from the measured microstructural state variables of internal dislocation density, subgrain size, and misorientation between subgrains using a physically-based model. The texture evolution is modeled using artificial neural networks.

Void swelling behavior in electron irradiated Fe–Cr–Ni model alloys under temperature variation

We examined the effect of short HVEM pre-irradiation at lower or higher temperature on void swelling behavior in electron irradiated austenitic model alloys, Fe–15Cr– xNi ( x = 15, 20, 25 or 30 wt%). By 6 dpa irradiation at constant temperature, voids were formed at 200–400 °C in 15, 20, and 25Ni alloys but did not form above 350 °C in the 30Ni alloy. In the temperature variation irradiation, an initial 0.6 dpa irradiation at 200–500 °C was followed by irradiation of 5.4 dpa at 400 °C. The effect of pre-irradiation at lower temperature was a slight increase in the void number density, and that at higher temperature was a suppression of void formation. SFT induced by irradiation at lower temperature were not effective void nuclei in the subsequent irradiation at higher temperature.

The Effect of Multiple Deformations on the Formation of Ultrafine Grained Steels

A C-Mn-Nb-Ti steel was deformed by hot torsion to study ultrafine ferrite formation through dynamic strain-induced transformation (DSIT) in conjunction with air cooling. A systematic study was carried out first to evaluate the effect of deformation temperature and prior austenite grain size on the critical strain for ultrafine ferrite formation (e C,UFF) through single-pass deformation. Then, multiple deformations in the nonrecrystallization region were used to study the effect of thermomechanical parameters (i.e., strain, deformation temperature, etc.) on e C,UFF. The multiple deformations in the nonrecrystallization region significantly reduced e C,UFF, although the total equivalent strain for a given thermomechanical condition was higher than that required in single-pass deformation. The current study on a Ni-30Fe austenitic model alloy revealed that laminar microband structures were the key intragranular defects in the austenite for nucleation of ferrite during the hot torsion test. The microbands were refined and overall misorientation angle distribution increased with a decrease in the deformation temperature for a given thermomechanical processing condition. For nonisothermal multipass deformation, there was some contribution to the formation of high-angle microband boundaries from strains at higher temperature, although the strains were not completely additive.

Effects of titanium additions to austenitic ternary alloys on microstructural evolution and void swelling

Ternary austenitic model alloys were modified with 0.25 wt.% titanium and irradiated in the Fast Flux Test Reactor (FFTF) at dose rates ranging over more than two orders in magnitude. While lowering of the dose rate strongly increases swelling by shortening the incubation dose, the steady-state swelling rate is not affected by dose rate. Although titanium addition strongly alters the void microstructure, swelling at about 420°C does not change with titanium additions, but the sensitivity to dose rate is preserved.

Influence of Silicon and Phosphorus on Radiation-Induced Segregation of Chromium and Nickel in Austenitic Model Alloys

The influence of undersized solute additions on radiation-induced segregation (RIS) at grain boundaries was examined for austenitic model alloys. High-purity Fe-15Cr-20Ni alloys, doped with either 0.5 mass% silicon or 0.025 mass% phosphorus, were exposed to electron irradiation at several temperatures in a high-voltage electron microscope. The grain boundary depletion of chromium and segregation of nickel were significantly suppressed by the addition of silicon and phosphorus, and the addition of phosphorus significantly reduced the RIS at a grain boundary. It was shown by numerical simulation that the present results stem from the strong interaction between the additives and interstitial atoms produced by the electron irradiation.

Effect of Cr and Ni Contents on the Oxidation Behavior of Ferritic and Austenitic Model Alloys in Air with Water Vapor

Ferritic and austenitic model alloys with various contents of Cr and Ni ranging between 10–20% and 0–30%, respectively, were oxidized in air + 10% water vapor during 1 hr cyclic oxidation at 650°C and 800°C. Depending on the alloy composition and temperature, either a thin protective oxide scale was observed or accelerated attack occurred which sometimes included spallation. For austenitic model alloys, increasing either the Cr or Ni contents delayed the accelerated attack. For lower Cr and Ni contents at 800°C, accelerated attack, including spallation, occurred at short exposure times. No spallation was observed for the ferritic model alloys. However, accelerated attack can occur quickly with low Cr contents. Increasing the temperature delayed the breakaway observed on ferritic alloys whereas it reduced the protective-oxide-growth stage for austenitic alloys.

Fine Structure of Shear Bands Formed during Hot Deformation of Two Austenitic Steels

Shear bands formed during both cold and hot plastic deformation have been linked with several proposed mechanisms for the formation of ultrafine grains. The aim of the present work was to undertake a detailed investigation of the microstructural and crystallographic characteristics of the shear bands formed during hot deformation of a 22Cr-19Ni-3Mo (mass%) austenitic stainless steel and a Fe-30 mass%Ni based austenitic model alloy. These alloys were subjected to deformation in torsion and plane strain compression (PSC), respectively, at temperatures of 900 � C and 950 � C and strain rates of 0.7 s � 1 and 10 s � 1 , respectively. Transmission electron microscopy and electron backscatter diffraction in conjunction with scanning electron microscopy were employed in the investigation. It has been observed that shear bands already started to form at moderate strains in a matrix of pre-existing microbands and were composed of fine, slightly elongated subgrains (fragments). These bands propagated along a similar macroscopic path and the subgrains, present within their substructure, were rotated relative to the surrounding matrix about axes approximately parallel to the sample radial and transverse directions for deformation in torsion and PSC, respectively. The subgrain boundaries were largely observed to be non-crystallographic, suggesting that the subgrains generally formed via multiple slip processes. Shear bands appeared to form through a co-operative nucleation of originally isolated subgrains that gradually interconnected with the others to form long, thin bands that subsequently thickened via the formation of new subgrains. The observed small dimensions of the subgrains present within shear bands and their large misorientations clearly indicate that these subgrains can serve as potent nucleation sites for the formation of ultrafine grain structures during both subsequent recrystallisation, as observed during the present PSC experiments, and phase transformation.

The primary origin of dose rate effects on microstructural evolution of austenitic alloys during neutron irradiation

The effect of dose rate on neutron-induced microstructural evolution was experimentally estimated. Solution-annealed austenitic model alloys were irradiated at ≃400 °C with fast neutrons at seven different dose rates that vary more than two orders difference in magnitude, and two different doses were achieved at each dose rate. Both cavity nucleation and growth were found to be enhanced at lower dose rate. The net vacancy flux is calculated from the growth rate of cavities that had already nucleated during the first cycle of irradiation and grown during the second cycle. The net vacancy flux was found to be proportional to (dpa/s) 1/2 up to 28.8 dpa and 8.4×10 −7 dpa/s. This implies that mutual recombination dominates point defect annihilation in this experiment, even though point defect sinks such as cavities and dislocations were well developed. Thus, mutual recombination is thought to be the primary origin of the effect of dose rate on microstructural evolution.

The contribution of various defects to irradiation-induced hardening in an austenitic model alloy

The combination of charged particle irradiation using MeV energy range accelerators and an ultra-low load indentation is a potential technique to study the mechanical property changes of fusion structural materials due to high-energy neutron irradiation. This work was intended to examine the contribution of various defects to irradiation-induced hardening in an Fe–15Cr–20Ni model alloy by means of single/dual-beam ion irradiation and a micro-indentation. In single/dual ion-irradiated specimens, a significant reduction in Frank loop number density was observed in a region where indentation-induced dislocations evolved. It was observed that only Frank loops with diameters of about 20 nm partially survived in the plastically deformed region. In the dual ion-irradiated specimens, cavities shear-deformed along the mass-flow directions were observed in the region beneath the indent. It was concluded through the indentation hardness determination and post-irradiation indentation micro-structural examination that small Frank loops cause hardening to a larger extent than expected from their actual sizes and induce plastic instability by annihilating by interactions with moving dislocations, while clean cavities produced by ion irradiation are not strong deformation barriers as estimated from neutron data.

Formation and migration of helium bubbles in Fe–16Cr–17Ni austenitic alloy at high temperature

The formation and migration of helium bubbles in Fe–16Cr–17Ni austenitic model alloy at temperatures 400–1250°C have been studied by in situ electron microscopy, energy dispersive spectrometry (EDS) and EELS analysis. The nucleation of bubbles under 10 keV He+ ion irradiation was dominant at 400°C, while the growth was dominant above 600°C. It was revealed that the mean square of the migration distance was proportional to time, which is quantitative evidence of Brownian type motion and yielded diffusivities from 10−18 to 10−20 m2/s at 1185°C, depending on the diameter from 1 to 3 nm. These facts suggest that the mobility of helium–vacancy complexes or bubbles is an important factor governing the formation process of helium bubbles. The analysis by scanning transmission electron microscope-electron energy loss spectrometry (STEM-EELS) elucidated Ni precipitation even around small helium bubbles.

Macroscopic and microscopic subdivison of a cold–rolled aluminium single crystal of cubic orientation

An aluminium single crystal of cube orientation has been rolled to 15, 30 and 50% reductions under controlled homogeneous rolling conditions. The deformation structure of the rolled specimens was investigated by both scanning electron microscopy (SEM) and transmission electron microscopy (TEM) over several scales of magnification. The local crystallographic orientations have been measured by an automatic electron back scattering patterns (EBSP) technique and a semiautomatic TEM method. Orientation image maps based on the local orientation data have been used to reveal the evolution of the deformation structure during rolling. It is observed that by an opposite rotation around transverse direction (TD) the crystal was subdivided into four macroscopic bands, termed matrix bands in the present paper, which are parallel to the rolling plane. Between the four bands there are three transition bands in which the orientation changes continuously from that of a matrix band to that of the adjoining one. A model based on the idea of location–dependent shear strain caused by geometric and friction effects together with a plasticity analysis has been used to explain the macroscopic subdivision of the crystal. In addition to the macroscopic subdivision, a microscopic subdivision by the formation of cell–blocks within the matrix bands and a cell structure within transition bands has also been observed. A difference related to shear amplitude difference between the active slip systems changing continuously across the crystal has been observed. Both the macroscopic orientation of the dislocation boundaries and the misorientation angles and axes across dislocation boundaries are analysed and it is found that Frank9s formula is a useful tool in analysing the dislocation boundaries formed during deformation.

Microstructural evolution in Fe [sbnd]Cr [sbnd]Ni alloy irradiated with Ni ion under varying temperature

To understand defect processes under temperature variation irradiation, Fe 16Cr 17Ni austenitic model alloy was irradiated with heavy ions by changing the irradiation temperature in a stepwise manner from 473 to 773 K and from 573 to 773 K. Most of the interstitial loops formed by the pre-irradiation at 473 or 573 K disappeared in the initial stage of subsequent irradiation at 773 K. With increasing the dose at 773 K, the size and density of loops increased again. The behaviors of loops during 773 K irradiation were influenced not only by the dose but also by the temperature of pre-irradiation. These phenomena were analyzed using rate theory for defect clustering. Numerical calculation indicated that radiation induced resolution of small vacancy clusters brought vacancy-predominant condition temporarily by changing the temperature from low to high and resulted in shrinkage of interstitial loops.

Positron Annihilation Lifetime Measurement of Low Temperature Electron Irradiated Fe-Cr-Ni and Fe-Cr-Ni-P Alloys

Positron annihilation lifetime mesurements of the low temperature electron irradiated austenitic model alloys, Fe-16Cr-17Ni and Fe-16Cr-17Ni-O.IP, have been made, It has been shown that phosphorus atoms influence on the migration temperature of the self interstitial atoms. Moreover, it has also been shown that phosphorus atoms suppress the clustering of vacancies probably due to the precipitates Fe2P formation. The vacancy migration temperature was determined as about 350K and was not affected by the presence of phosphorus atoms, which suggestS that phosphorus atoms do not capture vacancies.

Effects of temperature variation on damage microstructures in Fe-Cr-Ni alloy induced by heavy ion irradiation

To study the effects of varying temperature on microstructural evolution, Fe-Cr-Ni austenitic model alloy was irradiated with 4 MeV Cu ions at constant temperatures of 473, 573, 773 K and stepwise temperature variation of 473/773, 573/773 K. Preirradiation at lower temperatures increased interstitial loop density in the subsequent irradiation at higher temperature, even if the dose of the former irradiation was low. However, most of the interstitial loops formed at 473 K or 573 K disappeared in the second step irradiation at 773 K. This phenomenon is not thought to be the result of thermal annealing but believed to be caused by reaction of defects or defect clusters formed at both lower temperature and higher temperature. A defect clustering model for interstitials and vacancies during irradiation indicates that the resolution of small vacancy clusters causes unexpected vacancy-rich condition temporarily in the case of temperature-variant irradiation.

A Computational Study of Defects Evolution during Irradiation in Single-Phase Austenitic Alloys

A composite model of the point defect processes and the extended defect evolution in irradiated materials was composed for a theoretical investigation of the response of fusion first wall materials to neutron bombardment. The point defects model calculates the concentration changes of single point defects including a transmutant helium, simple point defect clusters and complex clusters. The extended defect model consists of individual rate theory models of evolution of cavities, Frank faulted loops, network dislocation and other microstructural features. The model was calibrated based on the dual-ion experimental data on an Fe-15Cr-20Ni ternary austenitic model alloy. Using the calibrated model, the effects of helium generation on point defect processes, cavity nucleation, dislocation evolution and mechanical property change were investigated

The effect of titanium addition on microstructural evolution in austenitic steel irradiated with fast neutrons in FFTF

Austenitic model alloys were irradiated with fast neutrons in FFTF (Fast Flux Test Facility) to investigate fundamental mechanisms of microstructural evolution in simple austenitic steels under relatively high dose irradiation. Irradiations were performed in MOTA (Material Open Test Assembly) at 693, 793 and 873 K. Void swelling was reduced by the addition of minor elements such as titanium and carbon to a simple Fe-Cr-Ni ternary alloy at all the tested temperatures. In the titanium bearing alloys, very small cavities were detected on dislocation lines and MC precipitates. Fine dislocation loops were observed in the matrix of the irradiated materials and their size and density were not found to be affected by the solute additions.