High-temperature Tensile Deformation Research Articles

Slip Activity and Precipitation Behaviors of Al-Cu-Li Alloy During High Temperature Tensile Deformation

Slip Activity and Precipitation Behaviors of Al-Cu-Li Alloy During High Temperature Tensile Deformation

Effect of friction stir processing on grain refinement and superplastic properties of binary Al-8 wt.% Si to Al-30 wt.% Si alloys

ABSTRACT As-cast binary Al–Si alloys containing ∼8, 12, 20, and 30 wt.% Si, representing the hypoeutectic, eutectic (12 wt.% Si), and hypereutectic compositions, were subjected to severe plastic deformation by friction stir processing (FSP) for grain refinement to 2.3–2.7 μm by dynamic recrystallization. Tensile tests conducted by differential strain rate test technique over the strain rates of 10−4 – 10−2 s−1 and test temperatures in the range of ∼840–530 K led to strain rate sensitivity index (m) varying from ∼0.04 to 0.40 depending on temperature, strain rate, and alloy composition. The constant initial strain rate tests conducted at 10−4 s−1 and 840 K exhibited a maximum elongation of 250% in the hypereutectic Al–20Si alloy, but the same increased linearly with m irrespective of alloy composition. Generally, with increasing Si content, the activation energy for deformation increased from 104.7 ± 14.4 kJ/mol for eutectic Al–12Si to 310.4 ± 32.3 kJ/mol for hypereutectic Al–30Si, which increased further to 572 ± 148 kJ/mol over the higher temperature range of 840–800 K. Analysis of observed deformation and microstructure behaviour supports the occurrence of superplasticity, whereby the accommodation of grain boundary sliding by grain boundary migration led to enhanced grain growth or else the local high-stress concentration at the particle-matrix interface led to cavity formation. There was no evidence of dynamic recrystallization during high-temperature tensile deformation but the flow softening observed is ascribed to the occurrence of concurrent cavitation.

Portevin–Le Chatelier (PLC) effect induced by different deformation mechanisms in Ni–25Mo–8Cr alloy during high-temperature tensile deformation

Portevin–Le Chatelier (PLC) effect induced by different deformation mechanisms in Ni–25Mo–8Cr alloy during high-temperature tensile deformation

Investigation of microstructure evolution and its effect mechanism on high temperature tensile properties of 304L stainless steel processed by laser powder bed fusion

Laser Powder Bed Fusion (LPBF) 304L stainless steel has garnered significant attention due to its unique blend of strength and ductility at room temperature. However, there is limited understanding of its high-temperature tensile properties. In this study, we conducted tensile testing on LPBF 304L at temperatures ranging from 623K to 1023K in its as-built state. Additionally, we performed a comparative tensile test on traditionally hot-rolled (at 1433K) 304L stainless steel under equivalent test conditions. The results revealed that LPBF 304L steel exhibited high ductility at 973K (ultimate tensile stress (UTS) is 257.3 ± 3.6 MPa)-1023K (UTS is 218.7 ± 3.8 MPa), but its strength (yield strength (YS) decreased from 346.0 ± 4.5 MPa to 182.7 ± 3.8 MPa) and plasticity (UTS decreased from 405.7 ± 5.3 MPa to 218.7 ± 6.4 MPa) decreased slowly as the testing temperature increases. We also delved into the microstructure and its impact on high-temperature tensile deformation. The fine grains and ensuing dynamic recovery were found to be responsible for the reduced strength and ductility of LPBFed 304L samples at elevated temperatures. Furthermore, a sawtooth fluctuation emerged in the stress-strain curve at 873K, which could be attributed to dynamic strain aging (DSA) and the interaction between solute elements and dislocations.

High temperature tensile properties and deformation behavior of CoCrFeNiW0.2 high entropy alloy

High entropy alloys (HEAs) have been a new-type of structural materials, which show good performance in various extreme conditions. In the present study, we systematically investigated the high temperature deformation behavior of as-cast CoCrFeNiW0.2 HEA under different temperatures from 298 to 1173 K and various strain rate of 1 × 10−4 s−1 to 5 × 10−3 s−1. An obvious temperature dependence of yield strength can be found with a remarkable decrease from 362 MPa at 298 K to 119 MPa at 1173 K. The normal Portevin–Le Chatelier (PLC) effect was presented in the intermediate temperature range of 673 K to 1073 K, the serrated flow transformed in the sequence of A → A + B → A + C → B + C → C with the increasing temperature, which is caused by dynamic strain aging (DSA). W atoms played an important role in the interaction between solute atoms and dislocations. Fractography analysis suggests the failure mode transition from transgranular fracture to intergranular fracture. Electron backscatter diffraction (EBSD) demonstrates that the high temperature deformation mechanism of the HEA is dominated by dynamic recovery (DRV). Additionally, transmission electron microscopy (TEM) was performed to observe the dislocation configurations and the interaction between dislocations and μ phase precipitates at various temperatures, and these results further identify the high temperature deformation mechanism. To summarize, this study indicates the proposed HEA has excellent high temperature mechanical properties and a wide range of potential applications in many fields.

High-temperature tensile properties and deformation behavior of a multi-phase FeNiCrAlTi alloy

Single-phase multi-principal elements alloys (MPEAs) within the elevated temperature experience particularly severe strength loss, and even a distinct ductile-to-brittle transition. Herein, we report on a non-equiatomic, multi-phase FeNiCrAlTi MPEA which possesses remarkable combinations of mechanical properties across a wide range of temperatures from 400 °C to 800 °C, especially its specific yield strength possesses nearly 100 MPa/g∙cm−3 at 600 °C. The excellent mechanical behavior benefits from the synergistic effect of the special phase structure and precipitation strengthening by the ordered Ni (Al, Ti) B2 nanoparticles. The transformation of deformation mode at 600 °C should be considered the dominant of the resistance to intermediate or high-temperature embrittlement. This work provides a new methodology for regulating mechanical properties of superior alloys that involve high-temperature conditions.

High-Temperature Tensile Behavior of an As-Cast Ni-W-Co-Ta Medium–Heavy Alloy

High-temperature tensile experiments with tensile rates ranging from 0.01 s−1 to 10 s−1 were carried out at various temperatures ranging from 1000 °C to 1250 °C with a Gleeble-3800 thermal simulation tester to evaluate the physical properties of an as-cast Ni–W–Co–Ta medium–heavy alloy. The microstructure evolution of the alloy during high-temperature stretching was characterized by metallographic microscopy, scanning electron microscopy, and transmission electron microscopy. The results indicated the emergence of multiple slip lines and the parallel arrangement of dislocations in the grain of the alloy after high-temperature stretching, and typical characteristics of plane slipping were observed. The plasticity of the Ni–W–Co–Ta medium–heavy alloy increased, but its strength decreased with an increase in the deformation temperature. In contrast, an increase in the strain rate resulted in a noticeable increase in the strength and plasticity of the medium–heavy alloy. The experiments revealed that the maximum tensile strength of the as-cast Ni–W–Co–Ta medium–heavy alloy was 735 MPa (T = 1000 °C, ε˙ = 10 s−1). Additionally, the maximum reduction in area and elongation was 38.1% and 11.8% (T = 1250 °C, ε˙ = 10 s−1), respectively. The mode of fracture after high-temperature tensile deformation was brittle fracturing.

Analysis of Hot Tensile Fracture and Flow Behaviors of Inconel 625 Superalloy.

In this work, Inconel 625 alloy is explored regarding high-temperature tensile deformation and fracture behaviors at a strain rate of 0.005-0.01 s-1 under a deformation temperature ranging from 700-800 °C. The subsequent analysis focuses on the impact of deformation parameters on flow and fracture characteristics. The fractured surface reveals that ductile fracture is dominated by the nucleation, growth, and coalescence of microvoids as the primary failure mechanisms. The elevated deformation temperature and reduced strain rate stimulate the level of dynamically recrystallized (DRX) structures, resulting in intergranular fractures. The Arrhenius model and the particle swarm optimization-artificial neural network (PSO-ANN) model are developed to predict the hot tensile behavior of the superalloy. It indicates that the PSO-ANN model exhibits a correlation coefficient (R) as high as 0.9967, surpassing the corresponding coefficient of 0.9344 for the Arrhenius model. Furthermore, the relative absolute error of 9.13% (Arrhenius) and 1.85% (PSO-ANN model) are recorded. The developed PSO-ANN model accurately characterizes the flow features of the Inconel 625 superalloy with high precision and reliability.

The columnar dendrite and equiaxed dendrite transformation of high Nb-TiAl alloy by laser-directed energy deposition

Laser-directed energy deposition (LDED) is a significant method for shaping high Nb-TiAl alloys. The dendrite type has an impact on the mechanical properties of parts fabricated with LDED. This study establishes a dendrite transformation model for the alloy. The refinement mechanism of microstructure and the strengthening mechanism of high-temperature mechanical properties are analyzed through experiment and simulation. The results show that the scanning rate has a significant influence on the dendrite type. Modifying the scanning rate brings about a gradual decrease in the G/R (where G denotes temperature gradient and R is the growth rate) value of the molten pool, and a concurrent gradual increase in the value of G×R. These changes facilitate the transformation of columnar dendrites into equiaxed dendrites. Furthermore, the lamellar colony size is reduced by 72.8% and the tensile strength at 900 °C is increased by 16.2%. Microstructure refinement is the primary reason for the enhancement of high-temperature tensile characteristics, with dislocations and deformation twins also contributing to crack propagation resistance.

Microstructural evolution, high temperature tensile deformation behavior, and deformation mechanism in an Mg–Zn–Y–Ca–Zr alloy processed by multidirectional forging and hot rolling

To explore high temperature ductility, a new Mg-2.70Zn-1.34Y-0.37Ca-0.02Zr (wt.%) alloy has been fabricated by novel multidirectional forging (MDF) and hot rolling. The microstructure and mechanical properties were investigated. The average grain size of MDF + hot rolled alloy is 10.89 ± 0.90 μm refined from the as-cast average grain size of 60.03 ± 0.72 μm. The ultimate tensile strength of 260.51 ± 1.03 MPa, yield strength of 192.29 ± 1.21 MPa, and elongation of 17.83 ± 0.72 % were obtained at room temperature. For high temperature tensile behavior, microstructural examination revealed that continuous dynamic recrystallization and discontinuous dynamic recrystallization are the main softening mechanism in the temperature range of 573–673 K, while dynamic grain growth with bimodal grains and twins is discovered at 723 K in this alloy. X-ray diffraction and scanning electron microscopy –energy dispersive spectroscopy examinations revealed that the constituent phases are composed of α-Mg solid solution and intermetallic compounds of Ca2Mg6Zn3, Mg3YZn6 (I-phase), and Mg3Zn3Y2 (W-phase). The microstructural evolution, such as dynamic recrystallization and dynamic grain growth at desired tensile temperatures, is related to the thermal stability of constituent phases. The elongation to failure of 215.4 % was demonstrated at 673 K and 1.67 × 10−2 s−1, exhibiting high strain rate quasi-superplasticity. A power-law constitutive equation was established. The deformation activation energy of 177.948 kJ/mol and stress exponent of 4.494 revealed that the dominant deformation mechanism of this alloy at elevated temperatures of 573–723 K is dislocation climb controlled by lattice diffusion.

Tensile properties of Ti6.5Al2Zr1Mo1V titanium alloy fabricated via electron beam selective melting at high temperature

Here, near α titanium alloy TA15 (Ti-6.5Al–2Zr–1Mo–1V) samples were fabricated by electron beam selective melting (EBSM) and their high-temperature tensile deformation behaviors were studied at 773K–1023K. The results show that the tensile strength decreases gradually with the deformation temperature while the yield ratio increases as temperature increases from 773K to 923K, and then decreases gradually with further increasing the temperature. The deformation mechanism of EBSM-built TA15 samples at elevated temperatures can be ascribed to work hardening caused by the increase of dislocation density, and softening caused by the recovery and recrystallization. The studied TA15 alloy exhibits obvious work hardening behaviors at both 773K and 823K, dynamic recovery occurs during tensile deformation at 873K, and dynamic recrystallization plays a dominant role at 923K–1023K. The EBSM-built TA15 samples have excellent mechanical properties at medium temperatures, and the present results are helpful for the industrial applications of EBSM-built titanium alloy components in the aerospace fields.

Tensile properties and deformation mechanisms of a solution treated Ni–Fe-based alloy at high temperatures

This study investigated the high-temperature tensile properties and deformation mechanisms of a solution treated Ni–Fe-based alloy ranging from 600 to 750 °C. The solution treated GH4070P (HT700P) alloy exhibits superior yield strength of 490 MPa and ultimate strength of 593 MPa at 700 °C. The major carbides in the alloy are the blocky MC and discrete-distributed M23C6. The MC and M23C6 carbides at the grain boundaries (GBs) can impede crack propagation, yet concurrently intensify the stress concentration. The transgranular and intergranular fractures are observed. Above 700 °C, micro-cracks emerge around M23C6 carbides at GBs and then are interlinked to form lengthy secondary micro-cracks, resulting in the intergranular fracture. The precipitation of γ′ precipitates during tension at 700 and 750 °C confers the alloy to a higher strength.

Dynamic recrystallization-dependent high-temperature tensile properties and deformation mechanisms in Al-Mg-Sc-Zr alloys

This research elucidates discontinuous dynamic recrystallization (DDRX)-dependent high-temperature tensile properties and deformation mechanisms in simply extruded Al-7Mg alloys tailored by Sc/Zr ratios. The bimodal-grained Al-7Mg-0.4Sc alloy presents a high elongation to failure of ∼540% at 400 °C and 5 × 10−4 s−1. However, such superplastic behavior has not been observed in both coarse-grained Al-7Mg-0.1Sc-0.3Zr and Al-7Mg-0.3Sc-0.1Zr alloys. Thermally stable dispersed nano-sized Al3(Sc,Zr) particles strongly retard DDRX, accompanied by the retention of heterogeneous grain structure in Al-7Mg-0.1Sc-0.3Zr till fracture. The predominance of dislocation slip is proved by analyzing the evolution of texture and dislocations. In contrast, the bimodal grain structure evolves into a homogeneous fine one in Al-7Mg-0.4Sc with tensile deformation proceeding since coarsening Al3Sc precipitates play a weak role in restricting DDRX. The superplasticity in Al-7Mg-0.4Sc is achieved by cooperated mechanisms, i.e., grain boundary sliding (GBS) and dislocation slip as dominant mechanisms, which is accommodated by DDRX in the initial tensile deformation stage, followed by dislocation slip-accommodated GBS in the late stage.

Role of void nucleation at primary-γ'/γ interface on strain softening of nickel base superalloy 720Li

High-temperature tensile deformation behavior of polycrystalline nickel based superalloy 720Li used for manufacturing aero engine disks was studied by conducting uniaxial tensile tests in the temperature range of 25–800 °C and subsequent transmission and scanning electron microscopic examinations to decipher strain hardening/softening behavior. The alloy is strengthened by different sizes of γ'-precipitates named as primary, secondary, and tertiary-γ'. The sizes of primary γ' is in the range of 0.5–3 µm and most of them are incoherent with γ-matrix as confirmed by the TEM study. From engineering stress-strain curves, it was observed that specimens tested in the temperature range 25–650 °C exhibited a significant amount of strain hardening, whereas specimens tested at 720 and 800 °C softened after the onset of plastic yielding. From extensive SEM examinations, it is concluded that void nucleation at primary-γ'/γ interface at 720 and 800 °C was the main reason for strain softening after the onset of plastic yielding. Nucleation of voids at primary-γ'/γ interface is ascribed to (i) γ' size, which loses coherency with the matrix after reaching a critical value (rcrit); and (ii) difference in Schmid factor between γ' precipitate and γ matrix, which varies with test temperature. From the misfit data, the rcrit values were calculated at 25 and 800 °C and are found to be in the range of 1250–1500 nm and 150–200 nm, respectively. From the electron backscatter diffraction (EBSD) analysis, it is confirmed that at 720 and 800 °C when the difference in Schmid factors between γ' precipitate and γ-matrix is ≥ 0.1, the probability of formation of the void is high.

Enhanced thermoplasticity of 7Mo-0.4 N contained super austenitic stainless steel through pre-deformation

This study investigated the hot tensile deformation behavior of super austenitic stainless steel (SASS) with different pre-deformation degrees within 1150–1250 ˚C. The influence of pre-deformation on the number of annealing twin (AT), grain size, dislocation evolution, sigma phase precipitation, static recrystallization (SRX), and dynamic recrystallization (DRX) behaviors at hot tensile deformation stages is analyzed. Results show that the thermoplasticity of the SASS increases first and then decreases with the increase of pre-deformation degree at each deformation temperature. The area reduction of the samples pre-deformed for 0 %, 10 %, 20 %, and 25 % are 0.25, 0.32, 0.37, and 0.35, respectively at a tensile temperature of 1150 ˚C. When the pre-deformation degree is within the range of 0–20 %, the deformation storage energy (DSE) gradually increases with the pre-deformation degree. The SRX amount at the high-temperature holding stage and the DRX amount in the high-temperature deformation stage significantly increases, and the grain size gradually decreases. As a result, crack initiation and propagation are inhibited, and thermoplasticity improves. However, when the pre-deformation degree is increased to 25 %, the grains grow abnormally during the high-temperature tensile deformation, and a large number of sigma phases are induced, which deteriorates the thermoplasticity of the SASS.

Microstructure change characteristic and fracture mechanism of Ti-47.5Al-2.5V-1.0Cr-0.2Zr alloy during high temperature tensile deformation

Changes in the microstructure characteristics and fracture mechanism of Ti-47.5Al-2.5 V-1.0Cr-0.2Zr alloy during tensile deformation were studied by high temperature tensile tests at 750–900 °C and strain rate of 10−5–10−3 s−1. The experimental results indicate that when the tensile temperature increases or the strain rate decreases, the number of small dimples in the tensile fracture structure increases, the proportion of cross-layer fracture decreases, the dimples have torn edges, and the plastic deformation resistance increases. The higher tensile temperature corresponds to a higher dynamic recrystallization (DRX) ratio, and the number of dislocations near the DRX structure is reduced, which shows that the rheological softening effect is significantly enhanced. A lower strain rate corresponds to a higher proportion of DRX and a smaller proportion of lamellar microstructure. Dislocation slip is the main deformation mode; stacking faults can be observed in DRX grains. The changes in the hot deformation microstructure directly reflect decrease of high temperature tensile strength in macroscopic flow behavior. Under the action of tensile stress, holes usually form at the lamellar grain boundary, and the aggregation of multiple micro-holes will merge and connect to form micro-cracks. Cracks are generated and propagated between layers, and the tendency of holes or cracks generated at lower deformation temperatures or higher strain rates is greater. When the stress concentration reaches a certain critical value, the specimen will eventually fracture.

Study of Flow Stress Models and Ductile Fracture Criteria for CHN327 Nickel-Based Superalloy

The plastic deformation behavior of a CHN327 nickel-based superalloy under temperatures ranging from 600 °C to 700 °C and strain rates ranging from 0.001 to 0.1 s-1 was investigated using uniaxial high-temperature tensile tests. The stress-strain curves obtained by the tests showed that the maximum stress decreased as the temperature increased, while it increased as the strain rate increased. Based on the extensive data obtained in the experiment, three constitutive models (Hollomon, Swift, and the modified Voce equation) were employed to predict the constitutive relation. It was found that the modified Voce equation had the highest correlation coefficient and the best prediction accuracy. Thereafter, in order to predict the fracture of CHN327 during high-temperature tensile deformation, five ductile fracture criteria (Freudenthal, C&L, Brozzo, Ayada, and the R&T model), and the modified Voce equation obtained were incorporated into the finite element software (DEFORM). According to the results, except for the C&L and Brozzo models, all of the other ductile fracture criteria (DFCs) were suitable for predicting the damage distribution of the CHN327 alloy in tensile tests. For all of the DFCs considered, the R&T model provided the most accurate predictions, whose mean error was only 8.9%, far less than the values that other models predicted.

Room-Temperature Strengthening, Portevin-Le Chatelier Effect, High-Temperature Tensile Deformation Behavior, and Constitutive Modeling in a Lightweight Mg-Gd-Al-Zn Alloy

To explore room-temperature strengthening and high-temperature ductility, a lightweight novel Mg-1.85Gd-0.64Al-0.62Zn alloy was fabricated by innovative multidirectional forging and a hot-rolling technique. Microstructures and mechanical properties were studied at room and elevated temperatures with an optical microscope, an X-ray diffractometer, and a tensile tester. An ultimate tensile strength of 260 MPa, yield strength of 171 MPa, and elongation of 20.4% were demonstrated at room temperature. The room-temperature strengthening mechanisms were identified by strengthening the model estimation. A type C Portevin-Le Chatelier effect was discovered and elucidated in this alloy. X-ray diffraction analysis revealed that the phase composition is α-Mg solid solution and (Mg, Al)3Gd, Al7Zn3, and Al2Gd intermetallic compounds. Examination of the microstructure at elevated temperatures revealed that dynamic recrystallization and dynamic grain growth occur. In particular, it was discovered that bimodal microstructures or incomplete dynamic recrystallization microstructures exist in high-temperature deformation. A maximum quasi-superplasticity of 228.4% was demonstrated in this alloy at 673 K and 5.0 × 10-4 s-1. Flow stress curves showed that the present alloy exhibits Sotoudeh-Bate curves or a long intermediate strain-hardening stage followed by a strain-softening stage. A modified Zerilli-Armstrong constitutive equation incorporating the number of dislocations was established. The power-law constitutive equation was established to identify the deformation mechanism. Both constitutive models had good predictability. At 673 K and 5.0 × 10-4 s-1, the stress exponent was 4, and the average deformation activation energy was 104.42 kJ/mol. The number of dislocations inside a grain was 146. This characteristic evidence confirmed that dislocation motion controlled by pipe diffusion dominates the rate-controlling process under this condition.

Characterisation of tensile deformation behaviour of Fe–25Cr–35Ni-based alloy at high temperature

Fe–25Cr–35Ni-based alloy tensile deformation behaviour was studied through tensile test at strain rates of 5 × 10−4, 10−3, 5 × 10−3 and 10−2 s−1 from 900 to 1023°C in this paper. The flow stress has a good correlation with testing temperature and strain rate. TEM and SEM were used to analyse the microstructure evolution. The results show that DRV and DRX are the important factors for reducing the flow stress during hot tensile deformation. The occurrence of DRV and DRX was confirmed by TEM micrographs in high temperature tensile deformation. The hyperbolic sine-type equation with deformation activation energy and temperature describes Fe–25Cr–35Ni-based alloy hot tensile deformation, and the deformation activation energy is 5.24 × 102 kJ mol–1.

Quasi-superplasticity in the AlCoNiV medium entropy alloy with Heusler L21 precipitates

The high-temperature tensile deformation behaviors of the recrystallization annealed Al7(CoNiV)93 medium entropy alloy with a duplex hierarchical microstructure of face-centered cubic (fcc) and ordered body-centered cubic L21 Heusler-type grains were investigated. The alloy showed a remarkably high tensile strength of ∼1.1 GPa and good ductility of ∼17% at 923 K due to a good strain hardening capacity of 60 MPa. At 1073 K, the highest fracture elongation of ∼270% was displayed at a strain rate of 10−3 s−1. The strain rate sensitivity was estimated to be about 0.32, which is typical of alloys that show this quasi-superplastic elongation. The activation energy was also estimated to be ∼421 kJ/mol. With deformation at 1073 K at a strain rate of 10−3 s−1, the duplex microstructure transformed into a refined triplex fcc-L21-σ equiaxed microstructure. The development of the triplex equiaxed microstructure resulted from dynamic recrystallization (DRX), which assisted in the maximum superplastic-like elongation of 270%. The inability of the alloy to access true superplasticity was attributed to immense cavitation due to the high amount of the brittle σ phase, which served as de-cohesion sites for the early fracture of the sample. However, the observed quasi-superplasticity could still be useful in superplastic forming operations.