Intrinsic Stacking Fault Research Articles

Tensile Deformation Behaviors and Microstructure Evolution Under Various Temperatures for MAR‐M247 Nickel‐Based Superalloy

The study aims to analyze the tensile behavior and microstructural changes in MAR‐M247 nickel‐based superalloy across different temperatures. Tensile behavior is examined under a constant strain rate of 2.5 × 10−4 s−1 at varying temperatures. Results indicate a temperature‐dependent nature of the alloy's tensile properties. At 550 °C, a Portevin–Le Chatelier effect is observed, attributed to twinning nucleation and carbon atom diffusion. Dynamic recrystallization occurs at 950 °C, manifesting in a sinusoidal stress–strain curve. At room temperature, the primary fracture mechanism involves dislocation shearing in the γ matrix, with minor dislocation shearing in the γ′ phase. At 550 °C, carbides along grain boundaries and the γ′ phase impede dislocation motion. Concurrently, dislocations shear the γ′ phase, leading to the formation of superlattice intrinsic stacking faults and Kear–Wilsdorf locks, thereby enhancing tensile strength. However, at 950 °C, dislocation motion primarily involves climb, carbides decompose, the γ′ phase softens and enlarges, resulting in a notable decrease in tensile strength.

Compression and fretting wear studies of γ/α 2 duplex TiAl alloys at the nanoscale

The study investigated the nanofabrication behavior of TiAl alloys with a duplex structure of γ/α 2. The process included downward pressure followed by reciprocating friction with diamond grinding balls and was simulated using molecular dynamics (MD). It was found that a certain number of dislocations in the workpiece was low, and the resilience was high during the initial pressing stage. The dislocations increased, the resilience decreased, and the plastic deformation capacity was enhanced under continuous pressing. The α 2 phase did not deform significantly during the compression process. The presence of the α 2 phase increases the overall hardness of the material, and elastic-plastic deformation occurs mainly where the γ phase is present; the endowment layer dislocations generated during the intrinsic stacking fault rebound via the phase boundary to form V-shaped dislocations. During the reciprocating friction of the workpiece, forward friction produces V-shaped dislocations, and reverse friction makes the dislocations disappear. This process results in the forward average friction force being more significant than the reverse average. γ/α 2 phase boundary has an impeding effect on the downward proliferation of defects, and the phase boundary makes the temperature transfer appear discontinuous. During friction, the specific number of vacancy atoms in the γ-phase increases, and the transition between FCC and HCP occurs.

Chemical short-range order enhances fracture toughness of medium entropy alloy CoCrNi

Using hybrid molecular dynamics and Monte Carlo simulations, we examine the role of lattice distortion (LD) and chemical short-range ordering (CSRO) on the development of defects ahead of a mode I crack in medium entropy alloy CoCrNi. We show that CSRO noticeably increases fracture toughness. The result can be explained by the effect of CSRO on lowering LD and increasing intrinsic stacking fault energy and the direct impact CSRO has on the energetic barriers for emitting partial dislocations and forming nanotwins from CoCr clusters on the crack tip. CSRO allows the nanotwin domains to further support inelastic deformation, such as dislocation glide and amorphization, leading to stable crack-tip plasticity and postponement of softening. These findings imply that the superior fracture toughness in CoCrNi can be attributed to the non-negligible CSRO that naturally exists.

Effect of temperature on the mechanical properties of Ni-based superalloys via molecular dynamics and crystal plasticity

Study on the influence of temperature on the mechanical properties across multiple scales has been a focus on the research of Hastelloy-X (HX) alloys for the application in high-temperature structure components. In this work, Molecular Dynamics (MD) and Crystal Plasticity (CP) are put together to solve it from atomic scale to mesoscopic scale. MD research indicates that the deformation of HX alloy occurs in two stages at temperature below 300 K: initially, as stacking fault deforms, stacking fault can transform into twinning with increasing strain. When the temperature exceeds 300 K, deformation primarily forms a stacking fault. The twinning deformation path transforms from intrinsic stacking fault to extrinsic stacking fault and then to twinning. A mesoscopic-scale CP model was developed using atomic-scale deformation mechanisms to bridge the gap between deformation mechanisms and experimental results. The CP results indicate a functional relationship between the strength of HX alloy and temperature. This relationship appears insensitive to crystal texture and grain shape. Incorporating grain morphology and texture into the model significantly impacts the strength response of calculating HX alloy. After the tensile deformation of HX alloy at 300 and 1173 K, the atomic scale deformation results characterized by transmission electron microscopy are aligned with the MD simulation results. The relationship between strength and temperature predicted by CP results has also been validated. A thorough investigation into the deformation behavior of HX alloys across different scales, employing MD and CP models, introduces a novel approach for predicting the mechanical properties of superalloys.

A phase-field model of concurrent Suzuki segregation and partial dislocation glide

Co–Al–W-base superalloys with the classic γ−γ′ microstructure offer exciting high temperature property profiles. High temperature deformation in such alloys involves shearing of the γ′ phase by a a/3〈112〉 superpartial to create a superlattice intrinsic stacking fault (SISF) while the trailing a/6〈112〉 partial is pinned at the γ−γ′ interface. Experiments indicate that Suzuki segregation to such faults lowers their energy, which in turn lowers the resistance to partial dislocation glide. However, quantifying the extent of solute segregation and its impact on the dynamics of partial dislocation glide from experiments is challenging. In order to fulfill the purpose computationally, in this paper, we present a phase-field model of the phenomenon of concurrent solute segregation and partial dislocation glide. Our simulations reveal that under creep conditions in a representative pseudobinary alloy belonging to the Co–Al–W system, there is a marked increase in the partial dislocation velocity due to solute segregation. The rise in dislocation velocity due to segregation is found to be more significant for lower resolved shear stresses on the dislocation. Higher solute interdiffusivity also leads to enhanced partial glide velocities. Our model can be extended to multicomponent systems, which can, in turn, be used to accelerate the design of novel Co-base superalloys by predicting the kinetics of the displacive-diffusive shearing of the γ′ phase for prospective alloy systems.

Impact of Dilute Amounts of Fission Products on the Mechanical Behavior of Ni.

Fission products may interact with structural materials in various nuclear energy applications and cause their mechanical performance to deteriorate. Therefore, it is important to study the effects of different fission products on the mechanical properties of structural materials. In this work, nickel was chosen as a model structural material system and dilute amounts of uranium and fission product impurities X = (Tc, Te, Sb, Ce, Eu, and U) up to 4 at % were used. Density functional theory (DFT) calculations were utilized to assess the effects of these substitutional impurities on the elastic behavior of the metal. Additionally, DFT was used to investigate some aspects of plastic response by computing the generalized stacking fault energies on the {111} ⟨112⟩ slip system for all alloying elements and at varying distances away from the stacking fault plane. None of the dopants satisfied the Pugh or Pettifor criteria for embrittlement, and alloying with Tc led to a slight increase in the elasticity of nickel. The phenomenon of Suzuki segregation was observed for all alloying elements, and there was consequently a significant reduction in the intrinsic stacking fault energy. Finally, and based on the analysis of the stacking fault energies, dopants generally led to softening the nickel (except for Tc and Ce), and all of the dopants were correlated with a loss of ductility (except Eu). These findings may be useful to consider in the design of next-generation reactors and nuclear waste management systems.

Observation of broad triangular Frank-type stacking faults and characterization of stacking faults with emission wavelengths below 430 nm in 4H–SiC epitaxial layers

Frank-type stacking faults in 4H–SiC epitaxial layers were investigated using room-temperature photoluminescence mapping and high-angle annular dark-field high-resolution scanning transmission electron microscopy. Remarkably, the intrinsic Frank-type stacking fault (5,2) and intrinsic multilayer Frank-type stacking fault (4,2), which have been reported to have elongated triangular shapes to date, were found to have broader triangular shape, which was typical shape of Shockley-type stacking faults. The shaping of stacking faults could be explained based on the directions of the partial dislocations bounding the stacking faults. Formation models for the intrinsic (5,2) and (4,2) stacking faults were proposed. In addition, we observed Frank-type stacking faults (3,2,2,3) and (3,3) and Shockley-type stacking faults (3,3,3,2,3,2) and (3,2,3,3,3,2), which were not reported before. All of these stacking faults were composed with the stacking numbers 2 and 3 in the Zhdanov notation and showed very close characteristic photoluminescence emission wavelengths of 427–428 nm. Formation models for the stacking faults of (3,2,2,3), (3,3), (3,3,3,2,3,2), and (3,2,3,3,3,2) were proposed, also. Based on the proposed formation models of (4,2) and (3,3) stacking faults, formation models for the Frank-type stacking faults (4,1) and (3,2) were discussed. Finally, we pointed out that the (4,1) Frank-type stacking fault should be the intrinsic not the extrinsic fault, and the (3,2) Frank-type stacking fault is the extrinsic fault.

Achieving ultra-strong and ductile CoNi-based FCC multi-principal element alloys via alloying with refractory Mo and W

FCC single-phase multi-principal element alloys (MPEAs) often suffer from insufficient strength, which limits their wide applications. Here a CoNi-based FCC MPEA (Co44Ni44Mo9W3, at. %) was designed by alloying with refractory Mo and W. The Co44Ni44Mo9W3 alloy possesses an ultrahigh yield strength of ∼1050 MPa and good ductility of ∼30 % at ambient temperature. The excellent mechanical properties were correlated to its superior lattice friction stress and grain boundary strengthening compared to most reported FCC MPEAs and conventional alloys. By density-functional theory calculations, such extraordinary mechanical properties were further attributed to its high lattice distortion, low intrinsic stacking fault energy and high unstable stacking fault energy due to the addition of Mo and W. Transmission electron microscopy investigations revealed that the high strength-ductility combination arises from the synergistic effect of planar slip bands, stacking faults and deformation nano-twins.

Forest hardening and Hirth lock during grinding of copper evidenced by MD simulations

Through the use of molecular dynamics (MD) simulation, grinding process of a single crystal copper with two scratch configurations (i) near spacing (NS) between adjacent scratches, and (ii) far spacing (FS) between adjacent scratches were simulated and compared to the control samplei.e.,a single scratch (SS). FS configuration revealed the highest material removal, whereas NS configuration showed that the material removal is affected by various types of defects in the sub-surface which include FCC intrinsic stacking fault, a coherent twin boundary next to an intrinsic stacking fault and two adjacent intrinsic stacking faults. The formation of a Stair-rod 1/6 〈110〉 due to the reaction between two Shockley partial dislocations 1/6 〈112〉 was seen as a distinct feature of the NS configuration which forms the onset of hardening.

A combined EBSD and machine learning study of predicting deformation twinning in BCC Fe81Ga19 alloy

Compared to face-centered cubic (FCC) metals, body-centered cubic (BCC) metals demonstrate more intricate deformation behavior and microstructural characteristics because of the influence of intrinsic unstable stacking faults and defect structure. To investigate the unique phenomenon of deformation twinning in Fe81Ga19 alloy with BCC structure at moderate temperature and low strain rate, four twin nucleation models were constructed using a combination of electron backscatter diffraction (EBSD) technique and machine learning. These models are aimed to predict twin nucleation of grains and explore the relationship between twin nucleation and microstructural attributes. The study revealed that amongst the 235 grains analyzed during in-situ compressive deformation, 32 grains experienced twinning. Nine attributes influencing twin nucleation were selected and ranked according to their importance. Grain area, average image quality, neighboring grain count, and the Schmid factor of twinning systems exhibited significant influence on twin nucleation. Decision tree and tree ensemble (XGBoost) achieved 94% and 96% accuracy, respectively, on the test set, showcasing robust generalization capability. Due to the optimization of hyperparameters, support vector machine (SVM) and artificial neural network (ANN) exhibited outstanding performance. The ANN model achieved an accuracy of 97% and an F1 score of 0.91 on the test set, while the SVM model achieved an accuracy of 98% and an F1 score of 0.94, indicating superior performance. Furthermore, the study revealed that the twinning stress in Fe81Ga19 alloy exhibited weak responsiveness to temperature during deformation, and the intrinsic factors of grains were identified as crucial factors influencing twin nucleation.

Short-range ordering mechanics in FCC materials

Short-range order (SRO) has a crucial impact on the mechanical strength of metallic alloys. Recent atomistic investigations defined an average SRO and attempted to correlate it with the yield strength. We propose that the local change in SRO upon slip advance must dictate the strengthening, and we elaborate the methodology to establish the “SRO change” on a slip plane considering the Wigner-Seitz cell. The model captures the variation of lattice resistance (Critical Resolved Shear Stress; CRSS) in the crystal as the SRO changes depending on the probability of neighboring atoms. The methodology was applied to Ni-V binary alloys for a wide range of compositions and stacking fault widths. Dislocation core widths were determined as a function of SRO and energy parameters (unstable and intrinsic stacking fault energies; γus, γisf). The complex variation of CRSS with compositional variations shows good agreement with limited experimental results. The compositions corresponding to the transition from partial to full dislocations at higher vanadium contents are found depending on the SRO and the intrinsic energy levels.

Dependence of deformation mechanisms on micro-pores in the fourth-generation single crystal superalloy during out-of-phase thermal-mechanical fatigue

The high-temperature and long-term solution heat treatments of fourth-generation single crystal (SX) superalloys have sparked an increasing number of homogenization pores in the alloy. Out-of-phase (OP) thermal–mechanical fatigue (TMF) experiments were conducted to elucidate the effect of pores on the fatigue behaviors and deformation mechanisms. The results showed that the plastic deformation was mainly concentrated upon γ matrix in regions away from pores, more specifically, intrinsic stacking fault (ISF) and extrinsic stacking fault (ESF) as well as dislocation cross-slipping were detected. In areas near micro-pores, however, the γ/γ′-structure was subjected to severe twinning shearing. High-resolution observation illustrated that the amount of ESF and a/6 〈112〉 twinning dislocations had remarkably increased. Besides, the partial dislocations were more prone to move in different slipping systems at these regions with considerable stress concentration. These factors collectively enabled the formation of deformation twins from the edge of micro-pores. Moreover, the twinning nucleation could occur near both H-pores and S-pores, while the increasing pore size would enhance the nucleation and propagation of pore-induced twins by a large margin. These findings reported have provided important guidance for the future application of the fourth-generation SX superalloys.

Revealing complex planar defects and their interactions at atomic resolution in a Laves phase containing Co-base superalloy

In this study, extensive formation of different planar faults and substantial fault intersections were found and examined at atomic resolution in the cubic ZrCo2 based Laves phase formed in a Co-9Al-9W-2Zr alloy after annealing at 900 ºC. By analysis, it is confirmed that synchro-shear is the mechanism to form twins, extrinsic stacking faults and intrinsic stacking faults in the cubic Laves phase with three equivalent Burgers vectors 1/6<112> Shockley partials available for synchro-shear. But atomic shuffling in the dislocation core region is also required to fit the atomic arrangement in perfect and faulted lattices. Novel types of planar faults featured by a missing single Co-atom layer and a constant gap width are for the first time observed. The missing single layer is believed to be induced by the vacancy segregation on planes, especially on {111} twinning planes. The atoms below the gap shuffle into their new positions with different shuffling vectors to form different types of planar faults. Most intersections in the heat-treated specimen are quite gentle. Only occasionally, incident planar faults pass the barrier planar faults, and W is enriched in the intersection regions. In cases of more complex intersections, synchro-shear and atomic shuffling occur in extrinsic and intrinsic stacking faults as well as in twins, and vacancy diffusion and atom shuffling occur in the novel types of planar faults to form the observed intersection configurations. The extensive formation of planar faults might significantly alter the mechanical behavior and also influence functional properties of Laves phase containing materials.

Anisotropic deformation mechanisms in textured nanotwined Cu under shock loading

In this paper, a model of nanotwinted Cu with [111] texture is constructed, and the anisotropic response mechanism and microscopic deformation mechanism are investigated under shock loading with the impact velocity of 0.5 km/s, 1 km/s, 1.5 km/s and 2.5 km/s from 0°, 45° and 90° loading directions by molecular dynamics simulations. It is found that the textured nanotwinned Cu has obvious anisotropy under 0.5 km/s, 1 km/s and 1.5 km/s, and there are secondary yield and two-stage plastic relaxation under 90° loading. However, the anisotropic response is not obvious under hypervelocity impact (2.5 km/s). Under the shock loading, a large number of intrinsic stacking fault appear, among which the content of 0° and 45° is relatively higher than 90° shock. Under different loading direction, the behavior of multiple twins and dislocation motion varies. Specifically, at 0° loading, multiple twins emit dislocation motion simultaneously, leading to the formation of a large number of faults. At 90° loading, the twin boundary effectively blocks dislocation motion, resulting in less deformation. Under 45° loading, the deformation mode differs from both 0° and 90° loading, causing the original twin boundary to be destroyed and recombined into a new twin lamellar layer with varying thickness. This process creates intrinsic layer faults as well.

High Temperature Tensile and Compressive Behaviors of Nanostructured Polycrystalline AlCoCrFeNi High Entropy Alloy: A Molecular Dynamics Study

Abstract Molecular dynamics studies were performed to assess tensile and compressive behaviors at high temperatures up to 1200 °C for nanostructured polycrystalline AlCoCrFeNi high entropy alloy (HEA). As the temperature increased, the tensile yield stress, tensile/compressive ultimate strengths, and elastic modulus decreased, whereas the compressive yield stress remained constant. The temperature dependence of the phase structures (face-centered cubic (FCC) and hexagonal close-packed (HCP)) showed notable features between tension and compression. The HEA underwent FCC → HCP phase transformation when strained under both tension and compression. The evolution of the intrinsic stacking faults (ISFs) and extrinsic stacking faults (ESFs), which underwent FCC → HCP phase transformation, was observed. During compression, the ISFs → ESFs transition produced parallel twins. The evolution of mean dislocation length for the perfect, Shockley, and stair-rod partial dislocations was observed. Changes in the Shockley and stair-rod partial dislocations were observed after experiencing strain. The temperature dependence of the Shockley partial dislocation was high, whereas the stair-rod partial dislocation exhibited low-temperature dependence. From the simulation results, the structural usage of nanostructured polycrystalline AlCoCrFeNi HEA at elevated temperatures is recommended.

Ultrahigh grain boundary strengthening ability of VCoNi medium entropy alloy

Tensile tests of VCoNi medium-entropy alloy with different grain sizes were conducted at 77 K, 194 K and 293 K. The results show that the grain boundary strengthening ability characterized by the Hall-Petch slope of the alloy increases with decreasing temperature and is evidently higher than that of FCC pure metals, CuAl alloys, and some common FCC high-entropy alloys/medium-entropy alloys at room temperature. A new model correlated to intrinsic stacking fault energy and lattice friction stress is derived to describe the high grain boundary strengthening ability. The high grain boundary strengthening ability and its temperature dependence of the VCoNi medium-entropy alloy are mainly attributed to the high lattice friction stress of the VCoNi alloy. The findings provide a possibility for designing the single-phase FCC medium-entropy alloy with ultra-high yield strength and considerable plasticity.

Ultrahigh intermediate-temperature strength and good tensile plasticity in chemically complex intermetallic alloys via lamellar architectures

As a newly emerged class of materials, chemically complex intermetallic alloys (CCIMAs) with exceptional thermal and mechanical properties are a promising candidate for high-temperature structural use. However, serious intergranular embrittlement at intermediate temperatures (600∼800 °C) is frequently found in those CCIMAs, obstructing their large-scale engineering applications. In this study, through deliberately tailoring thermomechanical processing, we designed a lamellar-structured (LS) L12-type Co-Ni-Al-Ti-Ta-Nb-B-based CCIMA that effectively overcomes this critical issue. The LS-CCIMA exhibits an excellent yield strength (YS) of ∼1.0 GPa with a large tensile elongation of ∼17% at room temperature. More prominently, it also presents an anomalous YS of ∼1.2 GPa combined with an acceptable tensile elongation of ∼10% at intermediate temperatures ranging from 600 to 800 °C, outperforming those of many other simple ordered intermetallics and conventional superalloys. Such superb immediate-temperature strengths primarily originate from the high anti-phase boundary energy caused by the addition of multiple alloying elements (Ti, Ta, and Nb) and the pile-ups of geometrically necessary dislocations. Moreover, we attribute the acceptable tensile plasticity to the increased plastic deformation capacities from the activation of various deformation-induced substructures (e.g., dislocation pairs at 600 °C and superlattice intrinsic stacking faults at 800 °C) and the inhibiting mechanisms of the lamellar structures on oxygen-induced grain boundary damage and microcrack's propagation. This work provides a new pathway for the innovative design of strong-yet-ductile heat-resistant CCIMAs.

Temperature dependence of generalized stacking fault free energy profiles and dissociation mechanisms of slip systems in Mg

A machine learning-derived interatomic potential coupled with the Projected Average Force Integrator (PAFI) method was utilized to examine the temperature-dependent Generalized Stacking Fault Free Energies (GSFFE) profiles across all slip systems in Mg, fully accounting for anharmonic thermal vibrations. Intrinsic Stacking Fault Free Energies (ISFFE) and Unstable Stacking Fault Free Energies (USFFE) were assessed for temperatures ranging from 0 K to 800 K, marginally below the anticipated Mg melting temperature of approximately 900 K. The findings revealed that anharmonic vibrational effects are comparable to harmonic effects for Basal slip above 200 K, approximately half the Debye temperature. Anharmonic contributions were minimal for non-basal slip systems. The USFFEs of Pyramidal-I and II slip systems are predicted to crossover around 385 K due to vibrational effects. Marginally higher ISFFEs in Pyramidal-I across all temperatures also indicate possible cross-slip or SF transformation between the systems. A comprehensive examination of slip trajectories and atomic displacements unveiled intricate atomic displacements across all Mg slip systems and complex shuffle mechanisms for non-basal slips. In addition to corroborating previously documented dislocation dissociation mechanisms for Basal, Pyramidal-〈a〉, and Pyramidal-I slips, a new dissociation mechanism was discerned for dislocation slip in Pyramidal-II. This study offers crucial insights into the temperature-dependent behavior of SFFEs in Mg, the significance of intrinsic anharmonic thermal contributions to Basal slip, and the intricate atomic displacements and dissociation mechanisms of hexagonal close packed slip systems.

Anisotropic plasticity mechanisms in a newly synthesised high entropy alloy investigated using atomic simulations and nanoindentation experiments

This work used atomic simulations and nanoindentation experiments to investigate hardness, modulus alongside sub-surface crystal defects and dislocation mediated plasticity mechanisms leading to anisotropic pile up and local entropy variation in high entropy alloys. The experimental campaign began from Thermo-Calc phase prediction of Ni25Cu18.75Fe25Co25Al6.25 HEA which followed experimental synthesis of the material using arc melting method and experimental nanoindentation using a Berkovich indenter under load-controlled conditions. Through MD simulations, the value of hf/hmax in monocrystalline HEA was consistently found to be larger than 0.7 which suggested pile-up behaviour to dominate and sink-in behaviour to be unlikely. In the case of (110) and polycrystalline HEA substrates, the elastic work in the indentation hysteresis loop was seen to be larger than the (100) and the (111) orientations which explains that the (110) orientation substrate showed least elastic modulus and hardness while the (111) monocrystalline HEA showed the highest elastic modulus and hardness. From the simulations, a “lasso” type loop on the (110) orientation and cross-over of shear loops on the other orientations accompanied by dislocations of type 1/6 < 112 > (Shockley), 1/2 < 110 > (perfect), 1/3 < 001 > (Hirth), 1/6 < 110 > (Stair rod) and 1/3 < 111 > (Frank partials) were seen to manifest an early avalanche of competing plasticity events. The defects accompanying these dislocations in the sub-surface were identified to be FCC intrinsic stacking faults (ISF), adjacent intrinsic stacking faults (quad faults), coherent ∑3 twin boundary and a coherent twin boundary next to an intrinsic stacking fault (triple fault). The EBSD analysis applied to the MD data showed that the (210) orientation and the< 110 > family of directions were seemed to be preferable to plastically deform the FCC phased Ni25Cu18.75Fe25Co25Al6.25 HEA.

Selective Formation of Monodisperse Right Trigonal-Bipyramidal and Cube-Shaped CdSe Nanocrystals: Stacking Faults and Facet-Ligand Pairing.

Formation of monodisperse right trigonal-bipyramidal (rTriBP) and cube-shaped CdSe nanocrystals─both being encased with six (100) facets─is found to be dictated by type of stacking faults along the (111) direction of the zinc-blende structure and an ideal facet-ligand pairing for the (100) facets. During growth with little kinetic overdriving, seeds with single twin boundary (TB) and single intrinsic stacking fault (ISF) grow into rTriBP and cube-shaped nanocrystals, respectively, through two consecutive stages. During the facet-formation stage, each seed would grow rapidly into the smallest faceted one to contain the ∼3 nm seed, with cube-shaped ones growing much faster than rTriBP ones because of the stacking-fault-dependent seed location in the final faceted nanocrystals. In the following facet-growth stage, cube-shaped nanocrystals also grow faster, presumably due to the highly reactive stacking fault edges. Consistent with this hypothesis, growth of rTriBP nanocrystals can become faster than that of cube-shaped ones by intentionally introducing additional intrinsic stacking fault(s) in the seeds. Cube-shaped and rTriBP CdSe nanocrystals exhibit distinctive optical properties, representing two classes of optical materials.