Decreasing Deformation Temperature Research Articles

Supervised Machine Learning Approach for Modeling Hot Deformation Behavior of Medium Carbon Steel

Metal forming process parameters selection highly depends on the consistent and realistic characterization of material behavior under the combined effects of strain, strain rate, and temperature on the material flow stress. Hot deformation tensile tests are performed for AISI 1045 steel at deformation temperatures and strain rates ranges from 650 to 950 °C and 0.05 to 1.0 s−1, respectively. The received flow curves indicate that flow stress increases with a decrease in deformation temperature and an increase in strain rate. In this study, it is investigated the supervised machine learning techniques such as support vector regression, single decision tree, and random forest regression (RFR) models to characterize material‐flow behavior during hot deformation. Overall, the proposed RFR model results are in good agreement with the experimental observations. Besides, the proposed model's predictability is assessed using graphical and numerical validations. The numerical quantification confirms that the RFR models perform significantly better with a higher coefficient of determination (R 2), 0.9983, and low prediction error, 1.021%. Furthermore, it is revealed through the comparison with previous findings, that the proposed machine learning models can precisely calculate flow stress better than conventional models.

Effect of High‐Temperature Deformation on Bainite Transformation of a Low‐Carbon Bainite Steel

Unlike medium‐carbon and high‐carbon steels, the effect of the high‐temperature deformation process on the bainite transformation of low‐carbon steels is yet to be fully understood. Processes with varying high‐deformation temperatures and reductions are designed to investigate the bainitic transformation of low‐carbon steel. The results indicate that compared with the undeformed samples, for all deformation temperatures (850, 950, and 1050 °C) in the austenite temperature range, a small reduction of 5% accelerates the bainite transformation rate corresponding to the samples, and this can be attributed to the increase in the driving force. The promoting effect is increased with a decrease in deformation temperature. However, a relatively large reduction of the samples by 25% and 50% decreases the bainite transformation rate, and the inhibiting effect increases with decreasing deformation temperature. In addition, the amount of bainite increases at 1050 °C under all strains, and the amount is higher than the amount observed in undeformed samples. For the deformation temperature of 850 °C, the bainite amount increases under conditions of small reduction (5%), whereas the bainite amount decreases under large strain conditions (50% at 850 and 950 °C). The results provide a theoretical basis for optimizing the processing parameters associated with the production of low‐carbon bainite steels.

A Study of Hot Deformation Behavior of T15MN High-Speed Steel during Thermal Compression.

The hot deformation behavior of T15MN high-speed steel during thermal compression was studied by experiment and simulation. Specifically, the hot compression test was carried out on a Gleeble-1500 thermal-mechanical simulator at temperatures from 1273 to1423 K and strain rates from 0.01 to 10 s−1 with the deformation degree of 60%. It was found that all the flow stress curves were characterized by a single peak, indicating the occurrence of dynamic recrystallization (DRX), and flow stress will increase with increasing strain rate and decreasing deformation temperature. Based on the experimental data, the constitutive equations and thermal activation energy were obtained ( = 498,520 J/mol). Meanwhile, a cellular automaton model was established via the MATLAB platform to simulate the dynamic recrystallization phenomenon during hot deformation. The simulation results indicate that a good visualization effect of the microstructural evolution is achieved. Both increasing deformation temperature and decreasing strain rate can promote the increase in the average size and volume fraction of recrystallized grains (R-grains). Additionally, the calculated flow stress values fit in well with the experimental ones in general, which indicates that the established CA model has a certain ability to predict the deformation behavior of metal materials at elevated temperatures.

High-Temperature Deformation Behavior of M50 Steel

The hot deformation characteristics of M50 steel in the temperature range of 900–1150 °C and strain-rate range of 0.01–10 s−1 was investigated in this study using a Gleeble-3800 thermal simulation testing machine. The true stress–strain curves showed that the deformation resistance increased with decreasing deformation temperature, and increasing strain rate before the peak stress was reached. After the peak stress, dynamic reversion occurred, and consequently, the deformation stress decreased. The softening phenomenon was more obvious when the strain rate was low. The calculated values of the thermal deformation-activation energy Q and stress index n were 233,684.2 J/mol and 5.025568, respectively. On this basis, the Arrhenius-type constitutive equation was established, and in addition, a polynomial fit based on strain was performed to obtain the 9th-order strain-compensated constitutive equation with high fitting accuracy. By processing the flow stress curves, the processing maps of M50 steel were constructed, and the optimal processing range was predicted to be in the range of 1070–1150 °C and 0.01–1 s−1. The recrystallization behavior of M50 steel was also studied by constructing a dynamic recrystallization kinetic model and combining optical microscope (OM) and electron backscatter diffraction (EBSD) observation. The results show that with the increase of deformation temperature, the degree of recrystallization transformation increased accordingly, and the original grains were gradually replaced by recrystallized grains. Besides, in the optimal process zone for thermal processing, the recrystallized grains grew with decreasing strain rate and increasing temperature.

Microstructure and Thermal Deformation Behavior of Hot-Pressing Sintered Zr-6Al-0.1B Alloy.

Zr-6Al-0.1B alloy rich in Zr3Al phase is prepared by hot-pressing sintering. The thermal deformation behavior of sintered Zr-6Al-0.1B is analyzed by isothermal compression tests at deformation temperatures of 950, 1050, and 1150 °C with strain rates of 0.01, 0.1, and 1 s−1. The results indicate that at the early stage of thermal deformation, the stress increases rapidly with the increase of strain and then reaches the peak value. Subsequently, the stress decreases with the increase of strain under the softening effect. On the whole, the true stress-strain curve shifts to the high stress area with the increase of strain rate or the decrease of deformation temperature, so the sintered Zr-6Al-0.1B alloy belongs to the temperature and strain rate sensitive material. For the microstructure evolution of sintered Zr-6Al-0.1B during the isothermal compression, the high strain rate can improve the grain refinement. However, because sintered Zr-6Al-0.1B is a low plastic material, too high strain rate will exceed the deformation capacity of the material, resulting in an increase in defects. The increase of deformation temperature also contributes to grain refinement, but when the temperature is too high, due to the decomposition of Zr3Al phase, the deformation coordination of the material decreases, leading to the increase of the probability of the occurrence of defects. This study verified the feasibility of hot-pressing sintering to prepare Zr-6Al-0.1B alloy rich in Zr3Al phase and laid the foundation of “hot-pressing sintering + canning hot-extrusion” process of Zr-6Al-0.1B alloy components.

Enhancement of Uniform Elongation by Temperature Change during Tensile Deformation in a 0.2C TRIP Steel

It is important to control the deformation-induced martensitic transformation (DIMT) up to the latter part of the deformation to improve the uniform elongation (U.El) through the TRIP effect. In the present study, tensile tests with decreasing deformation temperatures were conducted to achieve continuous DIMT up to the latter part of the deformation. As a result, the U.El was improved by approximately 1.5 times compared with that in the tensile test conducted at 296 K. The enhancement of the U.El in the temperature change test was discussed with the use of neutron diffraction experiments. In the continuous DIMT behavior, a maximum transformation rate of about 0.4 was obtained at a true strain (ε) of 0.2, which was larger than that in the tensile test at 296 K. The tensile deformation behavior of ferrite (α), austenite (γ), and deformation-induced martensite (α′) phases were investigated from the viewpoint of the fraction weighted phase stress. The tensile test with a decreasing deformation temperature caused the increase of the fraction weighted phase stress of α and that of α′, which was affected by the DIMT behavior, resulting in the increase in the work hardening, and also controlled the ductility of α and α′, resulting in the enhancement of the U.El. Especially, the α phase contributed to maintaining high strength instead of α′ at a larger ε. Therefore, not only the DIMT behavior but also the deformation behavior of γ, α, and α′ are important in order to improve U.El due to the TRIP effect.

Springback prediction of 7075 aluminum alloy V-shaped parts in cold and hot stamping

In this paper, the springback behavior of high strength aluminum alloy 7075 is studied by experiments and finite element (FE) simulation. Firstly, an analytical model is established to predict the springback angles and analyze the springback trend. The springback experimental tests are conducted by using the V-shaped stamping dies. The influence of deformation temperature, punch radius, and blank holder force on the springback angles is studied. Finally, an FE simulation model is performed to investigate the deformation characteristics and springback process of the aluminum alloy sheet. The results show that the change of springback angles is direct proportional to the punch radius. The springback angles increase with the decreasing deformation temperature and the increasing blank holder force. The stress relaxation that occurs during the die holding stage is the primary reason of reducing the springback compared with cold stamping. Low blank holder force will cause side wall curl, which results in the deviation of forming size. The FE simulation model considering stress relaxation is capable of precisely predicting the change of springback angles, and the simulation results exhibit good consistency with the experimental results.

Deformation Behavior and Dynamic Recrystallization of Mg-1Li-1Al Alloy

In this paper, the hot workability of Mg-1Li-1Al (LA11) alloy is assessed through a uniaxial compression test in a temperature range from 200 to 400 °C and a strain rate, έ, of 1–0.01 s−1. The present study reveals that flow stress increases when the strain rate increases and deformation temperature decreases. Based on the hyperbolic sine equation, the flow stress constitutive equation of this alloy under high-temperature deformation is established. The average activation energy was 116.5 kJ/mol. Avrami equation was employed to investigate the dynamic recrystallization (DRX). The DRX mechanism affected by the deformation conditions and Zener–Hollomon parameters is revealed. Finally, the relationship between DRX volume fraction and deformation parameter is verified based on microstructure evolution, which is consistent with the theoretical prediction.

Formation of the Cenozoic Ailao Shan mid-crustal tectonic discontinuity: Role of Oligo-Miocene stratified sub-horizontal middle to lower crustal flow in the southeastern Tibetan Plateau

Structural, kinematic and geothermometry analyses are applied to unravel the structural evolution of the Ailao Shan Tectonic belt (ALTB) with a specific goal of understanding how crustal masses in southeastern Tibetan Plateau flowed during the Indian-Eurasian continental collision. A mid-crustal tectonic discontinuity that is characterized by a narrow ultramylonite belt separates the highly sheared high-grade metamorphic unit (HMU) from the low-grade metamorphic unit (LMU) along the ALTB. It is shown that rocks from the HMU and LMU possess deformation associations of identical structural geometries and kinematics, but at contrasting metamorphic conditions. Calculated mean kinematic vorticity numbers indicate that the ultramylonite belt was affected by dominant simple shearing, while the HMU and LMU on both sides of the belt were deformed by general shearing dominated by pure shearing. Geothermometry analyses on sheared two-mica schists from the HMU and quartz veins from the LMU reveal different deformation conditions, i.e., 610–777 °C, 0.4–0.6 GPa and 320–391 °C, respectively. There is also a sharp decrease in deformation temperature inferred from deformation microstructures and quartz c-axis fabrics, from 550–650 °C for the HMU to 350–400 °C for the LMU. These results suggest that there is an apparent temperature break of ca. 220 °C over a total structural distance of approximately 1 km between these two units at the present outcrop level. Our results show that the HMU and LMU are kinematically linked while rheologically decoupled, and were originally deformed by sub-horizontal shearing at different crustal levels in the same strain field. The ultramylonite belt between them potentially inherited a preexisting basement/cover contact along the ALTB and brought the two units into contact. We suggest that progressive stratified flow of middle to lower crustal masses during the extrusion of the Sundaland block at Oligocene to Early Miocene was responsible for the concurring high- and low-temperature fabrics at different crustal levels. Contemporaneous doming during the progressive crustal flow resulted in exhumation of the lower crustal rocks. During the process, the ultramylonite belt occurred as a tectonic discontinuity (TDC) that leads to the loss of a thick pile (ca. ∼6.3–7.8 km) of crustal masses in the middle crust.

Strengthening of low-cost rare earth magnesium alloy Mg-7Gd-2Y–1Zn-0.5Zr through multi-directional forging

Conventional rare-earth magnesium alloys have high strength but are not accepted by some fields because of their relatively high price. Magnesium alloys with the addition of small amounts of rare earth elements can improve the mechanical properties by severe plastic deformation while saving cost. In this paper, the effects of multidirectional forging (MDF) process on the microstructure and mechanical properties of low-cost rare earth magnesium alloy Mg-7Gd-2Y–1Zn-0.5Zr were investigated, and the plastic deformation mechanism, dynamic recrystallization mechanism and property strengthening mechanism during the MDF process were analyzed. The results showed that the yield strength, ultimate tensile strength and elongation of Mg-7Gd-2Y–1Zn-0.5Zr alloy were 395.2 MPa, 435.3 MPa and 17.6% after extrusion, MDF at reduced temperature conditions and T5 treatment. The increase in alloy strength is related to solid solution strengthening and is more attributed to the combined effect of grain refinement and aging precipitation. The dynamic recrystallization mechanism leading to grain refinement converts from discontinuous dynamic recrystallization to continuous dynamic recrystallization during MDF at reduced temperatures. And the decrease in deformation temperature leads to difficulties in prismatic and pyramidal slipping, where grain boundary sliding from fine grains and twinning in a few relatively coarse grains play a role in coordinating deformation.

Cryogenic Deformation Behavior and Microstructural Characteristics of 2195 Alloy

Cryogenic deformation can improve the strength and plasticity of Al–Li alloy, although the underlying mechanism is still not yet well understood. The effects of cryogenic temperature on the tensile properties and microstructure of an Al–Cu–Li alloy were investigated by means of tensile property test, roughness measurement, scanning electron microscope (SEM), optical microscope (OM), electron backscatter diffraction (EBSD), and transmission electron microscope (TEM). The results indicated that the strength and elongation of the as-annealed (O-state) and solution-treated (W-state) alloys increased with the decrease in deformation temperature, where the increasing trend of elongation of the W-state alloy was more significant than that of the O-state alloy. In addition, a temperature range was observed at approximately 178 K that caused the strength of the W-state alloy to slightly decrease. The decrease in temperature inhibited the dynamic recovery of the Al–Cu–Li alloy, which increased the dislocation density and the degree of work hardening, thus improving the strength of the alloy. At cryogenic temperatures, the internal grain structure was more involved in the deformation and the overall deformation was more uniform, which caused the alloy to have higher plasticity. This study provides a theoretical basis for the cryogenic forming of Al–Li alloy.

Investigation into Microstructure Evolution of Co–Ni–Cr–W–Based Superalloy During Hot Deformation

Hot compression tests were conducted using a Gleeble 3500 thermomechanical simulator at temperatures ranging from 1,000 to 1,200°C with the strain rate ranging from 0.1 to 10 s−1. Electron backscatter diffraction (EBSD) technique was employed by investigating the microstructure evolution during hot deformation. Microstructure observations reveal that deformation temperatures and strain rates have a significant effect on the DRX process. It is found that the fraction and grain size of DRX increase with the decreasing deformation temperature, along with the increasing high-angle grain boundaries (HAGBs). The fraction of DRX first decreases and then increases with the increase of strain rates. It is noted that there are both the nucleation mechanisms of discontinuous dynamic recrystallization (DDRX) and continuous dynamic recrystallization (CDRX) during the DRX process for Co–Ni–Cr–W–based superalloys. DDRX and CDRX are the primary and subsidiary nucleation mechanisms of DRX, respectively. It is also found that deformation temperatures and strain rates have almost no effect on the primary and subsidiary nucleation mechanisms of DRX. At the temperature above 1,150°C, the complete DRX occurred with the average grain sizes of about 25.32–29.01 μm. The homogeneity and refinement of microstructure can be obtained by selecting the suitable hot deformation parameters.

Hot Deformation Behavior of Ultralight Dual-Phase Mg-6li Alloy: Constitutive Model and Hot Processing Maps

High-temperature compression tests with dual-phase Mg-6Li alloy were conducted on the Gleeble-3500 thermal-mechanical simulator. Flow stress and micro-structure evolution were analyzed for temperatures (T = 423, 473,523 and 573 K) and strain rates (ε˙=0.001, 0.01, 0.1 and 1 s−1). On this basis, the constitutive model and hot processing maps were established. Besides, the dynamic re-crystallization (DRX) of α-Mg phase, grain orientation and texture composition under different deformation conditions were analyzed by EBSD technology. The experimental results show that the flow stress of Mg-6Li alloy increased with decreasing deformation temperature and increasing strain rate. In addition, the range of instability zone expanded with the increase of strain. The optimal thermal processing temperature was found to be in the range of 500 K–573 K, and the optimal strain rates were between 0.01 s−1–1 s−1. Model-predicted stress values were compared with experimental values for model verification. The 0.9954 correlation coefficient and the 5.48% average absolute relative error shown by the calculation indicate an acceptable accuracy of the model in predicting thermal deformation behavior of Mg-6Li alloy. Moreover, based on our EBSD data and maps analysis, the DRX proportion of α-Mg phase in Mg-6Li alloy was relatively low, and α-Mg phase formed <0001>//CD basal texture.

Hot deformation behavior and microstructure evolution of an Fe–30Cr–2Mo ultra-pure super ferritic stainless steel

The hot deformation behavior and microstructure evolution of an Fe–30Cr–2Mo ultra-pure super ferritic stainless steel were investigated at the temperature range of 950–1150 °C and strain rate varying from 0.01 to 10 s−1. A strain compensated constitutive equation based on the Arrhenius-type model was established to predict the flow stress. The hot processing map based on the dynamic materials model was achieved to identify the optimum processing parameters. In addition, the features of microstructure evolution combined with the processing map were systematically investigated. The experimental results revealed that the flow stress increased with decreasing deformation temperature or increasing strain rate. Dynamic recovery was confirmed to be the predominant softening mechanism. The values of flow stress predicted by the strain compensated constitutive equation agreed well with the experimental values. The extent of dynamic recrystallization and recrystallized grain size increased with increasing deformation temperature or decreasing strain rate, and the continuous dynamic recrystallization was attributed to be the predominant mechanism of recrystallization during hot deformation. The optimum hot working parameters were determined to be the deformation temperature of 1070–1150 °C and strain rate of 0.1–1 s−1 with a peak power dissipation efficiency of 42%.

Experimental study of grain structures evolution and constitutive model of isothermal deformed 2A14 aluminum alloy

Softening mechanism and microstructure evolution of 2A14 aluminum alloy were studied via isothermal deformation tests under the temperatures range from 573 K to 773 K and strain rates range from 10 −3 s −1 to 1 s −1 . Microstructures indicated that the grain structure was composed of massive fine sub-grains while only a small quantity of recrystallized grains formed along grain boundaries. The quantity of sub-grains increased with the decrease of deformation temperature or the increase of deformation strain rate, and the correlations between the deformation conditions and the geometrically necessary dislocation (GND) and stored strain energy were illuminated with quantification. The softening mechanism under different conditions was in compliance with the analysis of processing map. The microstructure indicated that the governing dynamic softening mechanism was the dynamic recovery for the temperature below 773 K, and dynamic recrystallization played the dominant role in the softening mechanism at low strain rates for the temperature at 773 K. The Arrhenius constitutive model and physical-based constitutive model considering lattice diffusion have been established, and both two models can well predict the flow stress and GND density of 2A14 aluminum alloy.

Influence of deformation parameters and network structure to the microstructure evolution and flow stress of TiBw/Ti64 composite

Titanium matrix composites (TMCs) play important roles in the weight reduction of the modern aero-space industry due to the excellent combination of strength and ductility. Especially, titanium matrix composites with network architecture have drawn much attention in recent years. To better understand the influence of deformation temperature, strain rate and the network size over the microstructure evolution and the flow stress of network structured TMCs, 8 vol.% TiBw/Ti64 composites with network sized 65 μm, 110 μm and 150 μm were prepared for thermal compression test. The results showed that the decrease in deformation temperature and the increase of the strain rate will prompt the dynamic recrystallization process, thereby producing refined equiaxed α grains. Multiple changes were observed accompanying the decrease of network size, including the refinement of the deformation microstructure, the drop of compressive flow stress, as well as the decrease of the texture intensity. It was also found that, the texture of specimens deformed in β region was inherited from the texture of body centered cubic (BCC) β phase through β→α transformation, abiding by Burgers relationship. Moreover, preference in variants selection during β→α transformation was observed, which is an important factor for the formation of texture in β phase deformation.

Analysis of Deformation Behavior and Microstructure Changes for α/β Titanium Alloy at Elevated Temperature

In this paper, the isothermal compressive behavior of Ti-6.5Al-3.5Mo-1.5Zr-0.3Si titanium alloy was investigated on a Gleeble-3500 simulator in the temperature range from 1073 to 1373 K at an interval of 50 K (while the phase transus temperature is approximately 1273 K) and the strain rate range of 0.001–10 s−1. Microstructure evolution and deformation behavior were investigated. The typical flow softening behavior during deformation is observed, which can be explained by the deformation heating effect and microstructure changes. The deformation heating effect is influenced by strain rate and deformation temperature, and it increases with the increasing strain rate and decreasing deformation temperature. In the α + β phase field, the fractions of the primary α phase decrease with the increase of deformation temperature and strain rate. In this case, dynamic recovery may be the main mechanism for microstructure evolution based on the electron back-scatter diffraction (EBSD) analysis. The fully phase transformation occurs above the β transus temperature, which is governed by Burgers orientation relations. The Zener–Hollomon parameter with an exponent-type equation was used to intuitively describe the effects of the deformation temperatures and strain rates on the flow stress behaviors. Furthermore, the influence of strain was incorporated in the constitutive analysis. A fourth-order polynomial was ideally matched to represent the influence of strain. In consequence, the constitutive equation of Ti-6.5Al-3.5Mo-1.5Zr-0.3Si titanium alloy including the phase transus and compensation of the strain was developed based on the experimental results throughout the deformation process. The results indicated that the correlation coefficient (R), root mean square error (RMSE), and the average absolute relative error (AARE) were calculated to be 0.987, 3.585 MPa, and 9.62% in the single-phase region and 0.979, 18.78 MPa, and 9.16% in the duplex-phase region, respectively. Hence, the constitutive model proposed in this research can provide accurate and precise theoretical prediction for the flow stress behavior of Ti-6.5Al-3.5Mo-1.5Zr-0.3Si titanium alloy.

On the Significance of Thermal Deformation and Dynamic Recrystallization on Flow Behavior of 25Mn2Si2Cr Bainitic Alloy

Thermal deformation characteristics, dynamic recrystallization behavior, and processing map of 25Mn2Si2Cr high strength bainitic steels are elucidated during wide temperature range (850–1100 °C) and strain rate (0.1–10 s−1) from flow behavior. Thermomechanical analysis by Thermo‐Calc software indicates that microstructure during thermal deformation is austenite, which is conducive to dynamic recrystallization. Phenomenological constitutive equations of thermal deformation, dynamic recrystallization equation, and processing map are established to predict flow behavior. The experimental results indicate that deformation temperature and strain rate have significant effect on thermal deformation process. Flow stress increases with decrease in deformation temperature and increase in strain rate. Increasing deformation temperature and decreasing strain rate promote dynamic recrystallization occurrence. However, the grain size increases with increase in deformation temperature. Thermal simulations predict that optimum thermal deformation parameters are deformation temperature (950–1000 °C), strain rate (0.25–5 s−1) and deformation temperature (850–950 °C), strain rate (1–5 s−1) for processing of Mn–Si–Cr bainitic steels.

High‐Temperature Deformation Behavior of a Novel Hot‐Rolled Transformation‐Induced Plasticity Alloy: Experiments, Constitutive Modeling, and Processing Maps

Hot deformation behavior of a novel hot‐rolled transformation‐induced plasticity (TRIP) alloy is studied by compression test in the temperature range of 1123–1323 K and strain rate range of 0.1–10 s−1, and the relation between flow stress and Zener–Hollomon parameter is systematically analyzed. The study indicates that the flow stress of experimental alloy increases with the decrease in deformation temperature and increase in strain rate. The value of stress exponent (n) and activation energy (Q) is 7.4 and 323.5 kJ mol−1. Processing maps developed using the dynamic material model at strains of 0.25, 0.40, and 0.55 predict optimum hot deformation temperature of 1273 K and strain rate of 0.8 s−1. The study of dynamic recrystallization (DRX) behavior under different deformation conditions by transmission electron microscope (TEM) suggests that two DRX mechanisms are operative—continuous dynamic recrystallization (CDRX) mechanism, which depends on high density of dislocations at the grain boundaries, and the second mechanism involves strain‐induced boundary migration (SIBM), governed by grain boundary bulging.

Significant transitions of mechanical properties by changing temperature in Cu-Al alloys with heterogeneous microstructures

Mechanical properties of Cu-5 at.%Al alloy with different recrystallization fractions ranging from 0% to 100% are studied by tensile tests at 77 K and 293 K, respectively. The relationship between uniform elongation and recrystallization fraction had been unveiled. The uniform elongation and recrystallization fraction have a transition from single-stage to two-stage relationship with decreasing deformation temperature from 293 K to 77 K, which is related to the strain gradient and deformation twinning in recrystallized grains. In addition, the conventional trade-off relationship between strength and ductility has been broken via tailoring the heterogeneous microstructures at 77 K.