Intrinsic Stacking Fault Research Articles

Deformation characteristics of Ti-Ni-Fe based multiphase intermetallic: Experiments and atomistic simulation

This study deals with the development of Ti-Ni-Fe based multi-phase intermetallic having high strength with enhanced ductility and elucidation of their deformation characteristics. The intermetallic exhibited a compressive strain and strength of ⁓ 11 % and ⁓ 2.1± 0.112 GPa at room temperature (RT), respectively. MD simulation at RT showed a higher dislocation density in the DO24 phase, where most of the dislocations were found to be superpartials 13101̅0 and 1311̅00 separated by super intrinsic stacking faults (SISF), which was further corroborated with TEM analysis. It was found that despite its low volume fraction, DO24 phase governed the plastic deformation. Further, hot compression of Ti45Ni50Fe5 exhibited yield strength anomaly (YSA), where yield strength decreased from room temperature to 373 K followed by an increase at 473 K and 573 K. MD simulation of hot compression showed lower dislocation density (⁓300–551×10−6Å−2) for B2 matrix at all temperatures, indicating strength is predominantly governed by it. The dislocation density of B2 matrix was found to increase with temperature up to 373 K followed by a decrease at 473 K and 573 K. This difference in dislocation density has been attributed to the cross-slip of 121̅11 screw partials from {110} plane to {211} plane, resulting in the formation of dislocation lock that led to YSA behavior.

Plastic evolution and phase transition mechanisms in γ-TiAl nano polycrystal by a molecular dynamics simulation

γ-TiAl based alloys are widely regarded as one important high-temperature structural material, however the deformation behaviors are still not clearly clarified. The microscopic plastic deformation and phase transition of γ-TiAl nano polycrystals under compression are investigated by molecular dynamics simulations. The results show that the initial plasticity is dominated by slip of partial dislocations whose continuous formation and secondary slip on the same slip plane result in the generation and disappearance of intrinsic stacking faults (ISFs). At the strain of about 0.2, a continuous structure transformation of “ISF → ESF → TB” is observed, during which the interaction between different ISFs causes the atomic misalignment corresponding to the change in atomic stacking order near ISF. As the compressive strain exceeds 0.2, the FCC-to-BCC phase transition occurs within polycrystals due to the high Mises stress, the high stress level within grains facilitates the expansion of BCC phase from initial FCC phase, and the newly formed BCC phase maintains the atomic-arrangement orderliness of original phase. The interface between FCC and BCC phases obeys the low-energy Kurdjumov-Sachs orientation relationship. This work is helpful for better understanding the atomic-scale deformation behaviors and provides a theoretical guidance for designing high-performance alloy materials.

Exploring stacking fault energy with the axial Ising model: A renewed approach

In this work we revisit the axial Ising model for computing intrinsic stacking fault energies (γISF), which are important in alloy design. We find that the supercell approach, which directly compares the energies of fcc stacking with and without a fault, is a specific solution of the Ising model and is the most elegant and efficient among the solutions of the same order of accuracy. Contrary to the traditional belief that only the supercell approach can capture the effect of local chemistry, we demonstrate that local chemistry also influences γISF in the axial Ising model. We also propose a new formula γISF=5EABABC−5EABC, which is similarly efficient, simpler, and more accurate compared to the widely used γISF=EAB+2EABAC−3EABC. We tested the new formula on pure Ni, Ni-Co alloys, and Cr-Mn-Fe-Co-Ni high entropy alloys and found satisfactory results.

Effects of grain size and stacking fault energy on twinning stresses of single-phase CrxMn20Fe20Co20Ni40-x high-entropy alloys

Deformation twinning was investigated in single-phase face-centered-cubic (FCC) high-entropy alloys of the CrxMn20Fe20Co20Ni40-x system (14 ≤ x ≤ 26 at.%). The intrinsic stacking fault energies (γisf) of these alloys varied systematically across this composition range, but other physical properties that have the potential of affecting deformation twinning remained constant allowing us to isolate the effects of γisf on twinning. Tensile tests were performed at 293 and 77 K, and the tests interrupted at several different strains to evaluate deformation-induced changes in microstructure by transmission electron microscopy. The critical stress above which deformation twinning occurs was thus determined. We also evaluated how grain size (19 ≤ d ≤ 237 µm) affects the yield stress (σy) and the uniaxial critical twinning stress (σtw), which allowed us to subtract the corresponding Hall-Petch contributions and compare our polycrystalline results with experimental and theoretical data reported for single crystals. Taking into account the critical resolved shear stresses for deformation twinning in our alloys (τtw) along with those of FCC pure metals and binary alloys reported in the literature, we derived the following relationship that rationalizes most of the experimental data: τtw = 10-3G + γisf/(3·bp), where G is the shear modulus and bp is the magnitude of the Burgers vector of a Shockley partial dislocation. This expression, involving only measurable material properties, can serve as a basis for the design and optimization of new HEAs with tunable twinning stresses and, in turn, exceptional strength-ductility combinations.

Effects of N, O, S on generalized stacking fault energies and dislocation movements in γ-Ni and γ′-Ni3Al

The effects of interstitial atoms (N, O and S) on generalized stacking fault energies in the γ-Ni and γ′-Ni3Al with were systematically elucidated by first-principles calculations. N, O and S atoms had the preference for octahedral interstitial sites in γ phase. N and S atoms had the preference for octahedral interstitial sites with 6Ni atoms in γ′ phase, while O atom had the preference for octahedral interstitial sites with 2Al4Ni atoms. With the adopted atoms, the intrinsic stacking fault energy in γ phase was decreased and the anti-phase boundary energy in γ′ phase was increased, which were attributed to the charge redistribution between the adopted atoms and neighbouring Ni atoms. The addition of N, S, especially O, hindered the extended dislocation movement and enhanced its formation probability. The addition of O and S atoms significantly enhanced the formation probability of Kear-Wilsdorf dislocation lock in γ′ phase, while the addition of N slightly reduced it.

The role of coherent nano precipitates on stacking fault and deformation twin formation during tensile deformation of wire arc additive manufactured nickel-aluminum-bronze

The strain hardening mechanisms and dislocation interactions of a wire arc additively manufactured (WAAM) nickel-aluminum-bronze (NAB) alloy was investigated using multi-scale characterization approach cross-correlating scanning electron microscopy (SEM), SEM-based electron back-scatter diffraction (EBSD), site- and crystallographic orientation-specific focused ion beam (FIB) nanofabrication, scanning transmission electron microscopy (STEM) and STEM-based energy dispersive X-ray spectroscopy (EDS). Flat, rectangular dog bone specimens were strained under uniaxial tension conditions. To understand the micro- and nanometer scale deformation mechanisms throughout the microstructure, tensile test specimens were interrupted near the yield strength and ultimate tensile strength. STEM microstructural characterization revealed that tensile deformation occurs by planar slip and multiple slip bands interacting on different {111} planes as well as the nucleation of dislocations from the interfaces associated with grains and secondary phase particles. To the authors knowledge for the first time the Center of symmetry (COS) analysis revealed that in both samples the shearing of nanoscale precipitates led to the formation of extended stacking faults including a combination of intrinsic stacking faults and 2-layer extrinsic stacking faults. At high strain levels more dislocations and stacking faults were nucleated, as well as intersecting slip bands further facilitating the activation of the stair-rod cross-slip mechanism and thickening of an ESF to create a 3-layer nano-twin. Activated mechanisms of plastic deformation and associated role of the precipitates are discussed with respect to the microscopic strength-plasticity characteristics as well as macroscopic mechanical properties of WAAM NAB alloy.

Deformation mechanism of Ni-based single crystal superalloy under ultrasonic surface rolling and subsequent thermal exposure

In present work, ultrasonic surface rolling (USR) is applied to DD6 Ni-based single crystal (NBSC) superalloys, and the thermal relaxation behavior of the pre-treated alloys are investigated at 650 °C. Combined with mechanical properties, the plastic deformation and recovery mechanisms of the hardened layers are revealed in detail. Results show that compressive residual stress amplitude of 1163 MPa, 75.1 % improvement of surface nano-hardness and 83.02 % reduction on surface roughness are achieved by USR. Both deformed layer thickness and slip density progressively increases with USR cycles, where initial octahedron {111}<110> slips transform to a mixture of octahedron {111}<110> and dodecahedron {111}<112> slips. Meanwhile, shear step length expansions form γ′ segment/γ terminal interface with superlattice extrinsic stacking fault (SESF) on 1¯11, which may have inferior energy threshold for γ dislocation invasions than primary ones through shortening mean free path. Also, this drives γ′/γ solute mingling to induce tortuous interfaces. While dislocation proliferation dominates γ channel hardening, the γ′ hardening derives from planar fault strengthening and net system energy augmentation. During thermal exposure, surface integrities of USRed DD6 are strongly degraded, attributable to slip trace elimination and local γ′ coarsening. Thermal exposure further compels the γ dislocation to migrate towards γ′ segment across γ′ segment/γ terminal interface. Then, they are probably consumed to motivate solutes to phase boundary to reshape the γ′ morphologies through interface rearrangements under the assistance of γ dislocation network. The orientation transformation of SESF on 1¯11 to 010 and the mixture of SESF and superlattice intrinsic stacking fault on 1¯11 are thought to be possible manipulated mechanisms for γ′ coarsening, significant impairing the work hardening of alloy due to interfacial coherency deterioration. This paper could provide guidance for USR strengthening designs of NBSC.

The effect of solute atom concentration on the generalized planar fault energy and twinnability of basal plane in magnesium alloys

The influence of 39 solute atoms under uniform distribution on the generalized plane fault energy (GPFE) and twinnability of the basal plane in Mg solid solutions at various concentrations has been investigated through first-principles calculations. Firstly, it is revealed that the interaction between solute atoms and the GPFs on basal plane of Mg extends beyond the fault layer to encompass multiple atomic layers. Moreover, it is observed that as the distance from the fault layer increases, the interaction energy is decreased. And the increase in the total interaction energy between the solute atoms and the deformed intrinsic stacking fault (SF) I2 results in a corresponding enhancement of their interaction with other key points of GPF, namely UI2, T2, and UT2, establishing a roughly linear relationship. Finally, based on this interaction, we employ a uniform distribution model to investigate the influence of solute atom concentration on the twinnability of basal plane in Mg solid solution alloys. It is observed that with an increase in solute atom concentration, several conventional elements such as K, S, Sr, Zr, As, as well as most rare earth (RE) elements including Y, Pr, La, Nd, Eu, Gd, Dy, Ce, Tb, Ho, etc., gradually enhanced the twinnability of Mg solid solution alloys. Conversely, certain elements like B, P, Cs, Se, Er, Tl, etc., were found to diminish the twinnability of Mg alloys. This study can provide valuable insights for designing high-strength and toughness Mg alloys.

Bidirectional Phase Transformations in Multi-Principal Element Alloys: Mechanisms, Physics, and Mechanical Property Implications.

The emergence of multi-principal element alloys (MPEAs) heralds a transformative shift in the design of high-performance alloys. Their ingrained chemical complexities endow them with exceptional mechanical and functional properties, along with unparalleled microscopic plastic mechanisms, sparking widespread research interest within and beyond the metallurgy community. In this overview, a unique yet prevalent mechanistic process in the renowned FeMnCoCrNi-based MPEAs is focused on: the dynamic bidirectional phase transformation involving the forward transformation from a face-centered-cubic (FCC) matrix into a hexagonal-close-packed (HCP) phase and the reverse HCP-to-FCC transformation. The light is shed on the fundamental physical mechanisms and atomistic pathways of this intriguing dual-phase transformation. The paramount material parameter of intrinsic negative stacking fault energy in MPEAs and the crucial external factors c, furnishing thermodynamic, and kinetic impetus to trigger bidirectional transformation-induced plasticity (B-TRIP) mechanisms， are thorougly devled into. Furthermore, the profound significance of the distinct B-TRIP behavior in shaping mechanical properties and creating specialized microstructures c to harness superior material characteristics is underscored. Additionally, critical insights are offered into key challenges and future striving directions for comprehensively advancing the B-TRIP mechanism and the mechanistic design of next-generation high-performing MPEAs.

Localized phase transformation strengthening in CoNi-based superalloys

Segregation processes during high-temperature deformation can be either detrimental or beneficial in γ′-strengthened superalloys depending on which elements segregate. In intermediate CoNi-based superalloys only softening due to extensive microtwinning was observed so far. By analyzing the creep behavior of a Ta-free and a Ta-containing CoNi-based superalloy at 750 °C and 620 MPa, we show that strengthening segregation behaviors along various planar defects, including microtwinning, can be achieved in this alloy class. While both alloys deformed under planar fault formation, the Ta-containing alloy outperformed the Ta-free alloy. Atomic-scale energy dispersive spectroscopy investigations revealed strengthening effects by local phase transformations along superlattice intrinsic and extrinsic stacking faults in both alloys. However, microtwinning is enhanced in the Ta-free alloy, while it is impeded in the Ta-containing alloy. Given the faster diffusivity of Ta compared to W, kinetic effects on localized phase transformation strengthening and their subsequent influence on the creep behavior are discussed.

Exploring defect behavior in helium-irradiated single-crystal and nanocrystalline 3C-SiC at 800°C: A synergy of experimental and simulation techniques

In this study, single crystal (sc) and nanocrystalline (nc) 3C-SiC samples were subjected to 30 keV He ion irradiation across various doses while maintaining a temperature of 800 °C. Employing techniques including Raman spectroscopy, transmission electron microscopy (TEM), and nanoindentation, the alterations in microstructure and hardness resulting from He irradiation with various fluences were examined.In sc-SiC, irradiation prompted the formation of He platelets, resulting in a hardness increase of 7 GPa. In contrast, nc-SiC, characterized by a higher stacking fault density, exhibited the formation of bubbles, primarily at grain boundaries (GBs), with fewer occurrences within the grain interior, leading to a hardness increase of 1 GPa. Notably, in both sc- and nc-SiC, hardness reached saturation and subsequently stabilized or declined with increasing He fluence.Through molecular dynamics (MD) cascade simulations, we discerned that various planar defects do not uniformly contribute to enhancing radiation resistance. For example, intrinsic stacking faults (ISF) and twins in SiC played a substantial role in altering defect density and configurations, thereby facilitating point defect annihilation. Conversely, extrinsic stacking faults (ESF) and Σ3 GBs had a limited impact on defect production during a cascade.Furthermore, calculations of cluster diffusivity revealed an accelerated movement of He-vacancy towards GBs compared to bulk material and other planar defects. Moreover, the scarcity of point defects and constrained mobility of He atoms towards stacking faults in nc-SiC elucidated the marked tendency of He to form platelets in sc-SiC.Additionally, our findings established a correlation between the calculated indentation hardness and the geometry of He defects, consistent with experimental results from nanoindentation. These results significantly contribute to ongoing efforts to design SiC materials with heightened radiation tolerance.

Tension-compression asymmetry in Ti50Ni45Fe5 B2 intermetallic

This work investigates the asymmetry under tension and compression for the homogenized Ti50Ni45Fe5 B2 intermetallic. B2 matrix undergoes localized stress induced martensitic transformation (B2 → B19') under compression, resulting in a visible nonlinear elastic regime in the stress-strain curve. This was absent during deformation under tension; which could possibly be ascribed to lower resolved shear stress required for slip under tension than compression and a higher stress required for stress induced martensite transformation under tension than compression due to different martensite variants. The plastic deformation of B2 matrix under both tension and compression was marked by the presence and movement of stable elongated super intrinsic stacking faults, oriented along particular direction, which could be attributed to the activation of ⟨111⟩ screw dislocations.

High-performance ultra-lean biodegradable Mg–Ca alloys and guidelines for their processing

High-performance ultra-lean binary Mg–Ca alloys are engineered by intelligent alloying and thermo-mechanical processing using hot-extrusion. With Ca-alloying contents as low as 0.2-0.6 wt.%, remarkable room-temperature tensile properties are obtained with tensile strength values as high as 380–420 MPa, or ductility values reaching a maximum of 36 %. By means of multiscale structural and chemical analysis using electron microscopy and energy dispersive X-ray spectroscopy, we show that multimodal strengthening mechanisms can be activated by modifying the spatial distribution of Ca as secondary phase. Our results indicate that strong precipitation strengthening is achieved when Mg2Ca phase particles are dispersed within the grains. On the other hand, preferential distribution of the Mg2Ca precipitates along grain boundaries imparts substantial grain-boundary strengthening by the Hall-Petch effect. Apart from secondary-phase precipitation, the role of Ca as solute atoms is paramount in promoting homogeneous deformation. The presence of Ca directly alters the intrinsic stacking fault energies and modifies the cross-slip energy barriers such that the slip-transition probability from pyramidal-to-basal and vice-versa becomes comparable. Both effects ensure competitive activation of basal and non-basal slip, thereby reducing the mechanical anisotropy. The mechanical performance in the current work, when compared to earlier reported studies of Mg alloys with similar or higher alloying content, shows a 2 to 10-fold increase in tensile strength without compromising ductility.

Ab initio study of helium behavior near stacking faults in 3C-SiC

First-principles calculations are used to investigate the effects of stacking faults (SFs) on helium trapping and diffusion in cubic silicon carbon (3C-SiC). Both extrinsic and intrinsic SFs in 3C-SiC create a hexagonal stacking sequence. The hexagonal structure is found to be a strong sink of a helium interstitial. Compared to perfect 3C-SiC, the energy barriers for helium migration near the SFs increase significantly, leading to predominant helium diffusion between the SFs in two dimensions. This facilitates the migration of helium towards interface traps, as confirmed by previous experimental reports on the nanocrystalline 3C-SiC containing a high density of SFs. This study also reveals that the formation of helium interstitial clusters near the SFs is not energetically favored. The findings from this study enhance our comprehension of helium behavior in faulted 3C-SiC, offering valuable insights for the design of helium-tolerant SiC materials intended for reactor applications.

Atomic Simulation of Deformation Behavior of Polycrystalline Co30Fe16.67Ni36.67Ti16.67 High Entropy Alloy Under Uniaxial Loading

Mechanical behavior and plastic deformation mechanism of a new type of Co30Fe16.67Ni36.67Ti16.67 high entropy alloys (HEAs) have been calculated by the molecular dynamics method. The results show that the polycrystalline Co30Fe16.67Ni36.67Ti16.67 HEA has remarkable tensile plasticity and anisotropy. When the crystallographic orientation of the grain is &lt;001&gt;, the plastic deformation mechanism is face‐centered cubic (FCC)→body‐centered cubic (BCC) transformation and deformation twins. Grain boundary and vacancy reduce the nucleation energy of FCC→BCC phase transition, making BCC phase nucleation easy and growing in a shear manner, eventually forming deformation twins in the BCC phase. When the crystallographic orientation of grain is &lt;110&gt; and &lt;111&gt;, the plastic deformation mechanism is stacking faults, FCC→hexagonal‐close‐packed (HCP) phase transformation, and deformation twins. The motion of Shockley dislocation leads to the stacking fault, intrinsic stacking fault leads to the FCC→HCP phase transition, extrinsic stratification fault leads to the twin deformation, and the grain refining occurs during the tension process. Temperature and strain rate also have strong effects on tensile strength and elastic modulus. These results will provide a theoretical basis for the development of the HEAs and expand their application.

Structure and exfoliation mechanism of two-dimensional boron nanosheets

Exfoliation of two-dimensional (2D) nanosheets from three-dimensional (3D) non-layered, non-van der Waals crystals represents an emerging strategy for materials engineering that could significantly increase the library of 2D materials. Yet, the exfoliation mechanism in which nanosheets are derived from crystals that are not intrinsically layered remains unclear. Here, we show that planar defects in the starting 3D boron material promote the exfoliation of 2D boron sheets—by combining liquid-phase exfoliation, aberration-corrected scanning transmission electron microscopy, Raman spectroscopy, and density functional theory calculations. We demonstrate that 2D boron nanosheets consist of a planar arrangement of icosahedral sub-units cleaved along the {001} planes of β-rhombohedral boron. Correspondingly, intrinsic stacking faults in 3D boron form parallel layers of faulted planes in the same orientation as the exfoliated nanosheets, reducing the {001} cleavage energy. Planar defects represent a potential engineerable pathway for exfoliating 2D sheets from 3D boron and, more broadly, the other covalently bonded materials.

Introducing high-density growth twins in aluminum alloys by laser surface remelting via templated nucleation of grains

It is difficult to generate coherent twin boundaries in bulk Al alloys due to their high intrinsic stacking fault energy. Here, we report a strategy to induce high-density growth twins in aluminum alloys through the heterogeneous nucleation of twinned Al grains on twin-structured TiC nucleants and the preferred growth of twinned dendrites by laser surface remelting of bulk metals. The solidification structure at the surface shows a mixture of lamellar twinned dendrites with ultra-fine twin boundary spacing (∼2 μm), isolated twinned dendrites, and regular dendrites. EBSD analysis and finite element method (FEM) simulations have been used to understand the competitive growth between twinned and regular dendrites, and the solidification conditions for the preferred growth of twinned dendrites during laser remelting and subsequent rapid solidification are established. It is shown that the reduction in the ratio of temperature gradient G to solidification rate V promotes the formation of lamellar twinned dendrites. The primary trunk spacing of lamellar twinned dendrites is refined by the high thermal gradient and solidification rate. The present work paves a new way to generate high-density growth twins in additive-manufactured Al alloys.

Optimized process and superior toughness of a (HfNbTaTiW)C high entropy carbide ceramic

In the present study, we optimized the fabrication process, characterized the microstructures and evaluated the mechanical properties of a high entropy carbide ceramic with novel composition, (HfNbTaTiW)C. Our results show that 1700 °C carbon thermal reduction (CTR) and 1800 °C spark plasma sintering (SPS) can yield a single phase (HfNbTaTiW)C with high relative density, low residual C/O and fine grain structure. The nanohardness of (HfNbTaTiW)C is on the lower end (28.16 GPa), but its fracture toughness (4.84 MPa m1/2) is superior to both its constituent monocarbides and most existing multi-cation transition metal carbides. The lattice distortion, bonding strength and stacking fault energy of (HfNbTaTiW)C are compared with (HfNbTaTiZr)C through First Principle Calculations to help understand the observed mechanical properties. The results indicate first, the reduced bonding strength contributes more importantly than the large lattice distortion to the hardness, leading to the observed low hardness of (HfNbTaTiW)C and second, the reduced bonding strength and low intrinsic stacking fault energy synergistically contribute to the observed high toughness of (HfNbTaTiW)C.

Symmetry breaking induced asymmetric dislocation-planar fault interactions in ordered intermetallic alloys

In this study, we present the first comprehensive examination of symmetry breaking in the interactions between dislocation and superlattice planar faults, including anti-phase boundary (APB), complex stacking fault (CSF), superlattice intrinsic stacking fault (SISF), to reveal the underlying asymmetric dislocation reaction mechanisms depending on the sense of applied stress, employing both large-scale atomistic simulations and continuum dislocation theory. Four ordered intermetallic alloy systems including γ−Ni/γ′−Ni3Al and γ−Ni/γ′−Ni3Fe, γ−Al/γ−TiAl, and α−Ti/α2−Ti3Al were selected as the representative model systems, with two primary symmetry breaking effects, i.e., translational and three-fold rotational symmetry breaking considered. Detailed atomic steps of asymmetrical dislocation reactions and the corresponding asymmetrical dislocation bypassing mechanisms of precipitation have been elucidated, shown to be highly dependent on the geometrical configuration of the precipitate and the relative magnitudes of APB, CSF and SISF fault energies. A continuum model framework was then developed, which, for the first time, provides accurate and quantitative predictions of the threshold conditions triggering critical asymmetrical dislocation slips, verified to be in good agreement with the simulation results. Our study also successfully reproduced the experimentally observed dislocation-induced APB-SISF transformation, with a new dislocation reaction mechanism proposed to explain the transformation process. The findings are expected to be a key enabling stepstone for future innovation in intermetallic alloys strengthened through ordered phases for advanced applications in aeronautic and automotive industries.

Elucidating the deformation characteristics of Ti–Ni–Fe based pseudo-binary B2 intermetallics

The room temperature uniaxial compression behavior of homogenized Ti50Ni50-xFex (5≤x ≤ 45) pseudo-binary B2 intermetallics has been investigated. The compressive stress-strain curves of these intermetallics exhibited three different characteristics (a) intermetallics with low Fe content (i.e., Ti50Ni45Fe5 and Ti50Ni40Fe10): exhibited stress induced martensite (SIM) transformation (B2 → B19 ') (marked by a plateau with non-linear elastic regime) followed by significant plastic deformation up to ∼ 40 % strain marked with significant strain hardening, (b) intermetallics with moderate Fe content (Ti50Ni35Fe15) showed no SIM transformation, but a typical smooth elastic-plastic transition with maximum strain to failure of ⁓ 11–12 % was observed, and (c) intermetallics with high Fe content (≥ 25 at. %) displayed a very small strain to failure (≤ 5 %) without visible plastic deformation. The TEM and XRD analysis of the deformed (15 % and 40 % strain) Ti50Ni45Fe5 and Ti50Ni40Fe10 intermetallics indicated the presence of internally twinned martensite variants (MVs) along with severely dislocated banded B2 matrix. The observed contrast in the deformation characteristics has been attributed to the variation in the values of elastic constants (C' and C44) and Zener anisotropy (A) with Fe content. The SIM transformation in Ti50Ni45Fe5 and Ti50Ni40Fe10 occurred at lower values of C' and C44 (due to elastic softening) and relatively low Zener anisotropy A. The significant plastic strain in these two intermetallics was accommodated by the presence and deformation of super intrinsic stacking faults (SISFs) formed by dissociation of 12⟨111⟩ screw super-partials. In contrast, the deformation in the intermetallics with high Fe content (≥ 25 at. %) was governed by the activation of ⟨100⟩ screw dislocations.