Deformation Twinning Mechanism Research Articles

Study on the hot deep drawing performance and microstructural evolution of AZ91D magnesium alloy sheets

Magnesium alloys are attractive because of their low density and high specific strength and stiffness; however, secondary phase strengthening and the low number of slip systems lower formability especially for the strongest magnesium alloys such as AZ91D. The formability of AZ91D sheets is investigated through orthogonal experiments, which show that AZ91D has no deep drawing properties at room temperature and that plastic forming is only possible under hot forming conditions. When the temperature is increased from room temperature to 400 °C, the limiting drawing ratio of the sheet increases significantly to 1.91, and the thinning rate reaches a minimum value of 6% at approximately 400 °C because of the improved plastic flowability. The optimum process parameters for hot deep drawing are a temperature of 400 °C, blank holder force of 5 kN, and punch speed of 3 mm s−1. The microstructural evolution shows that plastic deformation of the AZ91D magnesium alloy at high temperatures is markedly accommodated by twinning and that, with heating, this twinning deformation mechanism is activated and overcomes the hindrances of the reinforcing phases to achieve better forming properties.

Cross-slip of extended dislocations and secondary deformation twinning in a high-Mn TWIP steel

Twinning-induced plasticity (TWIP) steels have become important materials in industry owing to the good combination of strength and ductility and high strain hardening rate. The excellent mechanical properties are highly related to the glide of dislocation and deformation twinning. However, the cross-slip behavior of the extended dislocation and the mechanism of deformation twinning are still controversial. Here, the partial dislocation motion and austenite twinning of a high-Mn steel at the early stage of deformation were investigated using in-situ tensile transmission electron microscope (TEM) technique. Results show that a large number of plane glide and cross-slip of extended dislocations can occur at the early stage of deformation. Extended dislocation nodes can be formed as a result of the reaction between adjacent extended dislocations on the same glide plane. In-situ tensile TEM experiments confirm two cross-slip models of partial dislocation: (1) the Friedel-Escaig model, cross-slip based on constriction of extend dislocation and re-dissociation; (2) the Fleischer model, cross-slip involving Lomer-Cottrell dislocation. Based on experimental results and energy calculations, it can be confirmed that the formation mechanism of austenite primary deformation twin induced by partial dislocations is different from that of secondary deformation twin. Grain boundary emits partial dislocation into the grain to form stable stacking fault which induces austenite primary deformation twin. The formation of secondary deformation twin is related to cross-slip of extended dislocation. Only the cross-slip of an extended dislocation containing a 90° partial dislocation can induce the formation of secondary deformation twin by introducing the Frank partial dislocation.

A Review of Deformation Mechanisms, Compositional Design, and Development of Titanium Alloys with Transformation-Induced Plasticity and Twinning-Induced Plasticity Effects

Metastable β-type Ti alloys that undergo stress-induced martensitic transformation and/or deformation twinning mechanisms have the potential to simultaneously enhance strength and ductility through the transformation-induced plasticity effect (TRIP) and twinning-induced plasticity (TWIP) effect. These TRIP/TWIP Ti alloys represent a new generation of strain hardenable Ti alloys, holding great promise for structural applications. Nonetheless, the relatively low yield strength is the main factor limiting the practical applications of TRIP/TWIP Ti alloys. The intricate interplay among chemical compositions, deformation mechanisms, and mechanical properties in TRIP/TWIP Ti alloys poses a challenge for the development of new TRIP/TWIP Ti alloys. This review delves into the understanding of deformation mechanisms and strain hardening behavior of TRIP/TWIP Ti alloys and summarizes the role of β phase stability, α″ martensite, α′ martensite, and ω phase on the TRIP/TWIP effects. This is followed by the introduction of compositional design strategies that empower the precise design of new TRIP/TWIP Ti alloys through multi-element alloying. Then, the recent development of TRIP/TWIP Ti alloys and the strengthening strategies to enhance their yield strength while preserving high-strain hardening capability are summarized. Finally, future prospects and suggestions for the continued design and development of high-performance TRIP/TWIP Ti alloys are highlighted.

Molecular dynamics study of diffusionless phase transformations in HMX: β-HMX twinning and β-ɛ phase transition

We use molecular dynamics to study the mechanism of deformation twinning of β-1,3,5,7-tetranitro-1,3,5,7-tetrazocane (β-HMX) in the P21/n space group setting for the twin system specified by K1=(101), η1=[101¯], K2=(101¯), and η2=[101] at T=1 and 300 K. Twinning of a single perfect crystal was induced by imposing increasing stress. The following three forms of stress were considered: uniaxial compression along [001], shear stress in the K1 plane along the η1 direction, and shear stress in the K2 plane along the η2 direction. In all cases, the crystal transforms to its twin by the same mechanism: as the stress increases, the a and c lattice parameters become, respectively, longer and shorter; soon after the magnitude of a exceeds that of c the system undergoes a quick phase-transition-like transformation. This transformation can be approximately separated into two stages: glide of the essentially intact {101} crystal planes along ⟨101¯⟩ crystal directions followed by rotations of all HMX molecules accompanied by N-NO2 and CH2 group rearrangements. The overall process corresponds to a military transformation. If uniaxial compression along [001] is applied to a β-HMX crystal which is already subject to a hydrostatic pressure ≳10 GPa, the transformation described above proceeds through the crystal-plane gliding stage but only minor molecular rearrangements occurs. This results in a high-pressure phase of HMX which belongs to the P21/n space group. The coexistence curve for this high-pressure phase and β-HMX is constructed using the harmonic approximation for the crystal Hamiltonians.

Deformation mechanism and mechanical properties of a CoCrFeNi high-entropy alloy via room-temperature rolling, cryorolling, and asymmetric cryorolling

The deformation mechanism and mechanical properties of CoCrFeNi high-entropy alloys (HEA) processed by room-temperature rolling (RTR), cryorolling (CR) and asymmetric cryorolling (ACR) were studied. As the rolling reduction increased from 20 % to 80 %, a transition in the dislocation and twinning deformation mechanisms occurred. The cryogenic environment in rolling process can promote the activation of the twins in the HEA at a lower strain, improving the deformation efficiency. The introduction of differential speed ratio led to the finer micro-shear bands with a more uniform distribution, so that the grains were refined to a greater extent. In the ACR HEA after large deformation, the stacking faults accumulated inside the deformation twins further refined the twins along the direction of long axis. This increased the strength of ACR HEA together with Lomer-Cottrell locks and FCC/HCP boundaries, reaching the maximum value compared with CR and RTR. The ultimate tensile strength exceeded 1450 MPa, and the yield strength of ACR HEA increased by nearly 1200 MPa compared to the initial material, while it still maintained the same elongation as RTR and CR. ACR showed the best strength-ductility combination during the whole rolling deformation process. The microstructure evolution and the strengthening contribution mechanism of different rolling were discussed and calculated, and the specific mechanism of how the introduced differential speed ratio further improves the strength-ductility synergistic strengthening in CR was clarified.

Mechanical properties of an Al10Nb20Ta15Ti30V5Zr20 A2/B2 refractory superalloy and its constituent phases

Refractory superalloys (RSAs) are a new class of high-temperature structural materials consisting of a nanometer-sized mixture of two coherent A2 (disordered BCC) and B2 (ordered) phases. While these RSAs typically exhibit superior mechanical properties at elevated temperatures, they suffer from limited plasticity at lower temperatures, often attributed to the continuous ordered B2 matrix in these two-phase alloys. To knowledgably control the microstructure and mechanical properties of these alloys, the properties of the constitutive phases in RSAs should be known. Currently this information is absent. In the present work, the microstructure and mechanical properties (at 20–1200 °C) of a candidate Al0.5NbTa0.8Ti1.5V0.2Zr RSA and two single-phase alloys, which compositions correspond to the compositions of the A2 and B2 phases in the selected RSA, are reported. Interestingly, and quite unexpectedly, the intrinsic mechanical behavior of the constituent A2 and B2 phases, deciphered for the first time in the present study, revealed that they are softer and substantially more ductile than the parent two-phase RSA. Furthermore, the ordered B2 phase exhibits a deformation twinning mechanism leading to twinning induced plasticity (TWIP) coupled with a low APB energy and activity of 1/2<111> dislocations. These mechanisms are thought to be responsible for the high plasticity of the ordered B2 phase. The results presented here question the current assumption that the ordered B2 phase in these RSAs has limited plasticity and strongly indicate that the nature of the B2/A2 interface boundaries and coherency strengthening play an important role in controlling the mechanical properties of the two-phase RSA.

Mapping plastic deformation mechanisms in AZ31 magnesium alloy at the nanoscale

By coupling an improved speckle patterning method enabling high resolution digital image correlation (HRDIC) at nanoscale strain resolution (146 nm) with a scanning electron microscope allowing autonomous experimental control and image acquisition during in situ tensile straining, we have mapped the plastic deformation in AZ31 Mg alloy at the grain scale to significant plastic strains for the first time. Complemented by electron backscattered diffraction to reveal the underlying grain orientations this has delineated the activation of intragranular extensional twinning, slip, and intergranular deformation mechanisms. Using a new grain boundary (GB) strain mapping algorithm, displacements tangential to the GBs as large as 140 nm at 2% plastic strain have been recorded. Critically, this occurs not so much by sliding at the boundary but by slip over a mantle region extending 450 nm from the GBs. Within the grain interiors, dislocation activity is much less, taking place mainly by basal slip. Reverse dislocation slip and detwinning were found to occur on unloading emphasising the need for in situ measurements. The proposed methodologies (gold speckle pattern remodelling, automated in situ HRDIC tensile testing and GBS activity analysis) have the potential to characterise the real-time deformation behaviour of a wide range of engineering alloys at the grain scale at room and elevated temperatures.

The evolution of deformation twinning microstructures in random face-centered cubic solid solutions

The varied atomic arrangements in face-centered cubic (FCC) solid solutions introduce atomic-scale fluctuations to their energy landscapes that influence the operation of dislocation-mediated deformation mechanisms. These effects are particularly pronounced in concentrated systems, which are of considerable interest to the community. Here, we examine the effect of local fluctuations in planar fault energies on the evolution of deformation twinning microstructures in randomly arranged FCC solid solutions. Our approach leverages the kinetic Monte Carlo (kMC) method to provide kinetically weighted predictions for competition between two processes: deformation twin nucleation and deformation twin thickening. The kinetic barriers underpinning each process are drawn from the statistics of planar fault energies, which are locally sampled using molecular statics methods. kMC results show an increase in the fault number densities of solid solutions relative to a homogenized reference, which is found to be driven by the fluctuations in planar fault energies. Based on kMC relations, an effective barrier model is derived to predict the competition between deformation twinning nucleation and thickening processes under a fluctuating planar fault energy landscape. A key result from this model is a measurement of the length-scale over which the influence of local fluctuations in planar fault energies diminish and nucleation/thickening-dominated behaviors converge to bulk predictions. More broadly, the tools developed in this study enable examination of the influence of chemistry and length-scale on the evolution of deformation twinning mechanisms in FCC solid solutions.

A Brief Review on Theoretical Models of Deformation Twinning at Locally Distorted Grain Boundaries

A brief review of the theoretical models which describe mechanisms of deformation twinning in nanocrystalline and ultrafine-grained materials is presented. In the framework of the models, formation of nanoscale deformation twins occurs at locally distorted grain boundaries that contain fragments being rich with grain boundary dislocations due to preceding severe plastic deformation processes. Within the review, mechanisms of deformation twinning at locally distorted grain boundaries represent (a) the consequent emission of partial dislocation; (b) the cooperative emission of partial dislocations; and (c) the generation of multiplane nanoscale shear.

Theory of transformation-mediated twinning.

High-density and nanosized deformation twins in face-centered cubic (fcc) materials can effectively improve the combination of strength and ductility. However, the microscopic dislocation mechanisms enabling a high twinnability remain elusive. Twinning usually occurs via continuous nucleation and gliding of twinning partial dislocations on consecutive close-packed atomic planes. Here we unveil a completely different twinning mechanism being active in metastable fcc materials. The transformation-mediated twinning (TMT) is featured by a preceding displacive transformation from the fcc phase to the hexagonal close-packed (hcp) one, followed by a second-step transformation from the hcp phase to the fcc twin. The nucleation of the intermediate hcp phase is driven by the thermodynamic instability and the negative stacking fault energy of the metastable fcc phase. The intermediate hcp structure is characterized by the easy slips of Shockley partial dislocations on the basal planes, which leads to both fcc and fcc twin platelets during deformation, creating more twin boundaries and further enhancing the prosperity of twins. The disclosed fundamental understanding of the complex dislocation mechanism of deformation twinning in metastable alloys paves the road to design novel materials with outstanding mechanical properties.

Deformation-softening behaviors of high-strength and high-toughness steels used for rock bolts

In deep ground engineering, the use of high-strength and high-toughness steels for rock bolt can significantly improve its energy absorption capacity. However, the mechanisms and effects of rock loading conditions on this kind of high energy-absorbing steel for rock bolt remain immature. In this study, taking Muzhailing highway tunnel as the background, physically based crystal plasticity simulations were performed to understand the effect of rock loading rate and pretension on the deformation behaviors of twinning induced plasticity (TWIP) steel used for rock bolt. The material physical connecting to the underlying microscopic mechanisms of dislocation glide and deformation twinning were incorporated in numerical modeling. The rock loading conditions were mimicked by the real-time field monitoring data of the NPR bolt/cable equipment installed on the tunnel surrounding rock surface. The results indicate that the bolt rod exhibits pronounced deformation-softening behavior with decrease of the loading rate. There is also a sound deformation-relaxation phenomenon induced by the dramatic decrease of loading rate after pre-tensioning. The high pretension (>600 MPa or 224 kN) can help bolt rod steel resist deformation-softening behavior, especially at low loading rate (<10−1 MPa/s or 10−2 kN/s). The loading rate was found to be a significant factor affecting deformation-softening behavior while the pretension was found to be the major parameter accounting for the deformation-relaxation scenario. The results provide the theoretical basis and technical support for practical applications.

Microstructure evolution, tensile properties and deformation mechanism of Fe-6.5 wt.% Si steel doped with yttrium

Fe-6.5 wt.% Si steels doped with different contents of yttrium were fabricated by vacuum induction melting (VIM), forging, hot rolling and annealing. The microstructure evolution, ordered degree, tensile properties and deformation mechanism were investigated. The results showed that the amount of high-melting precipitates enriched in yttrium increased with the content of yttrium increasing. The solidification, hot rolling and annealing microstructures were refined due to the elevated nucleation rate and the effect of microstructure heredity. The steel containing the highest content of yttrium exhibited the highest tensile ductility and the most developed dimple patterns. Deformation bands appeared in the tensile microstructures at 400 °C. Most of the deformation bands in the steels without yttrium and doped with the intermediate content of yttrium possessed the special {112}<111> twinning mis-orientation with the matrix, implying the cooperation of the deformation twinning mechanism. Nevertheless, the smooth stress-strain curves and the absence of the deformation bands indicate the predominance of the dislocation sliding mechanism during the tensile test at 500 °C. The present work manifested that the doping of yttrium in Fe-6.5 wt.% Si steel could be utilized to raise the intermediate-temperature tensile ductility through purifying the matrix, refining the microstructure and reducing the ordered degree.

Achieving maximum strength-ductility combination in fine-grained Cu-Zn alloy via detwinning and twinning deformation mechanisms

• The FG Cu-30%Zn was prepared by ECAP and subsequent annealing. • The optimal strength and ductility combination was achieved. • Microstructural evolutions during tensile deformation were revealed by EBSD. • Deformation-induced detwinning of pre-existing ultrafine twin lamellae occurred. • Grain orientation with< 111 > // TD facilitates deformation twinning. Although deformation twinning has been demonstrated to improve the strain hardening and ductility of metals and alloys with low stacking fault energies (SFEs), the optimum grain size for maximum strength-ductility combination still needs to explore. In this work, we selected Cu- 30 wt% Zn alloy with extremely low SFE (7 mJm -2 ) acting as a model material. Specifically, Cu- 30 wt% Zn samples with different grain sizes ranging from 628 nm to 30.6 µm were prepared by equal-channel-angular pressing (ECAP) and subsequent annealing. Tensile test revealed that the maximum strength-ductility combination (ultimate tensile strength of 565 MPa and ductility of 20%) corresponds to a mean grain size of 3.8 µm. Electron backscatter diffraction (EBSD) indicated that pre-existing annealing twins in fine-grained Cu- 30 wt% Zn alloy annihilated at the initial deformation stage (<10% tensile strain) via detwinning of thin twin lamellae (<1 µm) and conversion of twin boundaries of thick twin lamellae (>1 µm) into conventional high-angle grain boundaries. In the later stage of deformation (>10% strain), deformation twinning occurred in grains with<111>orientation parallel to tensile direction, suggesting the combination of both detwinning and twinning deformations caused the maximum strength and ductility synergy. Our findings provide insights into optimization of strength and ductility of metals with low SFEs and detwinning-twinning deformation mechanisms.

Evolution of deformation twinning mechanisms in magnesium from low to high strain rates

We present a systematic investigation of {101¯2} extension twinning mechanism in single crystal magnesium micropillars deformed over seven orders of magnitude of strain rate, from 10–4 to 500 s−1, revealing how the accommodation of newly formed twins evolves with and depends on the kinetic compatibility of interfacial processes when high deformation rates are imposed. By combination of post-mortem 3D Electron Backscattered Diffraction, Transmission Kikuchi Diffraction and Transmission Electron Microscopy techniques, this work unveils the progressive evolution of the accommodating twin mechanisms from low to high strain rate, correlating differences in mechanical behavior with differences in twin crystallography. Away from quasi–static conditions, simple considerations of twinning shear do not suffice to describe unconventional twin morphologies, requiring the competition between newly activated dislocations and lattice distortions for allowing the evolution of the twin boundary along non–invariant twin planes. Under shock compressions, the basal/prismatic transformation establishing a lattice misorientation of 90° entirely governs the parent → twin conversion. The results illustrated here confirm that some of the recent interpretations deduced by particular twin morphologies are not universally valid and that deformation twinning is not only stress- but also strongly time–controlled.

Newer insights into the discrimination of pole mechanisms of twinning in a face-centered cubic high-Mn steel

Mechanism of deformation twinning during tensile straining of an Fe-0.013C-29Mn-2.4Al steel is reported. Transmission electron microscopy examinations revealed that twinning occurred through a modified pole mechanism proposed by Niewczas and Saada. In this mechanism, twinning Shockley partial dislocations and their stacking faults are formed through interaction among the prismatic dislocations with Frank partials of the Frank dipoles. The activation of the observed pole mechanism was explained through the length of the sessile Frank partials.

Texture effects and rate-dependent behaviors of notched magnesium bars

It is known that the mechanical behavior of many hexagonal close-packed (HCP) materials is sensitive to textural variations. In the technologically important magnesium (Mg) alloys, the anisotropic plastic behaviors and their dependence on different processing-induced textures have been reasonably well studied under relatively simple stress states. However, a detailed assessment of the rate-dependent microstructure–property linkages under triaxial stress states is relatively less explored. In this work, we present a computational investigation of the role played by strain rate, texture, and stress state in the macro–micro mechanical characteristics of Mg alloys. Three-dimensional boundary value problems using smooth and notched round bar geometries subjected to tensile deformation are modeled using a crystal plasticity framework that includes slip and twinning deformation mechanisms. We examine the responses over six orders of magnitudes of strain rates, for three different initial textures that span a broad spectrum of textural strengths, and three specimen geometries that induce varying levels of stress triaxiality. The simulations reveal a remarkable transition of the tensile response from a conventional power-law type to a sigmoidal type for certain combinations of textures, stress triaxiality levels, and strain rates. While the stress response is strongly rate-dependent, the macroscopic deformation anisotropy has a negligible effect from strain rate compared to the role of the stress state and texture. We discuss the potential implications of these observations on the rate-dependent failure of Mg alloys.

Unique transition of yielding mechanism and unexpected activation of deformation twinning in ultrafine grained Fe-31Mn-3Al-3Si alloy

Tensile mechanical properties of fully recrystallized TWIP steel specimens having various grain sizes (d) ranging from 0.79 μm to 85.6 μm were investigated. It was confirmed that the UFG specimens having the mean grain sizes of 1.5 μm or smaller abnormally showed discontinuous yielding characterized by a clear yield-drop while the specimens having grain sizes larger than 2.4 μm showed normal continuous yielding. In-situ synchrotron radiation XRD showed dislocation density around yield-drop in the UFG specimen quickly increased. ECCI observations revealed the nucleation of deformation twins and stacking faults from grain boundaries in the UFG specimen around yielding. Although it had been conventionally reported that the grain refinement suppresses deformation twinning in FCC metals and alloys, the number density of deformation twins in the 0.79 μm grain-sized specimen was much higher than that in the specimens with grain sizes of 4.5 μm and 15.4 μm. The unusual change of yielding behavior from continuous to discontinuous manner by grain refinement could be understood on the basis of limited number of free dislocations in each ultrafine grain. The results indicated that the scarcity of free dislocations in the recrystallized UFG specimens changed the deformation and twinning mechanisms in the TWIP steel.

Phase field study on the microscopic mechanism of grain size dependent cyclic degradation of super-elasticity and shape memory effect in nano-polycrystalline NiTi alloys

A non-isothermal and crystal plasticity based phase field model was established by incorporating various inelastic deformation mechanisms, i.e., martensite transformation, martensite reorientation, the austenitic plasticity caused by dislocation slipping, and the martensitic plasticity caused by dislocation slipping and deformation twinning, in which the deformation twinning mechanism was newly considered. The grain size (GS) effect of martensite transformation and reorientation was characterized by introducing a grain boundary energy, and the GS dependent resistances of dislocation slipping and deformation twinning were considered in the phase field simulation for the first time to describe the GS dependence of plastic deformation. Through a series of phase field simulations on the inelastic cyclic deformation of two-dimensional nano-polycrystalline NiTi shape memory alloy (SMA) systems, the microscopic mechanism of the GS dependent cyclic degradation of the super-elasticity (SE), one-way shape memory effect (OWSME) and stress-assisted two-way shape memory effect (SATWSME) of such alloy systems was newly clarified. The simulated results show that with decreasing the GS, the cyclic degradations of the SE and SATWSME are both mitigated gradually, while that of OWSME is aggravated first and then mitigated; when reducing the GS to 10 nm, almost no functional degradation occurs. Further analysis shows that the GS dependence of the cyclic degradations of SE and SATWSME can be attributed to that: as reducing the GS, the plastic deformation resistances gradually increase due to the Hall-Patch effect, and the GS effect of martensite transformation and reorientation reduces the deformation mismatch introduced in the polycrystalline system and leads to the decrease of local internal stress level (i.e., the driving force of plastic deformation decreases) (Factor I) during the cyclic deformation, therefore, the generation and accumulation of plastic deformation are inhibited. However, for the cyclic degradation of OWSME, besides Factor I, since the reorientation-induced plasticity is mainly concentrated on the grain boundaries and their nearby grain interiors, the increase of grain boundary proportion results in the concentration and accumulation of plastic deformation at more locations during the cyclic deformation (Factor II). The competition between Factors I and II makes the cyclic degradation of OWSME show a non-monotonic variation with the decrease of the GS .

Improvement of tension/compression asymmetry for high-performance ZK61 magnesium alloy rod via tailoring deformation parameters: Upsetting-extrusion temperature and upsetting ratio

Upsetting-extrusion (UE) process effectively prepared high-strength ZK61 magnesium alloy rods with tension/compression symmetry. The influence of upsetting-extrusion temperature and upsetting ratio on microstructure was studied. Microstructure evolution in the UE process included two stages: {10–12} tensile twinning significantly divided and refined the coarse grains in upsetting stage, forming a <0001>//ED (extrusion direction) texture component; grain refinement suppressed the inverse occurrence of {10–12} tensile twinning during extrusion stage, promoting multi-slip deformation and dynamic recrystallization (DRX) and leading to the diffused {0001}//ED texture. Lowering temperature and increasing upsetting ratio played positive roles in twinning division and DRX refinement. Both of them were beneficial to basal texture weakening which could effectively inhibit the activation of tensile twinning during compression, and promote basal slip contribution during tension, thus improving tension/compression asymmetry. The relationship between microstructure and tension/compression yield strengths was analyzed by texture-dependent Hall-Petch relationship composed of slip and twinning deformation mechanisms. Only slight weakening of the extruded fiber texture was required for tension/compression symmetry for fine-grained structure; but it was the opposite for coarse-grained structure. Correspondingly, only smaller critical upsetting ratio was needed at lower upsetting temperature.

Operando 3D imaging of defects dynamics of twinned-nanocrystal during catalysis

Using operando Bragg coherent x-ray diffraction imaging, we visualised three-dimensionally a single twinned-gold nanocrystal during the CO oxidation reaction. We describe the defect dynamics process occurring under operating conditions and indicate the correlation between the nucleation of highly strained regions at the surface of the nanocrystal and its catalytic activity. Understanding the twinning deformation mechanism sheds light on the creation of active sites, and could well contribute to the understanding of the catalytic behaviour of other catalysts. With the start-up of 4th generation synchrotron sources, we anticipate that coherent hard x-ray diffraction imaging techniques will play a major role in imaging in situ chemical processes.