Pre-deformation Temperature Research Articles

Cryogenic deformation tailors the dislocation accumulation mode to modify grain boundary character distribution of Incoloy 925

Cryogenic deformation was introduced in thermomechanical processing to restrict dislocation mobility and tailor the initial dislocation distribution at the deformation stage, subsequently followed by annealing treatment to alter the grain boundary characteristic distribution with a view to the feasibility of grain boundary engineering in Incoloy 925. The microstructural evolution deformed at cryogenic (77 K) and room temperature (298 K) under various rolling reductions and annealing conditions was systematically studied to optimize the grain boundary network, which is evaluated using multi-source quantitative statistics of the twin related domains. The results showed that cryogenic deformation can adjust the dislocation accumulation mode and plastic deformation mechanism. Geometrically dislocation distributions, reconstructed by HR-EBSD device, with fishbone-like, ribbon-like or lamellar characteristics can be observed for the deformed specimens. During the subsequent annealing, the samples with 5% and 15% pre-strain are incapable of producing the ideal twin related domains, while the sample annealed at 1075°C with 10% pre-strain show the optimization of GBCD with the highest Σ3n (n = 1, 2, and 3) boundary fraction being 69.67%. Compared with the room temperature pre-deformation, cryogenic deformation can effectively reduce the critical temperature of GBE, and also leads to the Σ3n grain boundaries proportion increasing from 31.47% to 67.46% in 10%/1050°C sample. Meanwhile, with the appropriate strain, cryogenic deformation is more conducive to optimizing the GBCD than room-temperature deformation.

Load partitioning behavior in a metastable austenitic stainless steel subjected to pre-deformation below and above Md temperature: Experimental and modeling studies

A physics-based model coupling the evolution of dislocation density in the austenite and strain-induced martensite phases with composite flow strength and kinetics of martensite transformation was developed to describe ambient temperature uniaxial tensile deformation behavior of mill annealed and pre-deformed 304 SS. The applicability of the model was evaluated by fitting the experimental stress-strain data and validated with the experimentally estimated dislocation density in the austenite phase of the steel. The modeling results revealed that the initial microstructure of the steel significantly influences the dislocation accumulation and annihilation rates as well as the load partitioning between austenite and martensite phases. Despite a lower initial dislocation density than the pre-deformed conditions, the mill annealed condition revealed a higher capacity (approximately two orders of magnitude) for dislocation accumulation in the austenite phase. The initial volume fraction of strain-induced martensite phase and the initial dislocation density of pre-deformed 304 SS are governed by the degree of pre-deformation and pre-deformation temperature, either below or above Md temperature (where Md is defined as the temperature above which plastic deformation does not result in austenite to martensite transformation). It is observed that storage, as well as annihilation rates in the austenite phase, are higher for the steel having a higher initial volume fraction of martensite and dislocation density. Though the pre-deformation altered the evolution of martensite volume fraction, the steel exhibited a minor variation in dislocation accumulation capability in the martensite phase for different conditions.

Effect of pre-deformation on microstructure characteristics, texture evolution and deformation mechanism of AZ31 magnesium alloy

According to the stress characteristics of the section of the wall reducing section in the cold rolling process of two roll pilger tube, the room temperature pre-deformation and secondary compression experiment of AZ31 magnesium alloy are carried out, and the effects of pre-deformation on the microstructure and plastic deformation mechanism of AZ31 magnesium alloy are studied by combining EBSD analysis with VPSC model. The results show that with the increase of pre-deformation, {0001} basal texture gradually disappears, and the twinning texture of c-axis parallel to ED direction gradually increases. When the secondary compression is 8%, it has completely transformed into the twinning texture of c-axis parallel to ED direction. Comparing the secondary compression properties under different pre-deformation amounts, it is found that the greater the pre-deformed texture strength produced by pre-deformation, the greater the yield strength in the corresponding secondary compression process. When the pre-deformation is 3%, the yield strength increases by 67%; When the pre-deformation is 5%, the yield strength increases by 130%. At the same time, the activation of {101‾1} compression twin greatly influences the secondary compression plasticity of magnesium alloy.

Effect of pre-strain temperature on the subsequent dynamic recrystallization of parent β phase and transformed α-variant selection in a novel near α titanium alloy

The microstructure and micro-orientation evolution of a novel high-temperature titanium alloy were characterized during a two-step deformation process. In the first step, the specimens were deformed at temperatures of 800, 850, 900 and 950 °C, and subsequently deformed in the same β-phase region of 1050 °C. The specimens pre-strained at 800 °C and 950 °C were fully recrystallized during the β-phase deformation process. By contrast, the specimens pre-strained at 850 and 900 °C were not fully recrystallized during the β-phase deformation process. This indicates that the extent of dynamic recrystallization did not increased monotonically with the pre-strain temperature. The extent of dynamic recrystallization was related to the stored energy accumulated in the α and β-phase during the pre-strain deformation. The change in stored energy was based on the difference in microstructure morphology produced by different pre-deformation temperatures,and stored energy was quantitatively estimated based on the local misorientation distribution. Meanwhile, the generation of dynamically recrystallized grains was beneficial to decreasing the generation possibility of the macro-zone. This decrease was attributed to the refinement of β-grains and randomly distributed orientation, which was also related to the decrease in the typically distributed “two parallel wings” orientation between the α and β phase induced by uncrystallized deformation. Additionally, the two-step deformation process with the pre-strain at the (α + β) middle regime was an effective strategy to promote the dynamic recrystallization and decrease the generation probability of the macro-zone.

Precisely Controlling the Output Force of Hydrogel Actuator Based on Thermodynamic Nonequilibrium Temporary Deformation.

Self-shaping hydrogel actuators have promising applications in various fields. However, one hydrogel actuator can generally access only one specifically predefined deformation and output force, which are determined by its thermodynamic equilibrium swelling state under external stimuli. Here, we present a simple yet versatile strategy for precisely programming the output force/energy of dual-gradient hydrogel actuators. The strategy is based on thermodynamic nonequilibrium snapping deformations occurring during the recovery process of predeformed dual-gradient hydrogel actuators in low-temperature water. The output force/energy of such thermodynamic nonequilibrium snapping deformation is highly associated with predeformation conditions of the hydrogel actuators, which increases with the increase of the predeformation temperature or time. In consequence, just by adjusting the predeformation conditions of the dual-gradient hydrogel actuators, their output force, energy, and power can be modulated precisely and continuously during the snapping deformation. The as-prepared hydrogel actuators can not only be used as smart lifters and grippers with ultrahigh accuracy of weight identification but also act as smart switches in the timing circuits with precisely adjustable operating time, paving the way for the design of a new generation of actuation materials.

Influence of Warm Predeformation Temperature on the Corrosion Property of Type 304 Austenitic Stainless Steel

The influence of predeformation temperature (100-400 °C) on the corrosion property of the predeformed 304 austenitic stainless steel (SS) in 3.5 wt.% NaCl medium was studied by electrochemical and microstructural characterization. The degradation of pitting corrosion resistance of the predeformed 304 SS can be inhibited by decreasing the deformation temperature. The increase in special (low-Σ coincidence site lattice) boundaries, the decrease in low angle and random grain boundaries improve the pitting corrosion resistance. The corrosion property of the predeformed 304 SS is controlled by the coupling effect of grain boundary and dislocation, while it is little affected by the residual stress. The microstructures induced by adjusting predeformation temperature change the characteristics of the passive film on the SS surface, which affects the pitting corrosion resistance. The achieved results distinctly demonstrate that high-strength and corrosion-resistant metallic materials are possible to be developed by controlling predeformation temperature.

Grain Refinement Assisted by Deformation Enhanced Precipitates through Thermomechanical Treatment of AA7055 Al Alloy

Fine-grained sheets of AA7055 Al alloy were produced by an improved double-stage rolling thermomechanical treatment (DRTMT) assisted by deformation-enhanced precipitates (DEPs). The DRTMT composed of a low temperature pre-deformation, an intermediate annealing, and a final hot rolling exhibited significantly superior tensile ductility to the conventional rolling thermomechanical treated alloy (CRTMT). Numerous fine spherical DEPs appeared after the pre-deformation. Those DEPs could exert a strong drag force on the migration of boundaries and dislocations. Dislocation cells were formed due to the drag force and dynamic recovery. These dislocation cells become polygon sub-grains by static recovery during the subsequent intermediate annealing. After the final hot deformation, with further deformation and rising temperature, low angle grain boundaries gradually stabilized and transferred to high angle grain boundaries. Due to the transformation, fine equiaxed grains were formed after DRTMT. The DRTMT alloys display superior elongation to the CRTMT alloy while maintaining high strength for grain refinement. Thus, DRTMT would be a good alternative to manufacture different heat-treatable Al alloys with fine grains efficiently.

Research on the plastic fluidity of high-strength steel 22MnB5 based on the thermal-mechanical-fluid multi-filed coupling

The effects of technological parameters such as pre-deformation, strain rate, and final forming temperature on the formability of high-strength steel 22MnB5 were studied. Firstly, the finite element model of thermal-mechanical-fluid coupling was established by finite element simulation software ABAQUS. Secondly, the simulation and experiment of the pure hydraulic bulging with the forming temperature are 20, 100, 300, 500, and 700 °C and were carried out, respectively. Finally, by setting the technological parameters such as the cavity pressure loading rate (2, 4, 6, 8, and10 MPa/s) and final forming temperature T (20, 100, 300, 500, and 700 °C), the numerical simulation and the experiment of the coupling of the initial room temperature pre-deformation and high-temperature final forming were studied, and the optimal combination of process parameters was optimized by making the bulging height as the objective function at the end of the forming process. Keeping the cavity pressure loading rate is invariable; the bulging height difference between the two processes is 12.02, 20.43, 26.18, 33.20, and 39.39 mm, respectively, when the temperature is 300 °C. Keeping the final forming temperature is invariable; when the cavity pressure loading rate is 8 MPa/s and the temperature is 300 °C, the difference of the bulging height difference between the hot hydraulic bulging and room temperature hydraulic bulging is maximum, the largest value is 2.17 mm. Taking the bulging height as the criteria of material forming performance, the combination of the final forming temperature of 300 °C and the cavity pressure loading rate of 8 MPa/s is optimum. Finally, the whole coupling process is divided into three stages, and by using the form of structuring sectionalized constitutive equation, the influence law of the dual action of work hardening and softening on the deformation and fluidity of the material in the compound-forming process is analyzed from the perspective of the evolution law of microstructure.

Ageing Behaviour of Al-Mg-Si Alloys After Cryogenic and Room Temperature Deformation.

The aim of this study is to investigate the effects of cryogenic and room temperature pre-deformation on subsequent artificial ageing of Al–Mg–Si alloys. Naturally aged and pre-aged samples were strained to 5%, 10% and 20% at RT (25 °C) and under liquid nitrogen, and artificially aged at 185 °C. Pre-deformation generally increases ageing kinetics for both the naturally aged and pre-aged alloys, which increase in proportion to the degree of pre-deformation, and which are slightly more pronounced for the cryogenic condition. The peak strength is constant, except for when a low degree of pre-deformation is used, in which case it is slightly reduced. Cryogenically deformed samples show an increased strength and hardness, compared to samples pre-deformed at RT, when subjected to an equal magnitude of strain. This difference is reduced during artificial ageing. Synchrotron measurements reveal that this behaviour can be linked to the greater dislocation density, which is not completely recovered even after prolonged ageing at 185 °C.

A constitutive model for strain hardening behavior of predeformed amorphous polymers: Incorporating dissipative dynamics of molecular orientation

A series of uniaxial compression tests are performed on amorphous thermoplastics poly (ethylene terephthalate)-glycol (PETG). The stress response shows a dramatic change with temperature and strain rate, including the modulus, yield strength and strain hardening. The strain hardening behavior also shows a strong dependence on predeformation, representing as a less pronounced hardening response during the reloading process with increasing the predeformation temperature and decreasing the predeformation rate. This phenomenon cannot be described by the models assuming a temperature-dependent hardening modulus. In this work, we develop a thermodynamic consistent model, which employs a dissipative back-stress tensor for molecular orientation. With increasing temperature, a stronger relaxation occurs, resulting in less molecular orientation and hardening response. A numerical scheme is also constructed to determine the model parameters. Comparison with experiments shows that the model is able to reproduce all the main features of the stress response at various strain rates spanning the glass transition region. The model can also accurately capture the strain hardening response of polymers predeformed to a strain of 30% or 60% at various temperatures and rates.

Evolution of Grain Boundary Character Distribution in Pure Copper during Low-Strain Thermomechanical Processing

In order to get optimal grain boundary character distribution (GBCD) and grain boundary properties, thermomechanical processing (TMP) is usually adopted in grain boundary engineering. However, the mechanism behind the TMP treatments and GBCD optimization is still unclear. The present study has conducted a series experiments involving low-strain TMPs to study the relationship between TMP parameters and the behind microstructural evolution. The experimental results indicate that in the scope of low-strain TMP, strain induced boundary migration (SIBM) is the most effective process for GBCD optimization. Besides, SIBM and grain growth would gradually transfer to recrystallization with the increase of pre-deformation level and annealing temperature. Further quasi in-situ EBSD results infer that SBIM is activated locally in some region with high stored energy, and further gradual initiation of SIBM from one region to another contributes to the gradual increase of special boundaries with annealing time.

Effects of Pre-Deformation Temperature on Nanobainite Transformation Kinetics and Microstructure

Effects of Pre-Deformation Temperature on Nanobainite Transformation Kinetics and Microstructure

Shape memory properties of olefin block copolymer (OBC)/poly(ɛ‐caprolactone) (PCL) blends

ABSTRACTConventionally, the chemically crosslinked shape memory polymer (SMP) blends are hard to recycle due to their network structure. Herein, the environmental SMP blends of olefin block copolymer (OBC), a unique thermoplastic elastomer, and poly(ɛ‐caprolactone) (PCL) were physically crosslinked. Dicumyl peroxide was used as the compatibilizer to improve their miscibility, as evidenced by the reduced dispersed domain size of PCL in the OBC matrix and the increased complex viscosity. The peroxide modified OBC/PCL blend conferred enhanced tensile properties, increased dynamic storage modulus, increased crystallization temperature, and higher recovery stress. The shape memory behaviors of OBC/PCL blends predeformed under two different predeformation temperatures (30 and 65 °C) were investigated. The recovery stress showed respective maximum peak values corresponding to their predeformation temperatures. In addition, the modified blends gave the better shape memory performance at 65 °C. Besides the peroxide modification approach, a precycle training process via prestretching the samples and reducing the mechanical hysteresis was implemented to improve shape memory performance further. This is the first work on the OBC‐based SMP blends to enhance shape memory performance by combining the chemical modification using added peroxide compatibilizer and the process modification using a precycle training process. © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2017, 134, 45475.

Effect of deformation induced precipitation on grain refinement and improvement of mechanical properties AA 7055 aluminum alloy

An improved TMT (thermomechanical treatment) double step hot rolling (DR) including low-temperature pre-deformation, intermediate short-term annealing and final hot rolling, was proposed to manufacture fine grained AA7055 sheets. Microstructural analysis results show that the low temperature pre-deformation can accelerate the formation and spheroidization of fine precipitates which can exert great drag force to the migration of grain boundaries and dislocations so as to promote the formation of high density dislocation cells and accommodate sufficient deformation storage energy subsequently. Then, these dislocation cells may turn into polygon sub-grains by short-period intermediate annealing. With the final hot rolling, low angle grain boundaries (LAGBs like sub-grain boundaries) were gradually transferred into high angle grain boundaries (HAGBs) and fine-grained structures were obtained. Tensile testing results reveal that samples processed by the optimal DR processing (300°C/60%+430°C/5min+430°C/50%) display superior tensile plasticity than the conventional hot rolling (CR) samples without strength loss, which could be correlated with the fine grained structure of the DR sample. It indicates that the present DR processing may be a good alternative to efficiently produce fine grain structured heat-treatable aluminum alloys.

Influence of high temperature pre-deformation on the dissolution rate of delta ferrites in martensitic heat-resistant steels

The dissolution process of delta ferrites and the influence of high temperature pre-deformation on the dissolution rate of delta ferrites in martensitic heat-resistant steel 10Cr12Ni3Mo2VN were studied by isothermal heating and thermal simulation experiments. The precipitation temperature of delta ferrites in experimental steel is about 1195 °C. M23C6-type carbides incline to precipitate and coarsen at the boundaries of delta ferrites below 930 °C, and can be rapidly dissolved by heating at 1180 °C. The percentage of delta ferrites gradually decreases with heating time. And a Kolmogorov-Johnson-Mehl-Avrami equation was established to describe the dissolution process of delta ferrites at 1180 °C. High temperature pre-deformation can markedly increase the dissolution rate of delta ferrites. Pre-deformation can largely increase the interface area between delta ferrite and matrix and thus increase the unit-time diffusing quantities of alloying elements between delta ferrites and matrix. In addition, high temperature pre-deformation leads to dynamic recrystallization and increases the number of internal grain boundaries in the delta ferrites. This can also greatly increase the diffusing rate of alloying elements. In these cases, the dissolution of delta ferrites can be promoted.

Annealing effect on the shape memory properties of polylactic acid (PLA)/thermoplastic polyurethane (TPU) bio-based blends

PLA and TPU were melt-blended to form shape memory bio-based blends with or without post-annealing effect. To the authors’ best knowledge, this is the first work to discuss the annealing effect on the PLA-based SMP blends. Annealed TPU showed regularly fractured surfaces unlike the macro-phase segregated domains for non-annealed TPU. After 3 h-annealing treatment, spherulites were observed for PLA, but not for TPU. The crystallinity of PLA increased, close to 3-fold increment, for annealed blends in comparison with non-annealed blends. The shape memory behaviors of PLA/TPU blends predeformed under three different predeformation temperatures (25, 80, 120 °C) were investigated. The annealing effect was helpful in enhancing the shape fixing ratio of the PLA/TPU (60/40) blend at high predeformation temperature of 120 °C in comparison with 25 °C. However, the suitable selection of the optimum predeformation temperature at 80 °C outweighed the annealing effect to attain the high shape fixing ratio, even in the case of non-annealed blends. The annealing effect often increased the perfection of crystal domains/interfaces and the larger crystal sizes, which would be detrimental to the molecular extensibility. The overall annealing effect on the shape recovery ratios were quite effective for both PLA/TPU blends of 80/20 and 60/40 without sacrificing the shape fixing ratios at the optimum predeformation temperature of 80 °C, attributing to the increased crystallinity of PLA and homogenized phase domains of TPU. Particularly, the annealing treatment did significantly increase the recovery ratio of the blends, more than 2-fold increment, especially for PLA/TPU (60/40) blend. At both lower or higher predeformation temperatures, the stress concentration between the increased crystalline domains and amorphous regions tended to dominate the annealing effect, leading to a negative contribution to the shape recovery processes.

Property Optimization for TWIP Steels – Effect of Pre-deformation Temperature on Fatigue Properties

The current work investigates the impact of pre-deformation temperatures on the microstructure evolution and the subsequent cyclic stress-strain response of high-manganese steel showing twinning-induced plasticity (TWIP) at room temperature (RT). Deformation at low temperatures increases the hardening rate at low to medium degrees of deformation through concurrent martensitic transformation. In contrast, high temperatures promote dislocation slip. Thus, employing pre-treatments at temperatures below and above RT leads to the evolution of considerably different microstructures. Low-cycle fatigue experiments revealed distinct differences for the pre-treated TWIP steels.

Influence of Pre-Deformation Temperature on Mechanics Performance of NiTiNb Shape Memory Alloy: First-Principles Calculation

Based on first-principles calculation, the mechanism of optimal pre-deformation temperature (Ms+30) for NiTiNb alloy was characterized and analyzed by several parameters, such as crystal constant, cohesive energy and the elastic constants. Simulation results showed that the shear modulus c′ and c44 of matrix NiTi phase was softened at 208K. At the same time, its cohesive energy obtained its local maximum value, which was originated from the easy transition ability of valence electrons in d orbital. In other words, the matrix NiTi phase was instable only when the pre-deformed temperature was not only for Ms but also to 208K (Ms+30K).

Effects of Mechanical Treatment and Pre-Deformation on Shape Recovery Stress of Ti<sub>50</sub>Ni<sub>47</sub>Fe<sub>3 </sub>Shape Memory Alloy

Effects of mechanical treatment, pre-strain and pre-deformation temperature on shape recovery stress of Ti50Ni47Fe3 shape memory alloys were investigated. Mechanical treatment, consisting of hot-rolling and hot-forging, improved σrmax σrmax (maximum recovery stress), dσr/dT (rate of recovery stress with temperature) and Ts (the temperature at which recovery stress rises sharply). Martensitic transformation and reverse transformation temperatures also increased with increasing the hot-rolling deformation. σrmax and dσr/dT increased with increasing pre-strain at first and then decreased with further increasing of pre-strain, and the maximum values of σrmaxand dσr/dT were obtained at a pre-strain of 10%. On the other hand, σrmaxand dσr/dT decreased with increasing pre-deformation temperature. Upon a hot-rolling of 88% area reduction and pre-deformation of 10% at-150°C, the Ti50Ni47Fe3 alloy exhibited excellent recovery stress property with the σrmax of 391MPa in combination with dσr/dT of 5.09.

Influence of Predeformation Temperature on Recovery Performance of Ni<sub>47</sub>Ti<sub>44</sub>Nb<sub>9</sub> Alloy Φ16mm Pipe Joint

Using the method of expanding diameter in low temperature and water bath heating, this paper studied the effect of pre-deformation temperature on shape memory recovery characteristics of Ni47Ti44Nb9 alloy Φ16 mm pipe joint. The results showed that the transformation hysteresis and the radial recoverable strain were relative stable along with the addition of pre-deformation temperature, which were 122°C±4°C and 7.1%±0.3% respectively. Moreover the radial recovery rate of inner diameter declined smoothly with the addition of pre-deformation temperature. Systems considering the three parameters of transformation hysteresis, recoverable strain and strain recovery rate, it seemed the excellent predeformed temperature for Ni47Ti44Nb9 alloy Φ16 mm pipe joint was in-65~-70°C.