Homologous Temperature Research Articles

Creep Deformation of Lithium Foil at Moderate Pressures (< 2 MPa) and Temperatures (30-50°C)

Lithium metal is receiving significant research attention as a potential next generation negative electrode material in lithium ion batteries because it can offer a ~50% increase in cell energy density compared to conventional graphite electrodes. There are, however, a number of mechanical challenges using lithium metal electrodes. Mechanical stack pressure is required to ensure good contact between solid-state electrodes and electrolytes. Moderate to high stack pressures (e.g. < 10 MPa) can also supress void formation during fast charging and discharging.1 Creep, the plastic deformation of a material under constant pressure, improves battery performance by preventing and filling voids, but can also lead to decreased performance if lithium is unable to sustain the required stack pressure, and potentially cell shorting if lithium is e.g. extruded beyond the current collectors.Creep deformation rates are highly dependent on homologous temperature and pressure. Lithium metal has a relatively low melting point (181°C) / high homologous temperature at common cell temperatures which should result in substantial lithium creep deformation. Previous studies have characterized both the tensile creep properties of lithium foils2 and compressive creep properties of bulk lithium rods.3 Significant barrelling of the high aspect ratio lithium rods (initial height:width of 4:1 to 1:1) during compression made it difficult to determine the applied pressure on the lithium, obscuring the relationship between pressure and creep rate.3 These important studies increased our collective understanding of the mechanical properties of lithium, but are not necessarily reflective of the compressive creep of lithium foils in a cell environment. Here, we report on the compressive creep of lithium foil within an electrochemical cell at commercially relevant stack pressures and temperatures.Compressive creep deformation of low aspect ratio (1:40 to 1:10; initially 0.3 – 2.5 mm thick, 13 mm diameter) lithium metal foils was measured in hermetically sealed, rigid but flexible cells at temperatures between 30 - 110°C and applied pressures between 1 - 2 MPa. Creep testing was performed by integrating a Conflat-style electrochemical cell4 featuring welded bellows and an optical window with an Instron 5966 Universal Testing System (which measures / controls force and displacement). Creep rates were determined using the time-dependent displacement data after reaching a constant applied force. Two methods to convert force to pressure will be described, including the use of an inline camera to measure the lithium-compressive piston contact area.Creep deformation was observed at all tested temperatures, and all pressures above 1 MPa. Creep rates were on the order of a few tenths of a micron per hour, which while small, implies very limited device lifetime with thin lithium layers. Lithium metal may not be physically strong enough for use in high energy density all solid-state devices. Figure 1

Tracer diffusion in the σ phase of the CoCrFeMnNi system

A single Cr-rich σ-phase alloy with a composition of Co 17 Cr 46 Fe 16.3 Mn 15.2 Ni 5.5 (at.%) and a tetragonal lattice structure was produced. The tracer diffusion coefficients of Ni and Fe were measured by secondary ion mass spectroscopy using the highly enriched 64 Ni and 58 Fe natural isotopes. On the homologous temperature scale, Ni and Fe diffuse in the σ phase faster as compared to the corresponding diffusion rates in the equiatomic and face-centered cubic CoCrFeMnNi alloy. In contrast, on the absolute temperature scale, these elements diffuse roughly at the same rates in both materials. Factors influencing element diffusion and phase stability of the σ phase compared to the equiatomic alloy are discussed.

Reducing hot tearing by grain boundary segregation engineering in additive manufacturing: example of an AlxCoCrFeNi high-entropy alloy

One major hindrance that alloy design for additive manufacturing (AM) faces nowadays is hot tearing. Contrary to the previous works which either try to reduce solidification range or introduce grain refinement, the current work presents a new approach of employing segregation engineering to alter the residual stress states at the interdendritic and grain boundary regions and consequently prevent hot tearing. Here, in situ Al alloying is introduced into an existing hot-cracking susceptible high-entropy alloy CoCrFeNi. It is found that within a certain range of compositions, such as Al0.5CoCrFeNi, the hot crack density was drastically decreased. During the solidification of this specific alloy composition, Al is firstly ejected from the primary dendritic face-centred cubic (FCC) phase and segregates into the interdendritic regions. Spinodal decomposition then occurs in these Al-enriched regions to form the ordered B2 NiAl and disordered body-centred cubic (BCC) Cr phases. Due to the higher molar volume and lower homologous temperatures of these B2/BCC phases, the inherent residual strain is accommodated and transformed from a maximum 0.006 tensile strain in CoCrFeNi to a compressive strain of ~0.001 in Al0.5CoCrFeNi. It is believed that this grain boundary segregation engineering method could provide a new pathway to systematically counteract the hot tearing problem in additive manufacturing of metals and alloys, using available thermodynamic and kinetic database information.

Solute repositioning to tune the multiple microalloying effects in an Al–Cu alloy with minor Sc, Fe and Si addition

Abstract For multicomponent Al-based alloys, one of the most valuable approaches of microstructural design is to find a path to maximize the multiple microalloying effects while overcoming their negative counterparts. In this paper, triple Sc-Fe-Si microalloying was performed in an Al–Cu alloy to assemble the co-existence of θ′-Al2Cu and Al3Sc precipitates. Besides, the mutual interactions among triple microalloying elements were utilized to maximize the positive effects on tailoring the dual precipitates, as illustrated in two goals of microstructural design: (i). As to θ′-Al2Cu plates, the multiple Sc-Fe-Si segregation at θ′-Al2Cu/matrix interface was preferentially created in the alloy before creep (as-aged condition). During subsequent high-temperature creep, such interfacial warden will be rapidly reinforced by solute repositioning, as a process of accumulating solutes diffusing from both inner θ′-Al2Cu precipitate and outer matrix to limit the interfacial migration. (ii). For Al3Sc precipitates, the beneficial Si microalloying effect on encouraging the nucleation of Sc-rich entities (precursor of Al3Sc) was successfully acquired during aging, while the detrimental Si effect on accelerating Al3Sc coarsening is generally prohibited by tuning Fe–Si synergy within Al3Sc interior. The establishments of (i) and (ii) enable the satisfactory dispersion as well as the outstanding thermal stability of dual precipitates in the current Al-Cu-Sc-Fe-Si system, leading to a good creep resistance at a high homologous temperature of 0.61Tm ~ 300 °C (where Tm is the melting temperature of the α-Al matrix).

ECAP of titanium alloy by sever plastic deformation: A review

Abstract Severe plastic deformation method is one of the most emerging methods in grain refinement techniques. It is known as mechanical or thermo mechanical deformation process of grain refinement. Because of its property like grain refinement without any changes in the dimensions of the specimen will makes this process as unique. In this research, a novel Severe Plastic Deformation (SPD) method has been proposed which imposes a higher amount of the plastic shear strain during the modified Equal Channel Angular Pressing (ECAP) process. ECAP is most suitable for materials used in industrial scale application, the process works by introducing a large shear strain by repeated extrusion steps. Among the different procedures in use for the production of nano-structured materials, only severe plastic deformation (SPD) processes exhibit a potential for producing relatively large samples suitable for industrial applications. Severe plastic deformation is metal forming processes in which an ultra-large plastic strain is introduced into a bulk metal in order to create nano- or ultrafine- grained structure at low homologous temperatures. In ECAP, such high degrees of plastic deformation are possible because one sample can be subjected several times to SPD in order to accumulate the total amount of plastic strain. Titanium alloy is used in this study of grain refinement because of its good bio compatibility in medical applications.

Thermal Aging Reliability of Socketable BGA Packages With Ni–Au-Coated SAC305 Spheres

Land grid arrays (LGAs) and ball grid arrays (BGAs) constitute the board-level interconnect technologies for a large number of packaged ICs. These technologies are used in socketing and surface mount (SMT) applications, respectively. The concept of socketable BGA packages, with solder spheres coated with a barrier layer/noble metal coating, was introduced to develop a single package design for processor ICs that are used in both socketing and SMT applications. While this concept has previously been demonstrated, the feasibility of these packages in socketing conditions has not yet been studied. This article assesses the applicability of these packages in socketing by subjecting the packages to thermal aging at a temperature of 120 °C. The change in microstructure with thermal aging along with the consumption of the barrier layer was understood. The joint shear strength evolution with thermal aging and the contamination of the top surface of the spheres was studied by the X-ray photoelectron spectroscopy (XPS) analysis. It was found that the trend for experimental consumption of the barrier layer closely follows the theoretical model predictions. Solid-state solder wicking of the Sn57.6Bi0.4Ag (SBA) solder joint due to creep was confirmed at high homologous temperature for the solder. This, along with grain coarsening over time, led to a decrease in the joint shear strength from 33.5 to 16 MPa. Polymer collars are suggested as a potential solution to prevent the solder wicking and make the package feasible for socketing applications.

Frustration of the dislocation density in NaCl and its implication on “Harper-Dorn” creep

Static annealing of (100)-oriented NaCl single crystals at a homologous temperature of ~0.92 showed a significant reduction in the etch-pit dislocation density during the first 100 h, which remained constant during subsequent annealing up to 225 h. The existence of the frustration of the dislocation density, at a value of ~109 m-2, in NaCl is similar to the observations in the high purity Al and LiF single crystals. The implication of the frustration dislocation density on the observation of a creep stress exponent of >1 in the Harper-Dorn creep regime is discussed.

Repulsion leads to coupled dislocation motion and extended work hardening in bcc metals

Work hardening in bcc single crystals at low homologous temperature shows a strong orientation-dependent hardening for high symmetry loading, which is not captured by classical dislocation density based models. We demonstrate here that the high activation barrier for screw dislocation glide motion in tungsten results in repulsive interactions between screw dislocations, and triggers dislocation motion at applied loading conditions where it is not expected. In situ transmission electron microscopy and atomistically informed discrete dislocation dynamics simulations confirm coupled dislocation motion and vanishing obstacle strength for repulsive screw dislocations, compatible with the kink pair mechanism of dislocation motion in the thermally activated (low temperature) regime. We implement this additional contribution to plastic strain in a modified crystal plasticity framework and show that it can explain the extended work hardening regime observed for [100] oriented tungsten single crystal. This may contribute to better understanding the increase in ductility of highly deformed bcc metals.

Tracer Diffusion in the Σ-Phase of the CrMnFeCoNi System

A single σ-phase alloy with a composition of Cr 46 Mn 15.2 Fe 16.3 Co 17 Ni 5.5 (at.%) and a tetragonal lattice structure was produced. The tracer diffusion coefficients of Ni and Fe were measured by secondary electron mass spectroscopy using the highly enriched 64 Ni and 58 Fe natural isotopes. On the homologous temperature scale, Ni and Fe diffuse in the σ-phase faster as compared to the corresponding diffusion rates in the equiatomic and face-centered cubic CoCrFeMnNi alloy. In contrast, on the absolute temperature scale, these elements diffuse roughly at the same rates in both materials. Factors influencing element diffusion and phase stability of the σ-phase compared to the equiatomic alloy are discussed.

Microstructure evolution and thermal stability of equiatomic CoCrFeNi films on (0001) α-Al2O3

Homogeneous face-centered cubic (fcc) polycrystalline CoCrFeNi films were deposited at room temperature on (0001) α-Al2O3 (c-sapphire). Phase and morphological stability of 200 to 670 nm thick films were investigated between 973 K and 1423 K.The fcc-phase persists while the original <111> texture of 30-100 nm wide columnar grains evolves into ~10 or ~1000 µm wide grains upon annealing. Only the metallic M grains having two specific orientation relationships (ORs) to the c-sapphire grow. These ORs are OR1 (M(111)[11¯0]//α-Al2O3(0001)[11¯00]) and OR2 (M(111)[11¯0]//α-Al2O3(0001)[112¯0])and their twin-related variants (OR1t and OR2t). They are identical to those reported for several pure fcc metal (M) films. Thus, the ORs in these fcc/c-sapphire systems appear not to be controlled by the fcc phase chemistry or its lattice parameter as usually assumed in literature.Upon annealing, the films either retain their integrity or break-up depending on the competing kinetics of grain growth and grain boundary grooving. Triple junctions of the grain boundaries, the major actors in film stability, were tracked. Thinner films and higher temperatures favor film break-up by dewetting from the holes grooved at the triple junctions down to the substrate. Below 1000 K, the film microstructure stabilizes into 10 µm wide OR1 and OR1t twin grains independent of film thickness. Above 1000 K, the OR2 and OR2t grains expand to sizes exceeding more than a 1000 times the film thickness. The grain boundaries of the OR2 and OR2t grains migrate fast enough to overcome the nucleation of holes from which break-up could initiate. The growth of the OR2 and OR2t grains in this complex alloy is faster than in pure fcc metals at equivalent homologous annealing temperatures.

Plastic Forming of Metals at the Nanoscale: Interdiffusion-Induced Bending of Bimetallic Nanowhiskers.

Controlled plastic forming of nanoscale metallic objects by applying mechanical load is a challenge, since defect-free nanocrystals usually yield at near theoretical shear strength, followed by stochastic dislocation avalanches that lead to catastrophic failure or irregular, uncontrolled shapes. Herein, instead of mechanical load, we utilize chemical stress from imbalanced interdiffusion to manipulate the shape of nanowhiskers. Bimetallic Au–Fe nanowhiskers with an ultrahigh bending strength were synthesized employing the molecular beam epitaxy technique. The one-sided Fe coating on the defect-free, single-crystalline Au nanowhisker exhibited both single- and polycrystalline regions. Annealing the bimetallic nanowhiskers at elevated temperatures led to gradual change of curvature and irreversible bending. At low homological temperatures at which grain boundary diffusion is a dominant mode of mass transport this irreversible bending was attributed to the grain boundary Kirkendall effect during the diffusion of Au along the grain boundaries in the Fe layer. At higher temperatures and longer annealing times, the bending was dominated by intensive bulk diffusion of Fe into the Au nanowhisker, accompanied by a significant migration of the Au–Fe interphase boundary toward the Fe layers. The irreversible bending was caused by the concentration dependence of the lattice parameter of the Au(Fe) alloy and by the volume effect associated with the interphase boundary migration. The results of this study demonstrate a high potential of chemical interdiffusion in the controlled plastic forming of ultrastrong metal nanostructures. By design of the thickness, microstructure, and composition of the coating as well as the parameters of heat treatment, bimetallic nanowhiskers can be bent in a controlled manner.

Simultaneous improvement in strength and ductility of severely deformed niobium alloy

The present study is on the improved monotonic response of a severely deformed niobium-based alloy. Nb-1Zr alloy was processed at room temperature using equal channel angular extrusion and cold rolling. After 16 passes, a more than 2.5 times increase in yield strength is accompanied by a ductility level exceeding the as-received state. Cold rolling of the processed billets resulted in tensile strengths exceeding 1 GPa. The enhancement in both strength and ductility is explained by the microstructural evolution and the low homologous processing temperature.

Using a composite flow law to model deformation in the NEEM deep ice core, Greenland – Part 2: The role of grain size and premelting on ice deformation at high homologous temperature

Abstract. The ice microstructure in the lower part of the North Greenland Eemian Ice Drilling (NEEM) ice core consists of relatively fine-grained ice with a single maximum crystallographic preferred orientation (CPO) alternated by much coarser-grained ice with a partial (great circle) girdle or multi-maxima CPO. In this study, the grain-size-sensitive (GSS) composite flow law of Goldsby and Kohlstedt (2001) was used to study the effects of grain size and premelting (liquid-like layer along the grain boundaries) on strain rate in the lower part of the NEEM ice core. The results show that the strain rates predicted in the fine-grained layers are about an order of magnitude higher than in the much coarser-grained layers. The dominant deformation mechanisms, based on the flow relation of Goldsby and Kohlstedt (2001), between the layers is also different, with basal slip rate limited by grain boundary sliding (GBS-limited creep) being the dominant deformation mechanism in the finer-grained layers, while GBS-limited creep and dislocation creep (basal slip rate limited by non-basal slip) contribute both roughly equally to bulk strain in the coarse-grained layers. Due to the large difference in microstructure between finer-grained ice and the coarse-grained ice at premelting temperatures (T&gt;262 K), it is expected that the fine-grained layers deform at high strain rates, while the coarse-grained layers are relatively stagnant. The difference in microstructure, and consequently in viscosity, between impurity-rich and low-impurity ice can have important consequences for ice dynamics close to the bedrock.

Effects of training on the thermomechanical behavior of NiTiHf and NiTiZr high temperature shape memory alloys

The effects of constant load thermal cycles (training) on the thermomechanical behavior of nano-precipitation strengthened Ni50.3Ti29.7Hf20 (NiTiHf) and Ni50.3Ti29.7Zr20 (NiTiZr) high temperature shape memory alloys (HTSMAs) were compared. Thermomechanical properties were determined as a function of the number of training cycles, which consisted of up to 2000 isobaric thermal cycles at 300 MPa, between lower and upper cycle temperatures of 35 and 300 °C, respectively. In addition, the stability of the trained alloys was determined after exposure to thermal treatments at temperatures above the upper cycle training temperature. Training at 300 MPa significantly improved the actuation strain capability of the NiTiHf HTSMA at low stresses (i.e., 50 MPa) and resulted in a two-way shape memory strain (TWSMS) up to 1.9%, but essentially had no effect on the 300 MPa response. Training had less notable benefits in the case of the NiTiZr, producing negligible TWSMS, and resulting in a decrease in actuation strain capability at 300 MPa with repeated cycling. The benefits of training to the NiTiHf HTSMA were maintained after aging at 400 °C but were lost after exposure to 500 °C and above. Since training was not notably beneficial to the NiTiZr alloy and resulted in a loss in strain capability at 300 MPa, the high temperature annealing treatment actually recovered strain capability in the alloy under high stresses. The superior TWSM response of the NiTiHf HTSMA as compared to the NiTiZr, was attributed to the higher melting temperature, and thus the lower homologous operating temperature of the former, when both alloys were tested over the same temperature range.

Internal oxidation of Ag-xIn alloys at low homologous temperature

Internal oxidation of Ag-xIn alloys (x = 10, 13, 16 at%) was studied in an air atmosphere at a relatively low homologous temperature (240 °C), where lattice diffusion is negligible. After exposure, metallic Ag nodules were present on the surfaces of all alloys, expelled to relieve stress due to internal oxidation, confirming that such coarse morphologies can be produced by short-circuit transport. Crystallographic orientation and grain boundary misorientation were found to influence nodule expulsion tendency, and the propensity for internal or external oxidation. Further analytical TEM characterization was used to study the nanoscale morphology and chemistry of nodules and internal oxide(s).

Grain growth at fragmentation stage in commercial purity aluminium deformed by high pressure torsion

The mechanism for grain boundary motion at the fragmentation stage of high pressure torsion still remains uncertain as measuring grain growth velocity in such stage is challenging. A new approach is proposed in this study to calculate the grain growth velocity in the fragmentation stage. The high grain boundary motion rate and the low apparent activation energy indicate the mechanism for coarse grain growth in commercial purity aluminium in the fragmentation stage at low homologous temperature is attributed to stress-induced boundary motion rather than discontinuous dynamic recrystallization.

Influence of processing route on the alloying behavior, microstructural evolution and thermal stability of CrMoNbTiW refractory high-entropy alloy

Abstract

Experimental and theoretical study of tracer diffusion in a series of (CoCrFeMn)[formula omitted]Nix alloys

Tracer diffusion of all constituting elements is studied at various temperatures in a series of (CoCrFeMn)100−xNix alloys with compositions ranging from pure Ni to the equiatomic CoCrFeMnNi high-entropy alloy. At a given homologous temperature, the measured tracer diffusion coefficients change non-monotonically along the transition from pure Ni to the concentrated alloys and finally to the equiatomic CoCrFeMnNi alloy. This is explained by atomistic Monte-Carlo simulations based on a modified embedded-atom potentials, which reveal that local heterogeneities of the atomic configurations around a vacancy cause correlation effects and induce significant deviations from predictions of the random alloy model.

Interdiffusion Studies in the Co-Sb System

CoSb based compounds have gained much importance in the fields of thermoelectric devices. In this work, we have conducted the solid–state conventional bulk diffusion couple experiments. To study the phase evolutions, Co/Sb diffusion couples are annealed at 450–550 °C. The interdiffusion zone is analysed using field emission gun equipped scanning electron microscope and the composition measurements are done in electron probe micro−analyser to confirm the growth of various product phases. The marker experiment indicates that the CoSb3 phase grows mainly by diffusion of Sb in the binary Co–Sb system. Growth of the CoSb3 phase is discussed based on assessment correlating the difference in mobilities of species with the high homologous temperature, crystal structure of the phase, and the concept of sublattice diffusion mechanism in line compounds.

Insights into dislocation climb efficiency in FCC metals from atomistic simulations

Dislocation climb is a mechanism fundamental to high temperature deformation processes, annealing of quenched-in defects, and irradiated defect absorption in metals. It is known from experimental observations and theoretical considerations that climb velocities in face-centered-cubic (FCC) metals can be slower than expected from diffusion kinetics, due to limited jog availability. Here we investigate the so-called climb efficiency, i.e., the ratio of the steady state climb velocity to that expected from diffusion-limited rates, employing atomistic simulations for FCC metals within the framework of a previously developed analytical model. The simulations consider the energetics of jog-pair formation and vacancy migration in the vicinities of edge-dislocations in Al and Cu. Calculated climb efficiencies for the lower-stacking-fault-energy Cu system are found to be substantially lower than those for Al over a wide range of homologous temperatures. These results agree qualitatively with experimental observations showing that the dislocation climb efficiency in FCC materials tends to be lower in systems with smaller stacking fault energy. Considerably higher energies and more complex kinetic pathways are reported for jog-pair formation in Cu than in Al, which correlates with the larger dissociation distance between partial dislocations in this system. This work highlights how atomistic modeling can be used to enhance mesoscale models of climb, and suggests alloy design considerations to tune climb efficiencies.