Compound Twins Research Articles

Structure of martensite in deformed Ti–Ni–Cu thin films

Microstructural evolution in a high-Cu-content Ti 50.2Ni 30Cu 19.8 shape-memory thin film deformed in the B19 martensite state was studied by transmission electron microscopy, and the deformation mechanisms were clarified. The thin film was prepared by magnetron sputtering deposition. In the undeformed film, the B19 martensite has mainly {0 1 1} B19 twins with a small number of {1 1 1} B19 twins. The tensile deformation involves a reorientation of the {0 1 1} B19 twin domains, de-twinning of the {0 1 1} B19 and {1 1 1} B19 twins, and a stress-induced B19–B19′ transformation with the production of (0 0 1) B19′ compound twins. The film shows a large recoverable strain of 5.5%, which is far beyond the stress plateau.

Morphology and Crystallography of Martensite Plate with Long Period Stacking Structure in Ti-Pd Shape Memory Alloy

The morphology and crystallography of martensite plates with a long period stacking structure in Ti-Pd shape memory alloy were investigated by transmission electron microscopy. In addition to B19 (2H) martensite, an 18R(2\\bar1)6 structure was observed in equiatomic Ti-Pd alloy quenched above 1373 K. Three fundamental plate variant combinations were identified in the plate group in the region consisting exclusively of 18R martensite. These were designated A:B, A:C and A:D types, and their morphologies and crystallographic characteristics are a wedge with type II, a spear with type I and a fork with compound twin relations, respectively. Duplex martensitic phases coexisting in the same area, namely, “two-in-one” structures, were also observed.

Crystallographic features of the primary Al 3Zr phase in as-cast Al-1.36 wt% Zr alloy

The habit and morphology of the primary Al 3Zr crystals formed during the solidification of Al-1.36 wt% Zr alloy were analyzed. It was observed that the crystals have a tabular habit. The longitudinal cuts of the smaller and larger sized crystals appear, respectively, in octagonal and rectangular forms. The primary crystals also appear in the dendritic form. A crystallographic study indicates that the smaller sized crystals are bound by {0 0 1}, {1 0 1} and {1 1 1} faceted planes, whereas the larger sized ones by {0 0 1} and {1 0 1} faceted planes. One compound twin with two variants is found in the dendritic crystals and the complete twinning elements are determined. The tabular habit of the primary crystals is attributed to the low roughness of {0 0 1} planes. The disappearance of {1 1 1} planes in the larger sized crystals result from their high roughness. The twin formation is realized by lattice shear plus local atom stacking fault accompanied by local atomic reshuffling. The dendritic crystals are the result of the formation of the compound twin.

On the elastic energy invariance of crystal twins

An analysis by Zanzotto (1992, 1996) showed that, if crystal twinning is considered as a material symmetry operation, the symmetry groups of many crystals that form twins correspond to the unimodular group, which is the symmetry group of elastic fluids. It is therefore evident that an elastic modelling based on an elastic energy that includes any possible combination of twin system activation must fail. However, one may try to construct an elastic energy which includes only the stressfree configurations of the parent and the first order twins. It is known that, even under this restriction, many twinning modes exhibit an elastic energy invariance, which renders them indistinguishable in terms of the elastic energy. In this article, it is demonstrated that this energy invariance holds for all compound twins. The implications of the latter are discussed. It is suggested to distinguish between crystallographically equivalent and crystallographically distinct compound twins, since different implications regarding the elastic energy invariance emerge.

Automatic twin statistics from electron backscattered diffraction data

A new computer code has been developed to automatically extract quantitative twin statistics from electron backscatter diffraction data. The new code is an improvement upon previous codes in that it handles materials of any crystal symmetry, type I, Type II and compound twins, and general stress states. Moreover, accuracy of the results has been greatly improved. In addition, twin statistics including number, area fraction, twin thickness and twinning dependencies on orientation, grain size and neighbourhood effects can be routinely analysed. The new code has been applied to scan data from deformed magnesium, zirconium and uranium, and can potentially be used for any twinning material for which reliable electron backscatter diffraction results can be obtained.

Transmission electron microscopy investigation of dislocation slip during superelastic cycling of Ni–Ti wires

Superelastic deformation of thin Ni–Ti wires containing various nanograined microstructures was investigated by tensile cyclic loading with in situ evaluation of electric resistivity. Defects created by the superelastic cycling in these wires were analyzed by transmission electron microscopy. The role of dislocation slip in superelastic deformation is discussed. Ni–Ti wires having finest microstructures (grain diameter <100 nm) are highly resistant against dislocation slip, while those with fully recrystallized microstructure and grain size exceeding 200 nm are prone to dislocation slip. The density of the observed dislocation defects increases significantly with increasing grain size. The upper plateau stress of the superelastic stress–strain curves is largely grain size independent from 10 up to 1000 nm. It is hence claimed that the Hall–Petch relationship fails for the stress-induced martensitic transformation in this grain size range. It is proposed that dislocation slip taking place during superelastic cycling is responsible for the accumulated irreversible strains, cyclic instability and degradation of functional properties. No residual martensite phase was found in the microstructures of superelastically cycled wires by TEM and results of the in situ electric resistance measurements during straining also indirectly suggest that none or very little martensite phase remains in the studied cycled superelastic wires after unloading. The accumulation of dislocation defects, however, does not prevent the superelasticity. It only affects the shape of the stress–strain response, makes it unstable upon cycling and changes the deformation mode from localized to homogeneous. The activity of dislocation slip during superelastic deformation of Ni–Ti increases with increasing test temperature and ultimately destroys the superelasticity as the plateau stress approaches the yield stress for slip. Deformation twins in the austenite phase ({1 1 4} compound twins) were frequently found in cycled wires having largest grain size. It is proposed that they formed in the highly deformed B19′ martensite phase during forward loading and are retained in austenite after unloading. Such twinning would represent an additional deformation mechanism of Ni–Ti yielding residual irrecoverable strains.

Martensite structure in Ti–Ni–Hf–Cu quaternary alloy ribbons containing (Ti,Hf) 2Ni precipitates

The martensite structure in a Ti 36Ni 44Hf 15Cu 5 ribbon annealed at different temperatures is investigated. When the annealing temperature is <873 K, spherical (Ti,Hf) 2Ni particles 20–40 nm in diameter precipitate in the grain interior. Transmission electron microscopy analysis shows that (0 0 1) compound twins are dominant in the ribbon containing homogeneously distributed (Ti,Hf) 2Ni precipitates. When the annealing temperature is 773 K, the boundaries between the martensite domains with the (0 0 1) twins are blurry and vague. When the annealing temperature is 873 K, four types of boundaries among the martensite domains are found: {1 1 1}, (0 0 1)//{1 1 1}, {1 1 3} and (1 1 0)//{1 1 3} types. When the annealing temperature is 973 K, the (0 1 1) twins become dominant, and the martensite variants show mainly spear-like and mosaic-like morphologies. However, martensite domains with (0 0 1) twins also exist around the coarse (Ti,Hf) 2Ni precipitates. Fine (Ti,Hf) 2Ni precipitates should be responsible for the improvement in shape memory effect and the superelasticity of Ti–Ni–Hf–Cu ribbons.

Two-Stage Martensitic Transformation in Thermal-Cycled Ti&lt;SUB&gt;40.5&lt;/SUB&gt;Ni&lt;SUB&gt;49.5&lt;/SUB&gt;Hf&lt;SUB&gt;10&lt;/SUB&gt; Shape Memory Alloy

Thermal-cycled Ti40.5Ni49.5Hf10 alloy exhibits B2↔B19′ martensitic transformation and two DSC transformation peaks appear after 10 thermal cycles. Transformation temperatures rapidly decrease during the early cycles. XRD tests reveal that both transformation peaks are associated with B2↔B19′ transformation. TEM observations indicate that as-homogenized B19′ martensite mainly consists of internal 〈011〉M type II twins. However, they are gradually replaced by internal (001)M compound twins with increasing the thermal cycles. This feature accounts for these two transformation peaks are associated with B2↔B19′ transformation with two different internal twinning modes. The microstructural evolution of B19′ martensite is ascribed to the dislocations induced by thermal-cycling as they can promote the formation of thin internal (001)M compound twins.

On the multiplication of dislocations during martensitic transformations in NiTi shape memory alloys

In situ and post-mortem diffraction contrast transmission electron microscopy (TEM) was used to study the multiplication of dislocations during a thermal martensitic forward and reverse transformation in a NiTi shape memory alloy single crystal. An analysis of the elongated dislocation loops which formed during the transformation was performed. It is proposed that the stress field of an approaching martensite needle activates an in-grown dislocation segment and generates characteristic narrow and elongated dislocation loops which expand on {1 1 0} B2 planes parallel to {0 0 1} B19′ compound twin planes. The findings are compared with TEM results reported in the literature for NiTi and other shape memory alloys. It is suggested that the type of dislocation multiplication mechanism documented in the present study is generic and that it can account for the increase in dislocation densities during thermal and stress-induced martensitic transformations in other shape memory alloys.

Microstructure and martensitic transformation behaviors of a Ti–Ni–Hf–Cu high-temperature shape memory alloy ribbon

The Ti36Ni41Hf15Cu8 melt-spun ribbon undergoes a B2 ↔ B19′ transformation upon cooling and heating. When the Ti36Ni41Hf15Cu8 melt-spun ribbon is annealed at 873 K for 1 h, the spherical (Ti, Hf)2Ni particles with a diameter of 20–40 nm precipitate in the grain interior. The fine (Ti, Hf)2Ni precipitates improve the stability of phase transformation temperatures and cause martensite domains, with (001) compound twins in three orientations dominant instead of (011) type I twins. {111}-, {113}- and (001)//{111}-type boundaries are observed among these martensite domains. When the (Ti,Hf)2Ni precipitates coarsen, (011) type I twins become main martensite structures in the ribbon annealed at 973 K for 1 h.

Cu content and annealing temperature dependence of martensitic transformation of Ti 36Ni 49− xHf 15Cu x melt spun ribbons

The martensitic transformation in Ti 36Ni 49− x Hf 15Cu x ( x = 1, 3, 5, 8) ribbons has been investigated. Only B2 to B19′ transformation was detected in all the present ribbons. The martensitic transformation temperatures do not change obviously with increase in the Cu content except that they decrease when the Cu content is 3 at.%. The lattice parameters of B19′ martensite, a and c increase, b almost remains constant, while the monoclinic angle β decreases with increase in the Cu content. For the ribbons with Cu content of 1 and 3 at.%, the martensitic transformation temperatures change slightly when the annealing temperature increases. For the ribbons with Cu content of 5 and 8 at.%, with increase in the annealing temperature, the martensitic transformation temperatures almost do not change and then decrease rapidly when the annealing temperature is higher than 873 K. TEM observation shows that the microstructure of the ribbons with Cu content of 1 and 3 at.% contains the martensite matrix and the (Ti,Hf) 2Ni particles with the size of about 150 nm, which does not change obviously when the annealing temperature increases. This results in that the martensitic transformation temperatures are not sensitive to the annealing temperature in the ribbons with 1 and 3 at.% Cu content. However, nano-scale (Ti,Hf) 2Ni particles precipitate in the ribbons with Cu content of 5 and 8 at.% when the annealing temperature is 773 and 873 K, and then the (Ti,Hf) 2Ni particles grow and coarsen rapidly with further increase in the annealing temperature. The coarsening of the (Ti,Hf) 2Ni particles should be responsible for the dramatic decrease of the martensitic transformation when the annealing temperature is higher than 873 K. For all the present ribbons, the substructure of B19′ martensite is (001) compound twins, and the inter-variant relationship is mainly (011) type I twinning.

Internal Defects of B19&rsquo; Martensite via R-Phase in Ti-Ni-Fe and Thermally Cycled Ti-Ni Alloys

Crystallography and morphology of B19’ martensite via R phase transformation in Ti-Ni-Fe alloy have been investigated by X-ray diffraction and transmission electron microscopy. The microstructure aspects of the B19’ martensite in the solution treated Ti-Ni-Fe alloy was the same as those in the solution treated binary Ti-Ni alloy. In the thermal cycled Ti-Ni-Fe alloy, the (001)B19’ compound twins frequently observed were the same as those in the thermal cycled binary Ti-Ni alloy. It is considered that the (001)B19’ compound twin was a kind of deformation twin to reduce the internal stress due to dislocations introduced during the thermal cycle, rather than a lattice invariant shear of the R to B19’ transformation.

Structure of martensite in sputter-deposited (Ni,Cu)-rich Ti–Ni–Cu thin films containing Ti(Ni,Cu) 2 precipitates

The martensite structure in sputter-deposited thin films of Ti 48.6Ni 35.9Cu 15.5 was studied. The Ti(Ni,Cu) 2 phase precipitates during the annealing process. Fine Ti(Ni,Cu) 2 precipitates can be deformed by the shear deformation of martensitic transformation, but they obstruct the movement of the twin boundaries to some extent. Coarse Ti(Ni,Cu) 2 precipitates seriously impede the growth of martensite plates and lead to a rectangular-cell-like structure of martensite in the film annealed at 873 K. The resistance of Ti(Ni,Cu) 2 precipitates to the growth of the martensite plates enhances with the coarsening of Ti(Ni,Cu) 2 precipitates, which is one of the reasons for the decrease in the maximum recoverable strain with increasing annealing temperature. B19′ martensite with (0 0 1) compound twinning is frequently observed near coarse Ti(Ni,Cu) 2 precipitates and grain boundaries in films annealed at 873 and 973 K. The local stress concentration should be responsible for the presence of B19′ martensite.

Stress-induced α″ martensitic (110) twinning in β-Ti alloys

A fully transformed α″ martensite with stress-induced nanoscale {110}, {021}-type compound twin or a 90° rotation twin has been experimentally explored and unambiguously characterized in traditional β-type Ti alloys, which usually undergo a partial martensitic transformation [a β grain partially transformed into α″ with internal (111) twin] by quenching. This newly observed twinning, which matches with the predication based on the deformation twinning theory of Bilby and Crocker [Proc. R. Soc. 288, 241 (1965)], can help to explain the deformation mechanism and aid future development of advanced materials.

Size effects on martensitic phase transformations in nanocrystalline NiTi shape memory alloys

Results of a systematic study are presented to review various effects of crystal size on the martensitic phase transformations in nanocrystalline NiTi shape memory alloys. The transformation temperatures and the transformed volume fraction strongly decrease with decreasing grain size less than about 100 nm. Transformation to martensite is not observed in grains smaller than a critical grain size of about 50 nm. The nanograins significantly impact the morphology of B19′ martensite composed of (001) compound twins that occur at an atomic scale and violate the well established theory of martensite formation. Self-accommodation occurs by a herringbone morphology of two twinned variants. Contrary to the martensite, grain size hardly impacts the transformation to the R-phase. The experimental results are explained by a size dependent transformation barrier that accounts for the suppression of the martensitic transformation, its thermal stability and unique morphology in the nanograins.

Crystallography and morphology of twins in equiatomic TiPt martensite

Twins in the B19 (2H) martensite in Ti–Pt shape memory alloy have been investigated by transmission electron microscopy. There are three twinning modes evident, i.e. {111}2H Type I, <121>2H Type II and {101}2H compound twins. In addition to the 2H martensite, various long period stacking structures are observed such as 21¯12¯ stacking and incommensurate like modulated martensites. It is frequently observed that two martensite structures derived from the identical closed packed plane of the parent phase coexist in the same area, namely, ‘two-in-one’ morphology. The crystallographic aspects of the ‘two-in-one’ morphology are also discussed.

Transmission electron microscopy study of the microstructure of B19 martensite in sputter-deposited Ti 50.2Ni 30Cu 19.8 thin films

The martensite structures in Ti 50.2 Ni 30 Cu 19.8 thin films were studied by transmission electron microscopy. The single-pair martensite morphologies with (0 1 1) or (1 1 1) twins are usually observed. (1 1 1) type I twins are dominant in the film annealed at 773 K, whereas (0 1 1) compound twins are dominant in that annealed at 973 K. The (0 1 1) twin width decreases dramatically when the annealing temperature changes from 773 to 973 K. The grain boundary precipitates in the films annealed at 973 K are responsible for these changes.

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, Ti 2Cu precipitates and spherical Ti 2Ni 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 Ti 2Ni precipitates, the amount of martensite with a single-pair morphology is less than that in thin films without precipitates. And in thin film with Ti 2Cu + Ti 2Ni 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 Ti 2Cu precipitates can change both the width and the direction of the martensite plates. Ti 2Ni 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.

Effect of special microstructure on the mechanical properties of nanocomposite

Al 2O 3 nanobelts were observed in the material Al 2O 3/SiC μ/SiC n nanocomposite with high-resolution transmission electron microscopy (HRTEM), the forming of ductile nanobelts is helpful for the improvement of fracture toughness. In addition, the special microstructure such as dislocations and compound twins were also observed in the nanocomposite with TEM, the complicated dislocation configurations and the forming of twins in larger SiC particles contributed to the improvement of fracture toughness by dissipating fracture energy. The Al 2O 3/SiC interface bonding was strengthened which also lead to the high mechanical properties of the nanocomposite.

Composition dependent crystallography of α″-martensite in Ti–Nb-based β-titanium alloy

Composition dependence of crystallographic features of α″-martensite in Ti–Nb-based alloy was systematically examined in Ti–Xmol%Nb–3mol%Al alloy, where X is 10–30. One of the lattice deformation strains, η3, became 0 at the critical Nb concentration of 23 mol% and the sign of η3 changed across the composition. The phenomenological theory of martensite crystallography (PTMC) predicted that the lattice invariant shear (LIS) would vanish at the critical composition. Transmission electron microscopy revealed that non-twinned martensite was formed with a habit plane of {755}b when η3 = 0, and that martensite with an internal twin of {111}o type I was formed with a {443}b habit plane for η3 = +0.008. Subscript b and o indicate β and α″ lattice, respectively. The crystallography of these two α″-martensites were well explained by the PTMC. On the other hand, non-twinned martensite with {111}b–{775}b habit plane was formed even when η3 was not zero. It was found that there is a critical value of |η3| for the formation of twinned martensite and that class B transformation, in which LIS is a {011}o compound twin, hardly occurs in β-titanium alloys.