Subgrain Boundaries Research Articles

Achieving recrystallization of prior β in Ti-6Al-4V alloy by electroshock

Fine grain strengthening is an important approach to improve mechanical properties in titanium alloys. The martensite (α′) of Ti-6Al-4V alloy generally shows fine acicular grain and has high strength. However, the grain size of martensite is influenced by the prior β grain, thereby affecting the overall strength-plasticity combination of the alloy. Generally, the prior β grains grow rapidly above β-transus temperature (∼1000 ℃). It is difficult to refine the prior β grain through traditional heat treatment alone if it coarsened. In this work, we found that electroshock treatment could directly recrystallize the coarse prior β grain in α+β titanium alloy without plastic deformation process. EBSD and TEM analyses found that the prior β grain size reduced from 44 µm to 29 µm and the remained textures had been eliminated after electroshock, therefore the tensile strength and elongation were simultaneously improved from 1196 MPa and 4.7 % to 1259 MPa and 10.4 %. The residual subgrain boundaries within prior β grain, electric current-induced extra driving force and extremely high heating rate (106 ℃/s) are the main reasons for the occurrence of recrystallization as well as no grain growth of prior β. These findings may be used for repairing the mechanical properties of coarse overheated microstructure or eliminating the anisotropy of columnar grain that produced in additive manufactured α+β titanium alloy without deformation processing.

Sample size dependence of high-temperature thermal stability of Ti2AlN MAX phase

The thermal stability of Titanium Aluminum Nitride (Ti2AlN) microcrystalline powder in vacuum has been studied by in-situ high-temperature X-ray diffraction. The limit of thermal stability for Ti2AlN phase is T < 850 °C. The process of thermal decomposition consists of the Al atoms sublimation and the formation of a new TiNx phase on the sample surface. An increase in sample size leads to an increase in thermal stability limit (850 °C for microcrystals and 1550 °C for dense bulk sample). Based on crystal chemical analysis and quantum chemical calculations, a mechanism of thermal dissociation was proposed. Kinetics of thermal dissociation of the Ti2AlN phase was modeled using the Avrami equation. Avrami exponent parameters (n = 1.09; 1.28) indicate that the process of germs formation occurs at a subgrain boundaries of the sample. The observed temperature dependence of the parameter n can be associated with a probability change of crosslinking formation during the growth of a new phase. The size dependence of Ti2AlN thermal stability was explained by the ratios of Al diffusion coefficients in the crystallite bulk and along the subgrain boundaries.

New insights into the microstructure of M2 high-speed steel

In this work, the microstructure of the heat-treated AISI M2 high-speed steel of different initial structural states, wrought and as-cast, was investigated in detail down to the atomic scale, using advanced electron microscopy techniques. Significantly different amount of the secondary carbide particles of a size up to 300 nm in the microstructure of both steels was established and described. In the tempered matrix, the formation of the ultrafine sub-grains with a size in the range of 3–15 nm and the substructure of the α-Fe consisting of nano-sized elongated plates about 200–300 nm in length and 7–30 nm in thickness were proven. Fine dispersions of the secondary hardening carbides along the matrix sub-grain boundaries and inside the carbide clusters were thoroughly investigated and identified as vanadium-rich MC carbide. Extremely fine precipitates at the boundaries of the nano-sized elongated plates in the substructure of the tempered α-Fe were revealed in both steels. The precipitates, regularly dispersed in the tempered matrix and containing carbon, were enriched primarily in iron, molybdenum, chromium, vanadium, and, to a lesser extent, in tungsten, as proven by the combined EDX and EELS analysis. At the atomic scale, the precipitates were found to be mostly disordered solids without a crystalline structure. However, some of the precipitates exhibited traces of crystalline structure, which likely corresponds to Fe21(W,Mo)2C6 phase.

The effect of secondary phases on microstructure and irradiation damage in an as-built additively manufactured 316 L stainless steel with a hafnium compositional gradient

Additive manufacturing (AM) or rapid prototyping has become a crucial tool for reducing both cost and time while increasing efficiency in qualifying structural materials for reactor use. In this study, directed energy deposition (DED) was used to develop an as-built 316 L stainless steel sample with three regions of increasing Hf-dopant to study the effects of Hf on the irradiation response of the material. Morphological and microstructural changes were analyzed before and after 2 MeV proton irradiation at 360 °C to a damage of 2.5 dpa at ∼ 10 µm below the surface. The addition of Hf effectively suppressed radiation-induced damage (dislocation loops, radiation-induced segregation) due to enhanced point defect recombination. The radiation damage seen in the as-built sample was further compared to a thermo-mechanically treated counterpart of the same fabrication and irradiation parameters which was found to behave superiorly. The increased radiation resistance of this material may be attributed to the as-built microstructure, which includes undissolved Hf particles, delta ferrite grains and cellular sub-grain boundaries that can hinder defect motion.

Development and characterization of a low-density TiNbZrAlTa refractory high entropy alloy with enhanced compressive strength and plasticity

A new refractory high entropy alloy (RHEA) is designed by combining refractory elements Nb and Ta with Ti, Zr, and Al, resulting in a (TiNbZr)89(AlTa)11 alloy composition with a low-density of 6.0 g/cm3. The novel RHEA features a BCC matrix with B2 and Zr5Al3 nano precipitates, exhibiting a compressive yield strength of ∼890 MPa and specific yield strength of ∼148.3 MPa g−1 cm3. Remarkably, the alloy demonstrates excellent compressive plasticity of ∼70% at 298 K. As the temperature increases to 873 K, 1073 K, and 1273 K, the yield strength of the RHEA gradually decreases to ∼610 MPa, ∼ 210 MPa, and ∼ 60 MPa, respectively. These results indicate that this novel RHEA showcases a combination of good specific strength and plasticity. Post-compression, B2, and Zr5Al3 phases are observed. The second-phase particles dispersed throughout the grains hinder dislocation motion and contribute to the formation of sub-grain boundaries during hot compression.

Corrosion behaviors of ODS-FeCrAl in oxygen-saturated lead-bismuth eutectic at 550°C: Effects of grain boundaries and minor elements

Long-term corrosion behavior of ODS-FeCrAl tube was investigated in oxygen-saturated static liquid lead-bismuth eutectic (LBE) at 550 °C. Different densities of sub-grain boundaries (sub-GBs) lead to different corrosion behaviors on the inside and outside surfaces of the tube. The oxide film formed on the inside surface consists of Fe3O4, Fe-Cr-Al spinel and Al2O3, and on the outside surface consists of Fe-Cr-Al spinel rich in Cr&Al and Al2O3. Excessively high density of sub-GBs accelerates the oxidation rate. The precipitation of W&Ti promotes the formation of Cr-rich and Al-rich oxides. The corrosion mechanism of ODS-FeCrAl in liquid LBE is discussed.

Effect of rapid solidification and post-processing on microstructure, magnetic and structural transition temperatures and magnetic properties in Ni50Mn29Ga21 magnetic shape-memory alloy

This study investigated the effect of rapid solidification and post-processing on microstructure, composition and magneto-structural properties of Ni50Mn29Ga21 (at.%) magnetic shape memory alloy (MSMA). Samples with varying cooling rates (1.1 ∙ 102 to 3.2 ∙ 103K/s) were prepared using arc melting for a comparative study in as-solidified condition and after two different heat treatments. A notable change from columnar-dendritic to cellular-dendritic solidification and a decrease in solidification length scale was observed at more rapid solidification. All samples showed Mn enrichment and corresponding Ga depletion at the sub-grain boundaries. The fine-scale solidification structure, chemical heterogeneity and lack of ordering resulted in broad martensitic transformations. At more rapid solidification, a decrease in saturation magnetization and abnormal slope changes in the hysteresis loop were observed. Heat-treatment resulted in grain growth, significant reduction in transformation breadth, and increase in magnetization saturation. Rapidly solidified samples with an estimated characteristic cooling rate of 1.1 ∙ 103K/s most closely resembled microstructures and magneto-structural properties from laser directed energy deposition (L-DED) processing. The results of this study help to understand cooling rate effects in L-DED additive manufactured Ni-Mn-Ga and provide post-processing routes that will result in microstructures and properties that resemble those of functional MSMA.

Achieving superior tensile strength of CoCrFeNiTi0.3 high-entropy alloy via in-situ laser powder bed fusion of CoCrFeNi and Ti

In this study, CoCrFeNiTi0.3 high-entropy alloy (HEA) was designed and prepared via the in-situ laser powder bed fusion (LPBF) of CoCrFeNi and Ti powders to improve the undesirable tensile strength of CoCrFeNi HEA. With the introduction of Ti, semi-elliptic melt pool characteristics became blurred with the concentration of Ti at melt pool boundaries, which was correlated with a Marangoni-convection-driven chaotic flow and insufficient diffusion of Ti during the rapid melting/solidification process of LPBF. During the solidification process, Ti acted as nucleation sites and triggered a columnar-to-equiaxed transition of grains in the printed CoCrFeNiTi0.3 HEA. Partial diffused Ti combined with Ni to form nanoscale Ni3Ti precipitates because Ti and Ni had the most negative mixing enthalpy compared with Co, Cr, and Fe. The nanoscale Ni3Ti precipitates were located at the sub-grains and showed a network structure. This was associated with the sub-grain boundary segregation driven by severe lattice distortion. The yield strength of the Ti-modified CoCrFeNi was enhanced from 509 MPa to 796 MPa, which suggested a 56.4% increase obtained by adding a small amount of Ti. The theoretical calculation was deduced to validate the strengthening mechanism mainly stemming from dislocation hardening and precipitation hardening. This study is anticipated to provide insights into the enhancement in the mechanical performance of HEA through composition modification via the in-situ alloying during LPBF.

Achieving ultra-fine grains and high corrosion resistance of Al–Zn–Mg–Cu alloy by ECAP and post cold rolling

The ultra-fine grain structure of Al–Zn–Mg–Cu alloy was prepared by equal channel angular pressing (ECAP) and post cold rolling (CR) combined deformation process. The effect of different rolling deformation on the microstructure, microhardness and corrosion behavior of the ECAP alloy were studied. The results show that with the increase of rolling deformation, the dislocation density increased, the grain and second phases of the deformed alloy were obviously refined. Grain refinement was mainly due to the gradual transformation of some subgrain boundaries and low angle grain boundaries (LAGBs) of deformed alloys into high angle grain boundaries (HAGBs), which eventually form fine grains. The microhardness improvement was attributed to dislocation strengthening, fine grain strengthening and second phase strengthening. Simultaneously, the corrosion behavior of the alloy was tested by immersion corrosion and intergranular corrosion, which revealed that high-density lattice defects, refined grain and second-phase particles can improve the corrosion resistance of the deformed alloys.

Effect of Grain Structure and Quenching Rate on the Susceptibility to Exfoliation Corrosion in 7085 Alloy.

The influence of grain structure and quenching rates on the exfoliation corrosion (EXCO) susceptibility of 7085 alloy was studied using immersion tests, optical microscopy (OM), scanning electron microscopy (SEM), electron backscatter diffraction (EBSD), and scanning transmission electron microscopy (STEM). The results show that as the cooling rate decreases from 1048 °C/min to 129 °C/min; the size of grain boundary precipitates (GBPs); the width of precipitate-free zones (PFZ); and the content of Zn, Mg, and Cu in GBPs rise, leading to an increase in EXCO depth and consequently higher EXCO susceptibility. Meanwhile, there is a linear relationship between the average corrosion depth and the logarithm of the cooling rate. Corrosion cracks initiate at the grain boundaries (GBs) and primarily propagate along the HAGBs. In the bar grain (BG) sample at lower cooling rates, crack propagation along the sub-grain boundaries (SGBs) was observed. Compared to equiaxed grain (EG) samples, the elongated grain samples exhibit larger GBPs, a wider PFZ, and minor compositional differences in the GBPs, resulting in higher EXCO susceptibility.

Superelasticity of Geometrically Graded NiTi Shape Memory Alloys

A stress plateau with a strain of 5–8% normally occurs during the stress-induced martensite transformation (SIM) of NiTi shape memory alloys. Within the stress plateau, the correlation between the stress and strain is lost, which limits their application in certain fields which require accurate control of inelastic deformation. In order to address this limitation, a series of step-like NiTi samples with graded cross-sectional area were designed and fabricated. Multiple stress plateaus were achieved by varying the sample width and adjusting the number of steps; for instance, two and three stress plateaus were obtained in the samples with two and three steps, respectively. Also, linear force–strain response was obtained by changing gradually the width of the samples. The functional stability of the geometrically graded samples was significantly improved by incomplete recrystallization annealing (600 °C) followed by low-temperature (250 °C) aging treatment. The incompletely recrystallized specimens contained many dislocations and grain and sub-grain boundaries, which promoted the uniform precipitation of Ni4Ti3 nanoparticles during aging treatment. The homogeneously and densely dispersed Ni4Ti3 nanoparticles were able to strengthen the matrix considerably and prevent plastic activities during stress-induced martensite transformation. As a result, the functional stability of the geometrically graded NiTi samples was much improved. After aging at 250 °C for 120 h, all the samples showed a small residual strain of &lt;1.0% after 20 loading–unloading cycles.

Simultaneous regulation of strength and ductility of laser powder bed fusion manufactured Haynes 230 alloy

Haynes 230 alloy is widely used in aero-engines with high strength, fatigue resistance, oxidation resistance and other properties. However, the powder metallurgy process limits its application due to hot cracks and low strength. In this work, crack inhibition and mechanical properties improvement of the laser powder bed fusion (L-PBF) manufactured Haynes 230 alloy using 25 groups of different laser energy densities and scanning strategies were investigated. Results show that the high density, nearly crack-free, fine microstructure, and excellent strength and ductility were achieved by a laser energy density of 59.4 J/mm3 and rotation angle of 90° for the L-PBF manufactured Haynes 230 alloy. The microstructure was composed of γ matrix, carbides (M6C, M23C6), and La2O3 nanoparticles. Interestingly, many carbides were found to be mainly uniformly distributed at the grain or sub-grain boundaries with the following orientation relation: [011]γ//[011]M23C6. Simultaneously, by adjusting laser energy density and scanning strategy, the liquid film, low-melting eutectic phase, hot stress concentration, and high-angle grain boundaries (HAGBs) were suppressed, and the hot cracks were reduced. In addition, yield strength and ultimate tensile strength enhancement of the L-PBF manufactured Haynes 230 alloy was attributed to the synergistic effect of solid solution strengthening, dislocation strengthening, Orowan strengthening, and grain refinement. Whereas ductility enhancement was the main effect of defect inhibition, grain refinement, and nanoprecipitate. This work has verified the feasibility of crack inhibition and mechanical properties improvement of the L-PBF manufactured Haynes 230 alloy by various laser energy densities and scanning strategies.

Microstructure and mechanical properties evolutions of 9Cr heat resistant steel deposited metals with different Si contents during 550 °C aging process

The stability of microstructure and mechanical properties of 9Cr heat resistant steel under high temperature and long term service conditions has been a hot issue of great interest. In the present study, the relationships between microstructure and mechanical properties evolutions of 9Cr heat resistant deposited metals with different Si contents (0.25 wt.% and 1.07 wt.%) at 550 °C aging for different times were investigated systematically. The results indicate that the degradation of mechanical properties during aging is mainly attributed to the coarsening of precipitates, transformation of precipitate types, decreasing of dislocation density and coarsening of martensite laths, and these factors are related to the Si content in the deposited metals and the aging time. It is worth mentioned that the sub-grain boundaries pinned by M23C6 forming after post weld heat treatment (750 °C/2.5 h) stabilize the tensile strength during aging. The recrystallization occurring after post weld heat treatment and high temperature aging is beneficial for the impact toughness but not for the strength. During aging process, the precipitate transformation follows the following sequence: M23C6→M6X→Laves phase. Moreover, the increase of Si content promotes the nucleation and coarsening of M6X, and then deteriorates the mechanical properties of the deposited metals.

Revealing hot deformation behavior of Ti2C-Ti composite and associated dynamic recrystallization mechanism of ceramic matrix

Titanium carbide based composites have attracted much attention owing to their light weight, high elastic modulus, and excellent wear resistance. However, the hot working of these composites has been facing great challenges, especially for the coordinated deformation of brittle phase. Here, soft Ti phase was introduced to improve the thermal deformation ability. In particular, the dynamic recrystallization (DRX) mechanism, crack propagation, and dislocation motion behavior of brittle Ti2C matrix were unveiled in this study. Firstly, a Ti2C-Ti composite was hot compressed at 770–1000 °C with strain rates of 0.012–0.0013 s−1. The results showed that the optimal processing temperature was 950 °C, at which the microstructure can be refined by DRX behavior without the occurrence of macro instability. In the later softening stage of hot compression, the recrystallized Ti2C grains were formed through both discontinuous dynamic recrystallization (DDRX) and continuous dynamic recrystallization (CDRX). The intergranular Ti phase, which acts as a binder, coordinated the deformation of the Ti2C-Ti composite, and the phase boundaries attracted dislocations on the side of the brittle Ti2C phase. This dislocation motion behavior mitigated stress concentration but inhibited the DRX behavior of the Ti2C phase near the binary phase interfaces. The lamellar Ti precipitates within Ti2C grains facilitated the occurrence of CDRX by promoting the formation of subgrains. The subgrain boundaries were formed when the average distance between dislocation on the dislocation wall was lower than 8.68 nm. Compared with the as-sintered composite, the nanohardness of the Ti2C matrix in the composite deformed at 950 °C increased by 17%. The enhanced hardness was mainly attributed to grain boundary and dislocation strengthening effect, as well as the reduction of Ti precipitates.

Mechanisms of necklace recrystallization in a BCC Fe-Al-Ta alloy with strengthening Laves phase precipitates

A necklace structure composed of fine grains formed by dynamic recrystallization was uncommonly observed at the pre-existing grain boundaries during the hot compression of a BCC Fe-25Al-1.5Ta alloy containing C14 - (Fe, Al)2Ta Laves phase precipitates. Two possible mechanisms for necklace formation were proposed; particle-stimulated nucleation and grain boundary bulging, depending on whether the original grain boundaries are occupied by C14 particles, or they are free of them. Recrystallization was initiated preferentially around the clusters of large particles at the boundaries containing particles. In contrast, the bulging of the original grain boundaries by strain-induced boundary migration was observed as a preliminary stage for necklace formation at the particle-free boundaries. The necklace structure expanded into the deformed volume in such a way that low-angle subgrain boundaries decorating the necklace layers transformed into grains with increasing deformation strain.

The Role of Grain Boundaries in Low‐Temperature Plasticity of Olivine Revealed by Nanoindentation

AbstractThe rheological properties of olivine influence large‐scale, long‐term deformation processes on rocky planets. Studies of the deformation of olivine at low temperatures and high stresses have emphasized the importance of a grain‐size effect impacting yield stress. Laboratory studies indicate that aggregates with finer grains are stronger than those with coarser grains. However, the specific interactions between intracrystalline defects and grain boundaries leading to this effect in olivine remain unresolved. In this study, to directly observe and quantify the mechanical properties of olivine grain boundaries, we conduct nanoindentation tests on well characterized bicrystals. Specifically, we perform room‐temperature spherical and Berkovich nanoindentation tests on a subgrain boundary (13°, [100]/(016)) and a high‐angle grain boundary (60°, [100]/(011)). These tests reveal that plasticity is easier to initiate if the high‐angle grain boundary is within the deformation volume, whereas the subgrain boundary does not impact the initiation of plasticity. Additionally, the high‐angle grain boundary acts as a barrier to slip transmission, whereas the subgrain boundary does not interact with dislocations in a measurable manner. We suggest that the distribution of grain‐boundary types in olivine‐rich rocks might play a role in generating local differences in mechanical behavior during deformation.

Recovery of deformation microstructure in the bulk interior revealed by synchrotron X-ray micro-diffraction

A synchrotron X-ray micro-diffraction based technique, namely differential aperture X-ray microscopy (DAXM), is used to investigate the subgrain evolution in the bulk interior during ex-situ recovery annealing of a lightly deformed Al sample. This is the first 4D DAXM study of recovery. Supplementary TEM observations reveal that DAXM enables proper reconstruction of the microstructure, with the important addition that it is in 3D. The DAXM result provides a direct observation of the 3D microstructural evolution during annealing and confirms that subgrain boundary migration is an important mechanism for subgrain growth, preferentially removing small subgrains. Moreover, it gives new evidence of subgrain coalescence as an alternative mechanism for subgrain growth—some neighboring subgrains are observed to merge due to the decrease of the misorientation angle across certain subgrain boundaries. A possible interaction of the two recovery mechanisms is discussed, as well as the advantage and limitation of the DAXM technique for recovery studies.

Irradiation damage and corrosion performance of proton irradiated 304 L stainless steel fabricated by laser-powder bed fusion

The irradiation-induced microstructure and corrosion performance of proton irradiated 304 L stainless steel (SS) fabricated by laser powder bed fusion (LPBF) is investigated. Irradiation-induced dislocation loops are observed in both LPBF and traditionally manufactured (TM) 304 L SS following 2 MeV proton irradiation at 360 °C. A comparison analysis of the oxide scales formed on proton irradiated LPBF and TM 304 L SS is conducted along with the unirradiated counterparts. The irradiation-induced defects accelerate the corrosion by providing a rapid diffusion path to form a thicker and more porous film with a double-layer structure and herein the oxide film thickness of TM 304 L SS is approximately twice that of the LPBF 304 L SS. The unique microstructure of high dislocation densities, cellular sub-grains boundaries and dispersed nanoscale inclusions provides large amounts of interfaces in LPBF samples, which is benefit to absorb the irradiation-induced point defects. The irradiation induced fast diffusion paths and the nucleation points of oxides are thus reduced, resulting in a thinner oxide film thickness and a better corrosion resistance revealed by the more Cr enrichment of inner scale in LPBF 304 L SS.

Laser powder bed fusion of GH3536 nickel-based superalloys: Processing parameters, microstructure and mechanical properties

In this study, a GH3536 nickel-based superalloys was fabricated by the laser powder bed fusion technology (LPBF) with emphasis on its microstructure evolution and mechanical properties. Nearly fully dense specimens with the relative density of about 98.8% were obtained by parameter optimization. GH3536 nickel-based superalloys swirlers based on the optimized processing parameters show a high surface finish and no obvious macro-defects are observed. An excellent strength–ductility balance of the alloy with a yield strength of 675 MPa, an ultimate tensile strength of 823 MPa and an elongation of 22.3% was attained. The TEM results show the significant micro segregation of Mo and Cr at columnar sub-grain boundaries and promoted the formation of the chain-like carbides, improving the room temperature strength. The grains of the specimens show obvious bimodal grain structure and the crystals have a preferred crystallographic orientation along 〈001〉 direction and the texture orientations were fiber 〈100〉 and {001} 〈310〉. These findings provide some guidance for the industrial application of LPBF manufacturing high-quality GH3536 nickel-based superalloys components.

Hydrogen-induced failure in a partially-recrystallized Al-Zn-Mg-Cu alloy with different aging conditions: Influence of deformation behavior dominated by microstructures

Hydrogen-induced failure in a partially-recrystallized Al-Zn-Mg-Cu alloy with different aging conditions was studied by in-situ electrochemical hydrogen charging method, and the hydrogen-induced cracking mechanism was elucidated by investigating the microstructure-dominated deformation behavior. It was found that in naturally-aged and peak-aged samples, cracks initiate at sample surfaces, and deformation dominated by shearing facilitates hydrogen diffusion, resulting in a high hydrogen embrittlement susceptibility. However, in the over-aged sample, deformation dominated by bypassing cannot transport hydrogen efficiently, which exerts no effect on crack initiation at the sample center. In addition, the same cracking pattern exists on all samples in silicone oil and in-situ hydrogen charging environments. The grain boundary of recrystallized grains is prone to strain localization, leading to intergranular cracking. In unrecrystallized grains, transgranular cracks propagate along intersections of slip lines and subgrain boundaries. This study improves the understanding of hydrogen embrittlement in new generation Al-Zn-Mg-Cu alloy by linking the existing hydrogen embrittlement mechanism to the deformation behavior dominated by microstructures.