Twinning of CaMgSi phase in a cast Mg–1.0Ca–0.5Si–0.3Zr alloy

Two types of twins and an orientation domain (orientation twin) were found in CaMgSi particles formed in a Mg–Ca–Si alloy by transmission electron microscopy. The two types of twins were shown to be a (1 0 2) twin with the (1 0 2) plane serving as the twinning plane and a (0 1 1) twin with the (0 1 1) plane serving as the twinning plane. In addition to the frequently observed two-fold twin, two different three-fold twins were also observed in the (1 0 2) twinned CaMgSi particles; their crystallographic features are discussed in detail. The boundaries of both the (1 0 2) and (0 1 1) twins were found to be coincident with the twinning plane. The orientation domains were formed by one segment rotating 60° about the [1 0 0] axis relative to the other. The possible formation mechanisms of the twins and orientation domain are discussed based on a crystallographic consideration.     

Laser compression of monocrystalline tantalum

Monocrystalline tantalum with orientations [1 0 0] and [1 1 1] was subjected to laser-driven compression at energies of 350–684 J, generating shock amplitudes varying from 10 to 110 GPa. A stagnating reservoir driven by a laser beam with a spot radius of ～800 μm created a crater of significant depth (～80 to ～200 μm) on the drive side of the Ta sample. The defects generated by the laser pulse were characterized by transmission and scanning electron microscopy, and are composed of dislocations at low pressures, and mechanical twins and a displacive phase transformation at higher pressures. The defect substructure is a function of distance from the energy deposition surface and correlates directly with the pressure. Directly under the bottom of the crater is an isentropic layer, approximately 40 μm thick, which shows few deformation markings. Lattice rotation was observed immediately beneath this layer. Further below this regime, a high density of twins and dislocations was observed. As the shock amplitude decayed to below ～40 GPa, the incidence of twinning decreased dramatically, suggesting a critical threshold pressure. The twinning planes were primarily {1 1 2}, although some {1 2 3} twins were also observed. Body-centered cubic to hexagonal close-packed pressure induced-transformation was observed at high pressures (～68 GPa).The experimentally measured dislocation densities and threshold stress for twinning are compared with predictions using analyses based on the constitutive response, and the similarities and differences are discussed in terms of the mechanisms of defect generation.     

Characterization of mobile type I and type II twin boundaries in 10M modulated Ni–Mn–Ga martensite by electron backscatter diffraction

Mobile type I and type II twin boundaries mediating the magnetic field-induced strain in five-layered modulated (10M) Ni–Mn–Ga martensite were analyzed by electron backscatter diffraction. Taking into account the slight monoclinic distortion of the pseudo-tetragonal lattice, the electron backscatter diffraction study reveals domains of 0.01–1 mm thickness adjacent to the type I and type II twin boundaries. The domains differing in the modulation direction are {1 0 0) compound twins and their effect on twinning stress is discussed. Detailed analysis of type II twin boundary reveals that the domains are further internally twinned by compound {1 1 0) twins 1–15 μm in size. An additional example of a complex twin microstructure combining type I and type II twin boundaries is presented.     

Cyclic twin nucleation in tin-based solder alloys

The microstructure and Sn crystal orientations of lead-free solder alloys such as near-eutectic SnAgCu have a significant influence on the mechanical response of a solder joint to service conditions. Thus solidification processes were examined in SnAgCu solder joints. Distinct evidence of sixfold cyclic growth twinning of Sn during solidification from the melt was observed in Sn–Ag, SAC and Sn–Cu solders. Three orientations of Sn grains, each having a common 〈1 0 0〉 direction, were found in each of these systems, though the morphologies of these cyclic twinned microstructures differed. Analysis of dendrite arm spacing in cyclically twined structures with a beach ball morphology implies that the common 〈1 0 0〉 axis intersects with the region of the nucleation event. Models are presented for two pseudo/metastable hexagonal unit cells based upon {1 0 1} or {3 0 1} twins that introduce the cyclic twinning structure at the nucleation stage. Formation of these hexagonal unit cells may be facilitated by the presence of alloy elements. Subsequent epitaxial growth of the tetragonal unit cell on this nucleus can account for all three types of morphologies observed in microstructures of Sn-rich solder alloys.     

Twinning and texture development in two Mg alloys subjected to loading along three different strain paths

The evolution of twinning and texture in two Mg-based (+Al, Mn, Zn) alloys was investigated using uniaxial tension, uniaxial compression and ring hoop tension testing at temperatures from ambient to 250 °C and a strain rate of 0.1 s⁻¹. The results indicate that the initial extrusion texture plays an important role in the formation of different types of twins and that the twinning behavior also depends on the strain path. Contraction and double twinning are the dominant twinning mechanisms in uniaxial tension, while extension twinning prevails in uniaxial compression and ring hoop tension testing. Schmid factor analysis indicates that only components that are favorably oriented (i.e., with the highest SF values) can undergo rapid and complete twinning. The different twinning behaviors are shown to be responsible for the sharply contrasting strain hardening characteristics of the experimental flow curves and dramatic texture changes.     

The impact of peak shock stress on the microstructure and shear behavior of 1018 steel

As-received and shock-prestrained 1018 steel specimens were subjected to forced shear experiments in a split-Hopkinson pressure bar (SHPB) at room temperature and a strain rate of 3800 s⁻¹ to determine the influence of shock-prestraining on the shear behavior of ferrite. Shock-loading was performed below (12.5 GPa) and above (14 GPa) the pressure-induced epsilon phase transition occurring at 13 GPa. Using electron microscopy and electron backscatter diffraction, twinning and microbanding were observed only in the shock-prestrained specimens. Quasi-static compression tests showed an increase in yield and compressive strengths with increased peak shock stress. SHPB tests produced shear localization in all specimens, with shear banding occurring only in the shock-prestrained specimens. Transmission electron microscopy revealed that, at the shear band edge, elongated cells dominate the microstructure, with more shock-induced twins remaining intact in the 12.5 GPa specimen than in the 14 GPa specimen.     

In situ TEM study of deformation twinning in Ni–Mn–Ga non-modulated martensite

The straining of non-modulated (NM) Ni–Mn–Ga martensite was studied by in situ transmission electron microscopy (TEM). Initially, the self-accommodated NM martensitic structure consists of internally twinned domains. During straining, the detwinning process starts within these domains. The internal twin variant more favorably oriented to the stress grows at the expense of the other one. In the detwinned, single-variant domain, a new twin variant can form, gradually replacing the existing variant via the twinning process. Both processes—detwinning and new twinning—proceed by the same mechanism, namely by the movement of twinning dislocations along the twin boundary. Lattice dislocations are also created in the detwinning process. While the boundaries between the internal twins are coherent and mobile, the boundaries between the internally twinned domains are incoherent, strained and not mobile. The planes of the coherent twin boundary are {2 0 2) planes and the Burgers vectors of the twinning dislocations are parallel to the 〈1 0 1] direction. The magnitude of the Burgers vector determined from the TEM observations disagrees with the calculation from the lattice constant measurement by X-ray diffraction. Possible reasons for this discrepancy are discussed.     

Effect of texture on load partitioning in Ti-6Al-4V

Neutron diffraction has been used to characterize the evolution of residual elastic strain in grains with different orientations due to room temperature plastic deformation in two plate product forms of Ti–6Al–4V. The evolution of lattice strains has been rationalized using a two-phase elastic–plastic self-consistent model using only the texture difference between the two product forms. It is found that the elastic properties of both the bulk and individual orientations can be reproduced quite satisfactorily, with a C′ modulus of the β phase of 15 GPa. The residual microstrains produced are generally greater in the unidirectionally rolled material than the cross-rolled, but are smaller than in Ti-834. The residual strains accumulated in the (0 0 0 2) orientation are near-zero, which can only be reproduced in the modelling by assuming a critical resolved shear stress for 〈c + a〉 slip only 1.5× that for 〈a〉 slip, compared to the 3× factor found for isolated single crystals. The implications of this for our understanding of deformation in these materials are discussed.     

Critique of mechanisms of formation of deformation,annealing and growth twins: Face-centered cubic metals and alloys

The formation of deformation, annealing and growth twins in face-centered cubic materials is discussed. Slip precedes deformation twinning, and twins form from the interaction between primary and secondary slip dislocations having co-planar, but different, Burgers vectors. The influence of several metallurgical variables on twinning can only be rationalized in terms of the model. Annealing twins form due to growth accidents on differently inclined {1 1 1} facets present on a migrating grain boundary. Growth twins also form by growth accidents on the {1 1 1} planes.     

Laser compression of nanocrystalline tantalum

Nanocrystalline tantalum (grain size ～70 nm) prepared by severe plastic deformation (high-pressure torsion) from monocrystalline [1 0 0] stock was subjected to shock compression generated by high-energy laser (～350–850 J), creating pressure pulses with initial duration of ～3 ns and amplitudes of up to ～145 GPa. The laser beam, with a spot radius of ～1 mm, created a crater of significant depth (～135 μm). Transmission electron microscopy revealed few dislocations within the grains and an absence of twins at the highest shock pressure, in contrast with monocrystalline tantalum. Hardness measurements were conducted and show a rise as the energy deposition surface is approached, evidence of shock-induced defects. The grain size was found to increase at a distance of 100 μm from the energy deposition surface as a result of thermally induced grain growth. The experimentally measured dislocation densities are compared with predictions using analyses based on physically based constitutive models, and the similarities and differences are discussed in terms of the mechanisms of defect generation. A constitutive model for the onset of twinning, based on a critical shear stress level, is applied to the shock compression configuration. The predicted threshold pressure at which the deviatoric component of stress for slip exceeds the one for twinning is calculated and it is shown that it is increased from ～24 GPa for the monocrystalline to ～150 GPa for the nanocrystalline tantalum (above the range of the present experiments). Calculations using the Hu–Rath analysis show that grain growth induced by the post shock-induced temperature rise is consistent with the experimental results: grains grow from 70 to 800 nm within the post-shock cooling regime when subjected to a laser pulse with energy of 684 J.     

Deformation twins in nanocrystalline body-centered cubic Mo as predicted by molecular dynamics simulations

This work studies deformation twins in nanocrystalline body-centered cubic Mo, including the nucleation and growth mechanisms as well as their effects on ductility, through molecular dynamics simulations. The deformation processes of nanocrystalline Mo are simulated using a columnar grain model with three different orientations. The deformation mechanisms identified, including dislocation slip, grain-boundary-mediated plasticity, deformation twins and martensitic transformation, are in agreement with previous studies. In 〈1 1 0〉 columnar grains, the deformation is dominated by twinning, which nucleates primarily from the grain boundaries by successive emission of twinning partials and thickens by jog nucleation in the grain interiors. Upon arrest by a grain boundary, the twin may either produce continuous plastic strain across the grain boundary by activating compatible twinning/slip systems or result in intergranular failure in the absence of compatible twinning/slip systems in the neighboring grain. Multiple twinning systems can be activated in the same grain, and the competition between them favors those capable of producing continuous deformation across the grain boundary.     

Deformation twinning in Ni2MnGa

Deformation twinning is investigated in the martensitic phase of a Ni_46.75Mn₃₄Ga_19.25 (at.%) alloy. X-ray and electron diffraction are used to establish the crystallography of the non-modulated tetragonal martensite, and transmission electron microscopy is employed to deduce the twinning parameters. It is convenient to define the twinning parameters with respect to a “monoclinic” unit cell, designated 2M: then K₁, η₁, K₂, and η₂ are (0 0 1), [1 0 0], (1 0 0), and [0 0 1] respectively. The Burgers vector of the active twinning disconnections is close to 1/6[1 0 0] and the disconnections are associated with steps of height d₍₀₀₂₎. These defects are expected to be highly mobile since their motion does not require atomic shuffling. It is shown that periodic arrangements of two layer twins produce modulated crystal structures, such as 14M.     

The influences of temperature and microstructure on the tensile properties of a CoCrFeMnNi high-entropy alloy

An equiatomic CoCrFeMnNi high-entropy alloy, which crystallizes in the face-centered cubic (fcc) crystal structure, was produced by arc melting and drop casting. The drop-cast ingots were homogenized, cold rolled and recrystallized to obtain single-phase microstructures with three different grain sizes in the range 4–160 μm. Quasi-static tensile tests at an engineering strain rate of 10^?3 s^?1 were then performed at temperatures between 77 and 1073 K. Yield strength, ultimate tensile strength and elongation to fracture all increased with decreasing temperature. During the initial stages of plasticity (up to ～2% strain), deformation occurs by planar dislocation glide on the normal fcc slip system, {1 1 1}〈1 1 0〉, at all the temperatures and grain sizes investigated. Undissociated 1/2〈1 1 0〉 dislocations were observed, as were numerous stacking faults, which imply the dissociation of several of these dislocations into 1/6〈1 1 2〉 Shockley partials. At later stages (～20% strain), nanoscale deformation twins were observed after interrupted tests at 77 K, but not in specimens tested at room temperature, where plasticity occurred exclusively by the aforementioned dislocations which organized into cells. Deformation twinning, by continually introducing new interfaces and decreasing the mean free path of dislocations during tensile testing (“dynamic Hall–Petch”), produces a high degree of work hardening and a significant increase in the ultimate tensile strength. This increased work hardening prevents the early onset of necking instability and is a reason for the enhanced ductility observed at 77 K. A second reason is that twinning can provide an additional deformation mode to accommodate plasticity. However, twinning cannot explain the increase in yield strength with decreasing temperature in our high-entropy alloy since it was not observed in the early stages of plastic deformation. Since strong temperature dependencies of yield strength are also seen in binary fcc solid solution alloys, it may be an inherent solute effect, which needs further study.     

Study of internal lattice strain distributions in stainless steel using a full-field elasto-viscoplastic formulation based on fast Fourier transforms

In this work, the evolution of internal lattice strains in face-centered cubic stainless steel under uniaxial tension is studied using a recently developed full-field elasto-viscoplastic formulation based on fast Fourier transforms. The shape of the diffraction peaks is simulated, and the predicted lattice strains (peak shift and broadening) are compared with the experimental measurements obtained by in situ tensile neutron diffraction. Detailed analysis of the lattice strain distributions reveal that {1 0 0} and {1 1 0} transverse families exhibit a bimodal nature, and that transverse lattice strains are more sensitive to local grain interactions compared with longitudinal lattice strains. A comparison with the results of a mean-field formulation indicates that type III (intragranular) stresses play a much larger role than type II (intergranular) stresses in diffraction peak broadening.     

Structure of martensite in Ti-rich Ti–Ni–Cu thin films annealed at different temperatures

After annealing at different temperatures, there are different types of precipitates in Ti-rich Ti–Ni–Cu thin films: plate-like Guinier–Preston (GP) zones, Ti₂Cu precipitates and spherical Ti₂Ni precipitates. The (0 1 1) compound twins and (1 1 1) type I twins are dominant in Ti-rich Ti–Ni–Cu thin films annealed at different temperatures, which suggests that the precipitates do not change the twinning modes of the B19 martensite. However, the amount of the (0 1 1) compound twin increases with increasing annealing temperature due to its small twinning shear. In thin films with GP zones or Ti₂Ni precipitates, the amount of martensite with a single-pair morphology is less than that in thin films without precipitates. And in thin film with Ti₂Cu + Ti₂Ni precipitates, hardly any martensite with a single-pair morphology is observed. For the different types of precipitates at the different annealing temperatures, the obstacle of the precipitates to the growth of the B19 martensite plate also varies. The GP zones slightly hinder the growth in the width of martensite, resulting in wavy twin boundaries at the martensite variant tip. The Ti₂Cu precipitates can change both the width and the direction of the martensite plates. Ti₂Ni precipitates also significantly disturb or impede the growth of the martensite variants. These effects lead to a decrease in the maximum shape recoverable strain with increasing annealing temperature.     

Mechanism of deformation and crystal lattice reorientation in strain localization bands and deformation twins of the B2 phase of titanium nickelide

Optical metallography and transmission electron microscopy were used to investigate the formation and microstructure of strain localization bands and deformation twins in single crystals of a TiNi alloy deformed by rolling. On the basis of these investigations, with the use of the theory of martensitic transformations based on the concept of cooperative thermal vibrations of coherent objects (close-packed planes) in crystals, a new mechanism of twinning in these alloys is suggested, namely, through the forward plus reverse (B2 → B19 → B2) martensitic transformations with the reverse transformation developing via an alternative transformation system. It is shown that using this mechanism one can describe on the same grounds the twinning in the crystal lattice of the B2 phase with twin planes of different ({1 1 2}, {1 1 3}, and {3 3 2}) Miller indices and the formation of strain localization bands with low-angle misorientations.     

Adiabatic temperature increase associated with deformation twinning and dislocation plasticity

We studied local deformation and temperature effects associated with mechanical twinning in Fe–3 wt.% Si at room temperature. During tensile testing, two large stress drops occurred. They were accompanied by local strain and temperature bursts, which we mapped via simultaneous displacement and temperature field characterization. To identify the microstructural origin of these phenomena, we performed high resolution electron backscatter scanning diffraction and electron channeling contrast imaging measurements. The microstructure at the positions where strong adiabatic heating occurred was characterized by the formation of primary twins and high dislocation activity within a range of about 10 μm around the twin–matrix interface. We suggest that the local temperature and strain jumps result from the formation and dissipative motion of lattice dislocations that accommodate twinning.     

Low strain rate deformation behavior of a Cr–Mn austenitic steel at −80 °C

The deformation behavior of a Cr–Mn austenitic steel during interrupted low strain rate uniaxial tensile testing at ?80 °C has been studied using X-ray diffraction (XRD), electron backscatter diffraction and transmission electron microscopy. Continuous γ → ε → α′ martensite transformation was observed until failure. High dislocation densities were estimated in the austenite phase (～10¹⁵ m^?2), and for the α′-martensite they were even an order of magnitude higher. Dislocation character analysis indicated that increasing deformation gradually changed the dislocation character in the austenite phase to edge type, whereas the dislocations in α′-martensite were predominantly screw type. XRD analyses also revealed significant densities of stacking faults and twins in austenite, which were also seen by transmission electron microscopy. At low strains, the deformation mode in austenite was found to be dislocation glide, with an increasing contribution from twinning, as evidenced by an increasing incidence of ∑3 boundaries at high strains. The deformation mode in α′-martensite was dominated by dislocation slip.     

The mechanical properties and the deformation microstructures of the C15 Laves phase Cr2Nb at high temperatures

Compression tests between 1250 and 1550 °C and 10⁻⁵ and 5 × 10⁻³ s⁻¹ and transmission electron microscopy have been employed to investigate the high temperature mechanical properties and the deformation mechanisms of the C15 Cr₂Nb Laves phase. The stress-peaks in the compression curves during yielding were explained using a mechanism similar to strain aging combined with a low initial density of mobile dislocations. The primary deformation mechanism is slip by extended dislocations with Burgers vector 1/2〈1 1 0〉, whereas twinning is more frequent at 10⁻⁴ s⁻¹. Schmid factor analysis indicated that twinning is more probable in grains oriented so as to have two co-planar twinning systems with high and comparable resolved shear stresses. Twinning produced very anisotropic microstructures. This may be due to synchroshear: a self-pinning mechanism which requires co-operative motion of zonal dislocations.     

Effect of the Zener–Hollomon parameter on the microstructures and mechanical properties of Cu subjected to plastic deformation

Pure Cu was deformed at different strain rates and temperatures, i.e. with different Zener–Hollomon parameters (Z) ranging within ln Z = 22–66, to investigate the effect of Z on its microstructures and mechanical properties. It was found that deformation twinning occurs when ln Z exceeds 30, and the number of twins increases at higher Z. The average twin/matrix lamellar thickness is independent of Z, being around 50 nm. Deformation-induced grain refinement is enhanced at higher Z, and the mean transverse grain size drops from 320 to 66 nm when ln Z increases from 22 to 66. The grain refinement is dominated by dislocation activities in low-Z processes, while deformation twinning plays a dominant role in high-Z deformation. An obvious increment in yield strength from 390 to 610 MPa was found in deformed Cu with increasing Z, owing to the significant grain refinement as well as the strengthening from nanoscale deformation twins.