Initial Microstructure Conditions Research Articles

Metallurgical and Mechanical Investigation on Single-Step Quenching and Partitioning Thermal Treatments on Commercial Low Alloyed 30MnV6 Steel

Characterization of the effect of different Quenching and Partitioning treatments on retained austenite stabilization, on the final microstructure, and on tensile properties was performed on 30MnV6 microalloyed steel. The amount of retained austenite and its location into the final microstructure were evaluated through X-ray diffraction and Electron Backscattered diffraction. Moreover, different initial microstructural conditions were employed to preliminarily investigate the effect of a starting mixed austenite–ferrite microstructure on the effectiveness of the treatment, and to tailor the material final tensile properties according to its actual utilization. Amounts of retained austenite between 4.5 and 6.3 pct were observed. High tensile properties were obtained, reaching 1675 MPa of UTS, with an improved elongation with respect to the reference quenched and tempered condition, which reached 14.8 pct. Hardenability is increased by the presence of retained austenite. Intercritical condition shows lower tensile strength but an increased uniform elongation with respect to the reference quenched and partitioned condition. Therefore, quenching and partitioning appears to be a promising treatment for the selected commercial steel.

Longitudinal age effects in microstructures of the hippocampal subfields.

AbstractBackgroundThe human brain undergoes considerable morphologic change with age, which is one of the primary risk‐factors for Alzheimer’s disease (AD). Age‐related microstructural changes in the brain may be quantified by advanced diffusion MRI (dMRI) with compartment modeling. In this longitudinal study, we aimed to investigate the effects of age on the hippocampal subfields, one of the brain regions vulnerable to the early stages of AD.MethodThirty‐seven cognitively normal older adults (70.6 ± 6.7 years) were recruited from the Indiana ADRC. Participants received a baseline and a follow‐up MRI scan over 24 ± 11.7 months with a Siemens 3T Prisma scanner using a 64‐channel head coil. We applied grey‐matter‐specific multi‐compartment diffusion model (cortical‐neurite orientation dispersion and density imaging) to derive diffusion microstructural metrics, namely orientation dispersion index (ODI), volume fractions of fast isotropic diffusion (VFISO), and intracellular‐space (VFIC) in the hippocampal‐subfields (CA1‐3, CA4 and dentate gyrus (CA4‐DG), and subiculum) [1]. To study the longitudinal age effects, we investigated the rate of change in the diffusion metrics and the association between the rate of change and age. Linear regression analysis was performed using SPSS software and adjusted for sex, education, APoE e4 status, and subfield volumes (Table 1).ResultAll subjects had normal cognition (i.e., MoCA and MMSE, Table 1). The volumes of CA1‐3 and subiculum significantly decreased between the baseline and follow‐up MRI scans, (p<0.05, Table 1). During this period, while significantly increased ODI within subject was observed in CA4‐DG, significantly decreased VFISO in all the subfields and increased VFIC in the subiculum were noted (Figure 1). The decreasing rate of VFISO in CA4‐DG was significantly associated with age (Table 2), suggesting the reduction in VFISO accelerated over age.ConclusionWe demonstrated that diffusion metrics are sensitive to microstructural alterations in the hippocampal subfields of cognitively normal older adults. The cornu ammonis region 4 and dentate gyrus is the most vulnerable region affected by aging with increased tissue dispersion and age‐accelerated reduction in fast isotropic diffusion. Future studies will focus on the effects of the initial microstructural condition to its rate of change with aging.

Leveraging chemical heterogeneity in steels heat treated to retain metastable austenite

The assumption of chemical homogeneity simplifies heat treatment design, particularly for processes like quenching and partitioning (Q&P) aimed at retaining metastable phases in advanced high strength steels. However, real microstructures contain chemical heterogeneities (e.g., banding from solidification or phase partitioning); these effects on final microstructures are not well understood. To investigate the role of chemical heterogeneity, inter-band and intra-band chemical heterogeneities were measured across starting ferrite/pearlite microstructures to predictively model final Q&P microstructures produced by heat treatment. Two models were developed to investigate effects across chemical bands and within prior pearlite regions, supported by experimental measurements of solute distributions. These models quantify the impact of initial microstructural condition on final microstructures with heat treatment, are applicable to other alloys and heat treatments where retaining metastable phases and solute profiles is desirable, and could be used to intentionally design heat treatment processes to exploit chemical heterogeneities and create otherwise unobtainable microstructures.

Influence of Friction and Particle Morphology on Triaxial Shearing of Granular Materials

The present paper addresses the influence of shape, size, and frictional characteristics of granular materials by utilizing X-ray computed tomography (CT) imaging and FEM. High-fidelity triaxial shearing simulations are conducted on a realistic assembly of sand grains. Applicable boundary conditions are represented, including the latex membrane for applying confining pressure. For this research, two poorly graded clean sands (SP) with distinct grain morphologies of round and angular particle shapes are considered. The effect of the particle size is naturally embedded in two materials with a difference of small size fraction grains present in angular sand. The deviatoric stress and volume change response increase up to the value of friction coefficient (μ) equal to 0.5 for rounded sand, and the strength-deformation response is unaffected by a further increase in friction. The axial strain, corresponding to peak deviatoric stress, did not depend on the friction value for identical initial microstructure and boundary conditions. The microscale investigation suggests a similar mean coordination number and percentage of grains with a coordination number less than two for friction values producing similar responses. The angular sand resulted in higher strength and low dilation compared to the rounded sand for friction value that best reproduces experimental results. The strength increase due to the particle shape effect becomes less pronounced for smooth grains. The grain-scale analysis indicates angular sand exhibits less dilation in contrast to the general observation in the literature due to smaller grain fractions absent in rounded sand, highlighting the need to introduce additional subclassifiers in classifying SP sands. The localized deformation pattern remains unchanged with friction and varies with grain shape. An attempt is made to link micromechanical insights to the macroscale response.

Effect of Initial Microstructure Conditions on the Microstructure and Mechanical Properties of Hot Extruded Mg-6Nd-2Al Alloy

Effect of Initial Microstructure Conditions on the Microstructure and Mechanical Properties of Hot Extruded Mg-6Nd-2Al Alloy

A soft sensor for property control in multi-stage hot forming based on a level set formulation of grain size evolution and machine learning

Close-die forging usually has the goal of shaping a workpiece at reduced forming forces and to set the properties for the application at hand utilizing the microstructural changes occurring at high homologous temperatures. The evolution of the property-determining microstructure can be treated as a dynamic system during hot forming. It is controlled by the temporal evolution of the temperature and velocity fields at the material points in the workpiece. Variations in the initial microstructure, friction and heat transfer conditions lead to variations in properties. When forging on screw presses, the workpiece is brought into its final shape with several blows. Here it generally seems possible to control the dynamic system of microstructure evolution over the course of the blows. Adaption of impact energy and pause times between impacts could be used to compensate the disturbances caused by, e.g., a delayed transport of the workpiece. So far, no work is known which uses the impact energy and pause time as a control variable to directly control the microstructure and property formation during forging. Instead of trying to control the microstructure evolution at each point in the workpiece, the present paper proposes to define regions of a desired microstructure state, e.g., grain size, and to track the evolution of the domain boundary. The boundary of the domain in which the target grain size is attained is extracted as a level set. This work shows the existence and proposes a fast surrogate model to predict its location as a function of the control input. The model can thus be considered a soft sensor that estimates the microstructural state based on information that can be retrieved in the process.

The effect of Zr on dynamic recrystallization during ECAP processing of Mg-Y-RE alloys

Two magnesium alloys containing yttrium and rare-earth elements were processed by equal channel angular pressing (ECAP) to explore the effect of the small addition of zirconium on the resulting microstructures. The composition of the alloys was Mg–3.8wt%Y–2.6 wt%RE–0.45 wt%Zr (WE43) and Mg–4.1wt%Y–2.8wt%Nd (WN43), respectively. It is shown that small Zr addition facilitates the formation of small secondary phase particles. The particles become smaller, and their distribution is denser. Consequently, the grain nucleation processes are more active, grain growth is suppressed, and the final grain size is smaller. The faster nucleation rate affects the texture formation during ECAP as well. It is shown that this effect is independent of the initial microstructural condition concerning both the grain size and the presence/distribution of secondary phase particles. The positive impact of Zr was proven employing scanning and transmission electron microscopy, texture measurements by X-ray diffraction and further reflected in the improved strength of the material in terms of microhardness and compression testing. It was shown that, in this way, the microstructure and texture of Mg-RE alloys could be effectively tailored by small Zr additions.

Residual Ferrite Control of 9Cr ODS Steels by Tailoring Reverse Austenite Transformation

By tailoring the reverse austenite transformation behavior of 9Cr oxide dispersion strengthened (ODS) ferritic/martensitic steels, the residual ferrite in ODS steels can be controlled. The reverse austenite transformation behavior of ODS steels is closely related to the initial microstructure conditions prior to austenite transformation. For the spark plasma sintered steels, both the amount and size of residual ferrite decrease with increasing heating rate. Nevertheless, high heating rate will increase the amount and size of residual ferrite in annealed ODS steels. As an isothermal treatment is performed at temperatures above Ac1, lower isothermal temperature has a more evident effect on the ferrite distribution in spark plasma sintered steels than that in annealed ones.

Modified kinetic model for describing continuous dynamic recrystallization behavior of Al 2219 alloy during hot deformation process

A unidirectional thermal compression test was applied to study the flow behavior and softening mechanism of Al 2219 alloy during hot deformation with a temperature range of 623 K–773 K, and strain rate range of 0.01s−1-1s−1. The true stress-strain curves obtained show that flow stress increases with the increase in strain rate and/or the decrease in temperature. Moreover, dynamic softening occurs on Al 2219 alloy during the hot deformation process. The continuous dynamic recrystallization (CDRX) of 2219 aluminum was observed using electron backscattered diffraction (EBSD) technique. Mean misorientation angle deviation and the reduction percentage in subgrain boundaries were proposed to describe recrystallization fraction qualitatively and quantitatively. The results show that the recrystallization fraction increases with the increase in temperature and the decrease in strain rate. Furthermore, a modified kinetic model of Al 2219 alloy using the specified initial microstructure and hot deformation conditions was developed, which is based on the traditional Avrami model. The modified model shows higher prediction accuracy, and also makes the steady-state strain physically significant.

The Effect of Rapid Heating and Fast Cooling on the Transformation Behavior and Mechanical Properties of an Advanced High Strength Steel (AHSS)

The major goal of this work was to study the effect of rapid heating and fast cooling on the transformation behavior of 22MnB5 steel. The effect of the initial microstructure (ferrite + pearlite or fully spheroidized) on the transformation behavior of austenite (during intercritical and supercritical annealing) in terms of heating rates (2.5, 30 & 200 °C/s) and fast cooling, i.e., 300 °C/s rate, were studied. As expected, the kinetics of austenite nucleation and growth were strongly related to the heating rates. Similarly, the carbon content of the austenite was higher at lower intercritical annealing temperatures, particularly when slower heating rates were used. The supercritical temperatures used in this study were similar to those used during commercial hot stamping operations, i.e., 845 and 895 °C, respectively, followed by a fast cooling rate. The prior austenite grain size (PAGS) was not strongly influenced by the nature of the initial microstructure, heating rate, reheating temperatures (845 or 895 °C), at 30 s holding time. The decomposition of austenite using fast cooling rates was examined. The results showed that 100% martensite was not obtained. The observed low temperature transformation products consisted of mixtures of martensite-bainite plus undissolved Fe3C carbides and small amounts of martensite-austenite (M-A). At higher supercritical temperatures, i.e., 1000 °C and 1050 °C, the final microstructure showed an increase in the volume fraction of martensite and a decrease in the volume fraction of bainite. The Fe3C and the M-A microconstituent were not observed. The best combination of tensile properties was obtained on samples reheated in the lower temperature range (845 to 895 °C). Interestingly, when the samples where reheated at the higher temperature range (1000 to 1050 °C) and fast cooled, the results of the mechanical properties did not exhibit significantly higher strength levels independent of heating rate or initial microstructural condition. This can be attributed to the change in the microstructural balance %martensite+%bainite as the reheating temperature increases. The results of this study are presented and discussed.

Effect of initial microstructure and beta phase evolution on dynamic recrystallization behaviour of Ti6Al4V alloy - An EBSD based investigation

The use of Ti6Al4V alloy in critical aerospace applications among other titanium alloys is higher due to the high strength, toughness and corrosion resistance characteristics. Achieved combination of strength and toughness is directly related to the bimodal (equiaxed alpha plus transformed beta) microstructure obtained by hot working of this alloy in a narrow strain rate and temperature domain. Present study distinctly describes the evolution of microstructure during hot deformation as an effect of process conditions and initial microstructure characteristics of Ti6Al4V alloy. The initial microstructure and processing conditions significantly influenced the evolution of β phase and dynamic recrystallization behaviour. An increase in β phase fraction was observed due to the increased adiabatic temperature at higher strain rates during α+β working. The increase in volume fraction of stable β phase as result of vanadium diffusion during hot working revealed the existence of strain induced phase transformation. Continuous dynamic recrystallization (CDRX) led to the fragmentation of initial α grains, and the effect of initial grain orientation on the CDRX mechanism was studied. The initial grains with the basal plane parallel to the compression direction revealed large fragmentation due to the high resistance to deformation. A CDRX mechanism map was developed based on the effect of processing conditions and initial grain characteristics.

Texture Modification in a Magnesium-Aluminum-Calcium Alloy During Uniaxial Compression

Texture evolution in an Mg-Al-Ca alloy at varied levels of alkaline earth solute content and phase fraction of coarse intermetallic particles has been examined. Elevated temperature uniaxial compression tests have been conducted on thixomolded AXJ810 material in three different initial microstructural conditions. These conditions were selected to sample a spectrum of solute content and intermetallic particle phase fraction. Solute content alters the populations of dislocations present after deformation, similar to effects observed in rare earth-containing alloys. The presence of coarse intermetallic particles promotes dynamic recrystallization at lower strains. Additionally, the combination of a fine initial grain size and coarse particles decorating the grain boundaries results in a more diffuse deformation texture by creating large orientation spreads within individual grains, promoting a further weakened recrystallized texture.

A model for the microstructure behaviour and strength evolution in lath martensite

A new model describing the microstructure and strength of lath martensite is introduced. The packet and block size were found to linearly depend on the prior-austenite grain size when introducing relevant crystallographic and geometric relationships of their hierarchical arrangements. A mechanism for the lath boundary arrangement within a block is postulated to ensure complete carbon redistribution to the lath boundaries. Accordingly, the dislocation density is obtained by considering the lattice distortion energy within a lath being equal to the strain energy of the dislocation density at the lath boundaries. Tempering effects are introduced by estimating the extent of carbon diffusing away from the lath boundaries; this mechanism relaxes the Cottrell atmospheres of lath dislocations and coarsens the boundaries. The yield stress as well as the microstructure evolution during tempering are successfully predicted by combining these results. The model is further extended to describe the yield stress in dual-phase steel microstructures by employing the iso-work principle. The model predictions are validated against experimental data in seven martensitic and five dual-phase steels, where the prior-austenite grain size, carbon content, tempering conditions and martensite volume fraction are employed as input. These results cover wide composition, initial microstructure and tempering conditions.

Dissolution and Precipitation Behaviour during Continuous Heating of Al–Mg–Si Alloys in a Wide Range of Heating Rates

In the present study, the dissolution and precipitation behaviour of four different aluminium alloys (EN AW-6005A, EN AW-6082, EN AW-6016, and EN AW-6181) in four different initial heat treatment conditions (T4, T6, overaged, and soft annealed) was investigated during heating in a wide dynamic range. Differential scanning calorimetry (DSC) was used to record heating curves between 20 and 600 °C. Heating rates were studied from 0.01 K/s to 5 K/s. We paid particular attention to control baseline stability, generating flat baselines and allowing accurate quantitative evaluation of the resulting DSC curves. As the heating rate increases, the individual dissolution and precipitation reactions shift to higher temperatures. The reactions during heating are significantly superimposed and partially run simultaneously. In addition, precipitation and dissolution reactions are increasingly suppressed as the heating rate increases, whereby exothermic precipitation reactions are suppressed earlier than endothermic dissolution reactions. Integrating the heating curves allowed the enthalpy levels of the different initial microstructural conditions to be quantified. Referring to time–temperature–austenitisation diagrams for steels, continuous heating dissolution diagrams for aluminium alloys were constructed to summarise the results in graphical form. These diagrams may support process optimisation in heat treatment shops.

Grain-scale experimental validation of crystal plasticity finite element simulations of tantalum oligocrystals

In this work, the grain-scale elastoplastic deformation behavior of coarse-grained body centered cubic (BCC) tantalum was simulated using a crystal plasticity finite element method (CP-FEM) and compared to experimental measurements of intragranular strain and rotation fields. To mitigate the effects of unknown subsurface microstructure, tantalum tensile specimens with millimeter-sized grains provided nearly constant microstructure through the thickness of the tensile bar. Experimental validation was performed in three ways: (1) electron backscatter diffraction (EBSD) to map intragranular rotation, (2) high-resolution digital image correlation (HR-DIC) to map the surface strain field, and (3) surface profilometry to map the out-of-plane topographic distortion. To ensure a direct apples-to-apples comparison to experiments, the details of the initial microstructure and boundary conditions were carefully replicated in the model. The deformation predictions using this novel BCC CP-FEM model for tantalum agree reasonably well with the experimental measurements. In addition, the model successfully predicted the failure location of a specimen subjected to large plastic strains. Several model parameters were explored that influence the BCC CP-FEM predictions such as the mesh dependence, the choice of active slip planes in BCC metals and the assignment of initial crystal orientations.

Experimental Study of the Interaction between Recrystallization and Precipitation Processes of an AA8011 Commercial Alloy

Phase changes in a commercial AA8011 alloy from different initial microstructure conditions were studied using thermoelectric power (ΔS), differential scanning calorimetry (DSC), and transmission electron microscopy (TEM) techniques with the purpose of obtaining evidence of the interaction between recovery-precipitation and recrystallization-precipitation processes occurring during nonisothermal heating at different rates. Thermoelectric power and its thermal derivative reflect this evidence by a displacement of the characteristic precipitation peaks, the recovery and recrystallization contributions remaining masked by the strong incidence of the iron precipitation on that property, while DSC measurements detect the emergence of new peaks not observed on thermograms of homogenized samples. An exhaustive study of these peaks permits direct differentiation between precipitation and recovery-recrystallization contributions. TEM confirms the interaction between both processes by means of local observations.

Comparison of the Annealing Behavior of Three Cold Rolled (Ti-bearing, Nb-bearing and V-bearing) HSLA Steels

ABSTRACTThe annealing behavior of three HSLA steels is studied using the combined techniques of EBSD-KAM and Sub-grain Method. These techniques have been successfully used to assess the annealing behavior of AK, IF and other high strength steels. Stored energy maps in the hot band, cold rolled and after annealing are constructed and analyzed. The combined usage of the Sub-grain Method and EBSD-KAM techniques are employed to calculate and compare the evolution of the stored energy and recrystallization behaviour during the annealing of Ti-bearing, Nb-bearing, and V-bearing HSLA steels. Orientation dependent stored energy distribution maps at different annealing stages are constructed and analysed. The results show that the stored energy distribution through the thickness of the samples is not uniform and is independent of the steel composition. Similarly the recrystallization behaviour is strongly related to the initial microstructural condition and particularly to the grain boundary character distribution of the steels.

Influence of initial conditions on the mechanical behavior of ion nitrided AISI 4340 steel

The influence of ion nitriding with surface roughness and initial microstructure on fatigue behavior and mechanical strength of AISI 4340 steel have been investigated under different process parameters including time and temperature. The first group fatigue and tension test specimens were not heat treated before ion nitriding and received in hot-rolled condition while the second group specimens were quenched and tempered. While the surface of the reduced section was being mechanically polished for the fatigue specimens, some specimens in the second group were not mechanically polished before ion nitriding to obtain the third fatigue test specimens group. While the effects of initial microstructure, surface roughness and ion nitriding conditions on fatigue behavior were investigated, the effects of initial microstructure and ion nitriding conditions on mechanical properties were investigated. First improvement in fatigue and mechanical properties were obtained with quenched and tempered. The obtained case depth of QT AISI 4340 steel is thicker than that of the hot-rolled AISI 4340 steel at all ion nitriding conditions and a linear relationship was obtained between fatigue strength of steel and case depth for both steels. Although the subsurface fatigue crack formation is dominant in failure initiation mechanism in ion nitrided steels, surface roughness effect on fatigue strength continued in a decreasing rate.

Effect of processing routes on evolution of texture heterogeneity in 2014 aluminium alloy deformed by equal channel angular pressing (ECAP)

In the present study, the effect of the processing routes (A, BA, BC and C) of equal channel angular pressing (ECAP) on texture evolution has been investigated for different initial microstructural conditions of the 2014 Al alloy. The present study also features the issue of heterogeneity in texture evolution along the thickness of the ECAP processed sample as a function of the processing routes. A stronger texture evolution was observed in the solution treated (ST) conditions for most of the processing routes (A, BA and BC except for C) due to the increased contribution from solid solution hardening during ECAP. A comparatively weak texture evolution is observed for as extruded+annealed and solutionised+aged conditions because of the strain scattering due to the fragmentation and dissolution of precipitates. The texture evolution along the thickness direction for ST samples suggests that the inherent heterogeneity of the ECAP process is minimised when processed via route C.

Modeling Flow Stress and Microstructural Development during Hot Working

A cellular automaton (CA) model was established to predict and control the microstructural evolution and flow stress characteristic during dynamic recrystallization (DRX). Values of model parameters were identified by a flow stress-based inverse analysis method; their variations with deformation conditions were estimated by the least square regression method and then integrated into the CA model. The effect of initial microstructure and deformation conditions on the microstructural evolution and flow stress behavior were investigated. The simulation agrees well with the experiment, which demonstrates the availability of the CA model.