Subgrain Size Research Articles

Effects of sintering temperature on grain growth of NiTiCu shape memory alloy

Abstract Nickel-titanium based shape memory alloys (NiTi SMAs) are extensively used in the biomedical field due to its unique properties such as superelasticity and shape memory effect. The martensite transformation can be executed in SMAs using these unique properties. The martensite transformation can be mainly influenced by the size of the grains presented in the alloy. Therefore, the control of grain size as per the requirement is a crucial in smart materials development. In this work, the nickel-titanium-copper (NiTiCu) SMA has been developed using spark plasma sintering (SPS) process at different temperatures. The sintering temperature is a significant factor that influences the size of grains in the consolidated alloys. The formation of grains in the sintered alloys has been evaluated with respect to the temperature which led to the formation of precipitates such as Ni3Ti, Ni4Ti3 and Ni3Ti2 in the SMA. The effects of sintering temperature on the grain size have been investigated using computational thermokinetics at different temperatures such as 700 °C, 800 °C and 900 °C. Moreover, the same has been carried out in experimentally and evaluated using transmission electron microscope (TEM) analysis. The results of the simulation and experiment exhibited the trend of the growth of grain with an increase of sintering temperature. In addition to these, the dislocation density, sub-grain size and recrystallized fractions were investigated on the sintered alloy.

Microstructural equivalence between bending and uniaxial creep

Recently, bending creep has been shown to facilitate the evaluation of creep properties of materials in high throughput fashion. Here, electron back-scattered diffraction analysis was used to observe steady-state substructures formed under bending and uniaxial creep tests. At a given combination of stress and temperature, the sizes of the steady-state sub-grains formed in both the cantilever and the uniaxial test samples were identical. By establishing the microstructural equivalence between bending and uniaxial creep, this study unequivocally validates bending tests for studying the creep response of materials.

Research advances on homogenization manufacturing of heavy components by metal additive forging

Heavy forgings are the core components of major equipment and even play an indispensable role in national security and the national economy. Traditionally, heavy forgings are usually manufactured with ingots of more than 100 tons. However, there are serious macroscopic segregation, shrinkage porosity and other defects in heavy forgings due to size effect of metal solidification process, which seriously affect the quality of heavy forgings and have become a worldwide problem. Avoiding the traditional concept in which a heavy forging must be created using a large steel ingot, Institute of Metal Research, Chinese Academy of Sciences first put forward a novel technology to manufacture heavy high-quality forgings by additive forging (AF). Small-sized slabs with excellent quality are used as the base elements. After surface cleaning, stacking, assembling, vacuum packaging, and deformation process characterized by pressure-forging and multi-directional forging at high temperature, the homogenized heavy forgings can be obtained with completely healed interface, which realizes the new manufacturing technology of “by making greatly small”. Through in-depth research on the microstructural evolution and bonding mechanism of the interface, we found the self-decomposition phenomenon of interface oxides. With the decomposed oxygen ions diffusing toward the matrix, oxide particles precipitated around both sides of the interface, forming the particle precipitation zone (PPZ). As the holding time increased, the width of the PPZ increased and the oxide precipitates in the PPZ transformed from MnCr x Al2– x O4 to Mn x Al3– x O4 and finally to γ-Al2O3, depending on the local oxygen activity. After holding for 24 h, the interfacial oxides completely decomposed and only a few nano-scale γ-Al2O3 oxide precipitates remained dispersed far away from the bonding interface, leading to the recovery of the mechanical properties of the bonding joints. This recovery mechanism should be of great importance to the engineering application of Additive Forging. Besides, we illustrated the dynamic recrystallization (DRX) mechnism of the bonding interface. The bonding of joints is related with interfacial grain boundary (IGB) bulging process, which is considered as a nucleation process of DRXed grain under different deformation environments. During recrystallization process, the bonded interface moved due to strain-induced boundary migration (SIBM) process. Stored energy difference (caused by accumulation of dislocations or difference of subgrain size at the bonding interface) was the dominant factor for SIBM during DRX. This technology has been applied to hydroelectric alloy steel spindles (110 tons in weight) and nuclear power stainless steel giant rings (15.6 m in diameter), which plays an important role in promoting the rapid development of high-end equipment in China and ensuring the independence and control of core materials for major equipment.

Structure characterization within the vicinity of the fine granular area by synchrotron radiation nano‐CT

AbstractThe three‐dimensional (3D) microstructures within the vicinity of the fine granular area (FGA) were investigated with synchrotron radiation nano‐computed tomography (CT). In this investigation, the very high cycle fatigue test of a high‐strength steel was carried out with an ultrasonic fatigue testing machine. Then, focused ion beam cutting was used to cut a part near the surface of the FGA. Finally, the 3D structure of the sample was obtained with the nano‐CT imaging. The results indicated that the base material was composed of various subgrains that are tens to hundreds nanometres in size. Through analysis of the slices, small granular areas consist of several subgrains. The statistical analysis of the subgrain size indicated that the subgrain size near the surface of the FGA is similar to that far from the surface of the FGA and near the fracture surface except the FGA. It indicated that the refinement has not been found by nano‐CT.

Dynamic restoration of the ferrite and austenite phases during hot compressive deformation of a lean duplex stainless steel

The active restoration processes within the ferrite and austenite phases were investigated in a Ni-free duplex stainless steel with the 75/25 austenite/ferrite ratio performing compression tests over the wide range of temperature and strain rate (700–1000 °C and 0.001–0.1s−1, respectively). The substructural features under different Zener-Hollomon parameters (Z) were examined using the electron backscatter diffraction analysis. A significant portion of flow softening was attributed to the active softening mechanisms in the continuous network of ferrite phase. The ferrite was softened through continuous dynamic recrystallization (CDRX) at high Z values, but the dynamic recovery (DRV) surmounted CDRX at low Z parameters. Interestingly, the discontinuous dynamic recrystallization (DDRX) mechanism was testified by increasing temperature and strain rate, i.e. at medium Z values. DDRX occurred by the growth of high-angle subgrains near the austenite/ferrite interfaces. With decreasing Z values, an increase in subgrain and grain sizes was observed. Contrarily to ferrite, the austenite phase was softened mostly through the dynamic recovery mechanism. At higher Z values microband arrays were formed in austenite holding specific character resembling to band-like misorientation gradients. However, under the lower Z values, a small fraction of recrystallized grains was formed through the CDRX/DDRX mechanisms. This was justified considering the lower strain accommodation in the austenite phase during deformation of duplex stainless steel.

Microstructural Equivalency between Bending and Uniaxial Creep

Recently, bending creep has been shown to facilitate the evaluation of creep properties of materials in high throughput fashion. Here, electron back-scattered diffraction analysis was used to observe steady-state substructures formed under bending and uniaxial creep tests. At a given combination of stress and temperature, the sizes of the steady-state sub-grains formed in both the cantilever and the uniaxial test samples were identical. By establishing the microstructural equivalence between bending and uniaxial creep, this study unequivocally validates bending tests for studying the creep response of materials.

Strengthening mechanisms influenced by silicon content in high temperature tempered martensite and bainite

Strengthening mechanisms influenced by silicon (Si) content during tempering of martensite and bainite between 500 and 650 °C were investigated. Microstructural features such as dislocation density, subgrain size, cementite size, and microalloy precipitate volume fraction and size were evaluated in low and high Si alloys heat treated to form bainite or martensite and tempered at 500 or 650 °C for 1 h. Higher Si contents increase the hardness of both bainite and martensite more after tempering at 500 °C, as compared to 650 °C. The increase in Si content leads to a greater increase in hardness in bainite compared to martensite after tempering at 500 °C. A strength model was used to calculate the increase in hardness from the quantitatively measured microstructural features affected by Si content. The strength model suggests that after tempering at 650 °C, increased Si content primarily increases the hardness by solid solution strengthening. However, after tempering at 500 °C, increased Si contents result in higher dislocation densities in martensite and bainite. A recovery model based on solute drag was evaluated and the results indicate that increases in Si content can delay recovery and result in higher dislocation densities after tempering at 500 °C. Increases in Si content also lead to finer cementite sizes in bainite after tempering at 500 °C. A cementite coarsening model was evaluated, and the results indicate that cementite refinement in bainite tempered at 500 °C cannot be accounted for by Si diffusion controlled coarsening alone. The lower transformation temperature employed to form high Si bainite, indirectly influenced by Si decreasing the martensite start temperature, is suggested to result in refinement of both cementite and subgrain size after tempering at 500 °C. Microalloy precipitation is not significantly affected by Si content in the present study.

Microstructure and mechanical property of electropulsing tempered ultrafine grained 42CrMo steel

Microstructure and mechanical property of 42CrMo steel after electropulsing (EP) treatment were studied. The average prior austenite grains (PAGs) size was refined to 2.9 μm after EP austenitization, compared with conventional austenitization. EBSD test results indicated the size of sub-grains with high angle boundaries was refined from 1.53 μm to 0.53 μm. The quenched 42CrMo steel with ultrafine PAGs was tempered by EP with peak temperatures ranging from 524–670 °C. Carbides precipitated during EP tempering within the total duration less than 20s. The size of carbides was refined. Combined with grain refinement strengthening and higher precipitation strengthening effects, the strength of EP treated 42CrMo steel was dramatically improved. The effects of PAGs ultra-refinement on martensitic structures were discussed. And the reasons for carbides refinement were analyzed. One reasonable reason was due to the PAGs ultra-refinement. Another reason should be attributed to the athermal effect of high density pulse current.

A Model for Static Recrystallization through Strain-Induced Boundary Migration

Control over the inelastic deformation of polycrystals aimed at achieving desired performance characteristics of finished products is impossible without analyzing the structural evolution of the material at different scale levels. A major task in this respect is to develop physically sound mathematical models for describing the internal structure evolution as it determines the material properties. An effective tool for addressing this task is a multilevel approach to describing the thermomechanical processing of crystalline materials, in which carriers and physical mechanisms of processes are considered explicitly. Thermomechanical processing allows a good control over the defect and grain/subgrain structure of the material to achieve desired macrocharacteristics. This paper explores the subgrain/grain structure evolution during static recrystallization that occurs through strain-induced boundary migration. Modeling is carried out in two stages. The first stage models the plastic deformation of a crystalline material at room temperature. The second stage considers holding at an elevated temperature resulting in recrystallization. Both stages are modeled within a unified multilevel approach. The problem of modeling static recrystallization is formulated, and an algorithm for its numerical implementation is described. The modeling results are obtained for a bicrystal in which each grain is represented by a group of lower-scale elements (subgrains). Recrystallization causes profound changes in the subgrain structure geometry, the average size of subgrains, and their shape change. Recrystallized subgrains are more elongated compared to the original ones. During recrystallization more defective grains are replaced with less defective grains, and as a result the energy stored in the material decreases. The developed model qualitatively describes the release of stored energy. Numerical experiments revealed a critical plastic strain value below which recrystallization does not occur.

EBSD Investigation to Study the Restoration Mechanism and Substructural Characteristics of 23Cr–6Ni–3Mo Duplex Stainless Steel During Post-deformation Annealing

In the present study, microstructural analysis of post-deformation annealed sample has been carried out for 23Cr–6Ni–3Mo duplex stainless steel. It was observed that the restoration mechanism and substructural characteristics were different for austenite and ferrite during post-deformation annealing. In the initial stage of post-deformation annealing, the restoration mechanism of austenite envisaged meta-dynamic recrystallization (MDRX), while the dominating mechanism of softer ferrite showed static recovery (SRV) with a minimal of static recrystallization (SRX). In the intermediate stage, the austenite showed SRX followed by coarsening of meta-dynamically recrystallized grains, and ferrite revealed mostly SRV. The last stage of post-deformation annealing showed the disintegration of substructures in both austenite and ferrite. The microstructure after 20 min of post-deformation annealing displayed coarse subgrains. Finer austenite subgrains were observed as compared to ferrite subgrains after the post-deformation annealing of 20 min. High-resolution EBSD was used to study substructural characteristics of microstructure, and post-processing of microstructural results was made by HKL Channel 5 software. Misorientation angle distribution, subgrain size as well as restoration mechanism corresponding to both austenite and ferrite were determined over intensive sample area.

A Multiple Length-Scales Nanoimprinting Approach on Nanocrystalline and Strongly Deformed CuZn30 Alloys

Metallic Nanoimprinting is a new approach to form robust surface structures on metals at various length scales. The shape and size of the formed structures not only depends on the dimensions of the Nanoimprinting die but also the mechanical behaviour of the imprinted material and its microstructure. To characterise the Nanoimprinting process, a multi length-scale-approach was used by varying the cavities (widths between 20 nm and 2.76 µm) as well as the microstructure of the alloy. CuZn30 was used in different cold-worked and heat-treated conditions, with grain sizes from 100 nm up to 277 µm, thus, covering a wide range of hardening behaviours and grain size to cavity width ratios. Experimental results show that the work hardening behaviour as well as the subgrain or grain size have a major influence on the forming characteristics during Nanoimprinting and a nearly ideal plastic behaviour (no work hardening) leads to the largest extrusion heights. For materials with a pronounced work hardening, low extrusion heights were measured for all cavity widths. This work demonstrates the potential of a simple imprinting process to generate surface features on metallic materials with a width <300 nm and an aspect ratio >1.

Effects of inter layer time and build height on resulting properties of 316L stainless steel processed by laser powder bed fusion

Laser powder bed fusion (L-PBF) is the most prominent additive manufacturing (AM) technology for metal part production. Among the high number of factors influencing part quality and mechanical properties, the inter layer time (ILT) between iterative melting of volume elements in subsequent layers is almost completely unappreciated in the relevant literature on L-PBF. This study investigates the effect of ILT with respect to build height and under distinct levels of volumetric energy density (VED) using the example of 316L stainless steel. In-situ thermography is used to gather information on cooling conditions during the process, which is followed by an extensive metallographic analysis. Significant effects of ILT and build height on heat accumulation, sub-grain sizes, melt pool geometries and hardness are presented. Furthermore, the rise of defect densities can be attributed to a mutual interplay of build height and ILT. Hence, ILT has been identified as a crucial factor for L-PBF of real part components especially for those with small cross sections.

Flow modelling of Ti6Al4V under large strains

This work uses flow stresses obtained experimentally at different strain rates and temperatures to validate flow modelling results. Flow curves of Ti6Al4V are measured via torsion experiments with a Gleeble® 3800 up to effective strains of 8. A physically based model that describes the evolutions of microstructure and the flow stress in the β-phase field was developed. A model of continuous dynamic recrystallization (CDRX) based on the work of Gourdet and Montheillet [1] for aluminium alloys is combined in this work with elements taken from Kocks and Mecking [2]. The model consists of a detailed description of the microstructure, based on different dislocation density populations and grain boundaries. All these internal variables evolve according to a production and a recovery term correlated mathematically with the temperature and the strain rate. The modelled output variables besides the flow stress are the total, the interior and the wall dislocation densities as well as the subgrain and grain sizes developed by continuous dynamic recrystallization. The model describes the softening occurring during large strain deformations, which is partly produced by the formation of new high angle grain boundaries (HAGB). The fraction of HAGB was used to determine the recrystallization grade, validated with microstructural characterization.

Microstructure evolution of ferrite during intercritical deformation in low carbon microalloyed steels

Effects of strain, deformation temperature and strain rate on microstructure evolution of ferrite during intercritical deformation in a low carbon microalloyed steel have been studied via electron back-scattered diffraction analysis. The result shows that continuous dynamic crystallisation (CDRX) can be induced by a small strain of 0.25 when deformation temperature is 750°C and strain rate is 0.1 s−1. Increasing deformation temperature or reducing strain rate prompt the CDRX. However, CDRX is remarkably enhanced when deformation temperature is larger than 780°C even under large strain rate condition. Both the equiaxed grain sizes and sub-grain sizes of intercritically deformed microstructures are smaller than 3 μm at various process parameters. Intercritical deformation is an effect way to ultra-refine microstructure for steels.

Microstructure prediction in low and high strain deformation of Al6063 using artificial neural network and finite element simulation

The final microstructure of materials under interactive effects of critical elements of deformation: strain, strain-rate, and temperature, often follows complex trajectories. Capturing the existing process-structure linkages is fundamental for controlling product outcomes, yet it calls for establishing the constitutive relationships that describe material behavior. In this paper, a backpropagation Artificial Neural Network (ANN) is proposed for microstructure prediction for a wide range of strain, strain-rate, and temperature conditions. Microstructural changes in Al6063 are experimentally examined using (i) quasi-static universal testing apparatus, Drop Weight Impact Tester (DWIT), and Split Hopkinson Pressure Bar (SHPB) tests for low strain regimes at elevated temperatures, and (ii) Plane Strain Machining (PSM) for high strain, high strain-rate, and the accompanied temperature rise conditions. Two ANNs are established to predict microstructure responses, grain and subgrain sizes, in low and high strain regimes, respectively. Additionally, the stress-strain results obtained from the low strain regime are used to calculate the Johnson–Cook (J-C) material model constants, which are then incorporated in the finite element (FE) simulation along with the developed ANN algorithm, to predict microstructure response for different cutting conditions. The performance of the ANNs and the FE simulations was evaluated using statistical indices. The comparative assessment of the models’ outcomes indicates close agreements with the experimental results in both low- and high-level deformations. The accurate predictions from PSM conditions can potentially be applicable for microstructural prediction of the machined surface.

Microstructure and Crystallographic Texture of Silicon Iron Modified by Torsion Under Quasihydrostatic Pressure

The electron backscatter diffraction, X-ray diffraction analysis, electromotive force instantaneous measurement, microhardness and coercive force measurement techniques are used to explore the development of the microstructure, crystallographic texture and physico-mechanical properties of silicon iron (Fe-3% Si) alloy under quasi-hydrostatic pressure in a Bridgman anvil. It is found that the alloy deformation is accompanied by its significant hardening. In-plane torsion test shows that the average grain size rapidly decreases down to 185 nm after a quater turn of the movable anvil. With the number of turns increased up to six the grain size gradually decreases down to 150 nm. At the same time, the average size of subgrains is less dependent on the deformation degree. The crystallographic texture evolution involves the formation of the strong {001} texture component after comparatively small megaplastic deformation followed by the simple shear texture formation. Such a behavior can be correctly described in terms of non-equilibrium thermodynamics.

Macro- and Micromechanical Behavior of 316LN Lattice Structures Manufactured by Electron Beam Melting

This work focuses on the possibility of processing stainless steel 316LN powder into lightweight structures using electron beam melting and investigates mechanical and microstructural properties in the material of processed components. Lattice structures conforming to ISO13314:2011 were manufactured using varying process parameters. Microstructure was examined using a scanning electron microscope. Compression testing was used to understand the effect of process parameters on the lattice mechanical properties, and nanoindentation was used to determine the material hardness. Lattices manufactured from 316L using EBM show smooth compression characteristics without collapsing layers and shear planes. The material has uniform hardness in strut shear planes, a microstructure resembling that of solid 316LN material but with significantly finer grain size, although slightly coarser sub-grain size. Grains appear to be growing along the lattice struts (e.g., along the heat transfer direction) and not in the build direction. Energy-dispersive x-ray spectroscopy analysis reveals boundary precipitates with increased levels of chromium, molybdenum and silicon. Studies clearly show that the 316LN grains in the material microstructure are elongated along the dominating heat transfer paths, which may or may not coincide with the build direction. Lattices made from a relatively ductile material, like 316LN, are much less susceptible to catastrophic collapse and show an extended range of elastic and plastic deformation. Tests indicate that EBM process for 316LN is stable allowing for both solid and lightweight (lattice) structures.

Microstructure and texture evolution of pure molybdenum during hot deformation

Microstructure and texture evolution of pure molybdenum prepared by powder metallurgy during the thermal mechanical processing were studied. Uniaxial compression tests were carried out on Gleeble-1500 thermal mechanical simulator in range of temperature (1100 ºC-1300 °C), strain rates (0.1 s−1-10 s−1) and true strain (0.3–0.6), respectively. The results show that the difference in deformation mechanism caused by temperature, strain rate and true strain has a great influence on microstructure and texture. Especially at 1300 °C, pure molybdenum will have obvious geometric dynamic recrystallization, and as the deformation progresses, the jagged high angle grain boundaries are close to each other and forming an equiaxed crystal. This equiaxed crystal structure is separated by a high angle grain boundary and the size of the equiaxed crystal is close to the subgrain size. In addition, this paper focuses on the deformation behavior of pure molybdenum under different true strains. There are two typical orientations of fiber texture. One is <100>//CD fiber texture, including {001} <100> cube texture and {110} <001> goss texture; other is <111>//CD fiber texture, including {112} <111> copper texture and {110} <111> texture. The increased distortion of the deformation provides a certain driving force for the rotation of the grain, prompting it to the preferred slip system, thereby weakening the texture resulting from the deformation. What’s more, the dynamic recrystallization behavior can significantly weaken deformed textures, especially for <111>//CD fiber texture.

Mechanical properties and thermal stability of an ultrafine-grained near β titanium alloy

Comparative studies of the structure, mechanical properties and thermal stability of an ultrafine-grained near β titanium alloy of the transition class Ti-5Al-5Mo-5V-1Cr-1Fe obtained by the methods of multiple pressing and radial-shear rolling with subsequent aging have been carried out. It is shown that as a result of multiple pressing, the formation of a homogeneous micro-duplex ultrafine-grained structure with an average grain (phase) size of ∼0.2 μm is observed. Radial shear rolling followed by aging leads to the formation of an inhomogeneous structure with precipitates of the α phase particles and a subgrain size of the β phase of about 0.5 μm inside which thin-acicular precipitates of the metastable α” phase are observed. Despite the smaller imposed plastic deformation and the larger size of grain-subgrain structure, alloy samples after radial-shear rolling and aging demonstrate higher strength properties at room temperature compared to the alloy obtained by multiple pressing (the ultimate strength of the alloy after these treatments is 1695 and 1600 MPa, respectively.) At the same time, the alloy obtained by the method of multiple pressing shows higher ductility and thermal stability of the mechanical properties due to the micro-duplex ultrafine-grained structure. The drop in strength as a result of annealing is due to the development of recrystallization in the case of the alloy after pressing and the dissolution of the strengthening α” phase in the case of the alloy after radial-shear rolling and aging.

Subgrain dynamics during recovery of partly recrystallized aluminum

The recovery dynamics of 498 individual subgrains within the non-recrystallized matrix of a 50% recrystallized aluminum sample is studied using high resolution X-ray diffraction at a synchrotron source. The subgrains’ change in size, orientation, axial strain and internal disorder are monitored as functions of temperature. The subgrains are surprisingly active, and the individual subgrains exhibit markedly different growth kinetics. Contradictory to common knowledge we find that the individual subgrains’ sizes change significantly even though the average size remain constant. Furthermore we observe no correlation between subgrain size and growth rate. An analysis of the volume changes of the individual subgrains indicates a broad spectrum of critical temperatures above which the subgrains grow or shrink. Possible mechanisms are discussed and related to relaxation of local stresses.