High Temperature Tensile Deformation Behavior Research Articles

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.

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.

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.

High-temperature tensile deformation behavior and failure mechanisms of Al-10Si-Mn-Mg high-pressure die-cast alloy

High-temperature tensile deformation behavior and failure mechanisms of Al-10Si-Mn-Mg high-pressure die-cast alloy

Achieving excellent superplasticity in an ultrafine-grained QE22 alloy at both high strain rate and low-temperature regimes

The high-temperature tensile deformation behavior of ultrafine-grained (UFG) QE22 alloy developed by friction stir processing was investigated at various strain rates (5 × 10−4 – 1 × 10−2 s−1) and temperatures (300–450 °C). The UFG QE22 alloy exhibited excellent high strain rate superplasticity with the maximum elongation of 1630% at a temperature of 450 °C and strain rate of 1 × 10−2 s−1, which is the highest elongation reported till date in QE22 alloy. The UFG QE22 alloy also shows superior low-temperature superplasticity with a maximum elongation of 850% at a temperature of 350 °C and strain rate of 3 × 10−3 s−1. Thus, the developed UFG microstructure is found to demonstrate dual-mode superplasticity; both at high strain rate and low-temperature. The primary UFG microstructure and basal texture developed via multi-pass FSP, along with pinning action of Mg12Nd eutectic and fine nano precipitates of Mg12Nd2Ag at grain boundaries played a major role in contributing to the dual-mode superplasticity. The cavitation behavior of the UFG QE22 alloy during superplastic deformation in a dual-mode regime has been studied. The kinetics of superplastic deformation of UFG QE22 alloy was estimated and found to be significantly faster as compared to conventional magnesium alloys processed by various severe plastic deformation routes.

Micromechanism of High-Temperature Tensile Deformation Behavior of a Directionally Solidified Nickel Base Superalloy

High-temperature tensile deformation behavior of directionally solidified nickel base superalloy CM 247 DS is studied by conducting tensile tests in temperature range RT-955 °C employing a constant strain rate of 10−3 s−1 and carrying out extensive electron microscopic examinations to understand the concomitant substructural evolution. The alloy exhibits yield strength anomaly (YSA) behavior like many other superalloys, and the yield strength maxima occur at 750 °C. However, unlike in most of the superalloys, ductility continuously increases with temperature. The deformation behavior of the alloy changes significantly with temperature. Transmission electron microscopic examination confirmed that at lower temperature (≤ 750 °C), γ′ shearing is the dominant deformation mechanism; whereas at temperatures above 750 °C, thermally activated dislocation looping around γ′ precipitate is dominant. Substructures evolved during deformation at 750 °C consists mainly of superlattice stacking faults (SSFs) inside γ′ precipitates, whereas at 850 °C uniform dislocation tangles are observed in γ matrix. Superlattice stacking faults result from shearing of γ′ precipitate by a/3〈112〉 dislocations, which arise from the decomposition of a/2〈110〉 matrix dislocations. YSA in this alloy is attributed to dislocation interactions inside γ′; however, the enhanced ductility even at 750 °C is due to formation of SSFs.

High-temperature tensile deformation behavior of hot rolled CrMnFeCoNi high-entropy alloy

In the present study, high temperature (700–1100 °C) tensile deformation behavior of hot-rolled CrMnFeCoNi high-entropy alloy was investigated. A single face-centered cubic phase was retained in the hot deformed specimens under different deformation conditions. The flow behavior was significantly influenced by temperature and microstructure evolution. A strong temperature dependence of yield stress was observed with a drastic drop in yield strength from 413 MPa at 700 °C to 94 MPa at 900 °C. A profound increase in ductility was observed above 700 °C and a perceptive decrease in ductility was observed above 900 °C. Electron backscatter diffraction analysis indicates incomplete dynamic recrystallization at 700 °C leading to a significant difference in the flow behavior.

High temperature tensile deformation behavior and failure mechanisms of an Al–Si–Cu–Mg cast alloy — The microstructural scale effect

In this study the high temperature tensile deformation behavior of a commercial Al–Si–Cu–Mg cast alloy was investigated. The alloy was cast with two different cooling rates which resulted in average secondary dendrite arm spacing of 10 and 25 μm, which is typical of the microstructure scale obtained from high pressure die casting and gravity die casting. Tensile tests were performed at different strain rates (10− 4 s− 1 to 10− 1 s− 1) and over a wide temperature range from ambient temperature to 500 °C. The fine microstructure had superior tensile strength and ductility compared to the coarse microstructure at any given temperature. The coarse microstructure showed brittle fracture up to 300 °C; the fracture mode in the fine microstructure was fully ductile above 200 °C. The fraction of damaged particles was increased by raising the temperature and/or by microstructure coarsening. Cracks arising from damaged particles in the coarse microstructure were linked in a transgranular-dominated fashion even at 500 °C. However, in the fine microstructure alloy the inter-dendritic fracture path was more prevalent. When the temperature was raised to 300 °C, the concentration of alloying elements in the dendrites changed. The dissolution rates of Cu- and Mg-bearing phases were higher in the fine microstructure.

High temperature tensile deformation behavior of Grade 92 steel

Candidate structural materials for advanced reactors need to have superior high temperature strength and creep-rupture properties among other characteristics. The ferritic–martensitic Grade 92 steel (Fe–9Cr–2W–0.5Mo, wt.%) is considered such a candidate structural material. Tensile tests were performed at temperatures of 600, 650 and 700°C in the strain rate range of 10−5–10−3s−1. After analyzing the tensile results using the Bird–Mukherjee–Dorn (BMD) equation, a stress exponent of about 9.5 and an activation energy of about 646kJ/mol were obtained. In the light of high values of the stress exponent and activation energy, the threshold stress concept was used to elucidate the operating high temperature deformation mechanism. As a result of this modification, the true activation energy and stress exponent of the high temperature deformation in Grade 92 steel were found to be about 245kJ/mol and 5, respectively. Thus, the dominant high temperature deformation mechanism was identified as the high temperature climb of edge dislocations and the appropriate constitutive equation was developed.

Constitutive modeling for high-temperature tensile deformation behavior of pure molybdenum considering strain effects

In order to get a reliable constitutive equation for the accurate simulation, high temperature flow behavior of molybdenum sheet was investigated by performing uniaxial hot tensile tests in the temperature range of 993–1143K at an interval of 50K with strain rates ranging from 0.0005 to 0.02s−1. The true stress–strain curves revealed the characteristics of strain hardening and followed by flow softening under lower strain rates at temperature over 1093K, and the dominant role of work hardening at higher strain rates and lower temperatures. Considering the effects of strain rates and temperature incorporating strain on the flow behavior, the Arrhenius-type constitutive equation with material constants expressed by the polynomial fitting of strain was established, and the flow stress values predicted by the constitutive equation agreed well with the experimental results.

Modeling high-temperature tensile deformation behavior of AZ31B magnesium alloy considering strain effects

The hot tensile deformation behaviors of AZ31B magnesium alloy are investigated over wide ranges of forming temperature and strain rate. Considering the effects of strain on material constants, a comprehensive constitutive model is applied to describe the relationships of flow stress, strain rate and forming temperature for AZ31B magnesium alloy. The results show that: (1) The effects of forming temperature and strain rate on the flow behaviors of AZ31B magnesium alloy are significant. The true stress–true strain curves exhibit a peak stress at small strains, after which the flow stress decreases until large strain, showing an obvious dynamic softening behavior. A considerable strain hardening stage with a uniform macroscopic deformation appears under the temperatures of 523 and 573K. The strain hardening exponent (n) increases with the increase of strain rate or the decrease of forming temperature. There are not obvious strain-hardening stages when the forming temperature is relatively high, which indicates that the dynamic recrystallization (DRX) occurs under the high forming temperature, and the balance of strain hardening and DRX softening is easy to obtain. (2) The predicted stress–strain values by the established model well agree with experimental results, which confirm that the established constitutive equation can give an accurate and precise estimate of the flow stress for AZ31B magnesium alloy.

Effect of Microstructural Change on High-Temperature Deformation in Pre-Annealed Zr<SUB>65</SUB>Al<SUB>10</SUB>Ni<SUB>10</SUB>Cu<SUB>15</SUB> Bulk Metallic Glass

The effect of the microstructural change on the high-temperature tensile deformation behavior in the supercooled liquid region is investigated in the pre-annealed Zr 65 Al 10 Ni 10 Cu 15 bulk metallic glass. The microstructure before the tensile test on the specimen annealed at 673 K for 1.8ks shows that a small amount of icosahedral phase precipitates and a large amount of amorphous phase still remains. On the contrary, the specimen annealed at 673 K for 2.7 ks shows that a large amount of icosahedral phase precipitates and the presence of amorphous phase is hardly noticeable. From the fact that flow stress of the specimen annealed at 673 K for 2.7 ks increases in comparison with that of asreceived specimen as well as the specimen annealed at 673 K for 1.8 ks in the tensile test, it seems that the deformation behavior in the tension tests is greatly influenced by the difference in the degree of the precipitation of icosahedral phase. Since the m value is about 0.5 and many grain boundaries of icosahedral phase are formed in the specimen annealed at 673 K for 2.7 ks, the interaction between the icosahedral phase particles must be the cause of increase in initial stress. Accordingly, the microstructure of the gage section after tensile test in the specimen annealed at 673 K for 1.8 ks shows that the large amount of icosahedaral phase precipitates in the amorphous matrix. The strain hardening must be caused by the interaction between the icosahedral phase particles whose precipitation and growth are enhanced by deformation.

Ultrahigh-temperature deformation of high-purity HIPed Si3N4

High-temperature tensile deformation behavior of high-purity HIPed silicon nitride material was investigated in the temperature range between 1600°C and 1750°C. Recoverable anelastic and non-recoverable deformation was observed in high-purity HIPed silicon nitride. A power-law deformation model analogous to rheological models was used to distinguish the different deformation components. A stress exponent n = 1.64 and an activation energy Q 1 = 708 kJ/mol was determined for the non-recoverable deformation. For the anelastic deformation a stress exponent p = 4 and an activation energy Q 3 = 619 kJ/mol was observed. Diffusional creep and grain boundary sliding with the accomodation process responsible for the anelastic component are discussed as deformation mechanisms.

High temperature tensile deformation behavior of β-Ti alloys

The high temperature tensile deformation behavior of β-Ti alloys has been investigated as function of strain rate, temperature and type of alloying elements. It was found that the magnitude of the flow stress drop increases with strain rate and solute concentration and decreases with temperature. For a given temperature and strain rate the extent of the drop decreased with a decrease in the relative atomic size between the solvent (titanium) and solute (manganese and vanadium) and a decrease in the diffusivity of the alloying element in the β phase. In addition, it was found that prestraining to a stress below the peak stress significantly reduces the peak stress and the magnitude of the flow stress drop. On the basis of these results it is suggested that the initial flow stress drop is due to the rapid multiplication of dislocations. The steady state behavior after the initial flow stress drop was attributed to dynamic recovery leading to the formation of subgrains.