Furnace Cooling Research Articles

Experimental investigation on the post-fire mechanical properties of YSt-355FR (0.1 % Mo) low-alloyed fire-resistant steel

Fire-resistant steel (YSt-355FR (0.1 % Mo)) has become the most utilized structural steel due to its superior performance at elevated temperatures compared to traditional structural steel at both the material and structural levels. However, its performance after exposure to fire remains questionable. This study investigates the post-fire performance of cold-formed fire-resistant steel at the material level. A total of 132 steel coupons and 84 Charpy test specimens were exposed to temperatures ranging from 100°C to 1000°C. These temperature-exposed steel coupons were subjected to four different cooling conditions: furnace cooling (FC), air cooling (AC), water-jet cooling (WJC), and water cooling (WC). Tensile tests were performed on the cooled steel coupons to obtain post-fire stress-strain curves, from which the residual mechanical properties were determined. Charpy impact tests were also conducted on the cooled steel specimens to obtain impact energies and identify the fracture patterns. Key observations regarding the post-fire behavior of fire-resistant steel in terms of strength, deformability, stiffness, and toughness parameters were made. Both the exposure temperature and the cooling condition significantly influence the post-fire performance of fire-resistant steel. Fire-resistant steel demonstrates superior post-fire behavior compared to traditional structural steel. Constitutive equations were proposed to predict the mechanical properties of fire-resistant steel under various post-fire conditions, and a sound reliability analysis was performed to examine the reliability of the predicted design parameters compared to the test results. The findings presented in this paper are significant as no prior investigation has assessed the post-fire performance of fire-resistant steel; thus, they may encourage the steel construction industry to prefer fire-resistant steel over traditional structural steels and promote the reuse of temperature-exposed fire-resistant steel for sustainable construction.

Effect of cooling rates and heat treatment time on Si phase of AlSi10Mg alloy manufactured by selective laser melting: Microstructure evolution and strengthening mechanism

As a processing technique to change the organization and properties of materials, heat treatment was widely used to strengthen metallic materials. This study examines the impact of varying cooling rates and heat treatment durations on the mechanical properties and microstructure of AlSi10Mg alloys produced through Selective laser melting (SLM). The heat treatment process was Solution Heat Treatment (SHT, 525 °C), and the heat treatment time includes 1 h, 2 h, and 3 h. Cooling methods consist of Refrigerant Cooling (RC), Water Cooling (WC), Air Cooling (AC), Oil Cooling (OC), and Furnace Cooling (FC). As the cooling rate decreases and the heat treatment time increases, the size of the formed Si phase particles varies from small to large, and the distribution deviates from inhomogeneous to homogeneous. In addition, the number of precipitated particles increases and then decreases. To investigate the impact of cooling rates and heat treatment time on the mechanical properties of AlSi10Mg alloys, quasi-static tensile and compression tests were performed. On the other hand, the tensile strength of the heat-treatment samples decreases while the ductility increases, and the Vickers hardness of the samples also decreases with decreasing cooling rate and increasing heat treatment time. The results of the study showed that the cooling rate and heat treatment time of the material had a greater effect on its tensile properties than on its compressive properties. The samples treated at 525 °C - 2 h - AC exhibited the highest tensile and compressive properties. Finally, the analysis of the strengthening mechanism of AlSi10Mg alloys through the presence of Si phase particles of varying sizes was conducted using transmission electron microscopy.

Improved properties of additively prepared Inconel 718 alloy post-processed with a new heat treatment

In this study, an alternative heat-treatment route has been explored to optimize the mechanical as well as the fatigue properties of Ni-based 718 alloy fabricated using additive methods. The test coupons were prepared using the laser-based powder bed fusion technique and post-processed with a new heat-treatment condition (NHT): solution treatment at 1130°C for 2hr followed by water cooling (WC) and the artificial aging at 730°C for 13hr followed by furnace cooling (FC). It was found that the NHT parameters enhanced the mechanical properties as well as the fatigue properties as compared to the standard heat-treatment (SHT) conditions: solution treatment at 980°C-1hr-FC followed by two-step artificial aging 720°C-8hr-FC(55°C/hr up to 620°C) + 620°C-8hr-FC. The NHT condition resulted in a more homogenized and recrystallized microstructure with a higher volume fraction of γ′ and γ′′ precipitates, high-angle coincidence site lattice (CSL) as well as the twin boundaries and marginally reduced grain size. The microhardness, tensile strength, and fatigue strength after the NHT condition were improved by ∼15%, ∼16%, and ∼12.5%, respectively, and it was mainly accredited to the high volume fraction of fine strengthening precipitates and homogenized microstructure.

Hot deformation behavior and dynamic recrystallization of 2195 Al–Li alloy with various pre-precipitation microstructures

In this work, 2195 Al–Li alloys with various precipitation microstructures were obtained by homogenization treatment followed by air-cooling (AC), discontinuous cooling (DC), and furnace-cooling (FC), and then tested by hot compression at different temperatures. The results show that the flow stresses of all specimens decrease with temperature and the peak stresses of AC, DC and FC specimens decrease in order at the same temperature. For FC samples, coarse pre-precipitates diminish the deformation resistance due to the reduction of solid solution strengthening and precipitation strengthening. Dynamic softening at low temperatures is significantly greater than that at high temperatures, except for the anomalous softening of the FC specimen at 520 °C due to stress release by cracks. At 370–420 ​°C, the dynamic softening of the AC specimen is more significant than the other samples, resulting from dynamic precipitation and precipitate coarsening. Furthermore, there is considerable dynamic recrystallization (DRX) in FC samples and not in AC and DC samples, although their precipitates coarsen to similar levels at low temperatures. This suggests that DRX is associated with the particle-stimulated nucleation mechanism by regularly arranged precipitates. The banded or regularly arranged precipitates divide the Al matrix into multiple confined units, hindering dislocations and (sub)grain boundary movement, and thus promoting the development of sub-grains and DRX.

Enhanced strength-ductility-impact toughness synergy of CT20 titanium alloy by introducing a multi-level lamellar martensitic α structure

In this work, different morphologies and structures of lamellar α are obtained by controlling the cooling rates, i.e., water cooling (WC), air cooling (AC) and furnace cooling (FC), and their effects on tensile properties and impact toughness of a new near α CT20 alloy (Ti-3Al-2Zr-1Mo) are investigated. Results showed that a heterogeneous three-dimensional network lamellar martensite α structure was formed after WC, a mixed structure composed of fine lamellar α/β and a small amount of martensite was formed after AC, and a coarse lamellar α structure was formed after FC. The tensile strength gradually decreased with decreasing cooling rate, and AC had the superior strength-plasticity match (yield strength was 13% higher than FC while their plasticity is equivalent). However, the impact toughness of AC (comparable to that of WC) was about 16% lower than that of FC (90 J/cm2). During tensile deformation, the synergistic effect of the good plasticity of the α colony, the obstruction of dislocations by the colony interface and the plastic deformation of the martensite of the AC contribute to the best strength-plasticity matching. During impact, crack initiates and propagates rapidly at the V-notch root, and the finite element simulation results showed that the plastic deformation was concentrated in a tiny region near the crack tip, resulting in the coordination deformation among the microstructures in AC cannot be fully realized. By contrast, the coordinated deformation of multi-level martensite and the generation of nano twins in WC make its impact toughness comparable to that of AC. The highest impact toughness of FC was attributed to the superior plasticity of coarse lamellae, slip transfer between coarse α and β lamellae, the generation of twins and the formation of a small jagged and large wave-like composite crack propagation path.

Research on Hydrogen-Induced Induced Cracking Sensitivity of X80 Pipeline Steel under Different Heat Treatments.

X80 pipeline steel has played a vital role in oil and gas transportation in recent years. However, hydrogen-related issues frequently lead to pipeline failures during service, resulting in significant losses of properties and lives. Three heat treatment processes (furnace cooling (FC), air cooling (AC), and water cooling (WC)) were carried out to investigate the effect of different microstructures on hydrogen-induced cracking (HIC) susceptibility of X80 pipeline steel. The WC sample demonstrated the highest hydrogen embrittlement index, registering at 21.9%, while the AC and FC samples exhibited progressively lower values of 15.45% and 10.98%, respectively. Under equivalent hydrogen charging durations, crack dimensions with a maximum length exceeding 30 μm in the WC sample generally exceed those in the FC sample and AC sample. The variation is attributed to the difference in microstructures of the samples, predominantly lath bainite (LB) in water-cooled samples, granular bainite (GB) in air-cooled samples, and ferrite/pearlite (F/P) in FC samples. The research results demonstrate that the sensitivity of lath bainite (LB) to HIC is significantly higher than that of pearlite, ferrite, and granular bainite (GB). The presence of a large amount of martensite/austenite (M/A) constituents within bainite results in a multitude of hydrogen trap sites. HIC cracks in bainite generally propagate along the profiles of M/A constituents, showing both intergranular and transgranular cracking modes.

Effect of Microstructure on Corrosion Resistance of Ultralow‐Carbon Bainitic Steel

The effect of microstructure on the corrosion behaviors of ultralow‐carbon bainitic (ULCB) steel in 3.5 wt% NaCl solution is discussed. Four ULCB steel specimens with different microstructures are designed via controlling the cooling routes after austenization, namely, furnace cooling (FC), air cooling (AC), oil cooling (OC), and water cooling (WC). Their corrosion behaviors are investigated by alternating immersion test (including weight loss and rust layer observation) and electrochemical tests (including polarization curves and electrochemical impedance spectroscopic measurements). Results show that the corrosion resistance of ULCB steel specimens decreases with increase of cooling rate, that is, in the following sequence: FC > AC > OC > WC. Furthermore, the electron backscatter diffraction characterization and scanning Kelvin probe force microscopy are performed to build the relationship between microstructure and corrosion behaviors of ULCB steels. The reduced corrosion resistance of ULCB steels results from the increased number of microgalvanic couples because of the significantly increased density of grain boundaries with increase of cooling rate.

Effect of Heat Treatment Duration and Cooling Conditions on Flexural Strength and Surface Roughness of Cobalt-Chromium Alloys produced by Selective-Laser-Melting

Post-heat treatment may enhance mechanical properties of selective laser melting (SLM) Cobalt chromium (Co-Cr) alloys, however it is unknown what duration the heat treatment needs to last and how the rate of cooling impacts the alloy's flexural strength and surface roughness. In this study, SLM Co-Cr alloy specimens were heated at 1150 C for 1 or 6 hours and subsequently cooled by air cooling (AC), furnace cooling (FC), or water quenching (WC). Flexural strength and surface roughness were then tested. Aim of this study: Investigate the effect of heat treatment and different cooling conditions on the flexural strength and surface roughness of SLM Co-Cr alloy specimens. The heat treatment will be conducted at 1150 C for two different durations (1-hour and 6-hours) and under three cooling conditions (AC, FC, and WC). Materials and methods: A total of forty-two rectangular specimens, measuring (34×13×1.5 mm) were manufactured through SLM and divided into seven groups, six specimens for each group as follows: The first group denoted as control with no heat treatment and no cooling applied, second and third groups heated for 1 or 6 hours then cooled in the air (AH-1 and AH-6), fourth and fifth groups heated for 1 or 6 hours and left inside the furnace until they cooled (FH-1 and FH-6), sixth and seventh groups heated for 1 or 6 hours and cooled by water quenching (WH-1 and WH-6). Flexural strength and surface roughness tests were performed on the specimens. A flexural strength value of (2147.33 MPa) was indicated that the control group exhibited the highest flexural strength, whereas the FH-6 group exhibited the lowest flexural strength value (1282.66 MPa). The control group showed the worst surface roughness (1.63 μm) while all other groups demonstrated no significant differences in surface roughness ranging from (0.32 to 0.49 μm) with a slight increase in the 6-hours heated groups. Conclusions: Slow cooling rate inside the furnace affects flexural strength negatively. Therefore, a high cooling rate is recommended to get a better flexural strength. On the other hand, surface roughness results suggests that 1-hour is better than 6-hours in terms of heat treatment duration and time saving

The Effect of Cooling Rate on Crystallographic Features of Phase Transformations in Zr-2.5Nb.

Zirconium (Zr) alloys are utilized as structural components for the cores of nuclear reactors due to the excellent combination of their mechanical properties and corrosion resistance under intense neutron irradiation conditions in water. The characteristics of microstructures formed during heat treatments play a crucial role in obtaining the operational performance of parts made from Zr alloys. This study investigates the morphological features of (α + β)-microstructures in the Zr-2.5Nb alloy, as well as the crystallographic relationships between α- and β-phases. These relationships are induced by the β→α(α″) displacive transformation that occurs during water quenching (WQ) and the diffusion-eutectoid transformation that takes place during furnace cooling (FC). To conduct this analysis, samples solution treated at 920 °C were examined using EBSD and TEM. The experimental distribution of α/β-misorientations for both cooling regimes deviates from the Burgers orientation relationship (BOR) at a discrete set of angles close to 0, 29, 35, and 43°. The experimental α/β-misorientation spectra are confirmed by crystallographic calculations for the β→α→β-transformation path based on the BOR. Similar spectra of misorientation angle distribution in α-phase and between α and β phases in Zr-2.5Nb after WQ and FC point to similar transformation mechanisms and the significant role of shear and shuffle in β→α-transformation.

Introducing lamellar LPSO phase to regulate room and high-temperature mechanical properties of Mg-Gd-Y-Zn-Zr alloys by altering cooling rate

The mechanical properties of the extruded and peak-aged Mg-12.8Gd-4.7Y-2.6Zn-0.9Zr (wt.%) (GWZ1352) alloys at room and high temperatures were systematically investigated with respect to their microstructures. The microstructural features were tailored through altering the cooling rates after homogenization by water-quenching, air-cooling and furnace-cooling, respectively. As the cooling rate reduced, the number of lamellar long period stacking ordered (LPSO) phase increased substantially and its spacing decreased gradually. Additionally, solute segregation at grain boundaries occurred after furnace-cooling. After extrusion, the quenching (QE) alloy exhibited nearly full dynamic recrystallized (DRXed) grains, whereas the air-cooling (AE) and furnace-cooling (FE) alloys displayed a typical bimodal structure consisting of fine DRXed and coarse un-DRXed grains. The nucleation and development of the DRXed grains were hampered by the nano-spaced lamellar LPSO phase and the solute segregation of grain boundary. The excellent high temperature mechanical properties of the FE alloy were mostly due to the abundant lamellar LPSO phase. After aging treatment, plentiful fine β′ phase was precipitated in the quenching and aged (QEA) alloy. In contrast, the β′ phase with a larger size and lower density, accompanied with a wide precipitation-free zone (PFZ) at grain boundaries was formed in the air-cooling and aged (AEA) and furnace-cooling and aged (FEA) alloys, which severely deteriorated their mechanical properties both at room and high temperatures.

Effect of solution cooling rates on microstructure and mechanical properties of K648 high chromium superalloy additive-manufactured by the extreme high-speed laser metal deposition

High chromium superalloy K648, additive-manufactured by extreme high-speed laser metal deposition (EHLMD) process, was heat treated by solution and then cooled at different cooling rates achieved by different conditions in this study. The investigation focused on the impact of solution cooling rates (water-cooling (WC), air-cooling (AC) and furnace-cooling (FC)) on the microstructure and mechanical properties of EHLMD K648 superalloy. As the solution cooling rates decrease, it was observed that grain sizes, carbide, and α-Cr grew larger. Simultaneously, the carbides at the grain boundary evolved from a discontinuous granular shape to a chain-like shape, while α-Cr transformed from short needle to a short rod shape. Concurrently, the quantities of secondary γʹ phase diminished, but the size of secondary γʹ phases enlarged as the cooling rate decreased. Additionally, a significant amount of small granular tertiary γʹ phase emerged in the FC superalloy. The microhardness of the EHLMD superalloy exhibited a substantial increase with the decrease of the solution cooling rate, to the point that the microhardness of FC superalloy reached 429 HV. The ultimate tensile strength and yield strength increase, but the elongation decreases significantly, with the ultimate tensile strength of FC superalloy reaching 1058.3 MPa. The fracture of EHLMD K648 superalloy gradually transformed into ductile fracture to ductile and brittle mixed fracture as solution cooling rates decreased.

Effect of Magnesium Treatment and Cooling Method on Manganese Sulfide in U75V Heavy‐Rail Steel Deoxidized by Si–Mn

The effect of magnesium (Mg) on the morphology of sulfides in silicon–manganese deoxidized low‐sulfur and low‐aluminum steels is investigated by adding different amounts of nickel–magnesium (Ni–Mg) alloy to a heavy‐rail steel U75V melt and cooling the melt from 1600 °C to room temperature under different conditions. Herein, the effects of Mg content and cooling rate on MnS inclusions are investigated. It is indicated that with an increase in the Mg content in molten steel from 0 to 64 ppm, the main composition of inclusions changes following the path of CaO–SiO2–Al2O3 → MgO–Al2O3–MnS → MgO–MnS–MgS. Mg significantly inhibits the precipitation of long‐strip MnS and can decrease the average size and aspect ratio of the inclusions. Under furnace cooling, when Mg is 41 ppm, the minimum aspect ratio of the inclusions is 1.61. The higher the cooling rate, the smaller the inclusion size; however, compared to water and furnace coolings, with cooling rates of 126.3 and 0.12 °C s−1, respectively, the inclusion aspect ratio of the air‐cooling conditions with a cooling rate of 67.7 °C s−1 is low. Finally, thermal simulations verify the effect of Mg treatment on the suppression of MnS deformation during rolling.

Revealing complex precipitation behavior of multicomponent Al83Zn5Cu5Mg5Li2 alloy

We investigated the microstructural evolution and precipitation behavior of a multicomponent Al83Zn5Cu5Mg5Li2 alloy using transmission electron microscopy (TEM) and atom-probe tomography (APT) during solution treatment, cooling, and natural aging. The as-cast alloy consisted of high volume fractions of V-Al5Cu6Mg2 and η-Mg(Zn,Cu,Al) phases, which were partially dissolved during solution treatment at 450 °C. Slow furnace cooling (FC) after solution treatment resulted in the formation of polygonal V-phase, elongated η-phase, and platelet Y-phase precipitates. The Y-phase precipitates consisted of one Mg-rich central layer and three adjacent Zn/Cu-rich layers, both lying on the {111}Al planes. Instead of Y-phase precipitates, air cooling (AC) induced the formation of S- and Zn-phase precipitates with well-developed orientation relationships. No precipitation occurred during water quenching (WQ). The microhardness increased significantly during the natural aging of the AC and WQ alloys, whereas no hardening was observed in the FC alloy. TEM and APT analyses demonstrated that the natural aging induced the formation of fine solute clusters with a chemical composition of 52.3Al–21.3Li–18.3Zn–4.9Mg–3.1Cu (at%), contributing to the natural age-hardening of AC and FC alloys. The solute clusters were fully coherent with the Al matrix and exhibited an interfacial region rich in Zn and two central regions rich in Li and Zn.

Effect of Solution Treatment on Microstructure Evolution of a Powder Metallurgy Nickel Based Superalloy with Incomplete Dynamic Recrystallization Microstructure

In this paper, the powder metallurgy (P/M) Ni-based superalloy FGH4096 with an incomplete dynamic recrystallization structure was treated by a solution treatment at different temperatures, cooling methods, and holding times. The size, morphology, and distribution of grains and γ′ precipitates were characterized by an optical microscope (OM) and a scanning electron microscope (SEM). Research results showed that with the increase of solution temperature from 1060 °C to 1100 °C, the degree of recrystallization increased continuously, the distribution of grain became uniform, and a large number of annealing twins were found. At the same time, the degree of redissolution of the primary γ′ precipitates at the grain boundary increased, and the size of secondary γ′ phase reprecipitated within the grain decreased. The morphology of the secondary γ′ precipitates is mainly spherical with a single distribution under air cooling (AC), while the morphology is near-spherical, cuboids, octets, petaloid, and dendrites with a bimodal distribution under furnace cooling (FC). The size of the γ′ precipitates decreased and the volume fraction increased with the extension of holding time at a higher solution temperature (1100 °C).

Tailoring multi-scale hierarchical microstructure parameters to achieve super-high strength and considerable plasticity in a dual-phase Ti-5.2Mo-4.8Al-2.5Zr-1.7Cr alloy

In the present study, traditional treatment was designed specially and carried out on a dual-phase Ti-5.2Mo-4.8Al-2.5Zr-1.7Cr alloy, and a multi-scale hierarchical microstructure differed from the ordinary microstructure of titanium alloys was obtained. During the solution-treatment at 900 ℃ for 1 h followed by furnace cooling (FC), air cooling (AC), water cooling (WC) and at 920℃for 1 h followed by WC, three equiaxed microstructures composed of primary equiaxed α phase (αp) and β phase with different fractions, and a typical bimodal microstructure containing primary αp and acicular secondary α phase (αs) were formed. During the aging-treatment at 550 ℃ for 6 h followed by AC after solution-treatment at 900 ℃ for 1 h followed by WC, a multi-scale hierarchical microstructure composed of equiaxed αp, acicular αs with a thickness of dozens of nanometers, smaller acicular α phase (αs-s) with a thickness of ten nanometers, distributed in the space of two αs, and whisker β phase (βw) in the transformed β (βt) regions, was obtained. Moreover, with the solution temperature increased to 920 ℃, it was deliberately designed that equiaxed αp was more refined, and finer and denser acicular αs-s precipitated in the alloy. Contributing to the equiaxed αp refinement and nano-scale acicular α phases, the hierarchical microstructure exhibited super-high yield strength of 1228 MPa and ultimate tensile strength of 1389 MPa. Meanwhile, the refined equiaxed αp could also offset the negative effect of acicular α phases on plasticity, and the plasticity was maintain at an considerable level of 8%.

Crystallographic Features of Phase Transformations during the Continuous Cooling of a Ti6Al4V Alloy from the Single-Phase β-Region.

Crystallographic relationships between α- and β-phases resulting from phase transformations, which took place during the continuous water quenching (WQ), air cooling (AC) and furnace cooling (FC) of a Ti6Al4V plates solution treated at 1065 °C, were investigated by methods of electron backscatter diffraction (EBSD) and transmission electron microscopy (TEM). WQ, AC and FC resulted in typical martensite, basket-weave and parallel-plate Widmanstatten structures, respectively. The experimental distribution of α/β-misorientations deviated from BOR at set discrete angles close to 22, 30, 35 and 43°. The experimental spectra of angles were confirmed by theoretical calculations of the possible misorientations between the α and β phases through the βo→α→βII –transformation path based on Burgers orientation relationship (BOR). Joint analysis of the experimental data and theoretical calculations revealed that the secondary βII-phase was precipitated according to the sequence βo→α→βII during continuous cooling from the single-phase β-region. Similar spectra for α/β-phase misorientations for all investigated cooling rates acknowledged the similar transformation mechanisms and dominant shear component of the phase transformations.

Microstructure evolution and corrosion behaviour of a high Mo containing α + β titanium alloy for biomedical applications

In the present work, effect of heat treatment on microstructure and corrosion behaviour of a high Mo containing α + β titanium alloy (Ti-6Al-1 V-4Mo-0.1Si) has been investigated. Heat treatment results in the formation of wide variety of microstructure depending on the heating temperature (below or above the β transus) and cooling conditions. Martensite was observed after oil quenching (OQ), Widmanstatten α (αWS) + β after air cooling (AC) and lamellar α (αL) + β after furnace cooling (FC). The corrosion behaviour of the heat-treated specimens were studied in simulated body fluid (SBF) at 37 °C using open circuit potential-time (OCP), electrochemical impedance spectroscopy (EIS) and potentio-dynamic polarization tests. X-ray photoelectron spectroscopy (XPS) was used to investigate the chemical nature of the corroded surfaces. The study revealed that, in general, OQed samples had increased corrosion resistance than the ACed and FCed samples. XPS confirmed the presence of TiO2 and Al2O3 on the corroded sample. The alloy's improved corrosion resistance was attributed to stable inert TiO2 film. Samples heat treated at 950 °C were found to have better corrosion resistance in general.

Atomic ordering of Fe3Co and its impact on the mechanical properties of low-cobalt (18–24 wt%) iron-cobalt alloys

B2 ordering behavior and its impact on mechanical properties of Fe–Co alloys with Co content range of 29–50 at% have already been thoroughly clarified. For Fe-25 at% Co alloy, L60 Fe3Co have been predicted by first-principles calculations to be energetically stable, but lack of experimental evidence. In this work, L60 Fe3Co ordering was experimentally demonstrated in slowly cooled Fe-23.7 wt% Co alloy by X-ray diffraction analysis. Different cooling conditions of water quenching (WQ), air cooling (AC), furnace cooling (FC) and slow cooling (SC) were set to determine the effect of ordering behavior on mechanical properties. Fe3Co ordering significantly causes the poor tensile plasticity and impact toughness of Fe-23.7 wt% Co alloy, and a faster cooling rate (e.g. water quenching) is required to suppress the ordering behaviors to improve ductility. For Fe-21.5 wt% Co, Fe-19.8 wt% Co and Fe-18.4 wt% Co alloys, we surprisingly found that Fe3Co ordering has a negligible effect on tensile plasticity and yield strength, but prominently brings about impact brittleness. The average width of paired dislocations configurations in SC Fe-23.7 wt% Co alloy was determined via TEM observation to be ∼29 nm, while ∼8.8 nm for SC 1J22 alloy due to its higher antiphase boundary energies. Our experimental and theoretical widths display a satisfied consistency within a certain error range, further confirming the atomic ordering of L60 structure in slowly cooled Fe-23.7 wt% Co alloy. For these low-cobalt alloys, the suppression of Fe3Co ordering by increasing the cooling rates can significantly improve the mechanical properties.

Cooling-Rate Effect on Microstructure and Mechanical Properties of Al0.5CoCrFeNi High-Entropy Alloy

Al0.5CoCrFeNi high-entropy alloy (HEA) was prepared by spark plasma sintering (SPS) using Al0.5CoCrFeNi gas atomized powder and was treated with different cooling rates (furnace cooling, air cooling, water quenching). The phase composition, microstructure, tensile properties, Vickers hardness, compactness, and fracture morphology of the alloy were systematically studied. The results show that the cooling rate can change the phase composition and phase shape of Al0.5CoCrFeNi HEA from BCC + FCC phase to BCC + FCC + B2 phase, and the BCC phase coarsens. The ultimate tensile strength and yield strength of the heat-treated Al0.5CoCrFeNi HEA decreased with increasing cooling rate, but elongation and Vickers hardness increased with increasing cooling rate. The ultimate tensile strength and yield strength of the furnace cooling (FC) samples reached the maximum value of 985.2 MPa and 524.1 MPa, respectively. The elongation and hardness of the water quenching (WQ) samples reached a maximum value of 43.1% and 547.3 HV, respectively, and the compactness of the alloy is higher than 98.8%. Therefore, the properties of Al0.5CoCrFeNi HEAs can be greatly improved by treatment with different cooling rates.

Revealing the correlation between microstructure and fatigue crack growth behaviors in Ti–6Al–3Nb–2Zr–1Mo alloy

In the current work, the correlation between microstructural features and fatigue crack growth (FCG) behaviors in Ti–6Al–3Nb–2Zr–1Mo alloy is investigated by a combination of electron backscatter diffraction (EBSD), electron channeling contrast (ECC) and mechanical tests. Three kinds of samples with different microstructures are prepared by water quenching (WQ), air cooling (AC) and furnace cooling (FC) after isothermal holding for 1h in α+β phase region of the as-received material. The da/dN-ΔK curves indicate that the FCG resistance ability of the AC sample is the worst, resulting in the highest FCG rate (FCGR). The corresponding FCG paths of these samples are also examined systematically. And the results show that the crack bifurcation and deflection in both the WQ sample and the FC sample are more serious than that in the AC sample. For the sample with the WQ microstructure, boundaries with high misorientation angles between different α variants transformed from β phase may act as an obstacle to hinder the dislocation movement and slip transfer, favoring the secondary crack initiation. Similarly, the strain incompatibility generated at the boundaries between different α phases in the FC sample may also promote the occurrence of crack bifurcation. In contrast, in the AC sample, α colonies that contain α variants with the same crystal orientation are more likely to be formed inside β transformation grains leading to few secondary cracks forming. Furthermore, a modified mathematical model is developed to evaluate the co-effect of the crack deflection and bifurcation on the FCGR. It is suggested that the crack bifurcation is the main factor that can effectively reduce the stress intensity factor of the main crack tip, and in turn slow down the FCGR.