Internal Twins Research Articles

Explicit modeling of double twinning in AZ31 using crystal plasticity finite elements for predicting the mechanical fields for twin variant selection and fracture analyses

A number of studies have found that the formation of double twins in low symmetry metals can lead to the onset of strain localizations, leading further to void nucleation and ultimately fracture. This work extends a recently developed three-dimensional crystal plasticity finite element framework [1] to explicitly model kinematics and kinetics of nucleation/formation, propagation, and thickening/growth of a discrete double twin lamella ({101¯1}−{101¯2}) in a magnesium alloy AZ31. With this approach, morphological and crystallographic reorientations as well as the shear transformation strains associated with strain accommodation by the double twinning sequence are modeled during simple compression and tension. The simulations predict that the distribution of local stress-strain fields during formation and growth of primary contraction twin creates the driving force for the formation of a secondary extension twin variant, which is consistent with experimental observations in both compression and tension. In particular, the contraction twin variant (01¯11)[01¯12¯] is predicted to form an internal extension twin variant (011¯2)[01¯11]. Furthermore, the prediction of the underlining crystallographic slip deformation mechanism reveals a substantial activity of basal slip within the contraction portion of the double twin, causing strain localization in its vicinity. Finally, the simulations reveal a gradient in the traction force field across twin-parent interface, suggesting that contraction twin-parent boundaries are weak links in the microstructure where voids can nucleate.

Crystallography Study on the Internal Twinning of Ti–Nb‐Based Shape Memory Alloys

AbstractInternal twinning structure of several Ti–Nb‐based alloys is studied by coupling transmission electron microscopy (TEM) characterization and theoretical calculation based on the phenomenological theory of martensite crystallography (PTMC) and the theory of deformation twinning. The TEM study accords well with the theoretical calculations. Possible candidates that can serve as internal twins were predicted by the PTMC analysis, more specifically, (011) compound twinning for the Ti–16Nb–6Zr–1.2Mg alloy, while (011) compound and {111} type I twinning for the other alloys of the present study. The critical resolved shear stress for the {111} type I twinning is lower than that for the (011) compound twinning, because the magnitude of Burgers vector for the {111} type I twinning is more than six times as high as that for the (011) compound twinning. Thus, the internal twins observed preferably belong to the {111} type I twinning when both of the two twinning systems meet the criterion of PTMC.

″Complete twin″ and ″the Transgranular Twin Line″ of Common Steels

The microstructures of 20, 30, 40, 45, T9 and T11 commercial carbon steels were observed by optical microscopy, scanning electron microscopy and transmission election microscopy. It was found that the internal twins in martensite of common steels is basically a complete twin, and internal twin plane of mutually parallel martensite units in a martensite packet have a same crystallographic orientation, namely, forming ″the transgranular twin line″ or ″interpenetration twinline″. The underlying mechanism of ″complete twin″ and″ the transgranular twin line″ were explored. Both all are to form in the martensite transformation process. In order to promote the growth of martensitic nuclei in initial stage, the associated nucleus with twinning relationship had been formed at a semi-coherent end plane of nuclei which possesses high energy, and forming transformation twins through ″double change″. Therefore, transformation twins are usually all ″complete twin″. In the martensite packet, all the martensite plates of mutual parallelism are to form the associated nucleus with twin interface plane at a side of original ″ twin block″ of the martensite and grow up, so a martensite packet all have the identical ″trans-granular twin line″. In the formation process of ″complete twinning″ and″ trans-granular twin line″, don’t involve the mechanics properties of material, and mainly decide from nucleation work and nucleus growth work. It is thus clear that, having the complete twins in the martensite doesn’t mean that the critical resolved shear stress (CRSS) of slipping exceeded the CRSS of twinning.

Exploring on Stereo Morphology and Habit Plane of Martensite in Ferrous Alloy

Themicrostructures of 20, 30, 40, 45, T9, T10, T11, 20Cr, 40Cr, 45CrNi, 65Mn, 9CrSi, CrWMn commercial carbon and alloy steels were observed by optical microscopy, scanning electron microscopy and transmission election microscopy, meanwhile the habit plane, the substructure and stereo shapeof martensit have been proposed. It was verified that the packet martensite should be divided into packet thin sheet martensite and packet fine martensite, the latter principally can be divided into {557}A packet fine plate martensite, {1 1 1}A packet fine plate martensite and {2 2 5}A packet fine plate martensite according to the carbon content. Space morphology of low carbon packet martensite units is sheet-like but not lath-like; that of martensite medium and high carbon packet martensite units is plate-like. There is no noticeable difference between the substructures of lath and plate martensite. Martensite without internal twin is not certainly lath martensite, while all martensite with numerous internal twins is certainly plate martensite.

Attainment of an exceptionally high strength in low-carbon steel along with modest ductility through a novel heat treatment route

ABSTRACTOver the last few decades, the use of steel (the most significant structural engineering material) is facing a significant challenge due to its replacement by other materials (such as composites) possessing higher strength-to-weight ratio/specific strength. This necessitates further enhancement in the strength of steel. In particular, low-carbon steel, in the annealed condition, suffers from inherent problems of poor strength and discontinuous yielding. In this research work a novel heat treatment route of incomplete austenitisation-based cyclic ice-brine quenching has been adopted on low-carbon steel (AISI 1010 steel containing 0.1 wt.% C) without considering any costly alloying or thermo-mechanical treatment. Accordingly, exceptionally high strength (UTS = 1.7 GPa) and specific strength (226 MPa g−1cm3) are achieved after three cycles along with a modest ductility (% Elongation = 9). This is the highest strength reported so far for low-carbon steel containing 0.1 wt.% C. Yield point phenomenon is also eliminated. This is attributed to a novel microstructure consisting of highly sub-structured fine plate martensite crystals containing internal twin and dislocation tangles along with dispersion of nano-sized cementite particles and clusters of cementite particles.

Autocatalytic surface reduction and its role in controlling seed-mediated growth of colloidal metal nanocrystals

The growth of colloidal metal nanocrystals typically involves an autocatalytic process, in which the salt precursor adsorbs onto the surface of a growing nanocrystal, followed by chemical reduction to atoms for their incorporation into the nanocrystal. Despite its universal role in the synthesis of colloidal nanocrystals, it is still poorly understood and controlled in terms of kinetics. Through the use of well-defined nanocrystals as seeds, including those with different types of facets, sizes, and internal twin structure, here we quantitatively analyze the kinetics of autocatalytic surface reduction in an effort to control the evolution of nanocrystals into predictable shapes. Our kinetic measurements demonstrate that the activation energy barrier to autocatalytic surface reduction is highly dependent on both the type of facet and the presence of twin boundary, corresponding to distinctive growth patterns and products. Interestingly, the autocatalytic process is effective not only in eliminating homogeneous nucleation but also in activating and sustaining the growth of octahedral nanocrystals. This work represents a major step forward toward achieving a quantitative understanding and control of the autocatalytic process involved in the synthesis of colloidal metal nanocrystals.

Tracking the αʺ martensite decomposition during continuous heating of a Ti-6Al-6V-2Sn alloy

The effect of heating rate on the decomposition kinetics of α″ martensite is continuously studied for a β-quenched Ti-6Al-6V-2Sn α + β alloy using fast in situ high energy synchrotron X-ray diffraction. The investigations are complemented with ex situ as well as dynamic metallographic analysis to reveal the mechanisms of phase transformation during critical stages of heating. The initial microstructure consists in a homogeneous distribution of fine acicular αʺ plates subdivided into domains associated with spinodal decomposition of αʺ, formed during β → αʺ transformation upon quenching. Quantitative phase analysis based on the Rietveld method provides the continuous evolution of lattice parameters, revealing the progression of diffusion-driven processes controlling the decomposition of αʺ into stable α and β phases (αʺ → α + β). The onset of this transformation shifts to higher temperatures with increasing heating rate. During the first stage of stabilization, precipitation of fine β particles occurs along the boundaries of twin related αʺ plates, while formation of β and α phases in the interior of α″ plates is identified at higher temperatures. Early formation of α takes place along boundaries of internal {111} α″ nano-twins, propagates up the borders of α″ plates, and can eventually further extend towards internal twins of neighbour martensite plates. Internal nano-twinning of αʺ plates takes place during αʺ decomposition owing to stress concentrations associated with structural heterogeneities within αʺ plates, i.e. spinodal decomposition of αʺ. The results obtained point to a relevant role of twin interfaces during phase precipitation.

Suppressing the image smear of the vibration modulation transfer function for remote-sensing optical cameras.

In on-board photographing processes of satellite cameras, the platform vibration can generate image motion, distortion, and smear, which seriously affect the image quality and image positioning. In this paper, we create a mathematical model of a vibrating modulate transfer function (VMTF) for a remote-sensing camera. The total MTF of a camera is reduced by the VMTF, which means the image quality is degraded. In order to avoid the degeneration of the total MTF caused by vibrations, we use an Mn-20Cu-5Ni-2Fe (M2052) manganese copper alloy material to fabricate a vibration-isolation mechanism (VIM). The VIM can transform platform vibration energy into irreversible thermal energy with its internal twin crystals structure. Our experiment shows the M2052 manganese copper alloy material is good enough to suppress image motion below 125 Hz, which is the vibration frequency of satellite platforms. The camera optical system has a higher MTF after suppressing the vibration of the M2052 material than before.

Hybrid Tip-Enhanced Nanospectroscopy and Nanoimaging of Monolayer WSe2 with Local Strain Control.

Many classes of two-dimensional (2D) materials have emerged as potential platforms for novel electronic and optical devices. However, their physical properties are strongly influenced by nanoscale heterogeneities in the form of edges, twin boundaries, and nucleation sites. Using combined tip-enhanced Raman scattering and photoluminescence (PL) nanospectroscopy and nanoimaging, we study the associated effects on the excitonic properties in monolayer WSe2 grown by physical vapor deposition. With ∼15 nm spatial resolution, we resolve nanoscale correlations of PL spectral intensity and shifts with crystal edges and internal twin boundaries associated with the expected exciton diffusion length. Through an active atomic force tip interaction we can control the crystal strain on the nanoscale and tune the local bandgap in reversible (up to 24 meV shift) and irreversible (up to 48 meV shift) fashion. This allows us to distinguish the effect of strain from the dominant influence of defects on the PL modification at the different structural heterogeneities. Hybrid nano-optical spectroscopy and imaging with nanomechanical strain control thus enables the systematic study of the coupling of structural and mechanical degrees of freedom to the nanoscale electronic and optical properties in layered 2D materials.

Compatibility at Junction Planes between Habit Plane Variants with Internal Twin in Ti-Ni-Pd Shape Memory Alloy

Compatibility at Junction Planes between Habit Plane Variants with Internal Twin in Ti-Ni-Pd Shape Memory Alloy

A new nanoscale metastable iron phase in carbon steels.

Metastable ω phase is common in body-centred cubic (bcc) metals and alloys, including high-alloying steels. Recent theoretical calculations also suggest that the ω structure may act as an intermediate phase for face-centred cubic (fcc)-to-bcc transformation. Thus far, the role of the ω phase played in fcc-bcc martensitic transformation in carbon steels has not been reported. In previous investigations on martensitic carbon steels, extra electron diffraction spots were frequently observed by transmission electron microscopy (TEM), and these spots were historically ascribed to the diffraction arising from either internal twins or carbides. In this paper, an intensive TEM investigation revealed that the extra spots are in fact attributed to the metastable ω phase in particle-like morphology with an overall size of several or dozens of nanometres. The strict orientation relationships between the ω phase and the ferrite matrix are in good agreement with those of the hexagonal (P6/mmm) ω phase in other bcc metals and alloys. The identification of the ω phase as well as the extra diffraction spots might provide a clue to help understand the physical mechanism of martensitic transformation in steels.

Impact of P and Sr on solidification sequence and morphology of hypoeutectic Al–Si alloys: Combined thermodynamic computation and phase-field simulation

Even small amounts of Phosphorus and Strontium strongly affect the microstructure of hypoeutectic Al–Si alloys. P is an unavoidable trace element in commercial Al-alloys which causes formation of AlP particles as potent nucleation sites for eutectic (Si). Sr, in contrast, is purposely added to modify the morphology of eutectic (Si) towards fine coral-like fibers. It is hypothesized that Sr does not only alter the growth kinetics of (Si), but additionally prevents detrimental (Si) nucleation due to neutralization of AlP particles by Al2Si2Sr formation. This presumes that both AlP and Al2Si2Sr precipitate prior to (Si). Using a newly assessed thermodynamic database for the Al–Si–Sr–P system, critical P and Sr thresholds for pre-silicon formation of AlP and Al2Si2Sr were evaluated and mapped under equilibrium and Scheil conditions. The competitive precipitation of AlP, Al2Si2Sr and (Si) and its impact on the evolution of the eutectic morphology was further studied by 3D phase-field simulations. Effective anisotropy functions for the (Si) interface mobility considered Sr-induced internal twinning. Depending on whether subcritical or supercritical P and Sr contents were selected, either a fine lamellar structure, a coarse flaky structure, or the targeted fine fibrous eutectic structure was reproduced.

Coalescence of silver clusters by immersion in diluted HF solution.

The galvanic displacement deposition of silver on H-terminated Si (100) in the time scale of seconds is instantaneous and characterized by a cluster density of 10(11)-10(12) cm(-2). The amount of deposited Ag follows a t(1/2) dependence in agreement with a Cottrell diffusion limited mechanism. At the same time, during the deposition, the cluster density reduces by a factor 5. This behavior is in contrast with the assumption of immobile clusters. We show in the present work that coalescence and aggregation occur also in the samples immersed in the diluted hydrofluoric acid (HF) solution without the presence of Ag(+). Clusters agglomerate according to a process of dynamic coalescence, typical of colloids, followed by atomic redistribution at the contact regions with the generation of multiple internal twins and stacking-faults. The normalized size distributions in terms of r/rmean follow also the prediction of the Smoluchowski ripening mechanism. No variation of the cluster density occurs for samples immersed in pure H2O solution. The different behavior might be associated to the strong attraction of clusters to oxide-terminated Si surface in presence of water. The silver clusters are instead weakly bound to hydrophobic H-terminated Si in presence of HF. HF causes then the detachment of clusters and a random movement on the silicon surface with mobility of about 10(-13) cm(2)/s. Attractive interaction (probably van der Waals) among particles promotes coarsening.

Management of Haemophilia in Developing Countries: Challenges and Options.

There are significant challenges in managing haemophilia patients in developing countries. These challenges are (i) Lack of proper health care infrastructure and human resources suitable for haemophilia care (ii) Competing health care priorities of the government. (iii) Lack of penetrance of medical insurance in the population. (iv) Lesser visibility of the haemophilia patients in health care system (v) Low awareness across the medical profession, population and the policy makers about the condition (vi) Non availability of factor concentrates (vii) Inadequate utilization of knowledge for reducing factor concentrate use. (viii) Inadequate pain relief (ix) Challenges due to inhibitor developing (x) Viral hepatitis & (xi) Lack of research publications relevant to the country are some of the challenges faced by PWH for their management in developing country. The solutions are not easy but development of a strong patient organization with linkages with World Federation of Haemophilia is an important initial step. Following that internal and international twinning, use of internal sources, strong advocacy programme targeting government, doctors, opinion makers will solve many of the challenges in the time to come.

The secondary precipitates of niobium-alloyed M3:2 high speed steel prepared by spray deposition

The secondary carbides in spray-formed M3:2 high speed steels (HSSs) with or without Niobium (Nb) addition have been investigated. The results show that the fine, dense and coherent MC and M2C secondary carbides can be precipitated on the internal twin boundaries of the martensite plates and within the plates during tempering at 560–600°C. Ten layers of (112¯0)M2C stacking along (200)α during triple tempering at 580°C comprise the needle-like M2C precipitates with ~1.5nm thickness. Increasing the tempering temperature can promote the coarsening of M2C precipitates as well as their sparse distribution. Under same tempering conditions, the superior tempering resistance of 2% Nb alloyed steel is made by high-density M2C nanoparticles. It is found that the lack of Mo in primary M6C and MC carbides induced by 2% Nb addition can promote a larger number of Mo atoms to be dissolved into the matrix, and the increased Mo not only prevents the agglomeration of nano M2C particles but also delay the generation of M3C during aging process.

Effects of molybdenum on microstructural evolution and mechanical properties in Zr–Nb alloys as nuclear fuel cladding materials

The Zr–Nb alloys were modified by doping of Mo as a minor alloying element to seek for the nuclear fuel cladding materials with better characteristics. The effects of Mo on microstructural evolution and mechanical properties in Zr–Nb alloys were systematically investigated and elucidated. Results showed that the martensitic microstructure, a mixture of lath martensites and lens martensites with internal twins, was observed in the alloys quenched from β-phase. Width of the lath martensite reduced with the increasing Mo concentration, and the volume fraction of lens martensite increased with increase in the Mo concentration. After final annealing, a new kind of precipitate, namely β-(Nb, Mo, Zr), was identified in the Mo-containing alloys. It was also found that Mo reduced the growth of the precipitates but increased their number density. Furthermore, Mo addition retarded the recrystallization process strongly and reduced the grain size significantly. In terms of the mechanical properties, Mo addition enhanced the yield strength and the ultimate tensile strength at room temperature, however decreased the ductility. The grain size strengthening was presumed as the greatest contributor in this system.

Interface modeling in continuum dislocation transport

Predictive microstructural models of poly-crystalline materials require a correct description of the mechanical behavior of internal boundaries, e.g. grain, phase and twin boundaries. Dislocations are the carriers of plastic deformation and the presence of internal boundaries restricts their motion. Interactions between dislocations and the resistance to their motion caused by the interfaces give rise to hardening and size effects, which should therefore be considered. In this paper, a continuum dislocation transport model in single slip is used to model a two-phase laminated microstructure containing (plastically) hard and soft phases. The phase boundary constitutes an interface in the model. The transport equations require continuity of the dislocation flux throughout the domain. Expressions for the dislocation flux in the bulk as a function of the dislocation densities and their gradients are readily available in the literature. However, the interface requires an additional constitutive model for the dislocation flux passing through it. Such a model is derived here from the interactions of infinite dislocation walls on both sides of the parallel boundary. A qualitative analysis is performed to reveal the effect of interface, material and geometrical parameters on the overall response of a two-phase laminate. The presence of the interface in the two-phase laminate gives rise to the observed characteristic hardening of dual-phase materials as well as to size effects.

On the relation between orientation relationships predicted by the phenomenological theory and internal twins in plate martensite

The phenomenological theory of martensite crystallography predicts two equivalent solutions for a particular habit plane in the case of a Fe–Ni–C alloy. Those two solutions differ in the magnitude of the inhomogeneous shear and in the orientation relationship (OR) they hold with austenite. Only the OR associated to the low shear solution has been observed experimentally so far. In the present study, the orientation relationship associated to the high shear solution is assessed experimentally using TEM measurements.

α′ martensite Ti–10Nb–2Mo–4Sn alloy with ultralow elastic modulus and High strength

By alloying and thermo-mechanical treatment, a novel α′ martensite Ti–10Nb–2Mo–4Sn (wt%) alloy with ultralow elastic modulus and high strength was prepared. Coarse α′ martensite plates with internal twins are formed in the solution treated alloy. Upon a severe cold rolling deformation, the alloy exhibits fine microstructure consisting of nano-sized α′ and dislocation tangles, resulting in a remarkable increase in strength. Meanwhile, the cold rolled alloy exhibits an ultralow elastic modulus of 41GPa, which is lower than that of most biomedical β-type Ti alloys containing a considerable amount of expensive Nb and Ta. Therefore, the α′ martensite Ti–10Nb–2Mo–4Sn alloy has a high potential not only for biomedical applications, but also for industrial applications, due to its low elastic modulus, high strength and low cost.

Martensite modulus dilemma in monoclinic NiTi-theory and experiments

Correct assessment of the NiTi martensite moduli and its significance are the subject of this paper. Current experiments and published experimental data reveal the considerable difference between the macroscopic elastic moduli for twinned martensite and detwinned martensite (single crystal state). Although this difference is significant, it is not explained adequately, and often neglected in modeling. In this study, based on the atomistic simulations, we establish Voigt and Reuss bounds and Hill’s estimate for macroscopic moduli for both the internally twinned states and single crystal (detwinned) states. The predicted elastic moduli in the twinned state compare favorably to the experimental thermally induced martensite (with internal twin arrangement) moduli. The single crystal moduli predictions conform to the experimental moduli for deformed martensite or stress-induced martensite in tension and compression showing asymmetry. We draw attention to the need for correct interpretation of the elastic moduli for understanding a wide range of phenomena in shape memory alloys, such as the transformation stress–strain response, the fatigue behavior, the stored elastic energy, the stress hysteresis and others.