Coarse-grained Microstructure Research Articles

Microstructure evolution after hot working and heat treatment in 6082 aluminum alloy manufactured by horizontal continuous casting

In this paper, 6082 aluminum alloy manufactured by horizontal continuous casting was subjected to high-temperature plastic deformation under 20–70 % deformation levels with following T6 heat treatment process. Microstructural and phase changes were investigated by light and scanning electron microscopy together with energy dispersive spectroscopy, while grain, grain boundary, dislocation and texture evolution was studied by electron-backscattered diffraction. The results are showing that continuous dynamic recrystallization processes are active during the hot working while influence of static recrystallization is significant only for highest deformation degrees. The abnormally coarse-grained microstructure does locally occur in the subsurface layer after the 70 % deformation and T6 heat treatment. Crystallographic texture components are gradually evolving from the various types (Goss, Brass, D) for the low deformations (up to 40 %) into fiber-type textures (α-fiber, γ-fiber or C2-fiber) for high deformations due to the inhomogeneity of plastic deformation. The formation of AlMnSi dispersoids, CSL-boundaries Σ25b and an increase in fraction of GND, MgSi and Fe-based intermetallic particles was observed during the phase transformation which promotes ongoing nucleation of recrystallization processes.

Enhancing microstructural integrity and mechanical performance of thick wire-arc directed energy deposited eutectic aluminum-silicon solid structure through continuous multi-pass friction stir processing

In wire-arc directed energy deposition (waDED), fabricating larger 3D components with single-layer tracks is challenging due to limitations in track thickness, requiring the development of bead overlapping strategies. However, fusion-based 3D-printed aluminum structures often show porosity and coarse-grained microstructures, limiting their industrial applications. To address these limitations, this study employs continuous multi-pass friction stir processing (FSP) on a waDED-produced aluminum block (140 mm × 150 mm × 28 mm). The severe plastic deformation induced by FSP triggers dynamic recrystallization (DRX), promoting the formation of equiaxed grains with an average grain size 8.75 times smaller than the as-fabricated waDED sample (67.03 μm vs 7.66 μm). Transmission Electron Microscopy (TEM) revealed decreased dislocation loops post-FSP, indicating DRX and dislocation annihilation. X-ray micro-computed tomography (X-CT) examination showed complete elimination of porosity in the waDED sample, owing to the intense mechanical stirring induced during FSP. Scanning electron microscopy (SEM) images of the waDED samples showed fibrous eutectic networks disrupted by FSP, leading to a uniform distribution of broken Si particles within the α-Al matrix. Hardness decreased by approximately 38 % (86 HV0.1 vs 53 HV0.1), while the average ultimate tensile strength decreased by about 4 % in the FSP-treated samples (159.19 MPa vs 153.02 MPa). Additionally, the formation of alpha fiber and weak cube texture components during FSP enhances ductility, as evidenced by a sixfold improvement in elongation (1.93 % vs 11.59 %). This textural transformation contributes to the characteristic deeper and more uniform dimples in the FSP-treated samples, indicative of ductile fracture, compared to the premature failure patterns in as-deposited samples.

An Internal-State-Variable-Based Continuous Dynamic Recrystallization Model for Thermally Deformed TC18 Alloy.

Continuous dynamic recrystallization (CDRX) is widely acknowledged to occur during hot forming and plays a significant role in microstructure development in alloys with moderate to high stacking fault energy. In this work, the flow stress and CDRX behaviors of the TC18 alloy subjected to hot deformation across a wide range of processing conditions are studied. It is observed that deformation leads to the formation of new low-angle grain boundaries (LAGBs). Subgrains rotate by absorbing dislocations, resulting in an increase in LAGB misorientation and the transition of some LAGBs into high-angle grain boundaries (HAGBs). The HAGBs migrate within the material, assimilating the (sub)grain boundaries. Subsequently, an internal state variable (ISV)-based CDRX model is developed, incorporating parameters such as the dislocation density, adiabatic temperature rise, subgrain rotation, LAGB area, HAGB area, and LAGB misorientation angle distribution. The values of the correlation coefficient (R), relative average absolute error (RAAE), and root-mean-square error (RMSE) between the anticipated true stress and measured stress are 0.989, 6.69%, and 4.78 MPa, respectively. The predicted outcomes demonstrate good agreement with experimental findings. The evolving trends of the subgrain boundary area under various conditions are quantitatively analyzed by assessing the changes in dynamic recovery (DRV)-eliminated dislocations and misorientation angles. Moreover, the ISV-based model accurately predicts the decreases in grain and crystallite sizes with higher strain rates and lower temperatures. The projected outcomes also indicate a transition from a stable and coarse-grained microstructure to a continuously recrystallized substructure.

Low cycle fatigue properties of CoCrFeNiMn high-entropy alloy with heterogeneous microstructure

Heterogeneous microstructure is a hot topic in materials science which facilitates tackling the dilemma of strength-ductility trade-off in engineering materials including high-entropy alloys. The present investigation was conducted on the impact of different microstructures including heterogeneous microstructure, constructed by thermomechanical treatment including cold rolling followed by annealing, on the monotonic and cyclic behavior of a CoCrFeNiMn high-entropy alloy. The tailored microstructure contained three features: non-recrystallized as a hard domain, ultrafine-recrystallized, and fine-recrystallized regions. A desirable combination of strength and ductility including UTS of ∼1.1 GPa together with an elongation of ∼15 % was achieved due to benefiting from hetero-deformation-induced hardening and activation of deformation twins in the fine recrystallized grains. Regarding low-cycle fatigue behavior, at the strain amplitude of 0.4 %, heterogeneous microstructure exhibits remarkable fatigue properties, including high maximum stress and extraordinary lifetime (more than 2 × 105). The compelling reasons for these desirable properties are triggering deformation twins in fine recrystallized grain, the absence of brittle sigma phase as a good location of crack initiation, and the non-recrystallized area as a hard domain for suppressing crack propagation. The results suggested that the heterogeneous microstructure has significant benefits during applying low strain amplitude, however, at high strain amplitude, grain refinement is not recommended, and coarse-grained microstructure provides better properties. The present investigation is a step forward to understand the significance of heterogeneous microstructures to improve fatigue properties of high- or medium-entropy alloys.

Grain size effect on strain localization, slip-grain boundary interaction and damage in the Alloy 718 Ni-based superalloy at 650 °C

Grain size effects on the early plastic strain localization and slip transfer at grain boundaries were investigated for the Alloy 718 Ni-based superalloy at 650 °C. Three microstructures with different grain sizes underwent monotonic tensile tests at 650 °C, both in air and under vacuum, until rupture. All the microstructure variants exhibit fully intragranular fracture under vacuum and partially intergranular fracture in air. In this latter case, predominant intergranular fracture mode was found in the fine-grain microstructures. Interrupted tensile tests were also conducted under vacuum with ex-situ SEM high-resolution digital image correlation (HR-DIC) measurements to assess in-plane kinematics fields at the microstructure scale. Out-of-plane displacement jumps were obtained using laser scanning confocal microscopy. Both crystallographic slip within grains and near Σ3 twin boundaries (TBs) and morphological sliding happening at grain boundaries (GBs) were documented. Statistical analysis of all plastic events aimed at quantifying strain localization distribution as a function of the microstructure. The fine-grain microstructure was found to have extensive strain localization at grain boundaries, while the coarse-grain microstructure is more prone to intragranular slip development and slip localization near TBs. Different scenarios of slip band/grain boundary interactions were evidenced.

Regulation of cryogenic mechanical behaviors of C-added non-equiatomic CoCrFeNiMo ferrous medium-entropy alloy via control of initial microstructure

This study demonstrated the potential for customizing the desired properties of the Co18.5Cr12Fe55Ni9Mo3.5C2 (at.%) ferrous medium-entropy alloy by manipulating the deformation-induced martensite transformation (DIMT) behavior at liquid nitrogen temperature. This was achieved by modifying various initial microstructures through annealing at temperatures ranging from 900 to 1200 °C. The variations in DIMT kinetics were analyzed based on two main factors. (1) Inducing carbide precipitation by annealing at 900 and 1000 °C results in changes in the composition within the matrix, which may affect the stability of the face-centered cubic phase. Samples with a higher volume fraction of the carbide precipitates exhibit lower ΔGFCC→BCC and faster DIMT kinetics. (2) The onset and kinetics of DIMT are also affected by the use of martensite nucleation sites, which may vary depending on the presence of non-recrystallized regions or the grain size. In fine-grained structures, martensite primarily nucleated in the non-recrystallized regions and grain boundaries. However, in coarse-grained microstructures, martensite mainly nucleated along the in-grain shear bands and their intersections. This precise control of the microstructure results in superior properties. The samples annealed at 900 and 1000 °C with carbide precipitates and fine grains exhibit ultrahigh ultimate tensile strength, which may reach elevated values up to ∼1.8 GPa, while those annealed at 1100 and 1200 °C with larger grains and no precipitates exhibit a uniform elongation that exceeds 100 %.

Powder-size driven facile microstructure control in powder-fusion metal additive manufacturing processes

Microstructure control in metal additive manufacturing is highly desirable for superior and bespoke mechanical performance. Engineering the columnar-to-equiaxed transition during rapid solidification in the additive manufacturing process is crucial for its technological advancement. Here, we report a powder-size driven melt pool engineering approach, demonstrating facile and large-scale control in the grain morphology by triggering a counterintuitive response of powder size to the additively manufactured 316 L stainless steel microstructure. We obtain coarse-grained (>100 μm) or near-monocrystalline microstructure using fine powders and near-equiaxed, fine-grained (<10 μm) microstructure using coarse powders. This approach shows resourceful adaptability to directed energy deposition and powder-bed fusion with no added cost, where the particle-size dependent powder-flow preheating effects and powder-bed thermophysical properties drive the microstructural variations. This work presents a pathway for leveraging feedstock particle size distribution towards more controllable, cost-effective, and sustainable metal additive manufacturing.

A low-cost Mg–Al–Mn–Zn alloy for automotive road wheel applications

In this study, a new Mg–Al–Mn–Zn alloy (AMZ211) was designed, aided by thermodynamic modeling, for applications in automotive road wheels using forging and flow-forming processes. The AMZ211 alloy was cast and forged in an industrial scale to obtain a forged blank to evaluate its tensile properties and hot formability under high-strain-rates (simulating flow-forming conditions). Multiple characterization techniques, including optical, scanning electron and scanning transmission electron microscopy as well as electron backscatter diffraction, were applied to investigate the microstructural evolution during the processing route. As expected from the alloy design, nano-sized Mn-containing dispersoids formed in a homogeneous manner during homogenization heat treatment, contributing to the formation of refined dynamically recrystallized (DRXed) grains in as-forged microstructure. The as-forged AMZ211 alloy showed better hot formability than as-forged AZ80 Mg alloy under high-strain-rate condition as well as a comparable strength property in relative to the benchmark and more expensive Mg–Zn–Zr alloys. The lower ductility of as-forged AMZ211 alloy may be due to the high twinning tendency of coarse unDRXed domains which were formed as a result of the original coarse-grained microstructure from solidification. The present study provides a low-cost Mg alloy for high-volume automotive road wheel applications.

Enhancing Durability and Capacity Retention of Ultrafine-Grained Aluminum Foil Anodes in Lithium-Ion Batteries.

In this study, we present our successful fabrication of commercial-grade pure aluminum anode foil (99.5%, 2NAl) with an ultrafine-grained (UFG) microstructure and high hardness, achieved through cold rolling. Under identical rolling conditions, a coarse-grained microstructure with a low hardness was attained from the high-purity Al foil (99.99%, 4NAl). The UFG 2NAl foil exhibited enhanced lithium-ion diffusivity and reduced nucleation and activation overpotentials for forming the β-LiAl phase compared to the 4NAl foil. The high-density grain boundaries in the UFG 2NAl foil facilitated the rapid formation of a uniform β-LiAl phase layer on its surface, thereby mitigating mechanical damage within the β-LiAl phase layer caused by volume changes during the lithiation and delithiation processes. The high hardness of the UFG 2NAl sample effectively prevented macroscopic plastic deformation during cycling, thus preserving the integrity of the β-LiAl phase layer and inhibiting the formation of cracks within the unreacted Al matrix. The collective advantages of reduced overpotential, enhanced Li-ion diffusivity, and high resistance to mechanical damage and plastic deformation in UFG 2NAl contribute to its superior durability and capacity retention compared to the high-purity Al in electrochemical cycling.

Rapid Solidification Techniques for Metal Processing: Microstructure and Properties

In recent years, rapid solidification techniques have garnered considerable attention due to their capacity to yield metals with unique microstructures and properties unattainable through conventional processing methods. These techniques involve swiftly cooling the metal, resulting in a fine-grained microstructure, unlike the coarse-grained microstructures typically obtained through traditional methods. This paper offers an overview of several rapid solidification techniques, such as melt spinning, spray forming, and gas atomization. These methods entail rapidly cooling molten metal by either quenching it onto a swiftly rotating wheel or spraying it onto a substrate. The cooling rates achieved, typically ranging from 104 to 107 K/s, far exceed those of conventional casting methods. The distinctive microstructure resulting from rapid solidification techniques presents numerous advantages over conventional methods. These include enhanced homogeneity, increased strength, improved ductility, heightened thermal stability, and augmented corrosion resistance. Consequently, rapid solidification techniques hold significant promise for applications across aerospace, automotive, and medical industries. This review also underscores recent advancements in rapid solidification techniques and their potential applications. These encompass the production of high-strength, wear-resistant materials for cutting tools, the development of lightweight alloys for aerospace applications, and the manufacture of biocompatible alloys for medical implants. In summation, this review offers a comprehensive examination of the latest progress in rapid solidification techniques for metal processing and their diverse applications across various industries.

Improvement of intergranular corrosion resistance of SS 304L by grain refinement followed by gamma-ray irradiation treatment for the application in the back end of the nuclear fuel cycle

The severe plastic deformation (SPD) technique has been widely used for grain refinement to produce bulk ultra fine-grained (UFG) microstructure. In this study, an improved corrosion-resistant material of type 304L stainless steel (SS) has been designed, from the coarse-grain (CG) microstructure to the UFG microstructure using the SPD technique through cryo-rolling at liquid nitrogen (at 77K) temperature followed by gamma-ray irradiation treatment. In comparison to the effect of gamma-ray on both CG and UFG microstructure, both specimens were irradiated with an equal amount of doses. Gamma (γ) ray irradiation (dose rate: 2.83 kGy/h) affected the corrosion resistance of AISI type 304L stainless steel (in both CG and UFG) in concentrated nitric acid (65 wt%) medium. The correlation of irradiation effects on the microstructure and nitric acid corrosion behaviors has been discussed, which is important for designing materials for application in nuclear-spent fuel reprocessing plants.

Cyclic plastic response of rotary swaged Fe-14Cr-10Al-4Y2O3 ODS alloy at 1000–1200 °C

This study is dedicated to the investigation of the cyclic stress-strain response of a newly developed ferritic steel reinforced with a high-volume fraction of oxide nanoparticles. The mechanically alloyed powder was consolidated by hot rotary swaging into cylindrical rods with diameters of 15 mm and 12 mm. Subsequently, the rods were annealed at 1200 °C for 4 h to achieve a fully recrystallized coarse-grained microstructure optimal for high-temperature applications. The cyclic tests were carried out in controlled deformation mode in a symmetrical push-pull cycle with triangular waveform at 1000 °C and 1200 °C in air. The Fe-14Cr-10Al-4Y2O3 ODS steel exhibits extraordinary cyclic strength of up to 140 MPa at temperature of 1200 °C. The results show that rotary swaging to a smaller diameter does not affect the resulting stress-strain response. The coarsening of the Y2O3 nanoparticles occurs only very slowly even at 1200 °C. In areas with significant plastic deformation, a slight rotation of the coarse grains from the ideal [111] direction and partial dynamic recrystallization was observed.

Exceptional improvement in the yield strength of AZ61 magnesium alloy via cryo-stretching and its implications on the grain growth during annealing

This study explored the influence of pre-stretching temperature on the microstructural characteristics and deformation behavior of AZ61 magnesium alloy and its implications on grain growth during subsequent annealing. AZ61 alloy was stretched to 5% plastic strain along rolling (RD) and transverse direction (TD) at room (RT) and cryogenic temperature (−150 °C, CT) followed by annealing at 320 °C for 1 h to investigate the twinning and dislocation evolution and its consequent effect on the flow stress, plastic strain and strain hardening rate. Compared to RT-stretched samples, significant improvement in yield stress, strain hardening rate and moderate reduction in elongation to failure were witnessed for CT-stretched samples along RD and TD. The subsequent EBSD analysis revealed the increased fraction of fine {10−12} twins and nucleation of multiple {10−12} twin variants caused by higher local stress concentration at the grain boundaries in CT-stretched samples as manifested by the kernel average misorientation. This higher twin fraction and twin-twin interaction imposed the strengthening by restricting the mean free path of dislocations leading to higher flow stress and strain hardening rate. During annealing of the RT/CT-stretched samples, the residual strain energy and twin boundaries were decreased due to static recovery leading to a coarse-grained twin-free microstructure. Strain induced boundary migration (SIBM) was found to be the predominant mechanism governing the grain growth during annealing via movement of high angle grain boundaries.

Development and testing of coarse-grained models for ultrasonic simulations of cast austenitic stainless steel

Ultrasonic inspection of cast austenitic stainless steel (CASS) in the nuclear industry is particularly challenging because of sound field scatter and attenuation caused by the coarse-grained microstructure. Modeling and simulation are important tools in ultrasonic testing, as they can be used to help address key aspects of inspections, such as developing new probe designs, predicting inspection reliability, and testing phased-array focal laws. However, developing a useful and reliable CASS model is challenging due to the many grain interfaces and crystalline orientations that must be captured. We demonstrate a method of creating a realistic CASS model that is usable in CIVA, a commercially available modeling and simulation software platform. Using polished and chemically etched sections to highlight the grain structure and orientation, we generate models of a coarse-grained equiaxed specimen and a columnar specimen. We also test an alternative method of generating a coarse-grained model using Voronoi regions. We qualitatively compare sound field scatter and quantitatively compare sound field attenuation and beam partitioning in simulated sound fields to those of laboratory-measured sound fields. Results show that the Voronoi models perform as well as or better than the models based on actual grain morphology. We also show that model-to-model randomness in Voronoi grain structure can impact the magnitude of a simulated echo response by a factor of two or more. Although CASS models are potentially a good depiction of reality for a given scenario, they should not be considered representative since CASS morphology can change significantly from specimen to specimen or within the same specimen.

Effect of Modifier Form on Mechanical Properties of Hypoeutectic Silumin.

Aluminum-silicon alloys require modification due to their coarse-grained microstructures and resulting low strength properties. So far, research into the modification process has focused on the use of various chemical components and technological processes, the tasks of which are to refine the microstructure and, thus, increase the mechanical properties of the alloy. In this paper, the answer to the question of whether the form of the modifier influences the modification effect of the hypoeutectic silumin will be found. The tests were carried out using the popular silumin AlSi7Mg. To answer our research question, the alloy was modified under comparable conditions using the following elements: Ti, B, and master alloys AlTi1.5 and AlB1.5. Modifiers in the form of Sr and master alloy AlSr1.5 were also used. All mentioned modifiers were produced and introduced into the liquid alloy in the form of a powder and a rod. Master alloys AlSr1.5 were also produced via cooling from the liquid state through cooling in air and the second variant at a speed of 200 °C/s (in the form of powder and a thin strip). The microstructure and mechanical properties were analyzed based on the following measures: tensile strength, elongation, and hardness of silumin. Based on the conducted research, it was found that the form of the modifier also affects the modification effect visible in the form of changes in the microstructure and mechanical properties. For the powder-modified alloy, greater fineness in the eutectic phase (α and B phases) and an increase in all analyzed mechanical properties were obtained.

Hot Workability Investigation of an Fe-Al-Ta Alloy Using Deformation Processing Maps

Fe-Al-Ta alloys are expected to replace high-alloyed steels in steam turbine blades. However, the mechanical properties of the forged blades are still not optimal due to limited grain refinement during hot forging and the coarse-grained microstructure inherited from the as-cast precursor. It is, therefore, essential to investigate the hot deformation behavior of the alloy to identify the optimum range for the deformation parameters leading to good hot workability with significant grain refinement. The hot deformation behavior and hot workability of an Fe-25Al-1.5Ta (at.%) alloy were investigated in the present work using constitutive modeling and the concept of processing maps. Uniaxial compression tests were conducted in a strain rate range from 0.0013 s−1 to 1 s−1 and in a temperature range from 900 °C to 1100 °C, where a disordered A2 α-(Fe, Al) matrix phase along with a C14-(Fe, Al)2Ta Laves phase were confirmed by X-ray diffraction. The flow stress–strain curves showed a broad maximum followed by a slight drop in stress until a steady state was reached. The optimum processing window for the studied alloy was located at 910–1060 °C/0.0013–0.005 s−1, where the efficiency of the power dissipation (η) and strain rate sensitivity (m) reached 50% and 0.33, respectively. The material underwent a combination of dynamic recovery and dynamic recrystallization over the whole tested deformation range. No flow instabilities were predicted based on Prasad’s flow instability criterion when deformation was performed up to a true strain of 0.5 and 0.8, indicating a high degree of hot workability of the studied alloy over the entire deformation range tested. The current study reveals a well-suited parameter range for the safe and efficient deformation of Fe-Al-Ta alloys, which may contribute to the optimization of the thermomechanical processing of this alloy.

Deformation mechanisms and rheology inversion of jointly deformed marble and quartzite in natural thermal gradient

Amphibolite facies marble surrounded by locally pure but mostly impure quartzite with ∼10–14% of dispersed white mica and crosscut by granite veins were jointly deformed within an extensional shear zone. Both marble and quartzite show three microstructural stages, M1m-M3m and M1q-M3qm, respectively, correlated with the onset of extension Stage 1 at ∼600 °C, and retrograde extension Stage 2 at ∼380 °C and Stage 3 at ∼300 °C. The coarse-grained microstructures M1m (1200 μm) in marble and M1q (300 μm) in pure quartzite showed the activity of basal ⟨a⟩ slip and prism ⟨a⟩ − rhomb ⟨a⟩ slip, respectively. The finer grained M2m (∼50 μm) microstructure in marble is associated with strain localization, subgrain rotation recrystallization and persistence of basal ⟨a⟩ slip. The M2q (175 μm) microstructure in pure quartzite shows a crossed girdle c-axis pattern, while the M2qm (120 μm) in impure quartzite shows locally c-axes parallel to the lineation, indicating the possible activity of prism [c] slip. Stage 3 in marble is characterised by minor recrystallization M3m that occurs mostly along microcracks formed subparallel to foliation. The impure quartzite M3qm microstructure shows further reduction in quartz grain size (80–90 μm) related to pining of well-dispersed white mica. The microstructure M1m-M1q and M2m-M2q formed by dislocation creep, while the M2qm and M3qm suggest the Rachinger grain boundary sliding of quartz grains along micas. Piezometric calculations suggest differential stresses of 1–5 MPa and ∼20 MPa for the M1m and M2m microstructures and 7 MPa and 11 MPa for the M1q and M2q microstructures, respectively. Comparison of these data with experimental flow laws confirmed that during Stage 1 marble is weaker than quartzite. However, during Stages 2 and 3, mica in impure quartzite became more dispersed but also locally aggregated; thus, the bulk strength of quartzite became governed by mixture of quartz and weak mica behaviour. This change in microstructure and the resulting deformation mechanism facilitated fluid transport across quartzite, which resulted in embrittlement of dominantly coarse-grained and impermeable competent marble. This process is accompanied by the formation of stretching faults developed parallel to weak layers of altered granite dykes in the vicinity of the marble-quartzite contact. The rheology inversion thus results from a mixture of deformation mechanisms in the weak polyphase quartzite and fluid-induced drop of effective pressure of strong and brittle marble at late stages of the extensional deformation.

Near‐Threshold Fatigue Crack Propagation in Sintered 304L Stainless Steel Compact with Network Structure Composed of High‐Entropy CrMnFeCoNi Alloy

High‐entropy alloys (HEAs) have several superior mechanical properties. However, for these alloys, there is a trade‐off between tensile strength and fatigue‐crack propagation resistance, as is the case for materials with a homogeneous microstructure. The purpose of the present study is to achieve both high‐strength and fatigue‐crack propagation resistance by using a bimodal structure. CrMnFeCoNi alloy and AISI304L powders with different particles diameters are mixed and then consolidated using spark plasma sintering to fabricate sintered compacts with a network structure composed of coarse‐grained 304L surrounded by fine‐grained HEA microstructure. Stress intensity factor K‐decreasing tests are conducted at various force ratios in the ambient laboratory atmosphere to examine near‐threshold fatigue‐crack propagation in the sintered compacts. The ductility and threshold stress intensity range ΔKth tested at a high force ratio for the sintered compacts are higher than that for a homogeneous fine‐grained CrMnFeCoNi alloy. Fatigue cracks in the sintered compacts mainly propagate through the coarse‐grained 304L microstructure, which has high‐fatigue‐crack propagation resistance near the threshold. Therefore, ΔKth for the sintered compacts increases because the ΔKeff,th, which is derived from the 304L without degrading the tensile strength, is higher than that for homogeneous fine‐grained CrMnFeCoNi alloy.

Dependence of Charpy Impact Properties of Fe-30Mn-0.05C Steel on Microstructure

Fe-30Mn-0.05C steel specimens with cold-rolled, partially recrystallized, fine-grained, and coarse-grained microstructures were fabricated by means of 80% cold rolling followed by annealing at 550–1000 °C. The initial and deformed microstructures were characterized, and the Charpy impact properties were tested at room temperature (RT) and liquid nitrogen temperature (LNT). It was found that the Charpy absorbed energy increased with the annealing temperature, while the specimens showed different trends: parabolic increase at RT and exponential increase at LNT, respectively. Compared with the fully recrystallized specimens, those with a partially recrystallized microstructure exhibited lower impact energy, especially at LNT. This was because cracks tended to nucleate and propagate along the recovery microstructure where stress concentration existed. The grain size played an important role in the twinning behavior and impact properties. High Charpy impact energy (~320 J) was obtained in the coarse-grained specimen having the grain size of 42.1 μm at both RT and LNT, which was attributed to the activation of high-density deformation twinning. However, deformation twinning was inhibited in the specimen with the average grain size of 3.1 μm, resulting in limited work hardening and lower impact energy.

Temperature and microstructure dependent tensile behavior of coarse grained superelastic NiTi

In this work, effects of temperature and microstructure on tensile behavior of superelastic NiTi wires were analyzed. We analyzed cold-drawn NiTi wires of 1.78 mm that received four different final heat treatments using short direct electrical current pulses. Final coarse-grained microstructures ranged from recovered up to fully recrystallized microstructure with grain size of 25 µm. Effects of temperature in the range 30–130 °C were analyzed using monotonous tensile tests where a temperature gradient was induced along the sample axis. Deformation processes related to martensitic transformation and plastic deformation were characterized by temperature resolved stress–strain responses, deformation work, released heat, volume fraction of martensite. Deformation-induced microstructure changes were analyzed post-mortem using Electron Backscatter Diffraction and x-ray diffraction. Except the least recovered microstructure, the samples did not follow linear temperature trends according to Clausius-Clapeyron. With increasing temperature, the plateau stress of localized martensitic transformation stabilized at a constant stress while volume fraction of martensite decreased down to zero. Localized strains induced by martensitic transformation and ductility evolved non-monotonously with temperature, reaching maxima at temperatures that decreased with increasing microstructure recovery or recrystallization. Large ductility was positively correlated with occurrence of austenite deformation twins in deformed microstructure.