Prior Thermomechanical Treatment Research Articles

Enhanced creep resistance in Mg-Y-Nd alloy via regulating prior thermo-mechanical treatment

In this study, the tensile creep behavior of a hot-rolled Mg-4Y-3.5Nd alloy subjected to different prior thermo-mechanical treatments was investigated at 220 ℃. Five groups of samples were prepared using different combinations of the solid solution (S), aging treatment at 220 ℃ for 30 h (A), and hot compression at 490 ℃ to a true strain of 0.25 (C). The abbreviations for the samples are S, SA, SC, SAC, and SCA. Upon examining the yield strength and creep resistance, it was found that creep resistance could not be directly predicted by the yield strength. The stability of the deformation bands (DBs) induced by prior thermo-mechanical treatment plays an important role in determining the creep resistance. The dislocation of the DBs and demonstrated the best creep resistance in the SAC sample, which were prepared using a solid solution, aging treatment, and subsequent hot compression. However, despite the highest yield strength, frequent dislocation motions destroyed the stability of the DBs and deteriorated the creep resistance of the SCA sample, which were prepared using a solid solution, hot compression, and subsequent aging treatment. Among the thermo-mechanical treatments used in this study, the application of aging treatment was important to obtain the resultant creep resistance. When the aging treatment was performed prior to hot compression, the creep resistance could be further enhanced based only on hot compression. Accordingly, the sequence from the strongest to the weakest creep resistance was SAC > SC > S > SCA > SA.

The damping behavior of a Ni-50 at.%Ti shape memory alloy

Abstract In order to utilize the shape memory alloys (SMAs) for prospective damping applications, a near equiatomic NiTi alloy was investigated on a dynamic mechanical analyzer in the infrasonic frequency range. A high damping capacity stable to the variation of temperature can be obtained during a two-stage phase transformation and in the martensite state approaching to the martensite-parent phase transformation temperature in the NiTi SMA. The transient damping peaks associated with the phase transformation in the NiTi SMA shows a strong frequency dependence in both solutionized and annealed specimens. The frequency dependence parameter, n, was found to be in the range 0.3 – 0.6. For the applied strain amplitudes from 8.5 × 10–6 to 5.0 × 10 –5 a clear strain amplitude dependence was not observed in the damping peaks, except for the solutionized specimens. While the damping capacity of the martensite state, characterized at 300 K, showed obvious frequency dependences in both the solutionized and annealed specimens, only the annealed specimens manifested the strain amplitude dependence. The different damping behavior was attributed to the internal structure of martensite, including dislocations, the twinning types and variant interfaces, changeable with the prior thermomechanical treatments.

High-energy synchrotron radiation X-ray diffraction measurements during in situ aging of a NiTi-15 at. % Hf high temperature shape memory alloy

NiTiHf high temperature shape memory alloys (HTSMAs) have begun to show considerable progress in their ability to be manufactured and are now on the cusp of becoming viable alloys for use in the aerospace industry, mainly as actuation devices. By aging the alloys at intermediate temperatures between 450 and up to 700 °C, Ni-rich NiTiHf HTSMAs form the H-phase, a Ni-rich nanoscale precipitate which can be used to strengthen the alloy and improve the shape memory response; however, precipitate phase evolution and growth has not yet been studied in-situ nor have the effects of previous thermo-mechanical processing on their formation. In this study, H-phase precipitate formation is observed experimentally in situ during aging of a Ni-rich NiTiHf alloy using high energy synchrotron radiation X-ray diffraction (SR-XRD) measurements. The effects of prior thermo-mechanical treatments, i.e. hot and cold rolling, on its formation rate and coarsening rate are compared. It is shown that growth of the H-phase can be observed in situ using SR-XRD and the H-phase formation rate seems to exhibit linear growth when aging at 550 °C, while increasing the temperature to 650 °C shows a logarithmic trend that plateaus after approximately 1 h of aging. Other properties such as incubation time, austenite lattice strains, texture evolution, and overall comparison between hot and cold rolling are discussed. New potential peak age conditions are suggested based on the diffraction data and evaluated using three-point bending and TEM imaging.

Enhanced quasi-isotropic ductility in bi-textured AZ91 Mg alloy processed by up-scaled RD-ECAP processing

In the present work, an up-scaled rotational die equal channel angular pressing (RD-ECAP), which circumvented the limitation of the removing and re-inserting procedure of the conventional ECAP, was developed to make it possible to prepare large scale ECAP billets with dimension of 50 mm × 50 mm × 100 mm, and was successfully applied to an as-cast AZ91 alloy without a prior thermomechanical treatment. Benefiting from the large scale ECAP billet, the anisotropic tensile behavior of the ECAP alloy was examined, and was carefully linked to its microstructure and texture evolution. Our results shown that the large scale AZ91 RD-ECAP alloy exhibited simultaneously improved strength and ductility compared to as-cast alloy, while it possessed significantly anisotropic mechanical behavior. Interestingly, the anisotropy of ductility was nonsignificant, showing dramatically enhanced quasi-isotropic ductility. The improved strength and ductility were ascribed to the significant grain refinement, fine second phase, and texture strengthening. The tensile anisotropy was resulted by the bi-texture which was comprised of four prismatic texture components and three basal texture components. This study clearly highlights the potential for scaling RD-ECAP for an industrial scale implementation, while significant anisotropic mechanic behavior should be paid much attention.

Effect of Prior Thermomechanical Treatment on Annealed Microstructure and Microhardness in Cobalt-Based Superalloy Co-20Cr-15W-10Ni

AbstractMicrostructure evolution in Co-20Cr-15W-10Ni alloy with hot deformation and subsequent annealing has been studied using hot isothermal compression at different temperatures and strain rates followed by annealing at 1,423 K for 1 h. In the as-deformed condition, optical microstructure reveals deformation bands in specimens deformed at low temperature (1,323 K) and high strain rate (10 s–1). In deformation beyond 1,423 K, very fine recrystallized grains are observed at all the strain rates, which is seen to be fully developed at 1,473 K onwards especially at lower strain rates (0.1 and 0.01 s–1). EBSD maps reveal recrystallized grains and presence of deformation twins. Grain growth is observed at strain rate lower than 0.1 s–1 and temperature more than 1,473 K. After annealing of deformed samples, fully recrystallized grains and annealing twins have been observed. Samples subjected to prior deformation at higher strain rates showed mixed grains with small banding of recrystallized structure, whereas more uniform recrystallized microstructure is observed at slow strain rates. Microhardness decreases with increase in deformation temperature. However, annealed hardness is found to be increasing marginally with decreasing prior deformation temperature (especially for slower strain rates), attributing to relatively uniform and fine microstructure after annealing.

Two-way shape memory effect under multi-cycles in [001]-oriented Ni49Fe18Ga27Co6 single crystal

The effects of prior thermo-mechanical treatment on two-way shape memory effect (TWSME) and its stability under thermal cycles in [001]-oriented Ni49Fe18Ga27Co6 single crystals were investigated. In this study, TWSME in quenched, stress-free and stress-assisted aged crystals was induced by 100 isothermal loading/unloading cycles (ε = 6%, σmax = 140MPa). Maximum reversible TWSME strain up to 5.5% was observed in stress-assisted aged single crystals in which a full one-way transformation strain εtr is achieved under minimum stress levels and the favourably oriented internal stress fields are formed by the selection of certain precipitate variants during stress-assisted ageing. In quenched and stress-free aged crystals TWSME strains are less than 2.2%. The mechanisms of cycling degradation of TWSME (decrease of TWSME strain and change in martensitic start temperature Ms) have also been discussed. In quenched single crystals, 100 thermal cycles result in irreparable degradation of TWSME. In contrast, age hardening in stress-free and stress-assisted aged crystals results in unchanged Ms temperature during thermal cycles and the restoration of the TWSME strain due to additional training (10 isothermal loading/unloading cycles).

Evolution of microstructure and its influence on tensile properties in thermo-mechanically controlled processed (TMCP) quench and partition (Q&P) steel

Controlled hot rolling is performed on low carbon Q&P steel and is subsequently followed by a direct Q&P (DQP) treatment and a separate Q&P treatment (SQP). Two different levels of strain viz. 0.51 and 1.1 are accomplished in the thermomechanical controlled process (TMCP) prior to the Q&P treatment. The microstructures of the steels processed through both the DQP and SQP methods with different thickness reductions, contain primarily small lath packets with fine martensite laths and thin films of inter-lath austenite. Comparatively higher martensite volume fractions and fine lath packets is noticed in the DQP method. Tendency of higher retained austenite fraction is observed in the SQP method. Presence of a high fractions of high angle grain boundaries (HAGB) in the martensite laths indicates a fully recrystallized prior austenite grains in both steels. In general, prior thermo-mechanical treatment improves the partition kinetics which is supported by the high value of calculated carbon content of the austenite, Cϒ, in the DQP steels. Combination of TMCP and Q&P process has resulted in remarkable increase in strength with adequate ductility compared to a simple Q&P treatment alone. Maximum strength of about 1398MPa with a total elongation of 14% is achieved in the 1.1DQP steel. The study suggests that performing a TMCP prior to Q&P promotes grain refinement and formation of high fractions of HAGBs that are beneficial to improve the tensile properties.

Microstructural development during equal channel angular pressing of as-cast AZ91 alloy

Equal channel angular pressing (ECAP) was successfully applied to a typical as-cast AZ91 alloy with very coarse initial grains without the need for a prior thermomechanical treatment. A two-step ECAP tests were carried out in which the first pass was conducted at 623K to overcome cracking, and the next passes were performed at lower temperature to reduce grain growth. In order to evaluate the influence of ECAP temperature on the evolution of microstructure and mechanical properties, ECAP processing was carried out at different temperatures of 593K, 563K, and 533K at the second step. According to microstructural investigations, simultaneous occurrence of dynamic recrystallization (DRX) and dynamic precipitation during this two-step ECAP processing led to a noticeable grain refinement and formation of a homogeneous microstructure. It was demonstrated that lower ECAP temperatures accelerate dynamic precipitation and DRX resulting in more rapid formation of fine and uniform microstructures. The tensile properties of ECAPed samples were evaluated at room temperature, and it was proved that ECAP processing can simultaneously improve both ductility and strength of the alloy. It has been indicated that a combination of DRX ratio, and dynamic precipitation during ECAP plays a decisive role in improving the mechanical properties.

Structure Refinement of Medium Carbon Steel and Deformation Properties Respond

The work presents the results on grains refinement of steel containing 0,45 wt pct carbon resulted from severe plastic deformation (SPD). Different steel structures from prior solutioning and/or thermomechanical treatment were prepared for deformation experimental. A coarse grain ferrite-pearlite structure was achieved applying solutioning. By application of thermomechanical (TM) controlled forging process, performing multistep open die forging, the refined ferrite-pearlite mixture was prepared. Final structure refinement of steel, having different initial structure, was then accomplished applying warm Equal Channel Angular Pressing (ECAP) at 400°C. Employment of this processing route resulted in extensive deformation of ferrite grains and cementite lamellae fragmentation. Applying the highest shear stress (εef - 4) the mixed structure of subgrains and ultrafine grains was present within the ferrite phase. In pearlite grains, modification of cementite lamellae due to shearing, bending, twisting and breaking was found efficient. The coarse cementite lamellae spheroidization was more efficient in prior TM treated steel. The tensile deformation records confirmed strength increase and diversity in strain hardening behaviour.

Grain Refinement and Deformation Behaviour of Medium Carbon Steel Processed by ECAP

The work presents the results on grains refinement of steel containing 0,45 wt pct carbon resulted from severe plastic deformation (SPD). Different steel structures from prior solutioning and/or thermomechanical treatment were prepared for deformation experimental. A coarse grain ferrite-pearlite structure was achieved applying solutioning. By application of thermomechanical (TM) controlled forging process, performing multistep open die forging, the refined ferrite-pearlite mixture was prepared. Final structure refinement of steel, having different initial structure, was then accomplished applying warm Equal Channel Angular Pressing (ECAP) at 400°C. Employment of this processing route resulted in extensive deformation of ferrite grains and cementite lamellae fragmentation. Applying the highest shear stress (εef - 4) the mixed structure of subgrains and ultrafine grains was present within the ferrite phase. In pearlite grains, modification of cementite lamellae due to shearing, bending, twisting and breaking was found efficient. The coarse cementite lamellae spheroidization was more efficient in prior TM treated steel. The tensile deformation records confirmed strength increase and diversity in strain hardening behaviour.

Prior Structure Modification on Grain Refinement and Deformation Behavior of Medium Carbon Steel Processed by ECAP

The present work, deals with grain refinement of medium carbon steel AISI 1045 (0.45% C), having different initial ferrite–pearlite microstructure resulted from thermal and thermomechanical treatment (TM). The purpose of TM steel processing was to refine ferrite and modify pearlite lamellae structure. The final grain refinement of steel structure was then accomplished during warm Equal Channel Angular Pressing (ECAP) at 400°C. Employment of this processing route, in dependence of the applied effective strain ϵef, resulted in extensive deformation of ferrite grains and cementite lamellae fragmentation. When applying higher shear stress (ϵef = 4) the mixed structure of subgrains and ultrafine grains was formed within ferrite phase, regardless the initial steel structure morphology. In pearlite grains, modification of cementite lamellae due to shearing, bending, twisting, and breaking was found efficient as straining increased. Processes of dynamic polygonization and recrystallization in deformed structure also contributed to submicrocrystalline grains formation in deformed structure. Comparing results the course of lamellae cementite spheroidization was then more efficient in prior TM treated steel. The tensile deformation results confirmed the strength increase, however deformation behavior and strain hardening generally for different initial structural conditions of steel, showed diversity.

Finite element simulation of earing defect in deep drawing

Deep drawing is an extensively used press working process since it eliminates expensive machining and welding operations and enables the production of components at a very high rate. The workpiece material used in a deep drawing process is anisotropic in nature, due to a prior thermomechanical treatment. Earing is one of the major defects observed in a deep drawing process due to the anisotropic nature of the sheet material. Knowledge about the ear formation in deep drawing allows a prior modification of the process, which can result in a defect-free final product with financial savings. In this paper, a recently proposed anisotropic yield criteria by Barlat et al. for rolled sheets is used to model the anisotropy for simulating the earing defect in square and circular cup drawing processes. The effect of the tooling geometry and process parameters on the ear formation is studied. It is shown that, in the square cup, the uneven metal flow rate, rather than the material anisotropy, is mainly responsible for the flange earing. Finite element formulation, based on the updated Lagrangian approach, is employed for the analysis. The stresses are updated in a material frame and the logarithmic strain measure is used, which allows the use of a large increment size. Isotropic hardening is assumed, and it follows a power law. Inertia forces are neglected due to small accelerations. The modified Newton–Raphson iterative technique is used to solve the nonlinear incremental equations.

The damping behavior of a Ni-50 at.%Ti shape memory alloy

In order to utilize the shape memory alloys (SMAs) for prospective damping applications, a near equiatomic NiTi alloy was investigated on a dynamic mechanical analyzer in the infrasonic frequency range. A high damping capacity stable to the variation of temperature can be obtained during a two-stage phase transformation and in the martensite state approaching to the martensite-parent phase transformation temperature in the NiTi SMA. The transient damping peaks associated with the phase transformation in the NiTi SMA shows a strong frequency dependence in both solutionized and annealed specimens. The frequency dependence parameter, n, was found to be in the range 0.3 – 0.6. For the applied strain amplitudes from 8.5 × 10 –6 to 5.0 × 10 –5 a clear strain amplitude dependence was not observed in the damping peaks, except for the solutionized specimens. While the damping capacity of the martensite state, characterized at 300 K, showed obvious frequency dependences in both the solutionized and annealed specimens, only the annealed specimens manifested the strain amplitude dependence. The different damping behavior was attributed to the internal structure of martensite, including dislocations, the twinning types and variant interfaces, changeable with the prior thermomechanical treatments.

Sub-micrometer structures generated during dry machining of copper

Deformation structures developed in the material ahead of the tool tip in commercial purity copper samples that were subjected to an orthogonal cutting process were investigated using transmission electron microscopy (TEM). The effect of the prior thermo-mechanical treatment has been considered by performing orthogonal cutting tests on hot extruded and fully annealed samples. Results showed that the microstructure of the primary deformation zone (PDZ) was characterized by the formation of elongated dislocation cell structures that were composed of heavily tangled dislocation walls. In the hot extruded samples, this structure was generated as a result of the activation of the pre-existing dislocations, but nucleation of fresh dislocations was needed in the annealed samples. Grain refinement occurred in the chip as a result of the subdivision of the elongated cells into smaller equiaxed sub-micron size (220 nm diameter) grains. Evidence for dynamic recrystallization has been found in the microstructure of the machined chips (MCs). However, calorimetric studies and the microhardness measurements have shown that the sub-micron structures of the machined chips were stable at temperatures up to 180 °C.

Non-Symmetric Tension-Compression Behaviour of NiTi Alloy

Development of tensorial constitutive equations suitable to model the thermomechanical behaviour of shape memory alloys (SMA) would greatly help engineering design of sophisticated components. While a number of studies have focused on mechanical behaviour under tensile loading, only a few have been done to characterise properties under other stress states. The aim of our study is to report some new experimental results obtained on a equiatomic NiTi. The experimental study was carried out using both uniaxial tension and compression tests which were performed with the same form for the specimens (sheet samples of gauge length 40 mm and cross section 5.6 mmx2.7 mm) in order to avoid any geometrical effect. This choice allows to submit the material to the same prior thermomechanical treatment, i.e. a cold rolling leading to a thickness reduction of 18% followed by an annealing at 430°C for 1/2 hour. The temperature of the experiments were achieved using a silicon oil bath. A special device was designed in order to avoid buckling during compression. Preliminary tests were performed in order to study the effect of this device which was concluded as negligeable. The results show that tension and compression behaviours are not symmetric, especially for superelastic behaviour. Moreover, the amplitude of the one-way memory effect and the slope of the linear variation of the transformation stress with the temperature are compared for these two stress states.

Influence of Prior Thermomechanical Treatment on the Training Behaviour of a NiTi Alloy

It is shown that two-way memory effect (TWME) can be obtained in shape memory alloys by various types of thermomechanical cycling (training). Although much uncertainty still exists on the physical origin of the TWME, it is well recognised that dislocation arrays developed during training can have an important influence on the TWME. The investigation presented in this paper was designed to provide more information on the influence of initial dislocation structure on the TWME training behavior of a near equiatomic NiTi alloy. Differences in initial dislocation structure were created by cycling specimens through austenite ⇔martensite transformations in different modes: thermal cycling between 200 K and 370K, symmetric mechanical cycling to ±5% shear strain, and combined thermal-mechanical cycling. Subsequent TWME training was performed in tension at a temperature just above Ms by deforming to 5% strain followed by heating. It is found that, whereas the deformation behaviour during training was different for specimens of different prior treatments, the final TWME after training was always the same

Hydride embrittlement in ZIRCALOY-4 plate: Part I. Influence of microstructure on the hydride embrittlement in ZIRCALOY-4 at 20 °C and 350 °C

The hydride embrittlement in ZIRCALOY-4 was studied at room temperature and 350 °C. Sheet tensile specimens of two fabrication routes in the stress-relieved, recrystallized, andβ-treated states were hydrided with or without tensile stress. It was found generally that the effect on strength of increasing hydrogen content was not important. However, for the tensile tests at room temperature, there is a ductile-brittle transition when the hydrogen content is higher than a certain threshold. The prior thermomechanical treatment shifts this transition considerably.In situ scanning electron microscopy (SEM) tests, fractography, and fracture profile observations were carried out to determine the fracture micromechanisms and the microscopic processes. At 20 °C, the fracture surfaces are characterized by voids and secondary cracks for low and medium hydrogen contents and by intergranular cracks and decohesion through the continuous hydride network for high hydrogen contents. This phenomenon disappears at 350 °C, and the hydrogen seems to exert no more influence on the fracture micromechanism even for very high hydrogen contents (up to 1500 wt ppm). A full-coverage model is proposed to estimate the critical hydrogen content that makes ZIRCALOY-4 totally brittle. The effect of microstructure on hydride embrittlement in different metallurgical states is thus explained according to the modeling. Special attention is devoted to relating the micromechanisms and the modeling in order to propose the possible measures needed to limit the hydride embrittlement effect.

High strain rate superplasticity of a 25 Wt Pct Cr-7 Wt Pct Ni-3 Wt Pct Mo-0.14 Wt Pct N duplex stainless steel

Effects of prior thermomechanical treatments on the superplasticity of a 25 wt pct Cr-7 wt pct Ni-3 wt pct Mo-0.14 wt pct N δ/γ duplex stainless steel have been studied by means of hot tensile testing with constant crosshead speeds. The objective is to increase the strain rate suitable for superplasticity. The strain rate is found to be markedly increased by a special prior treatment,i.e., solution treatment at temperatures in the δ single-phase region with subsequent heavy cold-rolling. In hot tensile tests at 1273 K, elongations greater than 1000 and 300 pct were observed at initial strain rates (έ) of 10−3 to 10−1 s−1 and 1 x 100 s−1, respectively. The results for strain rates 〈10−1 s−1 can be explained in terms of a structural superplastic effect due to grain refinement. In the case of έ 〉 10−1 s−1, transformation superplastic effects due to γ-phase precipitation from the σ-ferrite matrix are also important, especially in the early stages of deformation. In the equiaxedδ/γ microduplex structures during stable superplastic deformation, there exists a mixture of two different structures,i.e., dislocated and recovered/ recrystallized δ grains with a homogeneous dispersion of dislocation-free γ particles. This result shows that dynamic recrystallization ofδ grains occurs locally and intermittently due to the dispersion of relatively hardγ particles. The apparent average grain growth rate during deformation is small compared to static grain growth, because grain refinement due to dynamic recrystallization reduces the superplasticity-enhanced grain growth.

Measurement of irradiation creep of zirconium alloys using stress relaxation

Bent-beam stress relaxation tests provide a simple means of assessing the in-reactor creep behaviour of zirconium alloys with different prior thermo-mechanical treatments. By assuming that stress relaxation is equivalent to creep under decreasing stress, the irradiation-induced creep rate can be derived from tests in which small elastically constrained beams are exposed to fast neutron irradiation. In-reactor creep rates for stresses below about 150 MPa have been observed to depend linearly on applied stress, namely ge = Cσφ, hence the stress relaxation behaviour can be described by the relation σ/σ 0 = D exp(−CEφt) where σ/σ 0 is the ratio of the unrelaxed stress (derived from the change in curvature of the unconstrained beam) to the initial stress to bend the beam to the shape of the jig, D describes an initial rapid stress drop, C is a material constant, E is Young's modulus, φ is fast neutron flux and t is time. We review stress relaxation test results that have been used to characterize the effects of operating parameters (such as temperature, fast neutron flux and fluence) and microstructural properties (such as anisotropy, dislocation structure, grain shape, thermo-mechanical treatments and alloy content) on the in-reactor creep behaviour of zirconium alloys. Correlations between stress relaxation and creep results are also presented with support the use of the relaxation test for gathering in-reactor creep information. Strain recovery in in-reactor and out-reactor annealing experiments suggest that in addition to anelastic strain, a significant part of the in-reactor relaxation strain is due to irradiation damage-related creep.

Influence of helium injection schedule and prior thermomechanical treatment on the microstructure of type 316 SS

The influence of different helium injection schedules on microstructure development in Ni + ion-irradiated 316 SS at 625°C is discussed. Injection schedules were chosen to either approximate the MFR condition or mimic the mixed-spectrum reactor condition. Dual-ion irradiation to 25 dpa produced strongly bimodal cavity size distributions in solution annealed and solution annealed and aged samples, whereas single-ion irradiation followed by dual-ion irradiation to the same dose produced a cavity size distribution with a substantial component of intermediate-size cavities. Dual-ion irradiation produced only very small cavities in 20% CW material, while single-ion followed by dual-ion irradiation produced some intermediate size cavities and greater swelling.