Body-centered Cubic Alloys Research Articles

Rapid assessment of solid solution hardening via atomic size misfit parameter in refractory concentrated alloys

Solid solution hardening (SSH) is one of the simplest methods to increase the strength of alloys, as only the concentration of the solute matters. It has been recently proposed that SSH in some concentrated body-centered cubic (BCC) alloys may be dominated by edge dislocation, unlike previously proposed SSH theories for dilute BCC alloys. In this study, we examine the dominant SSH mechanism in 14 binary refractory concentrated alloy (RCA) systems, by measuring the atomic size misfit and the modulus misfit parameters and applying them to the Labusch and Suzuki SSH-based models. Interestingly, we find a general trend that increasing the portion of the atomic size misfit parameter provides a better estimation of the measured SSH, which is a noticeable feature of the edge dislocation mediated SSH. While screw dislocation is expected to contribute to SSH in some RCAs with small atomic size misfits, this study offers a useful guideline on how to manipulate SSH of RCAs having large atomic size misfits.

The predicted rate-dependent deformation behaviour and multistage strain hardening in a model heterostructured body-centered cubic high entropy alloy

Heterostructured high-entropy alloys (HEAs) exhibit a good combination of strength and ductility. However, the deformation mechanisms and related mechanical properties of the heterogeneous nanostructured body-centred cubic (BCC) HEAs remain largely unrevealed. Here, we report the effect of the strain rate on the mechanical properties and microstructural evolution in the heterostructured Al3CoCrCuFeNi HEA with the gradient grain size ranging from 9 nm at the surface to 45 nm in the center using the large-scale atomic simulations. Based on the characterization of microstructure evolution from the atomic simulation, a microstructure-based constitutive model that utilizes a single parameter set is established to study the effects of the multiple strengthening mechanisms at various deformation stages on the strength and strain hardening of the heterostructured HEA under different strain rates. The results show that the plastic deformation mechanisms are strongly influenced by the strain rate in the range of 1 × 106 to 1 × 1010 s−1. The dislocation and deformation twin are the main deformation mechanisms at the strain rate less than 1 × 108 s−1. The deformation twinning cooperated with multiple phase transformations plays a pivotal role at the strain rate of 1 × 109 s−1. The multiple phase transformations are the dominant plastic deformation model at the strain rate of 1 × 1010 s−1, which is different from the classic plastic-deformation mode, including the dislocation and deformation twin in traditional alloys. This phenomenon is ascribed to various types of atomic interactions and atomic-size mismatches, which produce strong lattice distortion. In particular, a large number of edge dislocations are observed, which is extremely different from screw dislocations invariably generated in traditional BCC alloys, and thus, could be responsible for high strength. Furthermore, the stress-strain response of the Al3CoCrCuFeNi HEA is derived from the individual contributions of the lattice distortion, dislocation, grain boundary, deformation twin, phase transformation, and back-stress strengthening mechanisms. The heterogeneous grain boundaries and lattice distortion contribute the most to the yield strength, and the dislocation, deformation twin, phase transformation, and back stress dominate the strain hardening. Therefore, these results highlight a new design strategy of HEAs to tailor their mechanical properties based on the partition of the multistage strengthening mechanisms by the heterostructure.

Influence of carbon catalysts on the improvement of hydrogen storage properties in a body-centered cubic solid solution alloy

Body-centered cubic (BCC) alloys are considered as promising materials for hydrogen storage with high theoretical storage capacity (H/M ratio of 2). Nonetheless, they often suffer from sluggish kinetics of hydrogen absorption and high hydrogen desorption temperature. Carbon materials are efficient hydrogenation catalysts, however, their influence on the hydrogen storage properties of BCC alloy has not been comprehensively studied. Therefore, in this paper, composites obtained by milling of carbon catalysts (carbon nanotubes, mesoporous carbon, carbon nanofibers, diamond powder, graphite, fullerene) and BCC alloy (Ti1.5V0.5) were extensively studied in the non-hydrogenated and hydrogenated state. The structure and microstructure of the obtained materials were studied by scanning and transmission electron microscopes, X-ray diffraction (XRD), and Raman spectroscopy. XRD and Raman measurements showed that BCC alloy and carbon structures were in most cases intact after the composite synthesis. The hydrogenation/dehydrogenation studies showed that all of the used carbon catalysts significantly improve the hydrogenation kinetics, reduce the activation energy of the dehydrogenation process and decrease the dehydrogenation temperature (by nearly 100 K). The superior kinetic properties were measured for the composite with 5 wt % of fullerene that absorbs 3.3 wt % of hydrogen within 1 min at room temperature.

An approach to design single BCC Mg-containing high entropy alloys for hydrogen storage applications

Mg is a lightweight element that can increase the gravimetric hydrogen storage capacity of high entropy alloys (HEAs). This work presents a new approach to design single-phase Mg-containing HEAs with attractive hydrogen storage properties. The design method is based on four calculated parameters (ϕ, VEC, ΔH∞¯ and ΔHfo¯) that allow us to find alloy compositions that form single body-centered cubic (BCC) solid solutions with high hydrogen affinity. The method was tested in the Mg–Al–Ti–Mn–Nb system and the Mg12Al11Ti33Mn11Nb33 alloy was selected among 1326 calculated compositions. This alloy was produced by high energy ball milling resulting in a homogeneous single-phase BCC alloy that absorbed 1.7 wt.% of H by forming a BCC monohydride. Despite its H uptake being H/M = 1, the gravimetric capacity of the lightweight Mg12Al11Ti33Mn11Nb33 alloy was comparable to refractory BCC-HEAs with H uptake of H/M = 2.

Chemical short range order strengthening in BCC complex concentrated alloys

Atomistic methods are used to anneal two body-centered cubic (BCC) chemically complex alloys (CCAs) in order to assess the effect of chemical short-range order on alloy strength. The two alloys, a model quaternary Co16.67Fe36.67Ni16.67Ti30 and ternary Nb33.33Ti33.33Zr33.33, are represented using simple Zhou interatomic potentials. Chemically random cells are annealed at temperatures 65% of the average melting temperature of the individual elements using a Monte Carlo approach, and the critical stress required to move a/2[111] screw dislocations in the initial and annealed cells are estimated using molecular dynamics simulations. It is shown that annealing leads to a softening of the BCC quaternary alloy relative to the random state, especially for deformation at low temperatures. On the other hand, short-range order has minimal effect on solid solution strengthening in the ternary alloy. These results are modeled using an extension of the Suzuki model of substitutional solid solution strengthening developed for BCC chemically complex alloys. Here the model is modified to account for the effects of chemical short-range order. Good agreement is shown between the model results and direct atomistic simulation data. The model presented should be useful in predicting BCC CCAs with adequate high temperature strength, which will accelerate development of high temperature, high strength BCC alloys for aerospace applications.

Theory of solid solution strengthening of BCC Chemically Complex Alloys

An analytic model describing substitutional solid solution strengthening in body-centered cubic (BCC) complex, concentrated alloys (CCAs) based on a/2<111> screw dislocation mobility is developed and presented. At lower temperatures it is assumed that dipole dragging is the rate limiting strengthening mechanism, and as the temperature increases this changes to a jog dragging mechanism. While both mechanisms are contained in Suzuki's classical model for substitutional solid solution strengthening developed for BCC alloys significant modifications are required to apply such a model to the refractory CCA's. First, two approaches for incorporating the net effects of the complex solute chemistry are developed and contrasted using a ternary NbTaTi alloy and four quaternary alloys, MoNbTaTi, WNbTaTi, CrMoNbTi and CrMoTaTi . Our model is extended to higher temperatures and applied to analyze experimental yield data as a function of temperature (0.15–0.7TL, where TL is the absolute liquidus temperature) and strain rate (10−5 to 10−2/s) in a range of BCC chemically complex alloys including stoichiometric NbTiZr and HfNbTaTiZr. We find that the model, based on just these two mechanisms, gives excellent agreement with experimental yield data in both these BCC CCAs, NbTiZr and HfNbTaTiZr. The experimental yield data of these two BCC CCA's as well as NbTaTi and four quaternaries are also analyzed using Maresca's edge dislocation model developed for these types of alloys. The screw model presented is useful in predicting BCC CCAs with adequate high temperature strength, which will accelerate development of high temperature, high strength BCC alloys for aerospace applications.

Design of TiVNb-(Cr, Ni or Co) multicomponent alloys with the same valence electron concentration for hydrogen storage

Recently, it has been hypothesized that the hydrogen sorption properties of body-centered cubic (BCC) multicomponent alloys might be related to the alloy’s valence electron concentration (VEC). In this work, we employed CALPHAD method to design three BCC alloys with the same VEC to evaluate this hypothesis. The (TiVNb)85Cr15, (TiVNb)95.3Co4.7 and (TiVNb)96.2Ni3.8 multicomponent alloys with VEC = 4.87 were produced by arc melting. Although some amount of segregation could not be avoided, the as-cast microstructures were composed of a BCC phase with highly homogeneous chemical composition. The alloys quickly absorb hydrogen at room temperature and formed FCC hydrides with high capacities of 2 H/M (3.1–3.2 wt%). For the three alloys, it was observed a two-step hydrogenation sequence indicating the formation of an intermediate BCC monohydride prior to the formation of the FCC dihydride. Moreover, the three alloys presented very similar values of enthalpy and entropy of hydrogenation, showing that the initial hypothesis seems to hold for the thermodynamic properties. The alloys hydrogenation/dehydrogenation cycling behavior were also evaluated. (TiVNb)85Cr15 had a small and continuous drop in capacity over cycling that was addressed to incomplete transformation of the BCC monohydride to the FCC dihydride, whereas (TiVNb)95.3Co4.7 and (TiVNb)96.2Ni3.8 showed a more stable behavior achieving a reversible capacity of 1.76 H/M (2.77 wt%) after 20 cycles. The desorption is strongly affected by absorption/desorption cycling for all alloys, as revealed by thermo desorption spectroscopy (TDS). The onset temperature of desorption depends on the alloys’ composition as well as the number of hydrogenation/dehydrogenation cycles. The results suggest that cycling and desorption properties of the alloys are less related to their VEC.

Enhancement of First Hydrogenation of Ti1V0.9Cr1.1 BCC Alloy by Cold Rolling and Ball Milling.

In this study, we evaluated the effects of a mechanical treatment by cold rolling (CR) and ball milling (BM) on the first hydrogenation of Ti1V0.9Cr1.1 alloy. The as-cast alloy has a body-centered cubic (BCC) crystal structure, and the first hydrogenation at room temperature under 20 bars of hydrogen is practically impossible. However, the samples mechanically activated by CR or BM readily absorbed hydrogen. The sample cold-rolled for one pass exhibited faster kinetics than the sample ball-milled for five minutes, but both samples reached the same storage capacity of 3.6 wt % hydrogen. Increasing the amount of rolling or the milling time decreased the hydrogen capacity. CR is considered the best and most efficient method for the activation of Ti1V0.9Cr1.1 alloy.

Effect of partial Ni substitution in V85Ni15 by Ti on microstructure, mechanical properties and hydrogen permeability of V-based BCC alloy membranes

Vanadium-based alloy membranes with body-centred-cubic (BCC) structure are considered as one of the leading alternatives to Pd-based alloys for hydrogen separation applications due to their lower cost and higher permeability. As permeability and mechanical properties depend on what microstructure can be produced mainly by alloy composition under same processing conditions, the effect of alloy composition on microstructure, mechanical properties and hydrogen permeability has been investigated for the V85Ni15 and V85Ni10Ti5 (at%) alloys prepared by a same process route. All Ni atoms dissolve into the V-matrix to form a single highly supersaturated solid solution with dendritic segregation of Ni-solute atoms in the binary alloy. A part of Ni replacement with 5 at% Ti leads to the formation of small interdendritic phases NiTi and NiTi2 in addition to major phase of V-based solid solution. The mechanical property testing shows that the ultimate strength of the ternary alloy is higher than that of the binary alloy, but the elongation and rollability are lower due to a combination of solid solution hardening and particle strengthening effect. The addition of Ti can greatly increase permeability about 4 times greater than the binary alloy at a permeation testing of 400 °C. But the presence of small amounts of interdendritic compounds provides a barrier to hydrogen migration, resulting in a relatively lower hydrogen diffusion coefficient. In theory, the diffusivity and solubility of hydrogen atom in the presence of alloying element Ti is higher than that in the presence of alloying element Ni in vanadium. This is demonstrated using first principles calculation which further explains the mechanism of hydrogen permeation.

Nanoscratching and mechanical behaviors of high-entropy alloys with different phase constituents

High-entropy alloys (HEAs) exhibit unique microstructural features and properties in nanoscale and atomic scale because of their multi-element alloy system. The nanoscratching behaviors of three HEAs with different phase constituents, relative to the microstructure and mechanical properties of the HEAs, were investigated. Three typical phase constituents were selected: face-centered cubic (FCC) structure, body-centered cubic (BCC) structure, and a dual-phase structure containing both FCC and BCC phases. Despite the fact that the FCC alloy has the highest ductility and strain hardening capability, it exhibited inferior scratch resistance due to the over-softening of hardness. Due to the brittle failure mode, the BCC alloy hardly exhibited desirable scratch resistance despite its highest hardness. By contrast, the nanostructured dual-phase alloy exhibited the best scratch resistance because of its good combination of strength and ductility, as well as the ductile failure mode. This research suggests that the HEA with structure comprising nanoscale hard and soft phases is desirable for nanoscratch resistance, and possesses appropriate hardness for industrial applications.

Mechanistic origin of high strength in refractory BCC high entropy alloys up to 1900K

The body centered cubic (BCC) high entropy alloys (HEAs) MoNbTaW and MoNbTaVW show exceptional strength retention up to 1900K. The mechanistic origin of the retained strength is unknown yet is crucial for finding the best alloys across the immense space of BCC HEA compositions. Experiments on Nb-Mo, Fe-Si and Ti-Zr-Nb alloys report decreased mobility of edge dislocations, motivating a theory of strengthening of edge dislocations in BCC alloys. The strength of BCC HEAs can be controlled by edge dislocations, especially at high temperatures, because the random field of solutes in these high-concentration alloys creates large energy barriers for thermally-activated edge glide. A parameter-free theory for this mechanism of edge motion in BCC alloys qualitatively and quantitatively captures the strength versus temperature for both MoNbTaW and MoNbTaVW alloys and other BCC HEAs. A reduced analytic version of the theory, embodying the edge-strengthening mechanism, then enables screening over > 600,000 compositions in the Mo-Nb-Ta-V-W family to identify promising new compositions with high retained strength and/or reduced mass density. Overall, the theory reveals an unexpected mechanism - edge dislocation motion in the random alloy - as responsible for high temperature strength in BCC alloys and paves the way for theory-guided design of new high-performance HEAs.

Theory of screw dislocation strengthening in random BCC alloys from dilute to “High-Entropy” alloys

Random body-centered-cubic (BCC) “High Entropy” alloys are a new class of alloys, some having high strength and good ductility at room temperature and some having exceptional high-temperature strength. There are no theories of strengthening of screw dislocations in BCC metals that span naturally from the dilute limit to the multi-component, non-dilute concentrations typical of the high-entropy domain. Here, such a theory is developed and validated. Unlike low-temperature elemental BCC metals and very dilute BCC alloys, strength is not controlled by the kink-pair nucleation mechanism. Rather, screw dislocations naturally adopt a kinked structure as the minimum total energy configuration in the field of random alloying atoms. The characteristic length and energy scales for the low-energy kinked screw dislocation are derived for random alloys, leading to a characteristic spacing of both kinks and cross-kinks that depends on the kink formation energy and a characteristic collective solute/screw dislocation interaction energy parameter. Glide motion of this initially-kinked screw dislocation occurs via Peierls-type motion, lateral kink glide, and failure of cross-kinks. All these features are observed in molecular dynamics simulations. The resulting strength versus temperature, strain rate, and composition is analytic. The theory is validated by comparison to experiments on non-dilute Fe-Si, Nb-Mo, Nb-W, and Ti-Nb-Zr-based high entropy alloys versus composition and temperature. The theory provides a framework for tailored design of new high-performance BCC alloys.

Effect of addition of Zr, Ni, and Zr-Ni alloy on the hydrogen absorption of Body Centred Cubic 52Ti-12V-36Cr alloy

The effects of addition of Zr, Ni and Zr7Ni10 on the crystal structure, microstructure and hydrogen absorption of Body Centred Cubic (BCC) 52Ti-12V-36Cr were investigated. We found that addition of Zr and/or Ni led to the formation of a Ni/Zr rich secondary phase. This secondary phase is responsible for the much faster first hydrogenation of the alloys with additives compared to the bare BCC alloy. Zirconium addition had positive influence on incubation time and intrinsic hydrogenation kinetics while nickel addition improved the hydrogen capacity. Among the additives tested, Zr7Ni10 is the best for optimized hydrogenation kinetics and capacity.

Structure, Hydrogen Storage, and Electrochemical Properties of Body-Centered-Cubic Ti40V30Cr15Mn13X2 Alloys (X = B, Si, Mn, Ni, Zr, Nb, Mo, and La)

Structure, gaseous phase hydrogen storage, and electrochemical properties of a series of TiVCrMn-based body-centered-cubic (BCC) alloys with different partial substitutions for Mn with covalent elements (B and Si), transition metals (Ni, Zr, Nb, and Mo), and rare earth element (La) were investigated. Although the influences from substitutions on structure and gaseous phase storage properties were minor, influences on electrochemical discharge capacity were significant. The first cycle capacity ranged from 16 mAh·g−1 (Si-substituted) to 247 mAh·g−1 (Mo-substituted). Severe alloy passivation in 30% KOH electrolyte was observed, and an original capacity close to 500 mAh·g−1 could possibly be achieved by Mo-substituted alloy if a non-corrosive electrolyte was employed. Surface coating of Nafion to the Mo-substituted alloy was able to increase the first cycle capacity to 408 mAh·g−1, but the degradation rate in mAh·g−1·cycle−1 was still similar to that of standard testing. Electrochemical capacity was found to be closely related to BCC phase unit cell volume and width of the an extra small pressure plateau at around 0.3 MPa on the 30 °C pressure-concentration-temperature (PCT) desorption isotherm. Judging from its high electrochemical discharge capacity, Mo was the most beneficial substitution in BCC alloys for Ni/metal hydride (MH) battery application.

The reversible hydrogen storage performances of Mg50Co50 and Mg50Co45Pd5 alloys with body-centered cubic phase in electrochemical system

The Mg50Co50 and Mg50Pd5Co45 alloys with nano-crystalline body-centered cubic (BCC) structure were confirmed by means of XRD and TEM/SAED analyses. They are available in the anode of Ni–MH battery system, which can be reversibly charged and discharged with maximum capacity of 458 mAh g−1. They exhibit greatly enhanced reversible hydrogen storage, differing from their tough dehydrogenation performances in solid–gas system [Y. Zhang et al., Journal of Alloys and Compounds 393 (2005) 147–153]. This phenomenon can be ascribed to the structural stability of Mg50Co50-based binary and ternary BCC alloys in the electrochemical reactions, and also the electro-catalysis activity. Introducing Pd into Mg50Co50 alloy lifts not only the initial discharge capacity, but also the high-rate discharge-ability. The exchange current density and hydrogen diffusion mobility can also be improved by the Pd additive in the ternary electrode alloys.

Synthesis and hydrogen sorption properties of TiV(2−x)Mnx BCC alloys

We report here the effect of activation process of TiV2−xMnx samples (x=0.2,0.4,0.6,0.8,0.9). Crystal structures are investigated for the as-cast, hydrogenated and desorbed states. The as-cast samples crystallise with a Body Centered Cubic (BCC) structure, and the lattice parameter decreases as x increases in the TiV2−xMnx series. Upon hydrogenation, the BCC structure expands and turns into a Body Centered Tetragonal (BCT) structure for the monohydride phase and into a Face Centered Cubic (FCC) for the dihydride phase. Pressure–Composition-Isotherms (P–C–I) measurements were performed to study the thermodynamic of TiV2−xMnx alloys and their hydrogen capacities. The maximum hydrogen uptake is observed for TiV1.2Mn0.8 composition, which store 3.4wt.% of hydrogen at 100°C. The effect of a new activation process on the hydrogen storage capacities of these alloys is discussed.

Effects of additive Pd on the structures and electrochemical hydrogen storage properties of Mg67Co33-based composites or alloys with BCC phase

Mg67Co33 and Mg67Co33–Pd composites/alloys prepared by ball milling for 120h possess nano-crystalline with body-centered cubic (BCC) structure, which was verified by high resolution transmission electron microscopy (HRTEM) and selected area electron diffraction (SAED) analyses. The introduced 5.0at.% Pd significantly lifts the initial discharge capacity from 10mAhg−1 of Mg67Co33 to maximum 530mAhg−1. Pd also drives the Mg67Co33–Pd composite forming a full BCC alloy during ball milling. The distribution of Pd gradually becomes homogeneous with the augmentation of the ball milling time according to the analyses by scanning electron microscopy-energy dispersive spectrometer (SEM-EDS). Exchange current density increased with the milling time and can be ascribed to the homogeneously dispersion of Pd over the surface. The introduced Pd also enhances the hydrogen diffusion coefficient of the Mg67Co33–Pd composites/alloys.

Crystal structure and hydrogen storage properties of body centered cubic 52Ti–12V–36Cr alloy doped with Zr7Ni10

The crystal structure and hydrogen sorption properties of 52Ti–12V–36Cr doped with Zr7Ni10 have been investigated. Two doping methods were used: (i) melting all the components together (single melt) and (ii) melting the alloy and Zr7Ni10 separately and thereafter re-melting them together (co-melt). The XRD patterns of both single melt and co-melt samples showed that these alloys crystallize in a body centered cubic (BCC) crystal structure. Upon hydrogenation, both doped 52Ti–12V–36Cr BCC alloys turns into Face Centered Cubic (FCC) phase.The single melt alloy exhibits a zirconium and nickel rich phase at the grain boundaries. This phase is well-distributed into the matrix and makes a connected network. The same network microstructure was observed in the co-melt. The single melt alloy exhibits an incubation time before its first hydrogenation (activation) whereas the co-melt alloy immediately started to absorb hydrogen. For both samples, the subsequent hydrogenation kinetic is fast. The maximum hydrogen capacities of the single melt and co-melt alloy are respectively 3.4 and 3.2wt.%, but the high stability of these hydrides prevents them to desorb at room temperature. Single melt and co-melt alloys have similar thermodynamic parameters.

Hydrogen transport through V 85Ni 10M 5 alloy membranes

Despite their inherent high permeability, unalloyed body-centred cubic (BCC) metals are prone to brittle failure due to their excessive hydrogen solubility. The primary challenge for BCC metal membrane development is therefore to control the solubility to a point where embrittlement is inhibited, while increasing the rate of hydrogen diffusion through the alloy. This can be potentially achieved through alloying, with several V-based BCC alloys exhibiting much improved resistance to embrittlement compared to pure vanadium, while maintaining higher hydrogen permeabilities than palladium alloys. While several binary and ternary V-based alloys have been investigated, a systematic approach is needed to properly evaluate potential alloying elements. In response, alloys of the general formula V 85Ni 10M 5 (atom%), where M is Si, Mn, Fe, Co, Ni, Cu, Pd, Ag, or Al have been fabricated, coated with 500 nm of Pd, and their microstructure, hydrogen permeability and hydrogen solubility evaluated. The results obtained showed small compositional variations can lead to large changes in permeability, whereas diffusivity is less-dependent on composition. Formation of multi-phase microstructures can enhance the permeability by increasing the vanadium content and hydrogen solubility of the BCC primary phase. The multiphase V 85Ni 10Ti 5 alloy exhibited a hydrogen permeability of 9.3 × 10 −8 mol m −1 s −1 Pa −0.5 at 400 °C.

Non-Pd BCC alloy membranes for industrial hydrogen separation

Abstract With low cost raw materials and high hydrogen permeabilities, body-centred-cubic (BCC) alloys comprising Group IV and V metals are of considerable interest for high-temperature hydrogen separation applications. Until recently, their tendency to embrittle severely has tempered the enthusiasm for these materials in the membrane research community, but efforts to develop BCC alloy membranes suitable for industrial H2 separation processes have increased recently and significant gains have been made in overcoming the inherent instability of these materials in hydrogen. Compared to competing face-centred-cubic alloys, BCC alloys have much higher solubilities that provide them with a high driving force for hydrogen permeation. This high solubility, however, exacerbates the problem of hydrogen embrittlement. Given their low cost components and high permeabilities, the development of membranes with sufficient durability and embrittlement resistance remains the greatest barrier to the widespread uptake of BCC membrane technology. This review discusses the key scientific issues pertaining to the development of BCC alloy membranes in high-temperature industrial processes, including hydrogen solubility and diffusivity, embrittlement and manufacturing. Compositional modification to tailor the hydrogen solubility, maximize the rate of hydrogen diffusion and inhibit the onset of embrittlement is discussed.