High Temperature Deformation Mechanisms Research Articles

Enhanced high-temperature ductility without strength drop in a lean Co Ni-based superalloy

The conventional Ni-based IN738LC superalloy series are typically hardened by a fine distribution of closely spaced fine and coarse Ni3(Al, Ti)-type L12 ordered γ' precipitates, which is formed by solid-state nucleation and growth in the disordered γ matrix. However, outstanding high-temperature mechanical performance requires a high amount (∼9 wt%) of Co, which is the alloy's most expensive adding element. Here, we report a compositionally modified IN738LC alloy with a 50% reduction in Co content and a 20% increase in comparatively cheaper Mo content. This compositional modification caused more γ' precipitates with uniform distribution than those in the conventional counterpart subjected to the same heat treatments. The modified superalloy showed a twofold increase in high-temperature (750 °C) tensile elongation while maintaining the conventional one's strength (∼1.1 GPa) at the temperature. This remarkable ductility gain, while no loss in strength, was predominantly attributed to a transition in the high-temperature deformation mechanism, i.e., from multiple intersecting slip bands-indued stress localization for the conventional IN738LC alloy to unidirectional microtwins for the modified IN738LC material. More Mo and less Co contents could drive more coherent γ' precipitates in the annealed state and the high density of microtwins in the deformed state, resulting in stress delocalization and enhanced ductility at comparable strength. The findings of this study have significant implications for the development of cost-effective, strong, and ductile superalloys for high-temperature applications.

High-temperature creep behaviour and deformation mechanism of a high-concentration Re/Ru single-crystal nickel-based alloy

In this study, the deformation and damage mechanisms of a high-concentration Re/Ru single-crystal nickel-based alloy at ultrahigh temperatures are studied via creep performance tests, microstructural observations and dislocation configuration diffraction contrast analyses at 1170 °C. The results show that the alloy has good mechanical properties. In addition, the creep life is 155 h at 1170 °C and 120 MPa. During creep, dislocations moving in the γ matrix can react with each other to form a dense and complete dislocation network. This dense and complete dislocation network can effectively prevent dislocations from shearing into the γ′ phase and reduce the creep rate during the steady-state creep period. In the late stage of ultrahigh-temperature creep, stacking faults are found in the γ′ phase, indicating that the alloy has a low stacking fault energy and that the stacking faults that form in this alloy can serve as obstacles for dislocation movement. In addition, the dislocations that cut into the γ′ phase in the late stage of ultrahigh temperature creep can cross-slip from the {111} plane to the {100} plane, forming a KW lock, which can inhibit the slip and cross-slip dislocation movement modes. Therefore, the presence of dislocation networks, stacking faults and KW locks in alloys is the main reason for the relatively low strain rate and good creep resistance of these alloys under ultrahigh-temperature creep conditions. In the late stage of creep, the initiated dislocations slip in two orientations, resulting in kinking of the rafted γ′ phase and crack initiation at the top interface of the twisted rafted γ/γ′ two phases. Under the action of shear stress, the crack generates tensile stress perpendicular to the stress axis, and it expands in this direction. This crack is the damage mechanism of the alloy during ultrahigh-temperature creep.

Influence of anelastic recovery on the cyclic creep behavior of AlCoCrFeNi2.1 eutectic high-entropy alloy

The AlCoCrFeNi2.1 eutectic high-entropy alloy (EHEA) is regarded as a promising candidate for structural materials in high-temperature equipment, which may experience the cyclic creep and anelastic recovery caused by low-frequency peak-shaving loading. To investigate the influence of anelastic recovery on the microstructure and cyclic creep behavior of AlCoCrFeNi2.1 EHEA, the stress-varying (SV) tests at 800 °C are conducted to introduce the anelastic strain for comparison with the stress-constant (SC) test. The results show that the steady-state creep rate (SSCR) is accelerated with the increase in the cyclic-creep cycle and valley stress in SV tests. The anelastic recovery happened during the valley-stress holding (VSH) stage significantly alters the microstructure in the B2 phase of EHEA under SV conditions. This anelastic-recovery-related microstructural evolution is subsequently elucidated on the basis of back stress theory. Finally, the influence of microstructural evolution on the cyclic creep behaviour is revealed. The findings obtained will enhance the comprehension of high-temperature deformation mechanism of dual-phase EHEAs.

The high-temperature deformation and microstructural evolution mechanism of Mg-Gd-Y-Zn-Zr alloy formed by CMT arc additive forming

ABSTRACT The hot deformation behavior of the Mg-Gd-Y-Zn-Zr alloy formed by CMT arc additive under 400°C, 0.1 s−1, and 50% deformation were systematically studied by means of hot compression test, constitutive equation, numerical simulation and microscopic structure. After deformation, twinning appeared in the microstructure. A large number of high-density dislocations and stacking faults were distributed around it. This was the thermal stress generated during the compression process, causing the deformation of the grains and stress concentration within the grains. The deformation at the center of the specimen was severe, the grains were elongated, and the equivalent stress was high. The simulated peak stress was close to the experimental value. The effect of the high-temperature deformation on the crystal slip was independent of the deformation region. The slip occurred along the {10-10} crystal plane and the <11-20> crystal direction with the deformation, which was related to the Hcp structure of the alloy.

Thermomechanics of Al–Cu–Li alloy plastic flow

Flat compression tests of Al–Cu–Li alloys are made. Samples deformed in isothermal conditions at deformation value ε = 55%, in strain rate intervals 10–3–10–1 s–1 and temperatures 400–480°C. The empirical equations connecting thermomechanical parameters of Al–Cu–Li alloys deformation are received. The hightemperature deformation mechanism diagrams (DMD) Al–Cu–Li alloys are plotted. Temperature and strain rate mechanisms action areas of hot deformation and warm deformation are specified. It is shown that the warm deformation area on DMD is equal to the dynamic poligonization area on structural conditions diagram, and the hot deformation area is equal to the partial recrystallization area.

Hot deformation behavior and mechanism of cold deformed CoCrCu1.2FeNi high entropy alloy

The high temperature deformation behavior and mechanisms of the cold-deformed CoCrCu1.2FeNi high-entropy alloy (HEA) were investigated by optical microscope (OM), scanning electron microscope (SEM) and electron backscattered diffraction (EBSD) at temperatures and strain rates of 950–1100 °C and 0.001–1 s−1, respectively. The results show that the flow stress increases as the deformation temperature decreases and the strain rate increases, and there is significant dynamic recrystallization (DRX) during the deformation process. 1050–1090 °C/0.02–1 s−1 and 975–1050 °C/0.002–0.2 s−1 are the optional hot temperature deformation regions. Discontinuous dynamic recrystallization (DDRX) occurs at different temperatures when the strain rate is 0.001 s−1 with a maximum recrystallization percentage of 90.28 %. The DRX degree gradually increases as the temperature rises from 950 °C to 1000 °C and from 1050 °C to 1100 °C. The maximum polar density of texture sharply increases to 11.78 at 1050 °C. The deformed microstructure leads to an increase in dislocation density, which could be effectively reduced by DRX.

An insight into high-temperature deformation mechanism of magnesium in-situ composite through development of Johnson-Cook and constitutive model

The establishment of deformation mechanisms of Mg-metal matrix composite (Mg-MMC) is important to improve the high-temperature challenging applications. In this present work, an AZ91/TiC + TiB2 hybrid in-situ Mg-MMC was synthesized, and its deformation mechanisms were studied through a uniaxial hot compressive test at different temperatures and strain rates. The Johnson-Cook (JC) model and constitutive equation were established using experimental stress-strain data. Through the development of JC model, it was revealed that the TiC–TiB2 particles enhanced the yield strength parameter and increased the activation energy of the in-situ composite compared to the parent alloy. The load-shifting capability and grain refinement were found to be the dominating mechanisms, which effectively restricted dislocation movement during deformation, resulting in improved deformation resilience of the composite. A detailed study of JC model and constitutive equation parameters was analyzed with a focus on their microstructures.

Study on creep behaviors of nickel-based single-crystal alloys considering microstructure evolution

Purpose The purpose of this study is to describe the mechanism of single-crystal high-temperature creep deformation, predict the creep life more accurately and study the creep constitutive and lifetime models with microstructure evolution.Design/methodology/approachThe mechanical properties of nickel-based single-crystal superalloy are closely related to the γ' phase. Creep tests under four different temperature and stress conditions were carried out. The relationship between creep temperature, stress and life is fitted by numerical method, and the creep activation energy is obtained. The creep fracture surface, morphology and evolution of strengthening phase (γ') and matrix phase (γ) during different creep periods were observed by scanning electron microscope. With the increase of creep temperature, the rafting time is advanced. The detailed morphology and evolution of dislocations were observed by transmission electron microscope (TEM).FindingsWith the increase of creep temperature, the rafting time is advanced. The detailed morphology and evolution of dislocations were observed by TEM. Dislocations are mainly concentrated in the γ channel phase, especially at high temperature and low stress.Originality/valueA creep constitutive model based on the evolution of γ' phase size and γ channel width was proposed. Compared with the experimental results, the predicted creep life is within 1.4 times error dispersion band.

Deformation Behaviour and Microstructure Evolution of Titanium Alloys with Hot Working Process

Abstract: The behaviour of the high-temperature deformation mechanism of titanium alloys at different temperatures and strain rates and the associated changes in the microstructure have been studied. In addition to good heat transfer properties, titanium has a low density, can be reinforced with alloys, and can be deformed and formed to increase strength. Titanium is nonmagnetic and a good conductor of heat. Its coefficient of thermal expansion is slightly lower than that of steel and less than half that of aluminium. Titanium's combination of mechanical and physical properties, as well as its resistance to corrosion, m ake it an ideal material for critical applications in the aerospace, industrial, chemical and energy sectors. It has been found t hat the appropriate parameters of the titanium deformation process are different temperature conditions and strain rates. The influence of the micro-structural properties of the deformed specimen was studied and correlated with the test temperature, total strain and strain rate to develop a constitutive equation for the relationship between yield strength, strain rate and temperature. Micro-structural studies were performed on the sample and the results analysed..

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.

Revealing the ultra-high high-temperature compressive mechanical properties and deformation mechanism of a heterostructured AlNp/Al nanocomposite

Heterostructured materials have attracted increasing attention owing to their superior mechanical properties. In this work, the high-temperature mechanical property and deformation mechanism of a heterostructured AlNp/Al composite were revealed by isothermal compression tests. The results showed that the superior compressive strength of the AlNp/Al composite could be maintained at both room temperature and high temperature of 300–500 °C during the whole deformation process. The strong pinning effect of the dispersive AlN nanoparticle (AlNp) on the matrix grains could prevent grain boundary softening and grain growth, as well as the special layered heterogeneous structure and network structure could avoid high temperature softening. The hot deformation mechanism of the AlNp/Al composite could be affected by the strain rate and deformation temperature. It was confirmed that continuous dynamic recrystallization (CDRX) dominated the whole deformation process when deformed at low strain rates (0.0001–0.01 s−1) and high temperatures (400–500 °C); while dynamic recovery (DRV) was the main mechanism at high strain rate (0.1–1 s−1) and low temperature (300–350 °C) Furthermore, the strain rate sensitivity exponent (m) was used to assess the relationship between peak stress and deformation temperature and strain rate, which could predict the instability region of the composite. It shows that the simultaneous action of high temperatures and high strain rates should be avoided during hot deformation. This work shed light on the microstructure design of the high-strength heat-resistant aluminum matrix composite.

The effects of Ti and Ta on the microstructure and high temperature deformation mechanisms of the CoNi-based superalloys

The effects of Ti and Ta on the microstructure and high temperature deformation mechanisms of the CoNi-based superalloys

Microstructural evolution and high-temperature deformation mechanism of 4Cr5MoSiV1Ti steel

Hot-working die steel usually withstands complex loading conditions and higher service temperatures, and its mechanical properties will inevitably deteriorate because of dynamic recovery, recrystallization, and plastic deformation of tempered martensite during service. A systematic study was carried out on a newly developed 4Cr5MoSiV1Ti hot working die steel to clarify the related microstructure evolution and deformation mechanism by stretching deformation at different temperatures ranging from room temperature to 640 °C. Results obtained show that the ultimate and yield tensile strength values at room temperature are about 1904 MPa and 1691 MPa, respectively. With increasing testing temperature, the strength decreases, the plasticity increases, and the fracture mode changes from brittle to ductile. All of these are closely related to a combined effect of tempered martensitic dynamic recovery, recrystallization, and secondary phase re-dissolution. Deformation mechanisms change from dislocation slip to rotation or sliding of ferrite with increasing stretching temperature.

In-situ surface study of the mechanism of high temperature deformation in an Al-Cu-Li alloy

The 2195 Al-Cu-Li alloy is able to obtain an elongation of 203% by high temperature stretching. The microstructure evolution and deformation mechanism of non-recrystallized Al-Cu-Li alloy after high temperature stretching were studied. The results show that the inhomogeneous grain structure is the main factor affecting the elongation. High resolution surface studies with focus ion beam show that intragranular dislocation slip provides up to 57% of the total strain and grain boundary sliding plays an accommodating role.

Effect of Initial Microstructure on the High-Temperature Formability of STS 321 Alloy Using a Dynamic Material Model

General austenitic stainless steel has a problem with intergranular corrosion due to volatilizing chromium, which forms chromium carbide in a high temperature environment. By adding titanium as an alloying element, STS 321 stainless steel has excellent creep resistance and intergranular corrosion resistance at high temperatures, because the formation of chromium carbide is suppressed. It is important to find the optimal process conditions for STS 321 stainless steel used in the aerospace field, because high temperature processing is mainly applied, and defects or inhomogeneity of materials that occur during high temperature processing lowers the yield of products. In this study, to investigate the effect of the initial microstructure on the high-temperature deformation behavior of STS 321 stainless steel, a high-temperature compression test was performed on two types of STS321 alloys with different initial microstructures. The temperature range was set at 50°C intervals from 800°C to 1100°C, and the strain rate was set at 10-1/sec intervals from 1 × 100/ sec to 1 × 10-3/sec. Based on the experimental results, the thermal activation energy, which differed depending on differences in the initial microstructure, was calculated. In addition, by deriving flow stress and processing maps, the difference in energy dissipation efficiency depending on temperature and strain rate was explained, along with the initial microstructure and high-temperature deformation mechanism.

High temperature flow behavior and deformation mechanism of Nb-22.5Cr-2.5Mo alloy

The high temperature compression is performed by isothermal compression tests using Gleeble 3500 thermo-mechanical simulator at a constant strain rate for the hot-pressed Nb-22.5Cr-2.5Mo alloy. The deformation temperature ranges from 800 to 1200 °C and the strain rate ranges from 0.001 to 0.1 s−1. The high temperature flow behavior and deformation mechanism of the alloy are investigated based on the compression test data and TEM observation of the deformed microstructure. The results show that the ductile–brittle transition temperature of the alloy is about 800 °C, and the flow stress is sensitive to the deformation temperature and strain rate, in which the plastic deformation ability increases obviously with the increase of deformation temperature and the decrease of strain rate. Besides, the hyperbolic sine Arrhenius model with stress exponent of 3.131 and apparent activation energy of 402 kJ/mol is established for the peak flow stress. The value of apparent activation energy is higher than that of Nb metal (246 kJ/mol), which implies the existence of Laves phase NbCr2 makes the deformation of the alloy more difficult. Moreover, it is found that the deformation mechanisms in the Nbss matrix are mainly DRV controlled by the glide and climb of the dislocations and DRX formed by bulging nucleation mechanism, while the deformation mechanisms in the Laves phase NbCr2 particles are mainly twinning and synchroshear of the Shockley partial dislocation.

Changes in High Temperature Deformation Behavior by Differences in Energy Dissipation Efficiency of Ti–6Al–4V Alloy

Ti–6Al–4V alloy is widely used as a system material in high temperature and high pressure environments in various industrial fields and is difficult to mechanically machine. Thus, high temperature processing methods such as hot forging, rolling, and hot forming are usually applied. Among various methods for deriving optimal molding conditions during high-temperature processing, the dynamic material model suggests energy dissipation efficiency according to the flow stress of the material. However, the energy dissipation efficiency merely numerically represents the change in flow stress, not the metallurgical behavior of the material. Therefore, it is necessary to understand the difference in energy dissipation efficiency in relation to the high-temperature deformation mechanism. In this study, high temperature compression tests were performed on the Ti–6Al–4V alloy. The temperature range was set at 800°C∼1200°C at intervals of 50°C, and the strain rate was set at 1 × 100/sec∼1 × 10−3/sec at intervals of 10−1/sec. Based on the results of the experiments, flow stress and processing maps were derived, and the high temperature plastic deformation behaviors of Ti–6Al–4V alloy were analyzed in correlation with the microstructural changes and mechanical properties according to temperature and strain rate. And the prior beta grain size according to the difference in energy dissipation efficiency was explained for each condition.

Enhanced viscous glide creep in disordered β (Cu17Sn13) intermetallic phase with coarse grains

High-temperature deformation mechanisms and processing maps of the coarse-grained A2 (disordered BCC) β (Cu17Sn13) intermetallic phase alloy (with the chemical composition of Cu-24.6 wt %Sn) were examined in compression in the temperature range of 843–993 K and strain rate range of 10−3-10 s−1. The β phase exhibited a stress exponent (n1) of nearly 3 over almost the entire strain-rate range at these temperatures. Limited subgrain formation occurred during hot compression. This microstructural feature and the observation of n1 = 3 strongly suggest that solute drag creep is the main high-temperature deformation mechanism. A comparison of the high-temperature deformation mechanisms and behaviors of the BCC β phase and FCC α-solid solution phase (with the composition of Cu–10Sn alloy studied in previous work) shows that both β and α phases exhibit solute drag creep, but β is considerably weaker in strength, and the upper limit of strain rate for solute drag creep is much higher (over 100 times). Solute drag creep at high strain rates in the β phase can be attributed to the high breakaway stresses for dislocations from the solute atmosphere in the presence of a high concentration of Sn solutes in the matrix. The β phase exhibits superior hot workability compared with the α phase according to the processing maps, especially at high strain rates. This is because there is no flow instability regime for the β phase under the investigated testing conditions and the power dissipation efficiency at high strain rates is higher in the β phase compared with the α phase.

Modelling of the intergranular fracture of TWIP steels working at high temperature by using CZM–CPFE method

Grain boundaries (GBs) of metallic materials will become the weakest and most vulnerable place when the materials deform at high temperature. This could directly affect deformation flow, plastic strengthening, the initiation and propagation of microcracks, and finally the fracture failure of materials. In this research, to reveal the high temperature deformation mechanisms of twinning–induced plasticity (TWIP) steels, the crystal plasticity finite element method (CPFEM) was employed to model the mechanical response of grain interiors. The cohesive zone model (CZM) of the bi–linear traction separation law (TSL) was developed to describe the damage and failure of GBs. By combining CZM and CPFEM, an analysis method was proposed to investigate the effect of the microstructure morphology, orientation and size of grains, and temperature. Furthermore, the damage initiation, accumulation and fracture at GBs were represented by the representative volume element model based on the CZM–CPFE method to explore the influences of GB angle, grain size and local microvoids (including the location and size) on the GB cracking. The stress damage cloud maps, the distribution of quadratic stress damage initiation criterion (QUADSCRT) and the scalar stiffness degradation (SDEG) describing the damage and fracture along the GBs were used to articulate the relationships of GB angle, size effect, local microvoids and the expected failure strain. The results show that low angle grain boundaries (LAGBs), the small average grain size (<21.2 μm), microvoids at the horizontal GBs and microvoids with a diameter below 1.5 μm can delay the initiation of microcracks and fracture failure along the GBs. The predicted damage and failure of GBs and the intergranular cracking characteristics in plastic deformation of TWIP steels at high temperature were corroborated by microscopic in–situ SEM experiments, in which the deformation characteristics inside the grains, and microcrack initiation and propagation near GBs were observed at 500 and 750°C. This research enhances the understanding of intergranular fracture, mechanical response and microstructure evolution of TWIP steels worked at different temperatures, and also provides a new way and strategy for studying the deformation behaviours of TWIP steels considering GB properties.