L12 Alloys Research Articles

Effect of Alloying Elements on the High-Temperature Yielding Behavior of Multicomponent γ'-L12 Alloys.

The exceptional mechanical properties of Ni-based high entropy alloys are due to the presence of ordered L12 (γ') precipitates embedded within a disordered matrix phase. While the strengthening contribution of the γ' phase is generally accepted, there is no consensus on the precise contribution of the individual strengthening mechanisms to the overall strength. In addition, changes in alloy composition influence several different mechanisms, making the assessment of alloying conditions complex. Multicomponent L12-ordered single-phase alloys were systematically developed with the aid of CALPHAD thermodynamic calculations. The alloying elements Co, Cr, Ti, and Nb were chosen to complexify the Ni3Al structure. The existence of the γ' single phase was validated by microstructure characterization and phase identification. A high-temperature compression test from 500 °C to 1000 °C revealed a positive temperature dependence of strength before reaching the peak strength in the studied alloys NiCoCrAl, NiCoCrAlTi, and NiCoCrAlNb. Ti and Nb alloying addition significantly enhanced the high-temperature yield strengths before the peak temperature. The yield strength was modeled by summing the individual effects of solid solution strengthening, grain boundary strengthening, order strengthening, and cross-slip-induced strengthening. Cross-slip-induced strengthening was shown to be the key contributor to the high-temperature strength enhancement.

The Magnetic Band-Structures of Ordered PtxFe1−x, PtxCo1−x, and PtxNi1−x (x = 0.25, 0.50, and 0.75)

The electronic band structures of the ordered L12 and L10 phases of the PtxM1−x (M = Fe, Co and Ni) alloys were investigated using spin-polarized density functional theory (DFT). The relative contributions of both itinerant (Stoner) and localized magnetism at the high-symmetry k-points were determined and discussed qualitatively. Significant directional effects were identified along the A and R directions of the L10 and L12 alloys, respectively, and are discussed in terms of charge channeling effects.

Disorder strengthening of ordered L12 alloys by face centered cubic (A1) precipitates

We present a new theory of strengthening (disorder strengthening) of a crystallographically ordered L12 matrix by spherical, coherent crystallographically disordered fcc (A1) precipitates. There are two versions of the theory, each involving different approximations for the length of the chord, inside the precipitate, subtended by the trailing dislocation in the super-dislocation pair. Each of these versions incorporates two other assumptions involving the statistics of dislocation-precipitate interactions, which determine the average spacing of the precipitates along the super-dislocation. All versions of the theory predict a peak-strength condition at small particle sizes, followed by an extended period of overaging in which the strength decreases slowly with increasing particle size. We compare the quantitative predictions of the theory with both computer-simulated and experimentally generated data on inverse Ni-base γ′-type superalloy matrices strengthened by γ precipitates. In all cases at least one version of the theory is in excellent agreement with the data. The predicted strengthening is significant, the order of 85–110 MPa in the computer simulated results and 40–80 MPa in model precipitation strengthened inverse superalloys containing small volume fractions (≤0.04) of γ precipitates. The results are discussed in light of the statistics of dislocation-precipitate interactions and the resistance of individual γ particles to shearing by the super-dislocation pair; this resistance depends on the ratio of maximum force required to break free of the precipitate and the line tension of the dislocation pair.

Nanoscale patterning of CrPt3 magnetic thin films by using ion beam irradiation

We have successfully fabricated planar patterned CrPt3 ordered L12 alloy films by Kr+ ion irradiation. Planar-patterned CrPt3 nanodots with various bit sizes from 200nm to 50nm were successfully fabricated by 30keV Kr+ ion irradiation at a dose of 2×1014ions/cm2, where e-beam lithography was used for creating the resist mask. We have confirmed that the nanofabrication process didn’t change the magnetic properties of CrPt3 ordered L12 alloy films. As-prepared film exhibited perpendicular hysteresis loop with the coercivity of 5.5kOe. The typical perpendicular maze domain structure with the stripe structure was clearly seen in as-prepared CrPt3 film. Magnetic force microscopy (MFM) images of patterned CrPt3 nanodots indicated that each un-irradiated bit consists of localized perpendicular magnetic domain structures, which corresponds to perpendicular magnetization direction. Nanodots with bit size ⩽80nm show either dark or bright contrast, suggesting single domain structure. No magnetic contrast in irradiated space is due to the suppressing of the magnetization by Kr+ ion irradiation.

Bifunctional alloys for the electroreduction of CO2 and CO.

We use density functional theory to study the reduction of CO2 and CO to hydrocarbons through a formyl pathway on (111) and (211) facets of L12 alloys with an A3B composition. We find that several alloys may reduce the thermodynamic overpotential for CO reduction by more than 0.2 V compared to a copper step, however, these alloys are most often rather unstable in aqueous environment or have low alloy formation energies and may be susceptible to segregation destroying the active sites. Strategies to improve alloy stability against corrosion or segregation would likely be needed in order to realize the full potential of these alloys.

Creep behavior of Ni3Ge single crystals of the deformation axis orientation [001]: Experiment and modeling

The results from an experimental study of the creep of Ni3Ge single crystals of orientation [001] are presented. Calculated creep curves of alloys with an L12 superstructure and high energies of antiphase boundaries (APBs) are also presented. The calculations are performed using a model of creep for single crystals of L12 alloys that is based on the concept of strengthening and recovery.

Simulation of high-temperature superlocalization of plastic deformation in single-crystals of alloys with an L12 superstructure

The processes of the superlocalization of plastic deformation in L12 alloys have been studied numerically based on a combination of the model of the dislocation kinetics of the deformation-induced and heat-treatment-induced strengthening of an element of a deformable medium with the model of the mechanics of microplastic deformation described in terms of elastoplastic medium. It has been shown that the superlocalization of plastic deformation is determined by the presence of stress concentrators and by the nonmonotonic strengthening of the elements of the deformable medium. The multiple nonmonotonicity of the process of strengthening of the elementary volume of the medium can be responsible for the multiplicity of bands of microplastic localization of deformation.

Frank–Read source mechanism, mobile dislocation density exhaustion and flow stress anomaly in L12 alloys: a dislocation dynamics study

We present a dislocation dynamics simulation study on the effect of Kear–Wilsdorf locks formation on the Frank–Read mechanism at temperatures below the peak temperature of the flow stress in some L12 alloys. The simulation developed in this paper differs from previous ones by taking into account the detailed structure of the connection of the Kear–Wilsdorf lock with the glissile part of the dislocation, as well as by allowing glide in the cube plane. Assuming that the plastic strain is carried out by the motion and development of the kinks connecting the Kear–Wilsdorf locks, we analyse their behaviour under the effect of an applied stress at temperatures below the peak temperature. We show that the area swept by an expanding dislocation loop in the octahedral plane, starting from an edge dislocation segment, is the result of the competition between the locking rate and the velocity of its screw part, whatever the length of the initial segment. When the applied stress is smaller than a critical stress τc/ d , the length of a moving edge segment decreases until it eventually stops. For a stress larger than τc/ d , the length of the segment increases and Frank–Read sources may develop. The first case is referred to as convergence, the second as divergence. The simulation results are applied to the analysis of the micro- and macro-yield behaviour of these alloys as well as to some of their specific properties.

Ab initio calculations on elastic properties in L12 structure Al3X and X3Al-type (X=transition or main group metal) intermetallic compounds

As high-temperature structural materials, L12 intermetallic compounds have attracted the strong interest from both fundamental and industrial aspects. Understanding of elastic property is a basis for the complete investigations of mechanical behavior of L12 alloys. In an effort to explore the electronic origin of elastic properties of L12 intermetallics, we have performed a systematic study on elastic constants for single crystals, and Young's modulus, shear modulus, bulk modulus and Poisson's ratio for poly-crystals of 22 known Al3X and X3Al-type (X=transition or main group metal) intermetallics using the ab initio calculations. Based on the calculations of elastic constants and extreme (both positive and negative) Poisson's ratios, we found a pronounced correlation between the extreme Poisson's ratio and the elastic anisotropy, i.e., approximate 40% of the investigated L12 intermetallics exhibit intrinsic auxetic behavior. Furthermore, based on the distribution of bonding charge densities, we revealed that the ductility and extreme Poisson's ratios were attributable to the directionality of bonds of these alloys. Our findings provide a new method to predict mechanical behavior of intermetallics.

First-principles study of static displacements in Fe-Pd magnetic shape-memory alloys

AbstractThis contribution reports static ionic displacements in ferromagnetic disordered Fe70Pd30 alloys obtained by relaxation of the ionic positions of a 108-atom supercell within the framework of density functional theory. Comparison with a simple statistical model based on Lennard-Jones pair interactions reveals that these displacements are significantly larger than can be explained by the different sizes of the elemental constituents. The discrepancies are presumably related to collective displacements of the Fe atoms. Corresponding distortions are experimentally observed for ordered Fe3Pt and predicted by first-principles calculations for all ordered Fe-rich L12 alloys with Ni group elements and originate from details of the electronic structure at the Fermi level.

Characterization of Grain Boundaries in Recrystallized L12-Type Intermetallic Alloys

Microstructural feature of the recrystallized Co-based (Co3Ti) and Ni-based (Ni3(Si,Ti) and Ni3Fe) ordered alloys with L12 structure was investigated by the electron backscatter diffraction (EBSD) method, with emphasis on grain boundary character distribution (GBCD). For comparison, the GBCDs of the recrystallized Co-Ni, Ni-Fe and 70/30 brass disordered alloys, and also copper, nickel and aluminum pure metals with A1 (fcc) structure, which have widely different stacking fault energies, were also determined. The frequency of Σ3 boundary for the Co-based alloys was higher than that for the Ni-based alloys, regardless of ordered L12 alloy or disordered fcc alloy, indicating that the frequency of Σ3 boundary was primarily dominated by stacking fault energy. Furthermore, the effect of ordering energy on structure and energy of the grain boundaries including Σ3 boundary in the ordered L12 alloys was discussed.

Violation of Schmid's law and tension–compression asymmetry in L12 alloys: role of ordered structure

Hypostoichiometric Ni3Al(B) undergoes a structural transformation from its initial L12 structure to DO22 structure during cold rolling. It has been found that this DO22 structure reverts back to L12 structureduring annealing. The transformation to DO22 structure as well as back to L12 at higher temperature is very much dependent on the degree of cold reduction. Based on thesefindings, the possible reason for abnormal behaviour of these L12ordered alloys, i.e. violation of Schmid's law and orientation dependence of flow stress is hypothesised.

Simulation of the plastic flow in Ni3Al: Work hardening and strain-rate sensitivity

Abstract The origin of the anomalous temperature dependence of the giant work-hardening rate and of the modest strain-rate sensitivity observed in many L12 alloys is investigated by means of an ‘end-on’ simulation of dislocation dynamics in Ni3Al. The present model reproduces most of the atypical mechanical properties observed experimentally. The study of the evolution of the distribution of the Kear-Wilsdorf locks during plastic deformation indicates that the order of magnitude of the work-hardening rate stems from a preferred exhaustion of the weakest locks. In addition, the low strain-rate sensitivity measured is found to rely on the absence of a correlation between the dislocation properties which provide the plastic strain and those which determine the flow stress level.

Simulation of the plastic flow in Ni3Al: work hardening and strain-rate sensitivity

Abstract The origin of the anomalous temperature dependence of the giant work-hardening rate and of the modest strain-rate sensitivity observed in many L12 alloys is investigated by means of an ‘end-on’ simulation of dislocation dynamics in Ni3Al. The present model reproduces most of the atypical mechanical properties observed experimentally. The study of the evolution of the distribution of the Kear-Wilsdorf locks during plastic deformation indicates that the order of magnitude of the work-hardening rate stems from a preferred exhaustion of the weakest locks. In addition, the low strain-rate sensitivity measured is found to rely on the absence of a correlation between the dislocation properties which provide the plastic strain and those which determine the flow stress level.

Simulation of Dislocation Dynamics in Ni3Al: A Study of Velocity Autocorrelations

AbstractThe yield strength anomaly in some L12 compounds has been linked to the thermally assisted cross slip of screw superdislocations. This work continues earlier efforts to understand the yield strength anomaly in L12 alloys using computer simulations of dislocation motion. Dislocations are modelled within isotropic elasticity theory, and simple rules are used to model the cross-slip process in the two dimensional geometry of the simulation. The velocity of a single dislocation in Ni3Al is studied as a function of the applied stress. The observed velocities vary nonlinearly with the applied stress. Further, dislocations are observed to become immobile for small applied loads. At high stresses, the dislocations are observed to advance relatively unhindered by the thermally activated cross slip process. Fluctuations in the velocity of the dislocations are studied, and their autocorrelation function shows an increased correlation time near a threshold stress. This threshold stress is identified with the critical stress proposed in earlier works.

Comment on 'A simulation of dislocation dynamics and the flow stress anomaly in L1 alloys' 2

Abstract A recent simulation of dislocation dynamics in L12 alloys, by Devincre, Veyssière, Kubin and Saada (DVKS), brought some new insight into the controversial problem of strength anomalies in these materials. The results of the simulation are discussed and compared with those of previous analytical models. Some interesting consequences on strain rate sensitivity and work-hardening rate, which were not discussed by DVKS, are presented on the basis of a combination of pre-existing models.

Comment on ‘A simulation of dislocation dynamics and the flow stress anomaly in L12alloys’

Abstract A recent simulation of dislocation dynamics in L12 alloys, by Devincre, Veyssiere, Kubin and Saada (DVKS), brought some new insight into the controversial problem of strength anomalies in these materials. The results of the simulation are discussed and compared with those of previous analytical models. Some interesting consequences on strain rate sensitivity and work-hardening rate, which were not discussed by DVKS, are presented on the basis of a combination of pre-existing models.

The mechanical anomaly of L12 alloys: a review of recent experiments and models

Abstract The anomalous deformation behaviour of L1 2 alloys is reviewed with emphasis made on the contributions of the past few years. The present understanding of the microstructural organization is presented schematically and its principal components outlined. The competition between a potent exhaustion of mobile dislocation and the difficulty of multiplication is discussed based on dedicated mechanical tests aimed at emphasizing transients. The information brought about by computer simulations is analyzed. Recent findings on the lack of orientation dependence of the flow stress in binary alloys are presented.

The ductile versus brittle response of L12 intermetallic bicrystals

Abstract A micromechanical model for understanding the brittleness of L12 intermetallic alloys is proposed. Based on the Rice-Thomson concept that the brittle versus ductile response of a crack tip is controlled by the competition between dislocation blunting of the tip and cleavage decohesion, the model shows that when glissile CSF-coupled dissociation of superdislocations occurs, an L12 intermetallic alloy may be intrinsically ductile, while APB-coupled dissociation leads to an intrinsically brittle behavior. Converting the DO22 structure to the L12 structure does not guarantee a ductile fracture behavior. In order to develop a ductile L12 alloy, the alloying element must be able to change the atomic bonding nature so that the CSF-coupled dislocation dissociation is promoted.

Dynamics of free surfaces in model fcc, hcp and L12 structures

Abstract Dynamic properties of free surfaces in their relaxed configuration obtained by the static relaxation method, are studied in fcc Ni and Al, hcp Ti and the L12 alloy Ni3Al. The (001), (110) and (111) surfaces are analysed for the fcc and Li2 structures and the (0001), (1210) and (1010) surfaces for the hcp structure. Interatomic potentials of the embedded-atom method type are used to calculate vibrational eigenfrequencies obtained through the Einstein and ‘cluster’ approaches for the atoms on the first few layers.