High-temperature Tensile Deformation Research Articles

Microstructure evolution and fracture mechanism of H13 steel during high temperature tensile deformation

It is known that mechanical properties of hot working die steel deteriorate during service due to dynamic recovery, recrystallization and plastic deformation of tempered martensite. In order to clarify the associated microstructure evolution and fracture mechanism, a systematic investigation was carried out on a widely used hot working die steel. Samples were tensile deformed at different temperatures ranging from 25 ℃ to 640 ℃ and examined using SEM and TEM. Results obtained have shown that mechanical properties as well as fracture mode change with increasing testing temperature. These phenomena have been elaborated together with microstructure evolution during high temperature deformation. Deformation mechanisms and roles of different carbides in crack formation and propagation have been discussed.

Microstructure, Tensile, and Creep Behaviors of Ti-22Al-25Nb (at.%) Orthorhombic Alloy with Equiaxed Microstructure.

This article investigates the tensile and creep behaviors of the Ti-22Al-25Nb (at.%) alloy with equiaxed microstructure. The experimental results show that the equiaxed microstructures are formed by isothermal forging in the α2 + B2 + O phase region, and then heat treating in α2 + B2 + O and B2 + O phase regions. The equiaxed particles are determined by isothermal forging and solution heat treating, and the acicular O phase is obtained by adjusting the aging temperature. The strengths of the alloy are sensitive to the thickness of the secondary acicular O phase. Increase in aging temperature improves strength and reduces the ductility. Deformation of the alloy mainly depends on the volume fraction and deformability of the B2 phase. During the high-temperature tensile deformation, the flow stress decreases with the increasing deformation temperature and increases with the increasing strain rate. The microstructure obtained by higher aging temperature (HT-840) has better creep resistance, due to the coarsening of the secondary acicular O phase.

High-temperature tensile properties and deformation mechanism of polycrystalline magnesium alloys with specifically oriented columnar grain structures

A Mg–4.78Zn–0.45Y–0.10Zr (wt%) alloy with specifically oriented columnar grain structures (crystal growth direction 〈101¯0〉) was prepared using directional solidification. The columnar grain structures possessed parallel-growing primary arms and straight longitudinal grain boundaries. The results from an electron backscatter diffraction analysis demonstrated that the angle between the c-axis and the growth direction was 74–87°. As the tensile tests were performed along the 〈011¯0〉 growth direction, the Schmid factors for the <a> cylindrical and <c+a> conical plane slips were as high as 0.45. When the tensile test temperature was 300 °C, dynamic recovery was observed and the <a> cylindrical, <a> conical, and <c+a> conical slip systems were all activated. This phenomenon, together with the high orientation consistency between the adjoining grains, resulted in an elongation increase up to 42%. Fractography of the Mg alloy revealed extended or parabola-shaped dimples with specifically oriented columnar grain structures, suggesting good ductility for this material.

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 properties of hypereutectic Al-25Si based alloy

The high temperature tensile deformation of a hypereutectic Al-25Si based alloy fabricated by spray forming and subsequent hot extrusion was investigated. Tensile tests were conducted at various temperatures and strain rates. It was revealed that the ductility of the alloy is sensitive to both the test temperature and strain rate. At a given strain rate, the peak value of elongation was obtained at 500 °C. At 490 °C and above, the peak value of elongation was observed at a strain rate of 1.0 × 10-2 s-1, although the elongation increased with decreasing strain rate at 460 °C and below. The high elongation was exhibited when a high strain rate sensitivity index (m value) was attained and a liquid phase existed during deformation. The liquid phase appeared as a filament-like structure which is aligned with the tensile direction on the fracture surface of the tensile deformed specimen. A higher elongation (>35%) was obtained when the volume fraction of the liquid phase was 0.7%-1.7%. The maximum elongation of 75% was achieved when the volume fraction of the liquid phase was about 1%. The transition of the activation energy was observed at 430 °C when incipient melting occurred.

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 of a directionally solidified nickel base superalloy: Role of micro constituents

High temperature tensile deformation behaviour of directionally solidified nickel base superalloy CM 247 DS was studied by conducting tensile tests in the temperature range 25–955°C and extensive electron microscopic examinations to understand the influence of different phases on tensile behaviour. Micro-constituents, especially carbides and γ−γ′ eutectics influence the deformation and fracture behaviour of alloy CM 247 DS. Cracking of carbide precipitates was extensively observed during tensile deformation at 25°C. However, tendency of carbide cracking decreased and more carbide-matrix interface de-bonding were observed with increase in test temperature.

Deformation mechanisms for superplastic behaviors in a dual-phase high specific strength steel with ultrafine grains

The superplastic behaviors of a high specific strength steel (HSSS) with dual-phase microstructure and ultrafine grains have been investigated under a temperature range of 873–973K and at a wide strain rate range of 10−4–10−1/s. The ultrafine grained HSSS exhibits excellent superplastic properties. The microstructure observations at interrupted strains for tests under temperature of 973K and at strain rate of 10−3/s have provided evidences of different mechanisms for two stages. At the first stage (strain range from 0% to 400%), the superplastic flow is attributed to the diffusional transformation from fcc austenite phase to intermetallic compound B2 phase coupled with grain boundary sliding. While intragranular dislocation activities should be the dominant mechanism for the second stage (strain range from 400% to 629%) due to the increased realistic strain rate by diffusive necking. The grain sizes of both phases are observed to be relatively stable and remain always sub-micron level during the high temperature tensile deformation, facilitating the superplastic flow.

The stability of thermodynamically metastable phases in a Zr-Sn-Nb-Mo alloy: Effects of alloying elements, morphology and applied stress/strain

In this paper, a dual phase Zr-Sn-Nb-Mb alloy was studied with TEM after thermal treatment and high-temperature tensile deformation. Plate and pressure tube material, manufactured through different processing routes, were used in this study. The overall average concentrations of Mo and Nb in the β phase are higher in the pressure tube than in the plate. It was revealed that these concentrations have significant effects on the subsequent stability of the β and ω phases as well as on the precipitation behavior of the α phase from the β phase. That is, the higher the concentrations, the more stable the β and ω phases are, and hence there is a reduced tendency for precipitation of α phase. Aging treatments cause the transformation of athermal ω to isothermal ω, as expected. The most striking finding is the product of the decomposition of the isothermal ω particles during aging treatment is determined as not being α phase, even though the structure of it is, as-yet, not fully determined. The non-uniform morphology of the β grains in the plate material provides us a unique opportunity to investigate the effects of morphology on the aging response of the β phase. It was found that thin β filaments suppress the precipitation of isothermal ω particles but enhance the precipitation of α phase at α/β interfaces. The effect of the Burgers orientation relationship between α and β grains on the precipitation of the α phase at the α/β interface is discussed. Applied high-temperature stress/strain has been found to enhance the decomposition of isothermal ω phase but suppress α precipitation inside the β grains. The suppression of α precipitation by applied stress/strain is discussed in terms of the ω assisted α precipitation. Implications of these findings for the in-service application of the alloy are discussed.

In-situ SEM observations of fracture behavior of BT25y alloy during tensile process at different temperature

In order to characterize the fracture behavior of BT25y alloy at different temperature, in-situ scanning electron microscopy of this alloy was performed during high temperature tensile deformation process. The results showed that microcracks mainly initiated at α/α and α/β interfaces during both 600°C and 650°C tensile process. However, the path of crack propagation varied at different temperature, it was related to the rapid decline in interface strength with deformation temperature increasing. When deformed at 600°C, interface strength was higher than grain strength, cracks propagated into grain interior, the tensile specimen presented transgranular fracture characteristic. As the deformation temperature increasing to 650°C, interface strength was lower than grain strength, cracks tended to propagate along interfaces, and the tensile specimen presented intergranular fracture characteristic. Mechanical performance was sensitive to deformation temperature, with the temperature increasing from 600°C to 650°C, the ultimate tensile strength fell by 200MPa.

Influence of ferrite matrix and precipitation status on the mechanical properties of low carbon low alloy steel during high temperature tension

Utilizing the inhomogeneity of temperature and stress status during heavy plate rolling, with controlled cooling, polygonal ferrite and quasi-polygonal ferrite with two grain sizes for each were obtained, and their precipitation status varied with different microstructures. GLEEBLE 3500 was used to investigate the influence of microstructure and precipitation status during high temperature tensile deformation. The results revealed that ferrite matrix microstructure and nucleation of precipitates were determined by the overall cooling process, while the final precipitation status was mainly determined by the cooling rate within the temperature range of (Ti,Mo)C precipitation during the subsequent isothermal process. Also, dislocations and subboundaries acting as containers facilitated the growth of precipitates along some particular directions. Pre-existing substructures, such as low-angle boundaries and statistically-stored dislocations, along with small precipitates, influenced the formation of subgrains and dislocation cells during high temperature deformation. These newly-generated substructures involved rotation or rearrangement of dislocations, and finally translated into recrystallized grains. Additionally, the grain size was closely related to the probability of interactional of subboundaries. Although the polygonal ferrite matrix showed more stable stress status, the quasi-polygonal ferrite matrix with fine precipitates and small grains exhibited better mechanical properties.

Structure and magnetic properties of a Ni3(Al, Fe, Cr) single crystal subjected to high-temperature deformation

The structure and magnetic properties of the Ni3(Al, Fe, Cr) single crystal subjected to high-temperature tensile deformation to failure at 850–900°C have been studied. No recrystallized grains and metastable phases were found. The rupture zone of the alloy subjected to deformation (at 900°C) to the highest degree demonstrates the fragmentation accompanied by rotation of atomic layers and changes of the chemical composition in the nickel and aluminum sublattices. Magnetic studies of the alloy have shown the existence of two Curie temperatures for samples cut from the rupture zone. Samples cut away from the rupture zone exhibit no additional magnetic transitions; twines and planar stacking faults in the alloy structure. The alloy deformed to the lower degree of deformation (at 850°C) also demonstrates twins; no ferromagnetic state was found to form.

Direct Observation of Two-Dimensional Grain Boundary Sliding and Switching in ODS Ferritic Steel

High-temperature tensile deformation was performed using an oxide-dispersionstrengthened (ODS) ferritic steel,, which has grain structure largely elongated and aligned in one direction, in the perpendicular direction. In the superplastic region II, two-dimensional grain boundary sliding (GBS) was achieved, in which the material did not shrink in the grain-axis direction and grain-boundary steps appeared only in the surface perpendicular to the grain axis. In this condition, a classical grain switching event was observed. Using kernel average misorientation maps drawn with SEM/EBSD, dominant deformation mechanisms and accommodation processes for GBS were examined in the different regions. Cooperative grain boundary sliding, in which only some of grain boundaries slide, was also observed.

Effect of microstructure morphology on the high temperature tensile properties and deformation in directionally solidified NiAl-Cr(Mo) eutectic alloy

High temperature tensile properties and microstructure morphologies of directionally solidified NiAl-Cr(Mo) alloy under different compositions and withdrawal rates were investigated. Tensile strength of 513.8MPa was obtained in the directionally solidified NiAl-36Cr-6Mo hypereutectic alloy at 1000°C, which is the highest value reported in the NiAl-Cr(Mo) alloy until now. In addition, perfect cellular eutectic alloy solidified at fast withdrawal rate possessed higher tensile strength and elongation than planar eutectic alloy solidified at low withdrawal rate when they had the same composition. This is because bonding strength between eutectic cells in the perfect cellular eutectic alloy was high, which resulted in high crack propagation resistance and harmonious deformations in the intracellular and intercellular regions. The results above may change the locked-in view that only regular eutectic microstructure morphology grown with planar solid/liquid interface could be used in the eutectic in-situ composites. Perfect cellular microstructure morphology grown in a far fast rate could be also used, which would result in high efficiency in the industrial production. Refined microstructure, larger volume fraction of strengthening phase and dispersion strengthening of second phase particles were contribute to the improvement of high temperature tensile strength of the directionally solidified NiAl-Cr(Mo) alloy. However, the bonding strength of interface was obviously different in the alloys with different microstructure morphologies, which had significant effect on the tensile strength. When bonding strength of phase interface was low, the other strengthening factors could not be able to play their due roles, thus leading to relatively low tensile strength.

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.

Effect of Ta additions on microstructure and mechanical properties of a single-crystal Co–Al–W-base alloy

Microstructures and mechanical properties of Co–9.5Al–5W–16Ni–6Cr–xTa (at% where x=0, 1.8, 2.8, named as 0Ta, 1.8Ta and 2.8Ta) alloys are investigated. Microstructures of heat-treated samples are composed of γ and cubical γ′ precipitates. The γ′ solvus temperature increases obviously with Ta additions. The yield strength as well as creep strength is improved with the addition of Ta. Ta additions affect the high-temperature tensile deformation mechanism. The γ′ phase of the three alloys is rafted along the tensile axis direction during creep tests at 900°C. The creep strength of 2.8Ta alloy is approaching the level of a first-generation Ni-base single-crystal superalloy.

Effects of long term thermal aging on high temperature tensile deformation behaviours of duplex stainless steels

Tensile deformation behaviours of unaged and thermal aged duplex stainless steels (DSS) were investigated at 350°C in order to understand the effects of long term thermal aging on the high temperature deformation behaviours of DSS. After aging for 20 000 h, the strength of DSS has a slight increase, the plasticity has a considerable decline, and the tensile fracture transfers from ductile to brittle. Nanoindentation tests indicate that ferrite has a considerable increase in hardness, and austenite has only a negligible increase with aging time. Thermal aging embrittlement is primarily concerned with ferrite. After long term thermal aging, spinodal decomposition and G-phase precipitation occur in ferrite and these reactions result in the dramatic decline of the ferrite phases' deformation ability. Cleavage cracks can easily initiate and propagate in ferrite of long term thermal aged DSS.

High-temperature tensile and creep deformation of cross-weld specimens of weld joint between T92 martensitic and Super304H austenitic steels

The high-temperature mechanical behavior of cross-weld specimens prepared from a dissimilar weld joint between T92 martensitic and Super304H austenitic heat-resistant steels incorporating Ni-based weld metal was evaluated at temperatures up to 650°C. For both high temperature tensile and creep tests, failure took place in T92 due to its faster degradation with temperature increase. The heat-affected zone of T92 played a critical role during creep deformation, resulting in type IV failure under the long-term creep condition. For the creep specimens, the location of failure shifted from the base metal region to the fine-grained heat-affected zone as the creep duration time increased from the short-term to the long-term condition. The massive precipitation of Laves phase on the grain boundaries of the fine-grained heat-affected zone during creep deformation was observed and found to be responsible for the accelerated void formation in the area leading to the premature failure.

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.