Steady-state Microstructure Research Articles

Understanding variations of thermal hysteresis in barocaloric plastic crystal neopentyl glycol using correlative microscopy and calorimetry

Plastic crystals (PCs) exhibit solid–solid order-disorder first-order phase transitions that are accompanied by large correlated thermal and volume changes. These characteristics make PCs promising barocaloric solid-state working bodies for heating and cooling applications. However, understanding the variation of transition temperatures and thermal hysteresis in PCs with cycling is critical if these materials are to replace traditional gaseous refrigerants. Here, for the archetypal barocaloric PC neopentyl glycol (NPG), we correlate microstructure obtained from scanning electron microscopy with local and total thermal changes at the phase transition from infra-red imaging and calorimetry, respectively. We outline an evolution in microstructure as NPG recrystallises during repeated thermal cycling through its solid–solid phase transition. The observed microstructural changes are correlated with spatially inhomogeneous heat transfer, yielding direct insight into the kinetics of the phase transition. Our results suggest that the interplay of these processes affects the undesirable thermal hysteresis and the nature of the kinetic steady-state microstructures that are stabilised during cycling between the ordered and disordered phases. These observations have implications for using NPG and other PCs as technologically relevant barocaloric materials and suggest ways in which the hysteresis in these types of materials may be modified.

The effect of carbide decomposition and carbon loss on the steady state composition of WC-Cr3C2-Ni composite coatings at 900 °C.

The steady-state composition and microstructure of 42 wt% WC-42 wt% Cr3C2–16 wt% Ni coatings sprayed under “low”, “medium” and “high” thermal input conditions was explored following treatment at 900 °C for up to 30 days. None of the coatings achieved thermodynamic equilibrium compositions. Attempts at compensating for mechanisms of bulk carbon loss and selective carbon loss from the dissolution phases alone, were both unsuccessful in predicting the steady-state compositions based on equilibrium phase diagrams. The low thermal input HVOF coating underwent recovery of the dissolved Cr3C2 phase to form an interconnected, two-phase Cr3C2 + Ni-alloy matrix surrounding the retained WC grains, which did not achieve thermodynamic equilibrium with the precipitating phases. The high carbide dissolution plasma ArH2 coating generated a homogeneous distribution of metallic and carbon elements such that the metal: carbon ratio exceeded 1:1, favouring the formation of (W,Cr)2C. The remaining material formed (Cr,W)3C2 within a Ni(Cr,W) alloy binder. Chromium stabilised the (W,Cr)2C phase such that it did not decompose into W + WC, as predicted in the WC system. The plasma ArHe response was a mix of these mechanisms. A distinctive feature of the heat-treated coatings was the lack of significant grain growth, with submicron carbide grains retained out to 30 days at 900 °C.

Microstructure of a heavily irradiated metal exposed to a spectrum of atomic recoils

At temperatures below the onset of vacancy migration, metals exposed to energetic ions develop dynamically fluctuating steady-state microstructures. Statistical properties of these microstructures in the asymptotic high exposure limit are not universal and vary depending on the energy and mass of the incident ions. We develop a model for the microstructure of an ion-irradiated metal under athermal conditions, where internal stress fluctuations dominate the kinetics of structural evolution. The balance between defect production and recombination depends sensitively not only on the total exposure to irradiation, defined by the fluence, but also on the energy of the incident particles. The model predicts the defect content in the high dose limit as an integral of the spectrum of primary knock-on atom energies, with the finding that low energy ions produce a significantly higher amount of damage than high energy ions at comparable levels of exposure to radiation.

Significance of strain rate in severe plastic deformation on steady-state microstructure and strength

The microstructure and mechanical properties of materials saturate to steady states after severe plastic deformation (SPD). Despite the well-known effect of temperature on the steady-state microstructure, there is no general agreement on the significance of strain rate and the applicability of the Zener-Hollomon parameter in this regard. In this study, several pure metals (aluminum, copper, titanium, and iron) and a Cu–30Zn (wt%) brass alloy have been processed by a high-speed high-pressure torsion (HPT) equipment with controllable rotation speeds in the range of 0.06–60 rpm. It is found that crystallite/grain size, dislocation density, microhardness and shear stress at the steady state are reasonably rate-independent for the von Mises strain rates in the range of 0.004–20 s−1. Because both rates of grain refinement and of dynamic recrystallization are proportional to the strain rate, it is suggested that their balance, which determines the steady state, is rate-independent.

Comparing structure-property evolution for PM-HIP and forged alloy 625 irradiated with neutrons to 1 dpa

The nuclear power industry has growing interest in qualifying powder metallurgy with hot isostatic pressing (PM-HIP) to replace traditional alloy fabrication methods for reactor structural components. But there is little known about the response of PM-HIP alloys to reactor conditions. This study directly compares the response of PM-HIP to forged Ni-base Alloy 625 under neutron irradiation doses ∼0.5–1 displacements per atom (dpa) at temperatures ranging ∼321–385 °C. Post-irradiation examination involves microstructure characterization, ASTM E8 uniaxial tensile testing, and fractography. Up through 1 dpa, PM-HIP Alloy 625 appears more resistant to irradiation-induced cavity nucleation than its forged counterpart, and consequently experiences significantly less hardening. This observed difference in performance can be explained by the higher initial dislocation density of the forged material, which represents an interstitial-biased sink that leaves a vacancy supersaturation to nucleate cavities. These findings show promise for qualification of PM-HIP Alloy 625 for nuclear applications, although higher dose studies are needed to assess the steady-state irradiated microstructure.

Formation of nanostructures in α-uranium processed by high pressure torsion

High pressure torsion (HPT) was employed to fabricate nanostructured α-uranium successfully. The evolution of microstructures (grain size and dislocation density) and related mechanical properties (hardness and elastic modulus) varied with strain were quantified and analyzed. The steady-state microstructures and microtextures of the nanostructured α-uranium were characterized by transmission electron microscopy (TEM) and transmission Kikuchi diffraction (TKD) analysis in scanning electron microscopy (SEM), respectively. The grain/subgrain size of the nanostructured α-uranium is below 100 nm, and a weak microtexture with four components is presented. The microstructure evolution of α-uranium during severe plastic deformation (SPD) could be divided into three stages, and it resonates with the variation of creep resistance as a function of strain. On account of the unique crystal structure of α-uranium, the grain refinement limit was discussed and compared with 25 pure metals from a statistical view.

Steady-State Microstructures of Quartz Revisited: Evaluation of Stress States in Deformation Experiments Using a Solid-Medium Apparatus

Dynamically recrystallizing quartz is believed to approach a steady-state microstructure, which reflects flow stress in dislocation creep. In a classic experimental study performed by Masuda and Fujimura in 1981 using a solid-medium deformation apparatus, two types of steady-state microstructures of quartz, denoted as S and P, were found under varying temperature and strain rate conditions. However, the differential stresses did not systematically change with the deformation conditions, and unexpectedly high flow stresses (over 700 MPa) were recorded on some experimental runs compared with the applied confining pressure (400 MPa). Internal friction in the sample assembly is a possible cause of reported high differential stresses. Using a pyrophyllite assembly similar to that used in the previous work and setting up paired load cells above and below the sample assembly, we quantified the frictional stress acting on the sample and corrected the axial stress. The internal friction changed in a complicated manner during pressurization, heating, and axial deformation at a constant strain rate. Our results suggest that Masuda and Fujimura overestimated the differential stress by about 200 MPa in their 800 °C runs. Crystallographic fabrics in the previous experimental sample indicated that the development of elongated quartz grains, which are characteristics of Type-S microstructures, was associated with preferential growth of unfavorably oriented grains during dynamic recrystallization.

Deformation characteristics, kinematic analysis, and formation ages of gneissose Palala granite in the Palala Shear Zone, southern boundary of the Central Zone, Limpopo Belt, South Africa

The Palala Shear Zone (PSZ) is a c. 13 km wide part of a vertical, crustal-scale lineament that forms the southern boundary of the polymetamorphic high grade Central Zone of the Limpopo Belt in Southern Africa. We have studied a set of strongly sheared samples of Palala granite, taken at the southern edge of the shear zone at the Palala River transect, in order to determine the deformation patterns and evolution history of the PSZ as well as the age and petrogenesis of the granite. We studied the characteristics of macro- and microscopic deformations, analyzed the finite strain and quartz electron backscatter diffraction (EBSD) fabrics, and estimated the rheological parameters of these mylonitic gneisses. We also carried out zircon U–Pb dating and geochemical analysis. The former yielded a weighted mean 207Pb/206Pb age of 2053.5 ± 9.9 Ma which is within error identical to the age of the Bushveld Igneous Complex (BIC). The Palala granite is an alkaline A-type ferroan granite very similar in major- and trace element chemistry to the voluminous and well-studied Nebo Granite, considered to be a final differentiation product of the BIC magma. The deformed samples have uniform subhorizontal lineation and exhibit steady-state microstructures. The quartz CPO's in a non-coaxial deformation regime are promoted by pervasive dynamic recrystallization with a dominant prismatic <a> slip and rhomb <a> slip system. The main high-grade dextral shearing along the PSZ in the study area is associated with this high-medium temperature deformation at 500–600 °C. The strain measurement indicates that the strain types of all these SL-type and L = S-type tectonites were a flattening strain within a steady-state simple shear-dominant ductile deformation. Rheological parameter estimations suggest differential stress within the range of 15.67–17.50 MPa with the paleo-strain rate in a relatively high range of 3.97 × 10−12 to 1.06 × 10−13 S−1. Together with published research, these data impose a constraint on the initiating of the transpression stage of a Paleoproterozoic intracontinental orogenic event of the Limpopo Belt. The main ductile deformation was the product of an NNW-SSE directed shortening that produced flattening strain and horizontal lineations along dextral sense shear zones, and resulted from NNW-SSE compression of the area between the Zimbabwe and Kaapvaal cratons. This high-level ductile deformation and mylonitization occurred after the emplacement of the Palala Granite at 2.05 Ga, and is thought to have taken place at medium crustal levels under upper greenschist to low amphibolite facies conditions. A late low-temperature deformation phase might be related to low-temperature basal <a> slip which is identified by our EBSD analysis.

Tensile creep properties of a CrMnFeCoNi high-entropy alloy

Tensile creep tests were performed on a CrMnFeCoNi high-entropy alloy at temperatures from 1023 K to 1173 K. A uniform stress exponent 3.7 ± 0.1 was found across all temperatures. The apparent activation energies of creep under various applied stresses were determined to be around 230 kJ/mol and decrease with increasing stress, indicating a stress-assisted, thermally activated behavior. Steady-state creep microstructures feature no subgrain formation and high dislocation density within grains. Based on our results, the creep rate of CrMnFeCoNi is believed to be controlled by both dislocation-dislocation interactions and dislocation-lattice interactions.

Large-scale phase-field study of anisotropic grain growth: Effects of misorientation-dependent grain boundary energy and mobility

Three-dimensional grain growth behaviors under anisotropic (misorientation-dependent) grain boundary energy and mobility are investigated via phase-field simulations. Based on a multi-phase-field model and parallel graphics-processing unit computing on a supercomputer, very large-scale simulations with more than three million grains are achieved, enabling reliable statistical evaluation of anisotropic grain growth. The anisotropic boundary properties are introduced by the classical Read-Shockley and sigmoidal models; the threshold misorientation angle, Δθh, included in these models is used as a quantity to determine the anisotropy strength of the system. Systematic simulations are performed for different Δθh values, through which the correlations between the anisotropy strength and grain growth characteristics such as grain size and misorientation distributions are examined. The obtained results show that anisotropic grain growth reaches the steady-state regime irrespective of the Δθh value. However, the kinetics and microstructural morphology during the steady-state growth are largely dependent on Δθh. Furthermore, by comparison with the simulated results, the applicability of analytical grain growth theories to anisotropic systems are tested. The tests reveal that the steady-state microstructure in anisotropic growth cannot be well captured by the existing theories, which is likely because the basic assumptions of the theories do not hold for anisotropic systems.

Micromechanical characterization of wire-arc additive manufactured and cast nickel aluminum bronze: Ambient and intermediate temperatures

Instrumented indentation is a semi-destructive and robust technique to assess micromechanical characteristics (e.g., local properties) of metallic materials at ambient and elevated temperatures. In the present study, the instrumented indentation is employed to assess and compare the micromechanical response and fundamental mechanisms of plastic deformation (e.g., rate-controlling plasticity) in wire-arc additively manufactured (WAAM) and cast nickel aluminum bronze (Cu–9Al–4Fe–4Ni–1Mn) at ambient and intermediate temperatures. Two separate sets of indentation-based test schedules are employed to investigate these mechanisms. In the first set of experiments, load-controlled (peak load of 500 mN) indentation tests are performed under various loading rates (5–50 mN/s) at a constant (25 °C) temperature. In the second set of experiments, load-controlled indentation tests (peak load of 200 mN) at various temperatures of 25, 200, and 300 °C are performed under a constant loading rate. Indentation load versus indentation depth and time data are analyzed to collect a wealth of information including indentation stress, indentation strain rate, size effect, dislocation activation energy, and activation volume to assess fundamentals of governing mechanisms of plastic deformation. Optical, scanning, and transmission electron microscopy are utilized to provide microstructural evidence for the proposed micromechanical mechanisms of plastic deformation based on dislocation activities and interactions. The nanoindentation hardness results show that the strength of the WAAM alloy is higher than the cast counterpart. However, neither cast nor WAAM-NAB materials are rate-sensitive at ambient (room) temperature. When the temperature is raised, both alloys become softer, and the indentation size effect becomes less pronounced. The activation volume and the activation energy data show that a steady-state microstructure does not necessarily exist around the indentations, and the nature of rate-limiting obstacles (against dislocation activities) may vary in the course of the indentation.

Characterization of stress-rupture behavior of nuclear-grade C26M2 FeCrAl alloy for accident-tolerant fuel cladding via burst testing

FeCrAl alloys demonstrate excellent high-temperature oxidation resistance compared to zirconium alloys used in LWRs. FeCrAl C26M2 has been down-selected as one of the candidate ATF cladding materials for Gen II LWRs. This study investigates the stress-rupture behavior of the nuclear-grade FeCrAl C26M2 tubing in the temperature range of 753-923 K. Rupture time and uniform strain at rupture were determined as a function of the temperature and applied stress using burst tests. The rupture data was found to obey the Larson-Miller Parameter and the Monkman-Grant relationship. Furthermore, the stress exponent (n) and the activation energy (QC) at lower stresses were determined as 4.4 ± 0.3 and 289 ± 25 kJ/mol, respectively, confirming power-law behavior. Above the normalized stress (σ/E) of 2 × 10−3, the power-law creep transited into the power-law breakdown regime. The steady-state deformation microstructure was examined using transmission electron microscopy (TEM) to detect the rate-controlling creep mechanism(s). Observation of sub-grains with dislocations at the boundaries in the power-law regime along with a stress exponent of 4.4 suggests dislocation climb as the rate-controlling mechanism. Two different fracture modes were observed: the tubing failed either by direct open-up or small crack and pinhole formation. The outcome of this study would provide essential data for the development of informative mesoscale deformation models paving way for discovery of new creep resistant alloys.

The effect of cooling during deformation on recrystallized grain-size piezometry

AbstractMost exposed middle- and lower-crustal shear zones experienced deformation while cooling. We investigated the effect of the strengthening associated with such cooling on differential stress estimates based on recrystallized grain size. Typical geologic ratios of temperature change per strain unit were applied in Griggs Rig (high pressure-temperature deformation apparatus) general shear experiments on quartzite with cooling rates of 2–10 °C/h from 900 °C to 800 °C, and a shear strain rate of ∼2 × 10−5 s−1. Comparisons between these “cooling-ramp” experiments and control experiments at constant temperatures of 800 °C and 900 °C indicated that recrystallized grain size did not keep pace with evolving stress. Mean recrystallized grain sizes of the cooling-ramp experiments were twice as large as expected from the final stresses of the experiments. The traditional approach to piezometry involves a routine assumption of a steady-state microstructure, and this would underestimate the final stress during the cooling-ramp experiments by ∼40%. Recrystallized grain size in the cooling-ramp experiments is a better indicator of the average stress of the experiments (shear strains ≥3). Due to the temperature sensitivity of recrystallization processes and rock strength, the results may underrepresent the effect of cooling in natural samples. Cooling-ramp experiments produced wider and more skewed grain-size distributions than control experiments, suggesting that analyses of grain-size distributions might be used to quantify the degree to which grain size departs from steady-state values due to cooling, and thereby provide more accurate constraints on final stress.

Predicting Nonequilibrium Patterns beyond Thermodynamic Concepts: Application to Radiation-Induced Microstructures

In this work, we derive an analytical model to predict the appearance of all possible radiation-induced steady states and their associated microstructures in immiscible A_{c[over ¯]}B_{1-c[over ¯]} alloys, an example of a nonequilibrium dynamical system. This model is assessed against numerical simulations and experimental results which show that different microstructures characterized by the patterning of A-rich precipitates can emerge under irradiation. We demonstrate that the steady-state microstructure is governed by irradiation conditions and also by the average initial concentration of the alloy c[over ¯]. Such a dependence offers new leverage for tailoring materials with specific microstructures overcoming limitations imposed by the equilibrium thermodynamic phase diagram.

Steady-state characteristics of fcc pure metals processed by severe plastic deformation: experiments and modelling

ABSTRACTA series of experimental studies are carried out to investigate the steady-state microstructures and mechanical properties of Cu, Ni, and Al obtained by equal-channel angular pressing (ECAP) at homologous temperatures. Under 0.32Tm the steady-state grain sizes ds of the three metals are below 1 μm, and dominated by dynamic recovery mechanism. Discontinuous dynamic recrystallization (DDRX) occurs in Cu and Ni processed by ECAP at 0.4Tm. The variation of dislocation density at 0.32Tm (ρCu>ρNi>ρAl) shows a dependence on stacking fault energy (SFE), and it is consistent with the change of kernel average misorientation (KAM) values (Cu > Ni > Al). The SFE not only influences the strain hardening capacity of metals but also the critical strain needed to get steady-state. In order to understand the relationship between SFE and ds quantitatively, a steady-state grain size model based on the Mohamed’s dislocation model is proposed through considering dynamic recovery mechanisms. Two important factors temperature and SFE are involved in the model, and other parameters and constants are determined from our experimental data. The predictions of ds by the model are compared with ds obtained by experiments for five fcc pure metals (Au, Ag, Cu, Ni, and Al), and the characteristic and limitation of the model are discussed.

Severe plastically deformed commercially pure aluminum: Substructure, micro-texture and associated mechanical response during uniaxial tension

Severe plastic deformation (SPD) of metals to obtain ultra-fine or even nano-sized grains has proven to be an interesting concept explored over the last few decades. However, the mechanical behavior of SPD metals and the underlying microstructural phenomena are not fully understood yet. In present work, commercially pure aluminum was subjected to high pressure torsion (HPT) deformation with strains ranging from very low levels to values well in the steady-state microstructure regime. The mechanical properties of the HPT processed samples were determined using tensile tests on miniature samples using full-field strain mapping. Orientation imaging microscopy (OIM) was utilized to follow the progression of grain refinement and texture as a function of imposed SPD. Local orientation based misorientation gradients helped to perform statistical boundary analysis and determine the fractions of incidental and geometrically necessary dislocation (GND) boundaries and local GND densities.From probability density distributions of the misorientation gradients two different stages of microstructural evolution, namely, fragmentation and saturation, could be discerned. The strength increased monotonously and the uniform elongation, though lower than the value of the annealed material, enhanced with the imposed strain in HPT. The post-necking response was observed to be highly microstructure dependent, where a lower grain size augmented the resistance for micro-crack propagation and enhanced the elongation-to-failure. In addition, the work hardening response corresponding to the yield point displayed maxima coinciding with the onset of the saturation stage. Anisotropy in fracture strain, observed between the axial and radial directions in a disk-like HPT sample, reduced with the randomization of shear texture, while higher intensities of the C {100}<110> orientation was considered responsible for the lower elongation-to-failure along the radial direction.

Effects of ternary alloy additions on the microstructure of highly immiscible Cu alloys subjected to severe plastic deformation: An evaluation of the effective temperature model

Phase evolution in dilute, strongly immiscible Cu-Mo, Cu-Mo-Ni, and Cu-Mo-Ag alloys during severe plastic deformation at low-temperature has been experimentally investigated. For the Cu95Mo05 alloy, Mo nanoparticles are formed, ∼10 nm in diameter, as part of a steady state microstructure, with less than ∼1 at.% Mo dissolved in the matrix. Addition of 10 or 20 at.% Ni to this binary alloy results in a significant increase in the Mo solubility, whereas comparable additions of Ag has a corresponding little effect. The steady state microstructures of alloys during ball milling of elemental powders are very similar to those during HPT processing of initially homogeneous solutions. The results are discussed in terms of an effective temperature model. Model MD simulations are presented to help relate the predictions of the effective temperature model to atomistic mechanisms.

Influence of slip ratio on worn-surface microstructure and fatigue wear behavior of D2 wheel steel

To investigate the worn-surface microstructure and fatigue cracks in D2 wheel steel under the pure rolling and 0.5% slip ratio conditions, a rolling wear test using a GPM-40 wear machine to simulate the wheel/rail operation was performed. After testing, a transmission electron microscope, a scanning electron microscope with electron backscatter diffraction, and micro-hardness testers were used to characterize the microstructure and fatigue cracks. The surface microstructure and hardness of the pure rolling sample were in a steady state after 8 × 104 cycles; however, the 0.5% slip ratio sample reached a steady state after 7 × 103 cycles. Regardless of whether the test uses the slip ratio, the orientation of lamellar pearlites gradually became parallel to the surface and a portion of lamellar cementites was fragmented and dissolved during the formation of steady-state microstructure. The slip ratio accelerates this process. The hardening mechanism of the samples shows a decrease in the lamellar spacing of pearlite and the refinement of proeutectoid ferrite (PF). As the number of cycles increased, plastic deformation of samples became increasingly severe and the wear mechanism of the samples was fatigue wear in steady state. The sample surfaces formed shallow cracks, which gradually peeled off. The slip ratio accelerated the initiation and propagation of fatigue cracks because of the high friction stress on the contact surface. Most fatigue cracks initiated at the interface of pearlite and PF and in the PF region.

Effect of small addition Indium on transient and steady-state creep characteristics, microstructure and properties of Sn-9Zn lead-free solders

Complete enforcement of the new propagation of lead-free solders needed detailed knowledge and comprehension of their mechanical attitude. In the present study transient and steady-state creep tests, microstructure and characteristic of Sn-9Zn lead free solders and Sn-9Zn-2In ternary alloys were studied under different five applied stresses ranging from ranged from 3.62 to 11.22 MPa and in the temperature range from 323K to 403K. It is obvious that the additions of Indium alloy to the Sn-9Zn binary system lead to a reducing of the melting point to, therefore, the Sn-9Z-2In solder material shows maximum ductility (strain) and well-behaved mechanical realization than that of eutectic Sn-9Zn commercial solder. Microstructural analysis revealed that the ternary alloys are more fine and refining than binary due to existence of -InSn4. The activation energy of transient creeps have been found to be 20.16 - 26.32 and 16.80 - 22.44 kJ mol−1 in the low and high temperature regions for Sn-9Zn binary and Sn-9Z-2In ternary alloys respectively. The steady state creep was studied in the same temperature range under same loads. The strain rate sensitivity parameter (m) has been found to elevate by elevating the applied temperature and stresses. The activation energies of steady state creep of the alloy have been found to be 26.31-30.82 and 23.36 -27.13 kJ mol−1 in the low and high temperature regions for Sn-9Zn binary and Sn-9Z-2In ternary alloys, respectively.

Effects of Ternary Alloy Additions on the Microstructure of Highly Immiscible Cu Alloys Subjected to Severe Plastic Deformation: An Evaluation of the Effective Temperature Model

Phase evolution in dilute, strongly immiscible Cu-Mo, Cu-Mo-Ni, and Cu-Mo-Ag alloys during severe plastic deformation at low-temperature has been experimentally investigated. For the Cu95Mo05 alloy, Mo nanoparticles are formed, ~ 10 nm in diameter, as part of a steady state microstructure, with less than ~ 1 at.% Mo dissolved in the matrix. Addition of 10 or 20 at.% Ni to this binary alloy results in a significant increase in the Mo solubility, whereas comparable additions of Ag has a corresponding little effect. The steady state microstructures of alloys during ball milling of elemental powders are very similar to those during HPT processing of initially homogeneous solutions. The results are discussed in terms of an effective temperature model. Model MD simulations are presented to help relate the predictions of the effective temperature model to atomistic mechanisms.