Subsequent Microstructural Evolution Research Articles

Single crystal growth of CoTi2O5 by solid state reaction synthesis

AbstractMaterials in single crystal form are often sought after because the absence of grain boundaries can result in unique properties relative to the polycrystal, but producing these materials is typically a slow and complex process. In this work, pseudo single crystals of the pseudobrookite compound CoTi2O5 were synthesized by solid‐state reaction from a duplex grain mixture of CoTiO3 and TiO2. The size of the crystallites was >250 µm. The transformation and subsequent microstructural evolution of the CoTi2O5 was studied by scanning electron microscopy (SEM), electron backscatter diffraction (EBSD), transmission electron microscopy (TEM), scanning transmission electron microscopy (STEM), and X‐ray diffraction (XRD). A novel growth mechanism was identified whereby a single crystal CoTi2O5 front advances simultaneously along multiple CoTiO3/TiO2 diphasic boundaries. The single crystal domains were composed of subgrains approximately 5 µm in diameter; differences in the subgrain size and misorientation were related to the growth mechanism and the initial grain size of the duplex CoTiO3–TiO2 mixture. CoTi2O5 is a little characterized compound, and this study represents the most significant microstructural study of CoTi2O5 to date. The findings may be applied to similar pseudobrookite compounds such as MgTi2O5 and Al2TiO5.

On the influence of deformation mechanism during cold and warm rolling on annealing behavior of a 304 stainless steel

In the present study, a 304 stainless steel was deformed at different temperatures of room temperature (cold rolling) and 200 °C (warm rolling) for elucidating the distinct deformation mechanisms, and then, the influence of the deformed microstructures on the subsequent microstructural evolution and texture development during annealing treatments was studied. The results indicated that deformation-induced martensite transformation was the dominant deformation mechanism during cold rolling, and a mixed microstructure consisting of ~60% volume fraction of α'-martensite and deformed austenite was obtained. Deformation twinning of austenite dominated the deformation during warm rolling. The underlying reason was the increasing SFE with deformation temperature, which inhibited the martensitic transformation but promoted the formation of deformation twins. On annealing, the recrystallization of retained austenite in cold-rolled samples was retarded because of martensitic reversion compared to the warm-rolled samples. Completely recrystallized structure with average grain size of ~0.7 µm was obtained in both cold and warm-rolled samples after annealing at 800 °C. Texture evolution indicated that martensitic reversion enhanced the intensity of {110}< 112 > texture in cold-rolled and annealed samples. However, the final recrystallization texture components were consisted of {110}< 112 > and {113}< 211 > regardless of rolling process. Furthermore, structure-property relationship was established, and combination of high strength and excellent plasticity can be obtained in both cold and warm-rolled sheets by tuning the subsequent annealing parameters.

Strain-rate-dependent deformation behaviour of high-carbon steel in compression: mechanical and structural characterisation

Dual-phase high-carbon steels are of significant interest in mining industries particularly in comminution and rock handling applications where the strain rate varies from very low to very high. The effect of quasi-static strain rate on the deformation behaviour of austenite–martensite high-carbon low-alloy steel is investigated for compression loading in this paper. Experiments were conducted at different compressive strain rates (2.56 × 10−4 to 2.56 × 10−1 s−1), and the subsequent microstructural evolution was characterised by optical microscopy, X-ray diffraction (XRD), scanning electron microscopy and electron backscatter diffraction (EBSD) techniques to establish the structure–property correlation. The experimental results indicated an increase in the yield strength (σy) with the increase in the strain rate due to the rate-dependent dislocation velocities. The strain hardening rate of the material exhibited a decreasing trend with an increase in the true strain values for all the applied strain rates. XRD results indicated the phenomenon of deformation-induced martensitic transformation (DIMT) to be rate dependent, whereas EBSD results showed an increase in the Kernel average misorientation values with increase in the strain rate. The volume fraction of retained austenite was observed to be decreasing with an increase in the engineering strain values irrespective of the applied strain rate. The microscopic features of the fracture surfaces showed the presence of transgranular cracking at low strain rates, whereas predominant intergranular cracks were found at higher strain rates. TEM results indicated the decrease in the lath martensites with increase in the strain rate.

Mechanisms of spontaneous chain formation and subsequent microstructural evolution in shear-driven strongly confined drop monolayers.

It was experimentally demonstrated by Migler and his collaborators [Phys. Rev. Lett., 2001, 86, 1023; Langmuir, 2003, 19, 8667] that a strongly confined drop monolayer sheared between two parallel plates can spontaneously develop a flow-oriented drop-chain morphology. Here we show that the formation of the chain-like microstructure is driven by far-field Hele-Shaw quadrupolar interactions between drops, and that drop spacing within chains is controlled by the effective drop repulsion associated with the existence of confinement-induced reversing streamlines, i.e., the swapping trajectory effect. Using direct numerical simulations and an accurate quasi-2D model that incorporates quadrupolar and swapping-trajectory contributions, we analyze microstructural evolution in a monodisperse drop monolayer. Consistent with experimental observations, we find that drop spacing within individual chains is usually uniform. Further analysis shows that at low area fractions all chains have the same spacing, but at higher area fractions there is a large spacing variation from chain to chain. These findings are explained in terms of uncompressed and compressed chains. At low area fractions most chains are uncompressed (spacing equals lst, which is the stable separation of an isolated pair). At higher area fractions compressed chains (with tighter spacing) are formed in a process of chain zipping along y-shaped structural defects. We also discuss the relevance of our findings to other shear-driven systems, such as suspensions of spheres in non-Newtonian fluids.

An Alternative to Avrami Equation

This paper proposes an alternative to the Avrami equation capable of describing whole transformation curves with significant fitting-correlations. The model bears physically meaningful parameters which permit considering the initial transformation kinetics independently from the subsequent microstructural evolution. Data of martensite, bainite, recrystallization, and pearlite transformations validate the model. Further to the expeditious description of transformation curves, the model guides the modeling of specific mechanisms.

A simplified approach to predict performance degradation of a solid oxide fuel cell anode

The agglomeration of nickel (Ni) particles in a Ni-cermet anode is a significant degradation phenomenon for solid oxide fuel cells (SOFCs). This work aims to predict the performance degradation of SOFCs due to Ni grain growth by using a simplified approach. Accelerated aging of Ni-scandia stabilized zirconia (SSZ) as an SOFC anode is carried out at 900 °C and subsequent microstructural evolution is investigated every 100 h up to 1000 h using scanning electron microscopy (SEM). The resulting morphological changes are quantified using a two-dimensional image analysis technique that yields the particle size, phase proportion, and triple phase boundary (TPB) point distribution. The electrochemical properties of an anode-supported SOFC are characterized using electrochemical impedance spectroscopy (EIS). The changes of particle size and TPB length in the anode as a function of time are in excellent agreement with the power-law coarsening model. This model is further combined with an electrochemical model to predict the changes in the anode polarization resistance. The predicted polarization resistances are in good agreement with the experimentally obtained values. This model for prediction of anode lifetime provides deep insight into the time-dependent Ni agglomeration behavior and its impact on the electrochemical performance degradation of the SOFC anode.

In Situ Characterization of Inconel 718 Post-Dynamic Recrystallization within a Scanning Electron Microscope

Microstructure evolution within the post-dynamic regime following hot deformation was investigated in Inconel 718 samples with different dynamically recrystallized volume fractions and under conditions such that no δ-phase particles were present. In situ annealing treatments carried out to mimic post-dynamic conditions inside the Scanning Electron Microscope (SEM) chamber suggest the occurrence of both metadynamic and static recrystallization mechanisms. Static recrystallization was observed in addition to metadynamic recrystallization, only when the initial dynamically recrystallized volume fraction was very small. The initial volume fraction of dynamically recrystallized grains appears to be decisive for subsequent microstructural evolution mechanisms and kinetics. In addition, the formation of annealing twins is observed along with the growth of recrystallized grains, but then the twin density decreases as the material enters the capillarity-driven grain growth regime.

The effect of silicon nitride powder characteristics on SiAlON microstructures, densification and phase assemblage

The surface characteristics, particle size distribution and impurities of starting Si3N4 powders exert a very significant influence on the microstructure of sintered silicon nitride based ceramics. Even a change of the processing conditions such as milling liquid media (water or isopropyl alcohol) and milling time can have a substantial effect on particle surface groups, and hence on the microstructure of sintered samples. In this study, SEM, XRD, FTIR, BET, elemental analysis and laser diffraction techniques were used for the comprehensive characterization of Si3N4 powders which were produced by diimide, direct nitridation and combustion synthesis, in as received state, and after milling in different liquid media (aqueous or alcohol), for various milling durations. The correlation of the surface characteristics and properties of the Si3N4 powders with sintering behavior, and microstructural evolution, densification and phase assemblages of the resulting SiAlON ceramics were reported. The milling conditions affected the surface chemistry of Si3N4 powders and the subsequent microstructural evolution. The microstructures evolved from the coarser β-Si3N4 powders were coarser, but the fine β-Si3N4 powders yielded a bimodal microstructure. The critical particle diameter of the β-Si3N4 powder for the formation of needle like SiAlON grains was determined to be less than 0.5 µm.

Process optimisation and microstructural evolution of friction stir spot-welded Al6061 joints

The joint strength and subsequent microstructural evolution of a friction stir spot-welded AA6061-T6 alloy was investigated according to the process parameters: tool rotation speed, dwell time and pin angle of the tool. A maximum tensile shear load of 2.78 kN was obtained from the joints generated under a combination of process variables like 1000 and 1500 rev min−1, 5 s, 5°. Under a fixed pin angle of 5° and a rotational speed of 1000 rev min−1, an increase in dwell time from 1 to 5 s resulted in a considerable increase in tensile shear load. An empirical process map under a fixed tool design is proposed to determine a feasible range of process conditions.

Microstructure Evolution of Biphasic TiNi1+x Sn Thermoelectric Materials

The effects of thermal treatment on the microstructure of biphasic materials comprising half-Heusler (hH) and full-Heusler (fH) phases, as well as on their associated thermal conductivity, are discussed. The focus of this study was on a biphasic hH/fH alloy of nominal stoichiometry TiNi1.2Sn, synthesized by containerless (magnetic levitation) induction melting. The alloy samples were exposed to various heat treatments to generate microstructures containing second-phase precipitates ranging in size from ~10 nm to a few micrometers. The materials were characterized with regard to morphology, size, shape, and orientation relationship of the fH and hH phases, both of which were present as precipitates within larger regions of the counterpart phase. The solidification path of the alloy and its implications for the subsequent microstructure evolution during heat treatment were elucidated, and relationships with the ensuing thermal conductivity were characterized.

Physical mechanisms of copper-copper wafer bonding

The study of the physical mechanisms driving Cu-Cu wafer bonding allowed for reducing the bonding temperatures below 200 °C. Metal thermo-compression Cu-Cu wafer bonding results obtained at such low temperatures are very encouraging and suggest that the process is possible even at room temperature if some boundary conditions are fulfilled. Sputtered (PVD) and electroplated Cu thin layers were investigated, and the analysis of both metallization techniques demonstrated the importance of decreasing Cu surface roughness. For an equal surface roughness, the bonding temperature of PVD Cu wafers could be even further reduced due to the favorable microstructure. Their smaller grain size enhances the length of the grain boundaries (observed on the surface prior bonding), acting as efficient mass transfer channels across the interface, and hence the grains are able to grow over the initial bonding interface. Due to the higher concentration of random high-angle grain boundaries, this effect is intensified. The model presented is explaining the microstructural changes based on atomic migration, taking into account that the reduction of the grain boundary area is the major driving force to reduce the Gibbs free energy, and predicts the subsequent microstructure evolution (grain growth) during thermal annealing.

Alloy and process design of thermo-mechanically processed multiphase ductile iron

Abstract This work highlights the transformation kinetics, microstructure evolution, hardness and compression properties of four thermo-mechanically processed ductile irons (DIs) having from 0 to 1.7 wt.% aluminum. The DIs are subjected to different true strain values of 0, 0.3 and 0.5 by deformation in the austenite region. Additionally, four types of matrix were produced, namely martensitic, martensitic–ferritic, ausferritic and ferritic–ausferritic. The ferrite introduction is accomplished by isothermal holding in the intercritical region. Aluminum increase widened the intercritical region and shifted it to higher temperature range. The former effect rendered the intercritical annealing more controllable. The latter caused substantial discrepancy in the chemistry of the intercritical austenite by varying the aluminum content. The subsequent transformation kinetics, microstructure evolution and mechanical properties of the intercritically annealed DI are governed by the chemistry of the intercritical austenite. Whereas, those with fully martensitic and fully ausferritic matrices are governed by the aluminum variation. It is also shown that the kinetics of ausferrite formation is accelerated by both increasing the deformation and introducing ferrite to the matrix. The strength and hardness are increased by the former and declined by the latter factor. The fracture strain has not shown a continual increase by increasing the ferrite content.

Recent radiation damage studies and developments of the Marlowe code

Radiation damage in materials relevant to applications evolves over time scales spanning from the femtosecond – the characteristic time for an atomic collision – to decades – the aging time expected for nuclear materials. The relevant kinetic energies of atoms span from thermal motion to the MeV range.The question motivating this contribution is to identify the relationship between elementary atomic displacements triggered by irradiation and the subsequent microstructural evolution of metals in the long term. The Marlowe code, based on the binary collision approximation (BCA) is used to simulate the sequences of atomic displacements generated by energetic primary recoils and the Object Kinetic Monte Carlo code LAKIMOCA, parameterized on a range of ab initio calculations, is used to predict the subsequent long-term evolution of point defect and clusters thereof. In agreement with full Molecular Dynamics, BCA displacement cascades in body-centered cubic (BCC) Fe and a face-centered cubic (FCC) Fe\\bond Ni\\bond Cr alloy display recursive properties that are found useful for predictions in the long term.The case of defects evolution in W due to external irradiation with energetic H and He is also discussed. To this purpose, it was useful to extend the inelastic energy loss model available in Marlowe up to the Bethe regime. The last version of the Marlowe code (version 15) was delivered before message passing instructions softwares (such as MPI) were available but the structure of the code was designed in such a way to permit parallel executions within a distributed memory environment. This makes possible to obtain N different cascades simultaneously using N independent nodes without any communication between processors. The parallelization of the code using MPI was recently achieved by one author of this report (C.J.O.). Typically, the parallelized version of Marlowe allows simulating millions of displacement cascades using a limited number of processors (<64) within only few hours of CPU time.

Plane wave simulation of elastic-viscoplastic single crystals

Despite the large amount of research that has been performed to quantify the high strain rate response of Aluminum, few studies have addressed effects of crystal orientation and subsequent crystal-level microstructure evolution on its high strain rate response. To study orientation effects in single crystal Al, both a constitutive model and novel numerical method have been developed. A plane wave formulation is developed so that materials undergoing anisotropic viscoplastic deformation can be modeled in a thermodynamically consistent framework. Then, a recently developed high strain rate viscoplastic model is extended to include single crystal effects by incorporating higher order crystal-based thermoelasticity, anisotropic plasticity kinetics, and distinguishing influences of forest and parallel dislocation densities. Steady propagating shock waves are simulated for [100], [110], and [111] oriented single crystals and compared to existing experimental wave profile and strength measurements. Finally, influences of initial orientation and peak pressure ranging from 0 to 30GPa are quantified. Results indicate that orientation plays a significant role in dictating the high rate response of both the wave profile and the resultant microstructure evolution of Al. The plane wave formulation can be used to evaluate microstructure-sensitive constitutive relations in a computationally efficient framework.

Effect of strain path on dynamic strain-induced transformation in a microalloyed steel

Dynamic strain-induced transformation (DSIT) in a low-carbon microalloyed steel was studied by hot cyclic torsion to understand the interactions between DSIT and strain path reversals, and the subsequent microstructure evolution when subjected to continuous cooling. The critical strain for DSIT (εc,DSIT) can be determined by analysing the dynamic softening of the flow stress–strain curves. When deformed to the same total accumulative strain of 2.0, deformation with a small strain amplitude in each pass and multiple strain path reversals led to the suppression of DSIT compared to the extensive DSIT ferrite produced by deformation with a large strain amplitude and a single strain reversal. The results reveal that the amplitude of monotonic strain, not the total accumulative strain, in relation to εc,DSIT determines the occurrence of DSIT. The suppression or promotion of DIST can be attributed mainly to the increment of the austenite grain boundary area associated with deformation, especially the development of serration and bulging, and, to a lesser extent, to the generation of high-angle boundaries by austenite grain subdivision. The evolution of these planar defects, which are believed to be the primary ferrite nucleation sites, is strongly influenced by strain path changes, and lead to significantly different DSIT behaviours. DSIT ferrite also showed very limited coarsening after continuous cooling as the ongoing deformation produces further nucleation sites in the austenite matrix and causes orientation variation of the DSIT ferrite inherited from austenite parent grains. Based on these observations, it is believed that the transformation mechanisms for DSIT are essentially the same as the reconstructive mechanism during static phase transformations.

Effect of milling temperature on nanoclusters and ultra fine grained microstructure of oxide dispersion strengthened steel

The effect of ball milling temperature on the mechanical alloying and subsequent microstructural evolution of ferritic oxide dispersion strengthened steel was studied. The mixtures of Fe–14Cr–3W–0.4Ti powder and Y2O3 were ball milled for 40 h at room-temperature, −70 °C, and −150 °C. As milling temperature decreased, the fraction of low angle grain boundaries increased. Final grain size after consolidation was greatly affected by the ball milling condition. The size of nanoclusters seems to be dependent on milling temperature too. Ultra-fine grain size with homogenous dispersion of Y–Ti–O nanoclusters was achieved by very low temperature milling.

Modeling of peritectic coupled growth in Cu–Sn alloys

In directional solidification experiments on hypoperitectic Cu–Sn alloys at low velocity and high thermal gradient, both lamellar and fibrous coupled peritectic growth patterns have been observed. Two phenomena that had not been observed in previous experiments on other alloy systems are investigated here with the help of different modeling approaches. The mean volume fraction of primary phase α, g¯α, as determined by X-ray microtomography, decreases with solidification distance over the entire length of the coupled zone, but is always much larger than that expected from the equilibrium phase diagram. Moreover, oscillations in g¯α with a spatial periodicity approximately equal to the lamellar spacing are also observed. The first observation is explained semi-quantitatively by a simple 1D diffusion model, which reveals that the onset of coupled growth occurs during the initial transient of the primary phase planar front growth. A two-dimensional phase-field model is used to monitor the subsequent microstructure evolution, and shows that the lamellar structure exhibits collective 1-λ oscillations. In agreement with previous studies, it was found that these oscillations lead to stable coupled growth only for a limited range of the control parameters.

A molecular dynamics study on the orientation, size, and dislocation confinement effects on the plastic deformation of Al nanopillars

Molecular dynamics are employed to simulate the compression process of Al nanopillars with different orientations. The simulations show that the initial dislocations always nucleate at free surfaces but the compression orientation plays a decisive role in the subsequent microstructural evolution and stress–strain response of the pillars. For higher symmetry orientations of [001] and [111], frequent dislocation interactions make the dislocation-starvation state not easily achievable, and the more mean-field interaction condition leads to less serrated stress–strain response. Lower symmetry orientations behave in the opposite way. Simulation of the presence of a rigid coating on the pillar’s surface also shows that dislocations are trapped by the coating with smooth strain-hardening after the initial dislocations are generated. The results show that whether the dislocation-starvation state can be achieved is a crucial factor governing the stress–strain response of small crystals.

Gleeble experiments concerning dendrite re-melting and its role in microstructural evolution in spray formed high-alloy metals

An experimental approach was designed to understand dendrite fragmentation of atomized droplets deposition in spray materials, and to study the relationship between dendrite fragmentation behavior and subsequent microstructural evolution. A Gleeble 3500 physical simulator was used to create controlled thermal shock conditions in solid-liquid mixtures of Superalloy GH4169 and High Speed Steel (HSS) T15 atomised powders, which simulated the environment of droplet deposition during the spray forming of large diameter billets at Beijing Institute of Aeronautical Materials (BIAM). The experiment results support that the rapid remelting and thermal shock play an important role in the development of the fine equiaxed microstructure in spray forming high-alloy metals.

Modelling and Experiments Concerning Dendrite Re-Melting and Its Role in Microstructural Evolution in Spray Formed Ni Superalloys

This paper presents a combined experimental and modelling approach to understand dendrite fragmentation of atomised metal alloy droplets during deposition in spray forming, and to study quantitatively the relationship between this dendrite fragmentation behavior and subsequent microstructural evolution. A Gleeble 3500 physical simulator was used to create controlled thermal shock conditions in solid-liquid mixtures of Ni superalloy IN718 atomised powders, which simulated the environment of droplet deposition during the twin-atomiser spray forming of large diameter IN718 alloy billets at BIAM. The experiments were complemented by phase field modelling studies at Oxford. Experiment and modelling supported the hypothesis that the characteristic equiaxed spray formed microstructure depends critically upon the rapid remelting and thermal shock of fine-scale dendrites in solid particles in the spray to provide a high density of embryonic grains.