Substructure Evolution Research Articles

Characterization of Al3Zr precipitation via double-step homogenization and recrystallization behavior after subsequent deformation in 2195 Al-Li alloy

The processability and final mechanical properties of Al-Li alloys are strongly affected by the uniformity of solute element and the degree of recrystallization, which can be improved via homogenization treatments. It is essential to control the microstructure to achieve the desired properties. In this work, the optimal double-step homogenization (DH) parameters for as-cast 2195 Al-Li alloy were first determined as 460 °C/16 h + 515 °C/20 h according to the comparative analysis, which fitted well with the results predicted by the corresponding kinetics modeling. Compared with the conventional single-step homogenization (SH), DH not only prevented the occurrence of overburning but also provided a more favorable condition for Al3Zr dispersoids to precipitate uniformly. The size and number density of Al3Zr dispersoids in the DH sample are finer and higher than those in the SH sample based on quantitative statistics. Due to the sufficient driving force for nucleation, the precipitate free zone (PFZ) of Al3Zr dispersoids was much narrower after DH. As a result, the DH sample possessed a higher Zener drag pressure, leading to a remarkable inhibition on the continuous evolution of substructures. Therefore, the recrystallization behavior in the DH sample was impeded effectively during the subsequent solution treatment. These findings are expected to provide reliable theoretical guidance for the industrial processing of as-cast 2195 Al-Li alloy.

Order-Tuned Deformability of Bismuth Telluride Semiconductors: An Energy-Dissipation Strategy for Large Fracture Strain.

In addition to thermoelectric (TE) performance tuning through defect or strain engineering, progress in mechanical research is of increasing importance to wearable applications of bismuth telluride (Bi2Te3) TE semiconductors, which are limited by poor deformability. For improving dislocation-controlled deformability, we clarify an order-tuned energy-dissipation strategy that facilitates large deformation through multilayer alternating slippage and stacking fault destabilization. Given that energy dissipation and dislocation motions are governed by van der Waals sacrificial bond (SB) behavior, molecular dynamics simulation is implemented to reveal the relation between the shear deformability and lattice order changes in Bi2Te3 crystals. Using the disorder parameter (D) that is defined according to the configurational energy distribution, the results of strain rates and initial crack effects show how the proper design of the initial structure and external conditions can suppress strain localization that would cause structural failure from the lack of energy dissipation, resulting in large homogeneous deformation of Bi2Te3 nanocrystals. This study uncovers the essence of the tuning mechanism in which highly deformable Bi2Te3 crystals should become disordered as slowly as possible until fracture. This highlights the role of the substructure evolution of SB-defect synergy that facilitates energy dissipation and performance stability during slipping. The disorder parameter D provides a bridge between micro/local mechanics and fracture strain, hinting at the possible mechanical improvement of Bi2Te3 semiconductors for designing flexible TE devices through order tuning and energy dissipation.

Microstructure, texture and mechanical properties of a nickel-free high nitrogen duplex stainless steel processed through friction stir spot welding

A nickel-free high nitrogen duplex stainless steel holding high Mn-content was friction stir spot welded under the various rotational speeds. The microstructure/texture evolutions of the joints and the correlated mechanical properties were investigated in detail. The ferrite and austenite constituent massive phases were refined down to the mean grain size of 0.9 μm and 1.1 μm, respectively. The microstructure evolutions revealed the activation of continuous dynamic recrystallization as the main restoration mechanism. This was further verified through the formation of Β- and Α-fiber shear texture in the specimens processed under the various processing condition. Besides the activation of different restoration micro-mechanisms in both ferrite and austenite, dynamic phase transformation of ferrite to austenite was also contributed in grain refinement. The transformation was facilitated by providing more diffusional paths through substructure development and grain refinement. The local and bulk mechanical properties of the joints were also assessed. The outstanding tensile-shear peak load of 12.8 KN obtained for the sample welded at 400 rpm was discussed relying on the developed ultrafine-grained microstructure and the maximum peak extension of 1.7 mm achieved at 600 rpm was attributed to the role of ferrite phase in strain accommodation.

Analysis of monotonic and cyclic crack tip plasticity for a stationary crack tip in a FCC crystal

The fields produced at the tip of a Mode I crack in a ductile single crystal have been previously investigated using theoretical, experimental, and numerical methods. Limitations in the material model complexity used in theoretical approaches and difficulties in experimentally measuring in situ crack tip fields invite consideration of numerical crystal plasticity. Prior crystal plasticity analyses of crack tip plasticity have leveraged simple phenomenological constitutive forms and investigated limited loading scenarios. The current work extends prior approaches by using a recently developed face-centered cubic (FCC) crystal plasticity model that considers dislocation substructures and complex back stress evolutions during cyclic loading in concert with a finite element model of a stationary crack tip. The model is used to i) more thoroughly understand the conditions where theoretical analyses remain valid in the context of dislocation interactions and substructure development and ii) explore potential fatigue crack growth driving forces for microstructurally small and physically small cracks. Specific observations are made regarding the observed formation of alternating bands of forward and reverse shear, unique to the present model with dislocation substructure, as well as the influence of ratcheting strain on the cyclic irreversibility of slip and associated correlative fatigue crack growth relations.

Deformation substructure development in AlXCoCrFeNi (X = 0, 0.3) high entropy alloy under different influencing factors

Three distinct initial microstructures, including the coarse grains (Al0 sample), fine grains (Al0-T sample) and fine grains with particles (Al0.3-TA sample), are prepared to investigate the deformation substructure development in AlxCoCrFeNi (X = 0, 0.3) high entropy alloy under the different influencing factors (grain size, strain temperature, strain level and particle). Results suggest slip dislocation is recognized in all deformed samples under the different strain conditions, while the strain temperature and the strain level are the critical factors for producing deformation twins for Al0 sample and Al0.3-TA sample, respectively. The effect of slip dislocation on deformation substructure development in Al0 sample is investigated via analyzing how orientation spread builds up inside the grains, confirming three different kinds of dislocation induced deformation patterns. The main difference between annealing twins and deformation twins are systematically investigated, revealing the different morphological features and the different number of activated twin variants.

Assessment of Dislocation Density by Various Techniques in Cold Rolled 1050 Aluminum Alloy

This study examines the evolution of dislocation density in cold rolled 1050 Al alloy. Various techniques such as numerical approaches, indentation techniques, X-ray diffraction line profile analysis, and electron backscattering diffraction were employed for the characterization of the deformed state. These methods allowed us to determine the nature of the evolution of the dislocation substructure during cold rolling. The investigated material was subjected to thickness reductions varying from 5% to 47%, which resulted in a continuous increase in hardness while the estimated dislocation density showed a tendency towards a less intense increase after a ~30% straining level. The numerical approaches employed, such as the Kubin–Estrin and a modified version of this model, are capable of ensuring a reasonable estimation of dislocation density at low and moderate deformation levels (~5–30%), while the discrepancy between the measured and simulated data is negligible when the material has been exposed to more severe rolling reductions.

Evolution of strength properties and defect sub-structure of the hypoeutectic A319.0 alloy irradiated by a pulsed electron beam and fractured under tensile stress

The paper intends to investigate strength and plasticity properties and defect sub-structure of the hypoeutectic А319.0 alloy irradiated by a pulsed electron beam. The study has established and the revealed extinction contours point out that the irradiation of the А319.0 alloy by a pulsed electron beam brings about the formation of an up to 100 µm thick surface layer in the elastic-stressed state. The structure of the modified layer comprises nano-dimensional non-equiaxed cells. The reason for the non-equiaxity in a cellular structure is suggested to be the irregular heat removal from the surface of modification. X-ray microspectroscopy has found out that high-speed crystallization cells are formed by an aluminum-based solid solution and separated with thin layers, which are enriched mainly by silicon atoms and, to a smaller degree, by atoms of magnesium, iron and copper.The data obtained in the mechanical testing suggest that the irradiation was carried out in the following conditions: 50 J/cm2 and 50 µs results in the simultaneous enhancement of strength (by 22%) and plasticity (by 44%) properties of the А319.0 alloy – these are the best positive values in the set of conducted experiments. The study on А319.0 alloy samples irradiated and fractured under the tensile stress has highlighted that their fracture is dominated by the mechanism of intercrystalline quasi-brittle rupture caused by forming lengthy thin layers of the second phase along the boundaries of high-speed crystallization cells.

On the warm temperature strain accommodation mechanisms of Ti–6Al–4V alloy holding different starting microstructures

The strain accommodation mechanism of Ti–6Al–4V alloy with different initial microstructure; equiaxed α+β, lamellar α+β, dual phase α+ά, and fully ά martensite was studied in warm temperature deformation regime. Toward this end, a series of tensile tests were performed at room temperature, 400, 500 and 600 °C. The progressive substructure development during tensile deformation of the equiaxed α+β microstructure at 400 °C increased the strain-hardenability (n-exponent) of the material. However, activation of continuous dynamic recrystallization at higher temperature of 600 °C as main strain accommodation mechanisms increases the portion of non-uniform elongation. This was attributed to the resistance against localized deformation and the occurrence of diffused necking. In the case of initial lamellar microstructure, mechanical fragmentation and thermal disintegration of β phase, and globularization of α-phase via boundary splitting mechanism were dominant restoration mechanisms. Martensite reverse transformation and dynamic recrystallization also served as the primary source of softening and strain compensation in dual-phase α+ά initial microstructure. This was led to the development of bimodal/trimodal microstructure representing acceptable balance between strength and ductility. The microstructural evolution in fully ά martensite was relatively analogous to that of α+ά microstructure apart from activation of load transition mechanism. The correlation of strain accommodation capability of each microstructure and tensile formability was discussed in details.

Microstructure and deformation behaviour of austenitic low-density steels: The defining role of B2 intermetallic phase

The feasibility of significant weight reduction in conjunction with the superior strength-ductility combination makes Fe-Mn-Al-C-based steels the candidate material for automotive and structural applications. The alloy chemistry and processing conditions influence the microstructure in low-density steels (LDS); however, their role in deformation behaviour is not fully understood. In the present study, three different austenitic LDS grades viz. Fe-28Mn-9Al-0.9C, Fe-28Mn-9Al-5Ni-0.9C and Fe-15Mn-9Al-5Ni-0.9C alloys in hot-rolled condition were used for evaluating the role of secondary phases, especially B2 ordered Fe(Ni)Al phase on deformation characteristics. Ni-free LDS microstructure consisted of the γ-matrix with fine κ-carbides, whereas Ni containing low Mn alloy possessed coarser κ-carbides along with B2 in both bamboo-like stringer and polygonal particle morphology in the γ-matrix. The B2 containing alloy exhibited higher strength with reduced tensile ductility than the B2-free alloy. Hardness of the γ-matrix and B2 phase were similar with B2 grains exhibiting distinct pop-in events in the nanoindentation curves indicating incipient plasticity. The γ-matrix and B2 co-deform near yield, and a favourable orientation relationship (OR) between the γ-matrix and B2 facilitated easy slip transfer, while the non-favourable OR controlled ductility by strain accumulation and B2-cracking. The differences in the strain hardening behaviour of B2-free and B2 containing alloys were elucidated based on the changes in dislocation substructure evolution examined by an automated crystal orientation mapping in electron microscopy.

Substructure induced dendrite-fragmentation during thermomechanical processing of as-cast Mg-Sn-Li-Zn alloy

The present work deals with emphasizing the role of substructure development in fragmentation of the cast structure in Mg-Sn-0.3Li-3Zn alloy during thermomechanical processing. In this respect, a new micro-mechanism and a conceptual model were proposed for dendrite fragmentation, which was supported by 3D-tomography, electron back scattered diffraction, and angle selective back-scattered imaging analyses. Balancing of the dihedral angle of the non-equilibrium triple junctions led to the formation of grooves at the dendrite arms, followed by penetration of α-Mg matrix through the grooves and finally resulted in fragmentation of the specified dendrite arm. In fact, despite the conventional expectation regarding the high contribution of brittle fracture and shearing in fragmentation of the dendrites at low temperature regime, it was observed that the substructure development in a medium-to-high stacking fault energy material greatly participated in thermally-activated disintegration of the dendritic cast-structure.

Microstructural-constraint induced ferrite refinement during compressive deformation of a triplex ferrite-based low density steel

The present work deals with proposing a new refinement mechanism in triplex microstructure of a ferrite-based low density steel in the course of thermomechanical processing. Toward this end, the material was deformed in compressive mode at 500 °C down to various strain levels and the microstructures was studied step by step with the aim of advanced characterization methods. Owing to the partially transformation of austenite to martensite, the softer lamella-type ferrites were encapsulated between austenite and/or martensite laths. It was found that the imposed strain was primarily accommodated in ferrite phase, due to the strain incompatibility between ferrite and martensite and the constraints coming from martensite, and the higher load-bearing capability of ferrite. This was led to substructure development within the ferrite, and groove formation through boundary splitting mechanism. Finally, the stepped ferrites were fragmented at higher imposed strain, and such constraint induced ferrite refinement was led to the formation of the grains down to the size of 0.5–2 μm.

Micromechanical crystal plasticity back stress evolution within FCC dislocation substructure

Experiments and theories have suggested the evolution of intragranular back stress is primarily dependent on the characteristics and development of dislocation substructure in face-centered cubic (FCC) metals and alloys. Despite this, continuum modeling of complex cyclic loading phenomena such as inelastic ratchet strain accumulation typically appeals to one of several phenomenological back stress forms with weak connections to physical mechanisms. In the current work, a micromechanically-based back stress evolution law is derived and implemented in a crystal plasticity framework for FCC metals that directly considers the evolution of dislocation substructure. The model is used in a case study of stainless steel 316L (SS316L), and good agreement is found with experiments on single- and poly-crystalline specimens subjected to monotonic, fully-reversed cyclic loading, and stress-controlled cyclic loading with mean stress. Ratcheting is attributed to dislocation substructure formation, dissolution, and stabilization within this physically-based framework.

The role of thermomechanical processing routes on the grain boundary network of martensite in Ti–6Al–4V

The influence of thermomechanical processing (TMP) of a Ti–6Al–4V alloy on the transformation texture and intervariant boundary network were investigated by conventional EBSD mapping along with the five-parameter boundary analysis approach. The texture characteristics of Ti–6Al–4V subjected to deformation in the β regime followed by the β→α martensitic transformation were examined using visco-plastic self-consistent simulation and forward calculation of the transformation texture. Comparison of the simulated and experimental texture characterisitcs revealed that the transformed α′ texture was dominated by the variant selection associated with substructure development in the β parent phase and the occurrence of specific self-accommodating α′ variants in the microstructure, promoting the quadrilateral and/or V shape variant arrangement. This resulted in a progressive increase in the 63.26°/[10‾553‾] intervariant boundaries with the strain increment, at the expense of 60°/[112‾0]. Moreover, the grain boundary network for all conditions was dominated by the triple junctions (grain boundary network) terminating on 63.26°/[10‾553‾] and 60°/[112‾0] intervariants. It is shown that the elastic interactions among the variants during the martensitic transformation is the dominant parameter affecting the grain boundary network, despite the presence of dislocation based variant selection.

The effect of post-processing heat treatment on the microstructure, residual stress and mechanical properties of selective laser melted 316L stainless steel

Additively manufactured 316L austenitic stainless steel typically displays a hierarchical microstructure consisting of fine columnar grains, cellular dislocation tangles and nano-inclusions, which provides a combination of exceptional strength and ductility. However, the rapidly solidified microstructure often contains significant residual stress and various post-processing heat treatments are generally used to relieve the residual stress and to alter the microstructure and properties. In this work, a 316L austenitic stainless steel additively manufactured by a laser-based powder bed fusion process (selective laser melting, SLM) was for the first time subjected to various heat treatments to systematically study the evolution of residual stress, microstructure and mechanical properties. Significant compressive residual stress was revealed in the core volume of the as-built condition, whilst moderate to full stress relief of 24%, 65% and ~90% was achieved upon 2 h post-processing annealing at 400 and 650 °C and solution annealing at 1100 °C for 5 min, respectively. The extent of stress-relieving is closely associated with the evolution of substructure (i.e., dislocation tangles), which also affects the yield strength. Marked alteration from the as-built metastable microstructure was seen except for the low-temperature treatment at 400 °C. This includes the precipitation of embrittling σ phase or its precursors at 650–800 °C which results in a reduction in ductility. Unlike conventional wrought 316L, no carbide formation was seen in the treatment temperature regime. Recrystallization of columnar grains and coarsening of nano-inclusions took place over time upon solution annealing at 1100 °C, causing softening and unexpected reductions in ductility. This work led to the establishment of heat treatment-property relationships and corresponding microstructural changes, which are of great significance for the component design and structural application of SLM 316L.

Columnar to equiaxed transition during β heat treatment in a near β alloy by laser additive manufacture

Abstract Columnar β grains are widely present in titanium alloys fabricated by Additive Manufacturing (AM), which leads to significant anisotropy in mechanical properties. An outstanding challenge is to facilitate the columnar to equiaxed transition (CET) of the prior β grains. In this research, CET and the underlying grain growth mechanism during β heat treatment in Ti–5Al–5Mo–5V–1Cr–1Fe (Ti55511) alloy manufactured by selected laser melting (SLM) and laser metal deposition (LMD) were investigated. For the first time, the quasi-in-situ observation was employed in the AM near β titanium alloys by the electron backscatter diffraction technique to analyze morphology, crystal orientation and substructure evolution, especially in the situation of bamboo-like microstructure with columnar grains and equiaxed grains distributed layer by layer. It was found that the prior β grains enriched low-angle grain boundaries (LAGB) are in a high stored strain energy state in as-deposited SLM Ti55511 leading to a recrystallized nucleation-and-growth process during heat treatment, while the prior β grains containing a small amount or even no LAGB are in a relatively lower stored strain energy state in as-deposited LMD Ti55511 resulting in recrystallization and grain growth processes.

Microstructural effects and mechanical response of microalloyed ferrite and austenite subjected to metal forming at extremely different rates of deformation

The mechanical behaviors and microstructures of severely deformed microalloyed austenitic M_A and ferritic M_F steels were analyzed in wires drawn from wire rods (6.5 mm in diameter) down to the final diameter of 0.75 mm. The deformation induced microstructures in the cold drawn wires under “static” (low speed wire drawing) and “dynamic” (high speed wire drawing) conditions were compared and analyzed. The analysis has been focused on a detailed characterization of the effects of microalloying on the dislocation substructure development and the in-situ recrystallization. The correlations between mechanical behavior and dislocation substructure development with increasing deformation reveals that precipitation strengthening and microstructure refinement are the dominating mechanisms that control the mechanical behaviors of wires, especially in those subjected to deformation under dynamic loading conditions. An approach has been presented to model the evolution of work hardening process during complex deformation. Additional advantage of the presented approach is its capability of tracking evolution of dislocation density during the deformation process. The changes of dislocation density calculated for both materials show similar behaviour, but higher values of the dislocation densities were observed in the specimens of M_F than in the case of M_A.

Inconsistent creep between dendrite core and interdendritic region under different degrees of elemental inhomogeneity in nickel-based single crystal superalloys

The creep inconsistency between dendrite core and interdendritic region is investigated in a nickel-based single crystal superalloy under 1373 K and 137 MPa. Two specimens with higher and lower degree of elemental inhomogeneity on dendritic structures are compared. For specimen with higher inhomogeneity, stronger segregation of refractory elements reinforces the local strength in dendrite core, but damages the strength in interdendritic region. Creep strain is accumulated faster in interdendritic region giving rise to promoted dislocation shearing in γ′ phase, faster degradation of dislocation networks and facilitated topological inversion of rated structures. Although the segregation of refractory elements produces a high density of topologically close-packed (TCP) phase in dendrite core, faster accumulation of creep strain forms microcracks prior in interdendritic region that gives rise to final rupture of the specimen. In another specimen, increased solid solution time gives rise to overall reduced inhomogeneity. Creep inconsistency is relieved to show more uniform evolution of dislocation substructures and rafting between dendrite core and interdendritic region. The second specimen is ruptured by formation and extension of microcracks along TCP phase although the precipitation of TCP phase is relatively restricted under reduced inhomogeneity. Importantly, the balance of local strength between dendrite core and interdendritic region results in over 40% increase of creep rupture life of the second specimen.

Insights into the Micromechanics of Organic Aerogels Based on Experimental and Modeling Results

While the characteristics of the macroscopic mechanical behavior of organic aerogels are well known, the mechanisms responsible for the substructural evolution of their networks under mechanical deformation are not fully understood. Herein, organic aerogels from the aqueous sol−gel polymerization of resorcinol with formaldehyde are first prepared. Specifically, the resorcinol to water (R:W) molar ratio is varied for obtaining diverse highly open‐cellular porous structures with mean pore sizes ranging between 30 and 50 nm. The corresponding network structures are then characterized and exhibit different morphological and mechanical properties. Furthermore, a micromechanical constitutive model based on the pore‐wall kinematics is proposed. While the arrays of particles forming the pore walls are moderately connected, the pore walls are considered to behave as solid beams under mechanical deformation. Moreover, the damage mechanisms in the pore walls that result in the network collapse are defined. All model parameters are shown to be physically derived, and their sensitivity to the macroscopic network behavior is analyzed. The model predictions are shown to be in good agreement with the experimental stress−strain data of the different aerogels.

A novel simulation of continuous dynamic recrystallization process for 2219 aluminium alloy using cellular automata technique

Based on the previous experiments, continuous dynamic recrystallization (CDRX) was determined to be the main softening mechanism of Al 2219 alloy during the thermal deformation process. The mechanism of microstructural transformation for the CDRX process was firstly discussed, and the dislocation evolution model, subgrain growth nucleation model, low angle grain boundaries (LAGBs) migration model, and topological deformation model were established respectively. Using the probabilistic cellular automata technique, a two-step microstructure formation method of initial equiaxed grains + local subgrains dispersion was proposed, which can simulate the initial microstructure at any processing state. Then, according to the proposed micro-mechanism and models of CDRX, a novel simulation approach of the CDRX process for Al 2219 alloy during thermal processing was established on the Matlab platform to predict the substructure's evolution. The CA model was applied to extensively analyze the influence of deformation parameters and the transformation law of LAGBs. The results showed that high temperature, high strain, and low strain rate can effectively promote the development of continuous dynamic recrystallization, which is consistent with the experimental results.

Evolution of Dislocation Substructure in Deformed Ordered and Disordered Pd3Fe Alloys in Low-Stability State

Evolution of Dislocation Substructure in Deformed Ordered and Disordered Pd3Fe Alloys in Low-Stability State