Thermal vacancies and the yield anomaly of FeAl

We review here the George-Baker model for the yield strength anomaly of FeAl which is based on hardening by thermal vacancies at intermediate temperatures and dislocation creep at high temperatures. Results of up-quenching and down-quenching experiments, which corroborate the vacancy hardening mechanism, are discussed. Some implications of the model are compared with available experimental results.     

Bending embrittlement of as-welded FeAl alloys

The study on the liquid induced embrittlement in the as-welded six FeAl alloys was performed under constant applied loads using four-point beam bend apparatus. Exposure to two different liquid environments, i.e. relative humidity R.H. 98% water vapor and antirust oil WD40, was conducted. The weld metal in all FeAl alloys exhibits an excellent liquid embrittlement resistance greater than the base metals. The fractures in all FeAl alloy welds occurred at the base metal. The fractograph of WD40 induced brittlement is characterized by a transgranular cleavage fracture. However, the fractograph of R.H. 98% water vapor induced brittlement is characterized by an intergranular and transgranular mixed fracture mode.     

Point defects and their properties in FeAl and FeSi alloys

Positron annihilation as well as differential dilatometric measurements were performed to measure the vacancy concentration in FeAl and FeSi. The highest vacancy concentration is found at the stoichiometric B2 composition with more than 3.5% at the melting point in FeAl. Additionally, diffusion and isotope effect experiments were carried out in B2 FeAl to obtain information about the diffusion mechanism. The isotope effect decreases when the material transforms from D0₃ to B2. The effective formation volume of the defects in the B2 phases is larger (1.4Ω) than one atomic volume Ω and is related to a defect involving more than one vacancy. The effective migration enthalpy of 0.5–1.8 eV as well as the effective formation enthalpy of 0.7–1.02 eV vary with the concentration of the alloys in an opposite manner. The results for the FeAl system suggest that different defect types may operate as diffusion vehicles in the different phases. The mobility of the defects dominates the thermomechanical behavior of FeAl alloys. The defect production during mechanical deformation of FeAl alloys is divided into two parts. The annealing out of mechanically produced defects occurs on the same time rate as that of thermal defects.     

The relation between the shape of the stress anomaly and the structure of Fe3Al alloys

The stress anomaly for Fe₃Al intermetallics exhibits various shapes in the D0₃ phase region. Two in situ TEM investigations at high temperatures were performed to explain this behaviour—the growth of D0₃ domains and the measurement of the long range order (LRO) in the D0₃ phase during aging. It is shown, that LRO depends on the depth under the surface of the specimen and changes during aging. In the light of this fact is possible to explain the various shape of stress anomaly.     

The stress anomaly in FeAl–Fe3Al alloys

The anomalous stress peak observed near 500–600 °C in Fe–Al alloys has now been convincingly explained using a model of hardening by immobile thermal vacancies on the lower temperature side of the peak and the loss of hardening as these vacancies become mobile at higher temperatures. The large numbers of vacancies required for such hardening are associated with compositions close to stoichiometry, i.e. 40–50%Al, raising the question of whether such a vacancy hardening model can be adopted for Fe₃Al alloys, which show a similar stress peak anomaly. Examination of data on vacancy formation over the entire range of composition, Fe–Fe₃Al–FeAl, shows that, indeed, a vacancy hardening model appears capable of explaining the stress anomaly for both FeAl and Fe₃Al.     

Effects of deviations from stoichiometry on the strength anomaly and fracture behavior of B-doped FeAl

Tensile tests were conducted at temperatures ranging from 145–1123 K on four different FeAl alloys, containing 40, 43, 45, and 48 at.% Al, each doped with 0.12 at.% B. The alloys were initially heat treated to obtain a relatively large grain size (˜200 μm), after which they were given a long, low-temperature anneal (673 K for 5 d), to minimize, respectively, the effects of grain boundary strengthening and thermal vacancies on the measured yield strengths. Each alloy displayed bcc-type behavior at low temperatures (yield strength decreasing with increasing temperature), followed by a strength anomaly at intermediate temperatures (yield strength increasing with increasing temperature), and a sharp drop in yield strength at elevated temperatures (beyond the anomalous strength peak). Thermal vacancies that are generated during the hold time at the test temperature may contribute to the production of the strength anomaly. In specimens not given the vacancy-minimizing anneal, quenched-in vacancies were found to substantially increase low-temperature strength, thereby masking the yield strength anomaly. As the Al concentration of FeAl increased, the prominence of the yield strength anomaly decreased. Ductility also exhibited a peak at elevated temperatures, first increasing with temperature until it reached a maximum value and then decreasing with further increases in temperature. The peak in ductility occured at lower temperatures as the Al content increased. The fracture mode in all four alloys was mixed (intergranular + transgranular) at cryogenic temperatures, predominantly intergranular at around room temperature, dimpled rupture at peak ductility, and intergranular cavitation at elevated temperatures where the ductility dropped.     

Characterization of complex planar faults in FeAl(B) alloys

Complex planar faults were observed by diffraction contrast transmission electron microscopy in a B2 FeAl based alloy containing Ni and B. The comparison of experimental images to simulated ones revealed the detailed structure of these faults that lie on {001} planes with both in-plane and out-of-plane components of the displacement vector. It was deduced that these defects form by segregation of (B-Al vacancies) complexes on a<100> dislocations, leading to various defect structures depending on the screw or edge character of the dislocation.     

Nickel-induced strengthening of boron-doped FeAl (B2) alloys

The effect of nickel additions on yield stress temperature dependence as well as on deformation mechanisms was investigated for Fe–Al alloys containing 40% Al. A set of different nickel containing alloys (up to 10 at.%), B-doped or B-free, was deformed in compression up to 950 °C. Subsequent dislocation structures were examined. It was shown that the addition of nickel strongly decreases the solid state solubility of boron in FeAl (B2) alloys. On the microscopic scale, nickel was confirmed to clearly decrease the dissociation width of the 〈111〉 superdislocations. From a mechanical point of view, it was shown to induce a large and linear strengthening at low temperatures (<400 °C). Owing to that specific hardening, the yield stress anomaly, usually occurring in such materials, was observed to be blurred out, but was nonetheless revealed by a zero strain rate sensitivity.     

Modeling of site occupancies in B2 FeAl and NiAl alloys with ternary additions

The addition of ternary alloying elements to FeAl and NiAl based intermetallic compounds could potentially influence their mechanical properties in a significant manner. Knowledge of the site occupancies of different species in these ternary compounds is of importance since it influences the bonding characteristics and consequently the mechanical properties. In this paper a simple approach involving chemical rate theory has been used to calculate the site occupancies in the ternary B2 compounds. These calculations predict that with increasing atomic number of 3d transition metal additions to B2 FeAl, the site occupancy of the ternary addition increases on the Fe site with the exception of Cu. The predicted exception for Cu has been experimentally verified using ALCHEMI. In addition, the site occupancies in B2 Ni–Al–Fe compounds of different compositions have been predicted and experimentally verified. The role of the relative ordering or clustering tendencies of the three different binary pairs in the ternary compound on the site preferences has also been discussed.     

Processing of Fe3Al and FeAl alloys by reaction synthesis

The effects of powder particle size, alloy composition, and reaction atmosphere on reaction synthesis of binary Fe---Al alloys were studied. Reactions were observed in an open (air) furnace, under static vacuum (in an evacuated quartz tube) and in a dynamic vacuum furnace. Reactions occurring in the open furnace and in the evacuated quartz tube were recorded using high-speed video equipment. High-speed videotapes of reaction synthesis of compacts formed from 45 μm Fe and 10 μm Al particles reacted in air and under static vacuum revealed that an unusual ‘two-stage’ reaction exists in this system under these conditions. Compacts formed from 9 μm Fe and 3 μm Al powder particles do not exhibit a two-stage reaction under any of the conditions examined in this work. The first stage of the two-stage reaction lasts several seconds and starts at round 650 °C. The second stage begins at about 900 °C, reaching temperatures between 1250 and 1350 °C. The progress of the reaction to the second stage is sensitive to the alloy composition and reaction atmosphere. The reaction behavior is explained in terms of thermodynamics and heat transfer, which control the delicate balance between heat accumulation and heat loss during reaction synthesis.     

Evaluation of the Peierls stress for boundary dislocations

The Peierls stress for 1/6〈111〉 twinning dislocations and 1/2〈111〉 perfect dislocations in a bcc structure has been evaluated. The calculations have been performed using the Peierls-Nabarro formalism. The Peierls stresses have been determined from the migration energy of a twin boundary γ_tbm and the energy of an unstable stacking fault γ_us. The values of γ_tbm and γ_us have been calculated using a set of generalized many-body interatomic potentials. The potentials were defined so as to ensure different stability of the bcc structure relative to other structures. It has been shown that this approach provides realistic values of the Peierls stress. The values of the Peierls stress for 1/6〈111〉 twinning dislocations are very sensitive to the model parameters, unlike those for 1/2〈111〉 perfect dislocations.     

Determining the energy of interaction of impurity atoms with dislocations in titanium alloys

Conclusions Determining the energy of interaction of atoms of alloying elements in titanium alloys by measuring the internal friction, we found that alloying leads to an increase in the energy of interaction. This may explain the higher values of the ultimate tensile strength of alloy VT22 in comparison with that of commercial titanium VT1-0.Translated from Metallovedenie i Termicheskaya Obrabotka Metallov, No. 1, pp. 52–53, January, 1980.     

On the multiplication of dislocations during martensitic transformations in NiTi shape memory alloys

In situ and post-mortem diffraction contrast transmission electron microscopy (TEM) was used to study the multiplication of dislocations during a thermal martensitic forward and reverse transformation in a NiTi shape memory alloy single crystal. An analysis of the elongated dislocation loops which formed during the transformation was performed. It is proposed that the stress field of an approaching martensite needle activates an in-grown dislocation segment and generates characteristic narrow and elongated dislocation loops which expand on {1 1 0}_B2 planes parallel to {0 0 1}_B19′ compound twin planes. The findings are compared with TEM results reported in the literature for NiTi and other shape memory alloys. It is suggested that the type of dislocation multiplication mechanism documented in the present study is generic and that it can account for the increase in dislocation densities during thermal and stress-induced martensitic transformations in other shape memory alloys.     

Hydrogen migration in electron irradiated Pd based dilute alloys around the 50 K anomaly

The migration of hydrogen in Pd–1 at.% Fe–H and Pd–1 at.% Ag–H alloys is investigated by electrical resistivity measurement around 50 K. The disordered hydrogen atoms are introduced by electron irradiation with 0.5 MeV electrons below 15 K. The disordered atoms order by migration of hydrogen atoms during the heating-up of the specimens. The recovery curves of electrical resistivity have two sub-stages for electron irradiated specimens and one stage for fast cooled specimens. The migration energy of hydrogen is obtained from the kinetic analysis of the resistivity change due to the ordering, using the cross-cut method for the electron irradiated specimen and fast cooled specimen. The obtained value of the migration energy for the low temperature stage is smaller than that for the high temperature stage. The value for the high temperature stage is similar to the energy for the case of fast cooling. The difference between the hydrogen atoms disordered by irradiation and that by fast cooling is discussed for Pd based alloys.     

An athermal deformation model of the yield stress anomaly in Ni3Al

An athermal deformation model was proposed for the yield stress anomaly in Ni₃Al based on the concept that dislocation multiplication versus the immobilization by the Kear–Wilsdorf (KW) locking controls the plastic deformation. In the model dislocations, superkinks lying between two KW locks multiply through expansion in an athermal manner. Regarding the KW locking, the entire screw segment cross-slips onto (010) to form long KW locks in one thermal activation event. The model can explain the macroscopic characteristics of the deformation, the strain-rate independent of the yield stress and the compliance with Schmid's law, which were recently revealed in binary, stoichiometric Ni₃Al single crystals. Quantitative assessment proved that the model reproduces the temperature dependence of the yield stress with reasonable fitting parameters. The driving force for the KW locking, one of the most important parameters for the yield stress anomaly, was evaluated at 127 mJ m⁻². This value is in good agreement with those calculated from the anti-phase boundary energies on {111} and {100} reported in the literature.     

Electronic bonding and property of FeAl

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Solute strengthening of both mobile and forest dislocations: The origin of dynamic strain aging in fcc metals

A full rate-dependent constitutive theory for dynamic strain aging is developed based on two key ideas. The first idea is that both solute strengthening and forest strengthening must exist and must exhibit aging phenomena. The second idea is that a single physical aging mechanism, cross-core diffusion within a dislocation core, controls the aging of both the solute and forest strengthening mechanisms. All the material parameters in the model, apart from forest dislocation density evolution parameters, are derivable from atomistic-scale studies so that the theory contains essentially no adjustable parameters. The model predicts the steady-state stress/strain/strain-rate/temperature/concentration dependent material response for a variety of Al–Mg alloys, including negative strain-rate sensitivity, in qualitative and quantitative agreement with available experiments. The model also reveals the origin of non-additivity of solute and forest strengthening, and explains observed non-standard transient stress behavior in strain-rate jump tests.