Stacking Fault Energy Research Articles

Compositional effect on pressure-induced polymorphism in high-entropy alloys

Recently, pressure-induced polymorphic phase transitions were recently discovered in several fragmented high-entropy alloys (HEAs), offering a valuable opportunity to deepen our understanding of these materials. However, the chemical and physical factors that govern these transitions are still unclear. Here, we combined in situ high-pressure and high-temperature synchrotron X-ray diffraction, X-ray emission spectroscopy (XES), and high-resolution transmission electron microscopy (HRTEM) to systematically study the evolution of the atomic and electronic structures in the Cantor alloy and its face-centered-cubic (fcc) subset alloys (CoCrFeMnNi, CoCrFeNi, CoCrMnNi, CoFeMnNi, CoCrNi, CoFeNi, CoMnNi, CrFeNi, and FeMnNi). Surprisingly, diverse behavior was observed among these closely related alloys during compression and decompression, which includes irreversible, reversible fcc to hexagonal close-packed (hcp) phase transitions, or even no detectable phase transitions up to ∼40 GPa. HRTEM measurements confirmed that the fcc and hcp phases abided by the classic Shoji-Nishiyama orientation relationship during the transitions. XES data indicated that high-pressure suppresses the local magnetic moments in all the studied alloys, suggesting that magnetic states do not significantly influence the polymorphic transitions. By comparing the effects of the atomic size difference, entropy, valence electron concentration, and stacking fault energy across all the compositions studied, only the stacking fault energy shows a strong correlation with the phase transitions, indicating it plays a key role in inducing polymorphism in HEAs.

Twinning mediated cryogenic toughness in an additively manufactured high Mn steel by manipulating stacking fault energy

The insufficient cryogenic toughness of wire-arc additively manufacturing (WAAM) component hinders their application in cryogenic fields. We report that tungsten is effective in enhancing the impact toughness of WAAM components, increasing it from 42 to 106J at 77K. The outstanding cryogenic toughness depends on the interaction between dislocations and twins, where the thickness of twins plays a pivotal role in determining whether they hinder or facilitate dislocation movement. Twins below 6.1nm hinder dislocations, impeding deformation, while those exceeding 21.7nm coordinate with the matrix orientation, aiding dislocation movement and forming microbands. The W element reduces the critical twinning stress from 112.3 to 83.5MPa by manipulating the stacking fault energy and grain size, thereby altering the twinning structure. However, excessive W content forms large-sized precipitates, serving as crack initiation sites and reducing crack initiation energy.

Deformation pseudo-twinning of the L21-ordered intermetallic superlattice

Ordered intermetallic materials are promising candidates for applications in extreme environments due to the strong localized bonding schemes that stabilize the structures against degradation. Here, we investigate the propensity for deformation twinning modes in the L21-ordered MnCu2Al intermetallic by inducing a state of severe plastic deformation. Post-mortem microstructural analysis reveals the presence of meso-scale deformation twins accompanied by a high degree of nano-scale crystalline heterogeneity. The orientation relationship between the parent and twin structures is consistent with activation of the 〈111〉{112} deformation twinning mode. Ab initio generalized stacking fault energy curve calculations corresponding to 〈111〉{112} twinning shear suggest that the magnitude of the twinning partial is 14〈111〉{112} and results in the local formation of a metastable orthorhombic phase within the pseudo-twin. This work highlights the discrepancy between the deformation modes of ordered/disordered structures and promotes an increased awareness for the role that long-range chemical order plays in determining deformation modes.

Developing a novel phase balance of duplex stainless steel for exceptional mechanical properties and corrosion resistance

The classical ferrite-austenite balance of duplex stainless steel is still challenged by fulfilling an ultimate balance between mechanical property and corrosion resistance. In this work, a novel phase-balanced metastable structure is in-situ developed using additive manufacturing via para-equilibrium transformation controlled by interstitial nitrogen diffusion. The novel phase-balanced structure comprises a primary fine equiaxed intragranular austenite, an incomplete partitioning of alloying elements, a predominate Kurdjumov-Sachs orientation relationship of ferrite/austenite interface and a high-density dislocation. Synergistic breakthrough of both mechanical property and corrosion resistance is endowed, where excellent yield strength is attributed to the high-density dislocation and the fine austenite, high ductility is facilitated by triggering stacking fault and kinking deformation from the dramatic increase of stacking fault energy, and extraordinary corrosion resistance is credit to the incomplete partitioning of alloying elements. This work expands the microstructure design framework for high performance materials.

Moiré polar vortex, flat bands, and Lieb lattice in twisted bilayer BaTiO3.

Through first-principles calculations based on density functional theory, we investigate the crystal and electronic structures of twisted bilayer BaTiO3. Our findings reveal that large stacking fault energy leads to a chiral in-plane vortex pattern that was recently observed in experiments. We also found nonzero out-of-plane local dipole moments, indicating that the strong interlayer interaction might offer a promising strategy to stabilize ferroelectric order in the two-dimensional limit. The vortex pattern in the twisted BaTiO3 bilayer supports localized electronic states with quasi-flat bands, associated with the interlayer hybridization of oxygen pz orbitals. We found that the associated bandwidth reaches a minimum at ∼19∘ twisting, configuring the largest magic angle in moiré systems reported so far. Further, the moiré vortex pattern bears a notable resemblance to two interpenetrating Lieb lattices and the corresponding tight-binding model provides a comprehensive description of the evolution the moiré bands with twist angle and reveals the topological nature.

Complex irregular stacking faults within Zr3Ge secondary phase nanoparticle stimulated by face-centered cubic zirconium phase growth

Here we report, for the first time, complex irregular stacking faults were generated within tetragonal Zr3Ge secondary phase particles (SPPs) in the Ge-modified Zircaloy-4 (Zr-4) alloy when subjected to high-temperature compression at 600°C. Atomic-scale observations indicated that the complex irregular stacking faults (CISFs) exhibit a non-coplanar deformation configuration characteristic. When viewed along the [001]Zr3Ge direction, atomic shifts were observed on three different crystallographic planes: (100), (010) and (110) planes, in other words, this defect was developed by the repetition and connection of superlattice intrinsic stacking faults on three different planes of the Zr3Ge SPP. Based on the crystallographic orientations relationships among the Zr3Ge and its surrounding precipitates, a mechanical model was proposed to uncover the nature of such CISFs structure. It confirmed that the CISFs can be attributed to the combined effects of the relatively low stacking fault energy and the growth of two nearby FCC-Zr phases.

Decoupling irradiation effects on deformation-induced martensitic transformations in commercial Ni-Cr-Fe alloys

This study deconvolutes the roles of irradiation-induced cavities and dislocation loops on deformation-induced martensitic transformations in Ni-Cr-Fe alloys. Until recently, martensitic transformations were not thought possible in these materials due to their high stacking fault energy. But mechanical martensitic transformations can be activated by nanoindentation, prompting a need to understand the role of defects such as those which would be generated during service in high temperature irradiation environments, on the extent of the transformations. Here, commercial Alloy 625 and Alloy 690 are irradiated with neutrons, protons, or He ions at 400 °C to create microstructures containing loops and cavities, predominantly loops, or predominantly cavities, respectively. Nanoindentation on irradiated {101} grains are dissected, and their deformation microstructures are characterized using post-mortem transmission electron microscopy (TEM). In Alloy 625, all irradiation types either partially or fully suppress the fcc → hcp → bcc transformation, whereas in Alloy 690, irradiation promotes the transformation, with neutron irradiation exhibiting the most mature fcc → hcp transformation. Irradiation defects compete to influence the martensitic transformation through a competition between increasing free energy and increasing hardening. Cavities increase free energy more than they increase hardening, thus promoting transformation, whereas loops hinder transformation because they increase hardening more than free energy. These findings have significant implications on the evolution of mechanical behavior and thus the safety factor of Ni-Cr-Fe alloy components under irradiation.

Quasi-in-situ observation on high-strain-rate deformation of Ti-xO(x=0.2, 0.4) polycrystals

Under high strain rates, the effects of aluminum and vanadium on the deformation mechanisms of pure titanium have been revealed. However, the effect of oxygen, a prevalent interstitial element, remains less understood. In this study, we conducted dynamic uniaxial compressive loading on Ti-xO (x = 0.2, 0.4 wt%) polycrystals along the rolling direction (RD) using a split Hopkinson pressure bar (SHPB). Deformation mechanisms were analyzed via quasi-in-situ electron backscatter diffraction (EBSD) and slip trace analysis. The findings indicate that twinning and dislocation slip are significant in the Ti-0.2O polycrystal, with dislocation slip predominating in the Ti-0.4O polycrystal. Both polycrystals primarily activate {101‾2} tensile twins; however, twin nucleation and growth are markedly reduced in the Ti-0.4O polycrystal, a phenomenon associated with enhanced stacking fault energy (SFE) and oxygen segregation {101‾2} tensile twin boundaries. Unlike in quasi-static conditions, prismatic <a> slip is more probable in the Ti-0.4O polycrystal, while basal <a> slip, first-order pyramidal <a> slip, first-order pyramidal <c+a> slip, and second-order pyramidal <c+a> slip are inhibited to varying extents. These variations are attributed to increases in the ∣b<c+a>∣/∣b<a>∣ ratio, CRSSBasal<a>/CRSSPrismatic<a> ratio, and CRSSFirst-order pyramidal<a>/CRSSPrismatic<a> ratio with higher oxygen content. The Burgers vector is identified as more critical than the critical resolved shear stress (CRSS). Additionally, cross-slip between the <c+a> slip systems occurs in both polycrystals, facilitated by the complex core structure of the <c+a>-type dislocation. However, enhanced oxygen-dislocation interactions in the Ti-0.4O polycrystal result in reduced cross-slip planes.

Enhancing room and cryogenic temperatures mechanical properties of FeCoCrNiMn high entropy alloys with high content of inexpensive element P

FeCoCrNiMn high-entropy alloys (HEAs) exhibit excellent ductility, particularly at cryogenic temperatures. However, their yield strength is relatively low. In this study, an inexpensive element, phosphorus (P), was added to HEAs at a high concentration (0.4 %). The results indicate that adding P not only enhances both strength and ductility, but also avoids embrittlement. The P-added HEAs were single-phase random solid solutions, with P distributed inside the grains and partially segregated at the grain boundaries. Within the grains, regions rich in P and poor in P were observed, along with significant tensile and compressive strain fields in the P-rich regions. These strain fields may lead to large local internal stresses. The presence of P prevents brittleness due to its specific atomic distribution, whether in coarse or fine grains. The yield strength increased from 185.27 MPa in P-free homogenized HEAs to 296.1 MPa in P-added HEAs, representing an increase of approximately 60 % at 298 K. At 77 K, the strengths further increased, irrespective of grain size. Further, P added reduced the stacking fault energy. At room temperature (298 K), P-free HEAs formed dislocation cells and high-density dislocation wall structures, while P-added HEAs formed additional microbands, deformation twins, and stacking faults due to increased lattice frictional stress. P-added HEAs demonstrated excellent mechanical properties at cryogenic temperatures, with good strength-ductility synergy and strain-hardening capacity. These enhancements can be attributed to the synergistic effects of nano deformation twins, hierarchical nano-spaced stacking fault networks, and Lomer-Cottrell locks, as well as their extensive interactions.

Enhancing fatigue resistance of Cr-Mn-Fe-Co-Ni multi-principal element alloys by varying stacking fault energy and sigma (σ)-phase assisted grain-size reduction

This study investigates two key aspects of the low cycle fatigue (LCF) behavior of alloys from the Cr-Mn-Fe-Co-Ni system at room temperature: (1) the influence of stacking fault energy (SFE) in single-phase face-centered cubic (FCC) alloys and (2) a grain size reduction triggered by the precipitation of a small amount of σ-phase. The first effect is investigated using model alloys (Cr26Mn20Fe20Co20Ni14 and Cr14Mn20Fe20Co20Ni26 in at.%, grain size: ∼60 µm), which have distinct SFEs at room temperature. A reduction in SFE from 69 to 23 mJ/m2 results in a 10 to 20 % increase in tensile/compressive peak stresses, i.e., cyclic strength, across all examined strain amplitudes (±0.3 %, ±0.5 %, and ±0.7 %) while maintaining comparable fatigue lives. Despite its higher cyclic strength, the low-SFE alloy exhibits delayed, and less evolved dislocation substructures than the other alloy. In both single-phase alloys, fatigue cracks originated from the surface reliefs, surface-exposed coherent annealing twin boundaries, and occasionally from high-angle grain boundaries. However, the crack propagation rate was slower in the low-SFE alloy, contributing to its superior fatigue resistance. By aging the low-SFE Cr26Mn20Fe20Co20Ni14 alloy differently, we could induce the precipitation of ∼5 % σ-phase during recrystallization, which strongly reduced the FCC grain size to ∼5 µm. With this microstructure, the cyclic strength increased by 50–65 % and remained more stable during fatigue testing while maintaining a comparable life. The σ-precipitates were found to deflect and arrest fatigue cracks, while extensive deformation twinning around cracks complements slip activity and reduces crack propagation rate. Overall, the σ-phase-assisted grain size reduction is 3 to 5 times more effective in improving cyclic strength than SFE reduction.

Influence of Cold Drawing on Phase Transformation and Tensile Properties of FeCrMn Austenitic Stainless Steel 201

Manganese stabilizes the austenite and reduces the stacking fault energy in face‐centered cubic (FCC) structures, promoting the formation of hexagonal and cubic structures. This study investigates the phase transformation and mechanical behavior of extremely low‐carbon FeCrMn austenitic stainless AISI 201 steel subjected to cold drawing in three stages, ultimately reaching a true strain of 0.93. The objective is to examine the transformation from the parent FCC structure to hexagonal close‐packed and body‐centered cubic structures as a combination of transformation‐induced plasticity and twinning‐induced plasticity phenomena. X‐ray diffraction and electron backscatter diffraction analyses are utilized to investigate phase transformations under significant plastic deformation and the crystallographic orientation relationships between phases. Additionally, tensile and hardness tests are performed to assess the impact of plastic deformation on mechanical properties. Results indicate that a true strain of 50% is optimal for balancing strength and ductility in CrMnFe austenitic stainless steel. After applying a true strain of ε1 = 26.7%, yield strength (YS) increases by 192% and ultimate tensile strength (UTS) by 35%, while elongation reduces by 41%. With a further strain of ε2 = 57.5%, YS increases by 71% and UTS by 44%, but elongation drastically decreases by 94%. Applying the final strain of ε3 = 94.0%, YS and UTS only increase by 3% and 2%, respectively, while elongation further reduced by 40%. These findings suggest that a true strain of 50% shall be considered the maximum reduction for maintaining a balance between strength and ductility in CrMnFe austenitic stainless steel.

An in-situ investigation of microstructure neighborhood effects on hydrogen-induced dislocation activity

The interactions between hydrogen and crystallographic defects are the key to understanding hydrogen induced localized plasticity or damage mechanisms. However, unraveling such interactions based on the resulting mechanical effects alone can be challenging, due to the spectrum of various effects hydrogen can create in a given microstructure. Pre-deformation or post-mortem characterization of hydrogen segregation has similar limitations, as they can only provide snapshots of these complex processes with challenging spatial and temporal scales. To tackle this, we design in situ experiments that combine electron channeling contrast imaging (ECCI), electron backscattered diffraction (EBSD), and desorption spectroscopy, to investigate hydrogen defect interactions, under no external loading. We show that the inhomogeneous hydrogen distribution within microstructures can cause long-range (>200 nm) dislocation rearrangements. Interpretation of these results as manifestations of boundary-segregation induced stress suggests that microstructural characteristics on either side of the boundary should also play an important role. We provide quantitative assessments of these microstructure neighborhood effects and discuss additional contributions from hydrogen-induced changes in stacking fault energy and elastic modulus.

Enhancing dry sliding wear resistance of high-Mn austenitic steel by adding N

In this study, the wear resistance of two high-Mn austenitic steels, i.e., Fe−18.5Mn−7Cr−0.6C and Fe−18.5Mn−7Cr−0.6C−0.21N was tested in dry sliding friction on a disc friction and wear testing machine (MMU-5G). The wear behavior and microstructure evolution of the two tested steels were investigated. The results revealed that adding N enhanced the wear resistance of high-Mn austenitic steel due to the synergistic effects of hardness, wear hardening, and oxide layer. Interstitial N atoms increased the matrix hardness and decreased the stacking fault energy (SFE). The lower SFE facilitated wear hardening, and the active mechanical twins along with the higher dislocation density induced the formation of a thicker nanocrystalline layer with finer grains. The nanocrystalline layer with higher surface activity promoted the adsorption of a protective oxide layer. These factors decreased the surface roughness, coefficient of friction (COF), and wear rate of N-alloyed high-Mn austenitic steel. This study provided valuable insights into the application of N-alloyed high-Mn austenitic steels under dry sliding friction conditions.

Effect of hydrogen on the basal dissociation of 〈a〉-type screw dislocation core in titanium

The existence of basal dissociated screw dislocation core structure of titanium with different H concentrations is investigated by first-principles calculation. The calculation of stacking fault energy of the basal plane with H in different layers suggests that a high H concentration would be beneficial for basal dissociation. The direct calculation based on the dislocation core instead proved that a small amount of H cannot stabilize the basal dissociated core structure. When H concentration increases, the basal dissociated core configuration and some mixed cores with basal faults could be more stable than the pyramidal dissociated structure.

Deformation mechanism of a metastable medium entropy alloy strengthened by the synergy of heterostructure design and cryo-pre-straining

Face-centered cubic (FCC) medium entropy alloys (MEAs) have received considerable attention due to their impressive mechanical properties and responses. However, their practical application is limited by their modest yield strengths. The potential enhancement of the mechanical properties of single-phase MEAs was explored in this study through a synergistic approach combining heterogeneous structure design with subsequent cryo-pre-straining. A heterogeneous lamella structure was produced in a single-phase Fe55Mn20Cr15Ni10 MEA via two-step rolling and annealing. Cryo-pre-straining at varying degrees (6, 12, 21, and 36%) introduced hexagonal close-packed (HCP) phase, high-density dislocations, twins, and stacking faults, leveraging the reduced stacking fault energy at cryogenic temperatures. This process enhanced the alloy's yield strength from 353 MPa to 1.2 GPa (compared to the baseline uniform coarse-grained structure), while maintaining an acceptable total elongation of 8.4%. The impact of cryo-pre-straining on the microstructure and mechanical properties of the MEA was assessed using in-situ synchrotron X-ray diffraction analysis. Cryo-pre-straining (36%) achieved a higher dislocation density (6.1 × 1015m−2) compared to room-temperature straining (2.5 × 1015m−2). The stress contribution from HCP-martensite and the evolution of dislocation density during loading were quantified, along with observations of negative stacking fault probability and strain-induced HCP→FCC reverse transformation in cryo-pre-strained samples under loading conditions. Furthermore, the contributions of regulated microstructures to the enhancement of yield strength were quantitatively assessed.

Effect of cyclic loading on microstructure and plastic deformation in heat-treated Co–28Cr–6Mo alloy fabricated via laser powder bed fusion: An in situ synchrotron X-ray diffraction study

We present a systematic microstructure-oriented plasticity investigation of an additively manufactured cobalt-based alloy aiming to relate microstructural changes to the fatigue resistance. A load-controlled cyclic test in the low-cycle fatigue regime was utilized to study mechanical response and microstructural changes in the alloy after reverse phase transformation heat treatment. Deformation-induced FCC → HCP phase transformation was followed using in situ synchrotron X-ray diffraction combined with ex situ electron backscatter diffraction and scanning electron microscopy. Scanning transmission electron microscopy provided experimental evidence of the presence of precipitates in the solution annealed sample. It was found, that a stepwise increase in plastic deformation is attributed to nucleation and growth of different martensitic crystallographic variants. These insights into plasticity and microstructural changes suggest that the reverse phase transformation heat treatment is an effective approach for improving the fatigue resistance of low stacking fault energy alloys, that are susceptible to deformation-induced martensitic transformation.

Exceptional strength-ductility synergy in the novel metastable FeCoCrNiVSi high-entropy alloys via tuning the grain size dependency of the transformation-induced plasticity effect

In-depth knowledge of the coupling between grain refinement and the transformation-induced plasticity (TRIP) effect in metastable alloys is a viable approach for the improvement of strength-ductility synergy, which needs systematic research with consideration of commercial austenitic stainless steels and novel high-entropy alloys (HEAs). Accordingly, in the present work, two Si-containing metastable HEAs in the Fe47Co30Cr10Ni5V8-xSix system (x = 3 and 6 at.%) were designed, and the TRIP-assisted AISI 304L stainless steel was also considered for comparison. The alloys were processed by cold rolling and annealing to obtain different grain sizes. Reducing the stacking fault energy (SFE) through adjusting chemical composition contributes to minimizing the detrimental effect of grain refinement on the ductility of TRIP alloys, while extremely low SFE must be avoided owing to the fast kinetics of deformation-induced martensitic phase transformation, which leads to the deterioration of ductility. In contrast to AISI 304L stainless steel, a strong TRIP effect was maintained upon grain refinement in the Fe47Co30Cr10Ni5V2Si6 HEA due to the remaining apparent SFE in the appropriate TRIP range. The tuned kinetics of martensitic transformation was found to be responsible for the exceptional ductility (∼65%) of Fe47Co30Cr10Ni5V2Si6 HEA at an ultrahigh tensile strength of 1250 MPa. Therefore, considering the identical trend of SFE with grain size, an appropriate initial SFE value is important for tuning the grain size dependency of the TRIP effect. Moreover, the ultrahigh strength was attributed to the high volume fraction of α΄-martensite as well as the high strength of the martensite phase due to the high Si content. Accordingly, for achieving strong-yet-ductile HEAs, a high Si content is recommended to benefit from solid solution strengthening in the martensite phase, a specially-designed chemical composition is needed for attaining a high volume fraction of α΄-martensite, and SFE should be in a desirable range to tune the kinetics of martensitic phase transformation.

A machine learning potential for simulation the dislocation behavior of magnesium

Accurate predictions of the dislocation behavior of magnesium (Mg) by molecular dynamics (MD) simulations are essential for studying the fundamental mechanisms of deformation and designing high plasticity Mg alloys. However, existing atomic potentials in MD simulation for Mg are not sufficiently quantitative for many dislocations-associated phenomena, such as stacking fault energy (SFE) and dislocation core structures. Here, by combining 468 density functional theory (DFT) calculated data points and a machine learning method, we create a broadly applicable deep learning potential (DLP) to study the dislocation behavior of Mg. We demonstrate that our DLP reproduces the SFE, lattice constants, elastic constants, and surface energies in reasonable agreement with experimental or DFT data. Furthermore, the DLP predicted basal 〈a〉, prismatic 〈a〉, pyramidal 〈c + a〉 dislocations all agree well with DFT results on dissociation distance and core structures. Importantly, the DLP has a superior performance on distinguishing the pyramidal I and II 〈c + a〉 screw dislocation core structures. Our results show that the DLP is suitable for investigating the dislocation behavior of Mg, making it valuable for future realistic atomistic studies of general deformation problems.

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.

A new nanoindentation approach for determining both indentation size effect and continuous apparent activation volume during loading of nanocrystalline Ni and Ni-20 wt.% Fe alloy

A new nanoindentation approach is developed in this study to determine both indentation size effect (ISE) and continuous apparent activation volume () during loading of nanocrystalline nickel (NC Ni) and nanocrystalline nickel-20 wt.% iron alloy (NC Ni-Fe alloy). This approach involves conducting nanoindentation tests under loading modes controlled by both constant strain rate (CSR) and constant loading rate (CLR). Firstly, a new empirical expression for the parameter in the modified Nix-Gao model is established to study the strain rate dependence of the ISEs created in the CSR and CLR tests. Then, continuous data of each individual sample (indentation) during loading are obtained in the CLR test through the alternative use of continuous stiffness measurement technique. The stress dependence of is analyzed based on Duhamel et al.’s model, which considers the role of vacancies generated during inhomogeneous dislocation nucleation. Finally, based on the above experimental and theoretical work as well as transmission electron microscopy observation, the influences of loading mode, loading rate, and material properties (stacking fault energy) on resulting intrinsic hardness of NC Ni and NC Ni-Fe alloy are analyzed. The nanoindentation approach developed in this study provides profound insights into the mechanical behaviors during loading and underlying mechanisms of materials.