A strategy for designing stable nanocrystalline alloys by thermo-kinetic synergy

Aiming to design stable nanocrystalline(NC) materials, so far, it has been proposed to construct nanostructure stability maps in terms of thermodynamic parameters, while kinetic stabilization has seldom been considered, despite the synergy of thermodynamics and kinetics. Consequently, the thermodynamically stabilized NC materials may be easily subjected to grain growth at high temperatures due to the weakly kinetic stabilization. Starting from the thermo-kinetic synergy, a stabilization criterion is proposed as a function of intrinsic solute parameters(e.g. the activation energy for bulk diffusion and the segregation enthalpy), intrinsic solvent parameters(e.g. the intrinsic activation energy for GB migration and the GB energy) and processing parameters(e.g. the grain size, the temperature and the solute concentration). Using first-principles calculations for a series of combinations between fifty-one substitutional alloying atoms as solute atoms and Fe atom as fixed solvent atom, it is shown that the thermal stability neither simply increases with increasing the segregation enthalpy as expected by thermodynamic stabilization, nor monotonically increases with increasing the activation energy for bulk diffusion as described by kinetic stabilization. By combination of thermodynamic and kinetic contributions, the current stabilization criterion evaluates quantitatively the thermal stability, thus permitting convenient comparisons among NC materials involved by various combinations of the solute atoms, the solvent atoms, or the processing conditions. Validity of this thermo-kinetic stabilization criterion has been tested by current experiment results of Fe-Y alloy and previously published data of Fe-Ni, Fe-Cr, Fe-Zr and Fe-Ag alloys,etc., which opens a new window for designing NC materials with sufficiently high thermal stability and sufficiently small grain size.     

Role of atomic migration in nanocrystalline stability: Grain size and thin film stress states

As the length scale of materials decreases to the nanometer regime, grain boundaries occupy a relatively larger volume fraction. Consequently, they play an important role in stabilizing nanocrystalline systems. This review looks at the role of solute segregation to grain boundaries in stabilizing such systems. In recent years, grain size stabilization from solute segregation has led to new types of thermodynamic stability maps as a materials design tool. We propose to extend and adapt these concepts of grain boundary solute segregation as a stabilizing effect to thin film stress states. A recent study on Fe–Pt alloy films, where one species enriched the boundaries, was shown to manipulate the stress from tensile-to-compressive as a function of composition. This suggests that intrinsic segregation can be used as a tunable variable to manipulate stress states, analogous to changing film processing parameters, such as deposition rate and pressure. The application of such solute segregation is at the precipice of new opportunities in materials design of thin films.     

The thermal stability of nanocrystalline cartridge brass and the effect of zirconium additions

The thermal stability of nanocrystalline cartridge brass (Cu–30 at.% Zn) and brass–Zr alloys were investigated. The alloys were produced by cryogenic ball milling and subsequently heat treated to a maximum temperature of 800 °C. The grain size of pure brass was found to be relatively stable in comparison to pure copper, and a high hardness was retained up to 600 °C. When 1 at.% zirconium was alloyed with the brass, the grain size was stabilized near 100 nm even at 800 °C. At the highest temperature, hardness was retained above 2.5 GPa for 1 and 5 at.% zirconium alloys, but the pure brass softened significantly. The stabilization is believed to be dominated by Zn–Zr interactions as a second phase of these two was observed in X-ray diffraction and transmission electron microscopy. Thermodynamic modeling indicates a zero grain boundary energy may be achieved depending on the mixing enthalpy value used (i.e., calculated vs. experimental) under ideal conditions, but microstructural features such as twinning and second phase particles are thought to be the dominant stabilization mechanism. Zr worked well in stabilizing the brass in the nanocrystalline state to nearly 90 % of its melting temperature.     

Microstrain and growth fault structures in electrodeposited nanocrystalline Ni and Ni–Fe alloys

Nanocrystalline Ni and Ni–Fe alloys produced by electrodeposition were characterized using high-resolution transmission electron microscopy (HR-TEM) and X-ray diffraction (XRD). The grain sizes for these materials spanned a range of about 81–10 nm. HR-TEM analysis on a series of images revealed the presence of local strains at both high-angle and low-angle grain boundaries and twin boundaries. In addition to this, stacking faults and twins of the growth type (growth faults) were observed in both the nanocrystalline Ni and Ni–Fe alloys. The growth fault density increased with increasing Fe concentration, which is consistent with a decrease in the stacking fault energy. The microstrain for the samples was determined from XRD pattern analysis based on line broadening. A general increasing microstrain trend with decreasing grain size was observed and considered to be related to the local strains observed at grain boundaries in the HR-TEM image analysis. With respect to grain size, the microstrain values for the nanocrystalline Ni–Fe samples were noticeably higher than some of the Ni samples. Further XRD pattern analysis was performed to determine the growth fault probabilities for each of the samples and analyze their influence on the microstrain. Increasing Fe was accompanied by an increase in growth fault probability, which was consistent with the HR-TEM image analysis. In addition to the effect of grain size, there is likely a contributing effect on microstrain-induced XRD line broadening due to the presence of growth faults.     

Effects of sintering aids on the densification of Mo–Si–B alloys

The effects of seven sintering aids (0.5?at.% Ni, Co, Fe, Cr, Zr, Nb, and Pd) on the densification of Mo–Si–B alloys of six different compositions (Mo, Mo–0.2Si, Mo–0.2Si–0.02B, Mo–2.5Si–2.5B, Mo–7Si–5B, and Mo–8.9Si–7.7B?at.%) are systematically investigated. It was found that Ni, Co, and Fe are effective in enhancing densification of Mo–Si–B alloys, and Ni is the most effective sintering aid. This study supports a previously proposed hypothesis that activated sintering results from enhanced mass transport in the sintering-aid-induced quasi-liquid intergranular films (a type of grain boundary complexion). The relative effectiveness of these sintering aids can be rationalized by analyzing several key thermodynamic parameters that control the stability of premelting-like grain boundary complexions. Future studies are needed to develop interfacial thermodynamic models and methods for computing “grain boundary complexion (phase) diagrams” for multicomponent systems, which can be a useful component for the “Materials Genome” project that will enable better predictions of the activated sintering and other materials phenomena.     

Evaluation of glass forming ability in binary and ternary metallic alloy systems – an application of thermodynamic phase diagram calculations

Abstract

The glass forming ability (GFA) of a wide range of binary and ternary alloy systems (Au–Si, Pd–Si, Ti–Be, Zr–Be, Hf–Be, Cu–Ti, Co–Zr, Ni–Zr, Cu–Zr, Ni–P, Pd–P, Ni–Pd–P, Cu–Pd–P, Co–Ti–Zr, Zr–Be–Hf, Ti–Be–Hf, Ti–Be–Zr) was calculated using a combined thermodynamic and kinetic approach. There is good agreement between the predicted glass forming ranges and those experimentally observed. By using this combined approach it has also proved possible to estimate critical cooling rates for phases not observed in the equilibrium phase diagram. A significant advantage of the approach is that, for multicomponent alloys, the melting temperatures and thermodynamic input parameters for the kinetic equations are derived using the constituent binary thermodynamic phase diagram calculations and, therefore, it has the potential to predict GFA in multicomponent systems using information from mainly binary systems.

MST/788     

Thermal stability and mechanical properties of nanocrystalline Fe–Ni–Zr alloys prepared by mechanical alloying

The thermal stability of nanostructured Fe_100?x?y Ni _x Zr _y alloys with Zr additions up to 4 at.% was investigated. This expands upon our previous results for Fe–Ni base alloys that were limited to 1 at.% Zr addition. Emphasis was placed on understanding the effects of composition and microstructural evolution on grain growth and mechanical properties after annealing at temperatures near and above the bcc-to-fcc transformation. Results reveal that microstructural stability can be lost due to the bcc-to-fcc transformation (occurring at 700 °C) by the sudden appearance of abnormally grown fcc grains. However, it was determined that grain growth can be suppressed kinetically at higher temperatures for high Zr content alloys due to the precipitation of intermetallic compounds. Eventually, at higher temperatures and regardless of composition, the retention of nanocrystallinity was lost, leaving behind fine micron grains filled with nanoscale intermetallic precipitates. Despite the increase in grain size, the in situ formed precipitates were found to induce an Orowan hardening effect rivaling that predicted by Hall–Petch hardening for the smallest grain sizes. The transition from grain size strengthening to precipitation strengthening is reported for these alloys. The large grain size and high precipitation hardening result in a material that exhibits high strength and significant plastic straining capacity.     

The effect of grain size on the nanocrystalline growth in metals and its simulation by Monte Carlo method

In this study, the thermodynamic stability of the grain boundaries and the grain growth of nanocrystalline Palladium (Pd) at 800 K were investigated. For this purpose, the Gibbs free energy curves of grain boundaries were plotted in terms of the excess volume by the use of Song’s, quasi-harmonic Debye approximation (QDA), and equation of state (EOS) thermodynamic models. The results of the two EOS and Song’s models showed that the excess volume increase up to values more than the critical excess volume, can result in the thermodynamic stability of nanocrystalline Pd. Therefore, according to the prediction of these two models, the nanocrystalline growth in metals was stopped at the grain sizes less than the critical grain size. But, according to the results of the QDA model there was no possibility for the stoppage of the grain growth and thermodynamic stability of the nanocrystalline Pd. To investigate the validity of the mentioned predictions, the Monte Carlo atomic simulation method was employed. The results obtained from the simulation confirmed the grain growth of nanocrystalline Pd within the size of the grains larger than the critical grain size and stoppage within the size of the grains less than the critical grain size.     

Austenite grain growth prediction coupling with drag and pinning effects in low carbon Nb microalloyed steels

Abstract

A metallurgical model has been developed to predict the austenite grain growth in Nb microalloyed steels. The mutual effects of Nb(CN) particle pinning and Nb solute drag on grain growth kinetics are studied. The particle dissolution, the undissolved particle coarsening and the changes in Nb solute in solution during reheating or isothermal heat treatment process are taken into account in the model. It is shown that, besides the pinning exerted by the NbC precipitates, the solute drag of Nb in solid solution plays an important role in the inhibition of austenite grain growth in Nb microalloyed steels. The Nb solute drag effect on grain growth decreases with increasing temperature because the grain boundary can gradually break away from the solute atmosphere in the higher velocity region at high temperature. The mean austenite grain size sluggishly increases with temperature in the low temperature region, while it significantly increases in the relative high temperature region. The predicted austenite grain size concerning the combined effect of Nb drag and Nb(CN) pinning is in good agreement with the experimental results from the literature.     

Bimodal grain size distribution: an effective approach for improving the mechanical and corrosion properties of Fe–Cr–Ni alloys

The effect of nanocrystalline grain size and bimodal distribution of nano- and microcrystalline grain sizes on the oxidation resistance and mechanical properties of Fe-based alloys has been investigated. Nanocrystalline and bimodal Fe–10Cr–5Ni–2Zr alloy pellets, prepared by mechanical alloying route, have been compared with conventional microcrystalline stainless steel alloys having 10 and 20 wt% Cr. Zr addition has been shown to improve the grain size stability at high temperatures. A significant improvement in the ductility of bimodal alloys with respect to nanocrystalline alloys was seen presumably due to the presence of the microcrystalline grains in the matrix. The high temperature oxidation of nanocrystalline and bimodal alloys at 550 °C shows superior oxidation resistance over microcrystalline alloy of similar composition (Fe–10Cr–5Ni) and comparable to that of microcrystalline alloy having twice as much Cr (Fe–20Cr–5Ni). Secondary Ion Mass Spectroscopy depth profiling confirms the hypothesis that nanostructure facilitates the enrichment of Cr at the oxide metal interface resulting in the formation of a passive oxide layer.     

A constitutive model of nanocrystalline metals based on competing grain boundary and grain interior deformation mechanisms

In this work, a viscoplastic constitutive model for nanocrystalline metals is presented. The model is based on competing grain boundary and grain interior deformation mechanisms. In particular, inelastic deformations caused by grain boundary diffusion, grain boundary sliding and dislocation activities are considered. Effects of pressure on the grain boundary diffusion and sliding mechanisms are taken into account. Furthermore, the influence of grain size distribution on macroscopic response is studied. The model is shown to capture the fundamental mechanical characteristics of nanocrystalline metals. These include grain size dependence of the strength, i.e., both the traditional and the inverse Hall–Petch effects, the tension–compression asymmetry and the enhanced rate sensitivity.     

Grain growth in porous two-dimensional nanocrystalline materials

Grain growth in two-dimensional polycrystals with mobile pores at the grain boundary triple junctions is considered. The kinetics of grain and pore growth are determined under the assumption that pore sintering and pore mobility are controlled by grain boundary and surface diffusion, respectively. It is shown that a polycrystal can achieve full density in the course of grain growth only when the initial pore size is below a certain critical value which depends on kinetic parameters, interfacial energies, and initial grain size. Larger pores grow without limits with the growing grains, and the corresponding grain growth exponent depends on kinetic parameters and lies between 2 and 4. It is shown that for a polycrystal with subcritical pores the average grain size increases linearly with time during the initial stages of growth, in agreement with recent experimental data on grain growth in thin Cu films and in bulk nanocrystalline Fe.     

Zr65Al7.5Ni10Cu15Co2.5合金的玻璃转变温度

根据Ｚｒ６５Ａｌ７。５Ｎｉ１０Ｃｕ１５Ｃｏ２。５合金的纳米晶，晶体，液体和玻璃比热的测量结果，研究了合金的玻璃转变温度与全金的热力学函数，动力学参数以及加热速度的关系。结果表明，非晶态合金玻璃转变所需转变激活能很小，玻璃转变温度实际上是由于加热速度引起的不同状态的玻璃与液体的热力学平衡温度。     

Finite element simulation of interfacial segregation in dilute alloys

The finite element software Comsol is used to simulate surface or grain boundary segregation in dilute alloys. The model computes simultaneously the evolution of interfacial concentration and diffusion in the bulk. The solute exchange between bulk and interface is governed by Darken’s equation. The model is able to reproduce thermodynamic and kinetic aspects of the phenomenon, in particular the saturation segregation level and the short-time segregation kinetics expressed by the McLean approximation. It is also able to reproduce experimental trends in the case of surface segregation of sulphur in a Ni superalloy. In the case of the grain boundary segregation of impurities (P or S) in engineering alloys, the present approach provides a practical tool, as it can be coupled to other finite element simulations (heat transfer and/or mechanics). Thus, it becomes possible to predict the risk of synergetic segregation and thermomechanical damage during service or processing (forging, welding,...).     

The effect of calcium segregation on grain growth in nanocrystalline TiO2

Differential scanning calorimetry and transmission electron microscopy are used to monitor grain growth in nanocrystalline TiO₂ doped with Sn and Ca. TiO₂ powder is prepared by a solution chemical method, and consolidated samples have an initial mean grain size of 30–50 nm, with a density greater than 95% of theoretical. Calcium is an effective grain growth inhibitor; the present samples show more resistance to grain growth than other nanocrystalline TiO₂ reported to date. A Kissinger analysis of grain growth exotherms indicates an effective activation energy of 1.3–1.5 eV, and calcium additions do not have a large effect on this value. These observations are discussed in light of the size dependent segregation behavior that has been observed in these samples using STEM microanalysis (1).     

Origins of saccharin-induced stress reduction based on observed fracture behavior of electrodeposited Ni films

This research presents experimental results of an investigation aimed at understanding grain size driven mechanical processes in electrodeposited Ni thin films where saccharine additions are commonly used to improve mechanical properties. Ni films were fabricated using salfamate-based electro chemical baths, where it is empirically known that mmol/l concentrations of saccharine will reduce the observed tensile stress in addition to lowering the grain size up to a few nanometer scales. Some previous observations and several theoretical models suggest that saccharine incorporation results in sulfur segregation at grain boundaries. Since grain boundary formation is also associated with tensile stress evolution, a plausible hypothesis is that saccharine additions are directly altering grain boundary energetics. This suggests that saccharine additions should also have an observable effect on intergranular fracture in these films. To test this prediction, in situ stress measurements during film growth and fracture testing of these same films were compared. Lithographically patterned substrates were used to produce films with ordered arrays of uniform islands, which demonstrated island size effects on stress evolution, and enabled a well-defined notch geometry along one of the island boundaries to facilitate fracture experiments. In situ uniaxial tensile testing under in a scanning electron microscope was then used to obtain the fracture strength of such specimens. This technique provided real time recording of microscopic deformation during uniaxial tensile loading. The observed relationships among residual stress, grain size, and fracture strength were then analyzed with detailed models of both film growth and fracture.     

Nano structure of rapidly quenched Cu–(Zr or Hf) – Ti alloys and their devitrification process

The structure and primary devitrification process of the melt-spun Cu₆₀(Zr or Hf)₃₀Ti₁₀ alloys were investigated. It was confirmed that the compositional segregation in the diameter range of 5–10 nm exists in the as-quenched state. The nanocrystalline particles with cubic structure are observed in the glassy matrix in thehigh-resolution transmission electron microscopy images, of which size is corresponding to the scale of compositional segregation. Small-angle X-ray scattering measurement also indicates the development of nanoscale inhomogeneity with the same size as that of nanocrystalline particles. The nanocrystalline region has high Cu content. In contrast, Zr or Hf and Ti elements are enriched in the glassy region. These results are recognized as the formation of novel structure consisting of the glassy and nanocrystalline phases. It is suggested that the precipitation of bcc CuZr phase as a primary crystallization phase proceeds in the glassy phase remaining the nanocrystalline phase in the Cu–Zr–Ti alloy. Meanwhile, the glassy and nanocrystalline phases are transformed to an orthorhombic Cu₈Hf₃ phase at the initial crystallization stage in the Cu–Hf–Ti alloy. These differences of crystallization process are consistent with the results of thermodynamic and kinetic analyses of the transformation mode.     

Modeling the dependence of strength on grain sizes in nanocrystalline materials

Abstract

A model was developed to describe the grain size dependence of hardness (or strength) in nanocrystalline materials by combining the Hall–Petch relationship for larger grains with a coherent polycrystal model for nanoscale grains and introducing a log-normal distribution of grain sizes. The transition from the Hall–Petch relationship to the coherent polycrystal mechanism was shown to be a gradual process. The hardness in the nanoscale regime was observed to increase with decreasing grain boundary affected zone (or effective grain boundary thickness, Δ) in the form of Δ^?1/2. The critical grain size increased linearly with increasing Δ. The variation of the calculated hardness value with the grain size was observed to be in agreement with the experimental data reported in the literature.     

Grain growth and stabilisation of nanostructured aluminium at high temperatures: review

Nanostructured (NS) materials have a large stored energy due to their large grain boundary area and thus tend to be unstable with respect to grain growth during high temperature annealing or deformation. This problem can limit the application of NS materials at high temperatures (>0·5T_m, absolute melting temperature), especially Al alloys owing to their low melting points. Restoration processes and grain growth in NS Al based materials are critically reviewed, with emphasis on nanostructure grain stabilisation at high temperatures. The mechanisms of normal and abnormal grain growth during isothermal annealing are presented, followed by consideration of thermal stabilisation by the addition of solute atoms/impurities and/or dispersion of second phase particles. Grain growth is significantly facilitated by applying deformation at elevated temperatures during preparation or further processing of semifinished NS materials. The dynamic restoration processes, dynamic grain growth and dynamic particle coarsening are addressed in NS Al. Finally, grain growth during consolidation of nanocrystalline powders (one of the principal methods to fabricate bulk NS Al) is presented, and the effects of processing parameters on grain size stabilisation are discussed.     

The spectrum of atomic excess free volume in grain boundaries

The grain boundary (GB) excess volume is an important structural factor that is strongly correlated with various thermodynamic and kinetic properties of GBs such as GB energy, GB mobility, GB diffusivity, and GB segregation energy, etc. However, the excess volume is usually reported as an average value of the entire GB. Such simplification does not consider the spectral nature of the excess volume in a GB, which cannot be used to describe the atomic mechanisms of some kinetic process, such as GB migration, that involves only a few atoms at a time. Here, we explore the spectrum of atomic excess volume in representative nanocrystalline Ni and Al samples as well as 388 Ni bicrystals based on the Olmsted dataset by using atomistic simulations. It is found that the nanocrystalline Ni and Al models show a skew-normal distribution in the spectrum of both the atomic excess volume and the atomic excess energy in the GBs, which show a weak inverse correlation between them. This is in stark contrast to the widely reported positive correlation between GB energy and excess volume based on the average value. We further show based on the statistical analysis that the correlation between the atomic excess volume and excess energy strongly depends on the GB type and a universal trend between them does not exist. While low ∑ Ni GBs generally shows a strong inverse linear correlation between these two properties, such correlation is weak for high ∑ Ni GBs. Moreover, we find that the spectrum of the excess volume shows characteristics distribution in some special Ni GBs. For example, twist GBs generally show a symmetrical unimodal distribution while most surveyed ∑3 Ni GBs with anti-thermal behavior show an apparent bimodal distribution. Nevertheless, a strong correlation is found between the atomic excess volume and the segregation energy based on the nanocrystalline Al model with Mg impurity, which implies a possible universal trend between the two properties. The current study thus shows that the excess volume provides useful insights in revealing the elemental structure–property correlations in GBs, which may be used as a structural variable in future thermodynamic modeling of GBs.