EBSD studies of the stress-induced B2–B19′ martensitic transformation in NiTi tubes under uniaxial tension and compression

In situ electron backscattering diffraction (EBSD) investigations were conducted on polycrystalline NiTi tube specimens during tensile and compressive deformation. The long-range cooperative and catalytic martensitic transformation under tension induces the transformation to proceed in the form of helical Lüders band. Propagation of the band is closely related to the spatial distribution of the orientations of individual grains. In uniaxial compression, the larger variation in Schmid factors, and consequently the larger variation in the critical transformation stresses among grains, leads to a homogeneous martensitic transformation, and therefore the absence of the Lüders band. To interpret the observed tension–compression asymmetry, a crystallographic model of the critical transformation stress and transformation strain for polycrystalline NiTi under tension and compression is proposed. The model defines three crystallographic regions: tension-favorable, compression-favorable and neutral zones. The orientation population in which tensile strains are larger than compressive strains is much higher than that of orientations with higher compressive strains. For resolved shear stress, orientation populations favoring tension and compression do not show any great difference.     

Study of the martensitic transformation in the Co–9 at % Al alloy

Phase transformations in the Co–9 at % Al have been investigated after slow furnace cooling. It has been shown that the structure and phase composition of the alloy after slow cooling do not correspond to the equilibrium phase diagram of the alloy of this chemical composition. It has been established that the α → ε martensitic transformation does not require overcooling and occurs even during a slow cooling of the alloy. It has been found that the formation of 4H modulated martensite is a specific feature of the binary alloys of cobalt and is not connected with the rate of their cooling. The Curie temperatures for the B2, α, and ε phases have been determined.     

Analytical modeling of stress-induced martensitic transformation in the crack tip region of nickel–titanium alloys

A novel analytical method, which predicts the extent of the stress-induced martensitic transformation (SIM) in the crack tip vicinity of nickel–titanium (NiTi)-based shape memory alloys, as well as describing the stress distribution in both transformed and untransformed regions, is presented. The method has been validated by comparisons with results from finite-element simulations with good agreement. Furthermore, the method has been used to analyze the effects of various thermomechanical parameters on the extent of the transformation region near the crack tip. Finally, the effects of several thermomechanical loading conditions, in terms of both applied stress and temperature, on the crack tip transformation behavior have been analyzed. The results highlight a marked effect of temperature on the extent of the transformation region and, consequently, on the crack tip stress distribution. As a consequence, temperature plays a role in the fracture process of NiTi alloys.     

Enhancement of ductility in B2-type Zr–Co–Pd alloys with martensitic transformation

The relationship between the microstructures and the mechanical properties in the ternary Zr–Co–Pd alloys has been investigated. X-ray diffraction measurements at room temperature indicated that Zr₅₀Co_50?xPd_x alloys underwent martensitic transformation from B2 to B33 structures as a result of substituting Pd for Co at amounts greater than 12 at%. The yield and tensile strength increased at a substitution amount of 6 at%. The total elongation remarkably increased as a result of substituting Pd for Co at amounts up to 10 at%; in particular, the Zr₅₀Co₄₀Pd₁₀ alloy exhibited a considerably high total elongation of 22%. Deformation-induced martensite with a lenticular morphology was observed in the B2 parent phase near the fractured edge, in addition to dislocation with the <100>_B2-type Burgers vector. Consequently, we conclude that the remarkable enhancement of the ductility is due to the transformation-induced plasticity associated with the deformation-induced martensite.     

Prediction of crystallographic characteristics of martensitic transformation in a Ni–Mn–Ga based Heusler alloy

The energy minimization theory was applied to analyze the crystallographic features of the martensitic transformation in the alloy Ni_52.5Mn_23.5Ga₂₄ and the results obtained were compared with those determined by experiments. It is revealed that the index of the boundary between martensite variants belongs to the lattice plan family of {112}_M and the orientation relationship between the martensite variant 1 and 2 is characterized by a rotation of 82.1° about [1 0 0]_M1/[0 1 0]_M2. The orientation relationship between the parent phase and the martensite features a rotation of approximately 4° about the c axis of the parent lattice from the position of the conventional Bain relationship.     

Effect of grain boundary character on the martensitic transformation in Fe–32at.%Ni bicrystals

The effect of grain boundary character on the martensitic transformation was examined in two types of Fe–32at.%Ni bicrystals containing a 90°<211> tilt or a 90°{211} twist boundary focusing on the martensite-start temperature (Ms) during cooling, the morphology of the martensite and the variant selection. The Ms of bicrystals with a tilt boundary was higher than that of single crystals, while bicrystals with a twist boundary showed no significant difference from that of single crystals. Coarse lenticular martensites formed symmetrically in neighbouring grains around the tilt boundary. In contrast, tiny martensites were uniformly distributed in bicrystals with a twist boundary, and in single crystals. In the vicinity of the tilt and twist boundaries, some variants with the habit plane almost parallel to the boundaries were preferentially selected among 24 variants. Moreover, since the equivalent variants in neighbouring grains at the tilt boundary are selected, strain compatibility of the shape strain of martensite across the tilt boundary is satisfied, resulting in an increased Ms. The effect of pre-strain on the heterogeneous nucleation of martensite at the boundaries was also investigated.     

Kinetics of austenitization under uniaxial compressive stress in Fe–2.96 at.% Ni alloy

Differential dilatometry has been employed to study the kinetics of the massive ferrite (α) → austenite (γ) transformation upon isochronal heating (i.e. austenitization) of the substitutional Fe–2.96 at.% Ni alloy subjected to a range of applied constant uniaxial compressive stresses. A phase-transformation model, involving site saturation, interface-controlled (continuous) growth and incorporating an impingement correction for an intermediate of the cases of ideally periodically and of ideally randomly dispersed growing particles, has been employed to extract the interface-migration velocity of the α/γ interface and the transformation-induced deformation energy taken up by the specimen. The value obtained for the energy corresponding with the elastic and plastic deformation associated with the accommodation of the α/γ volume misfit depends on the austenite fraction and increases distinctly with an increase in the applied uniaxial compressive stress, which is compensated by, in particular, an increase in the chemical driving force corresponding to an increase in the onset temperature. The opposite effects of an applied uniaxial compressive stress on the α → γ transformation and on the γ → α transformation can be discussed as the outcome of constrained plastic deformation due to transformation-induced strain.     

Interaction between martensitic structure and defects in β ↔ β′ + γ′ cycling in CuAlNi single crystals. Model for the inhibition of γ′ martensite

The authors have previously reported an estimate of the energy associated with the inhibition effect of γ′ martensite after β ↔ β′ + γ′ cycling in CuAlNi single crystals. In this paper, a microscopic model is proposed to explain the γ′ inhibition, related to the localized interaction between a dislocation array and the twinned γ′ structure. Dislocations with Burgers vector [1 0 0]_β and line direction [1 1 1]_β in an isotropic β matrix are considered. The model takes into account the interaction between the martensitic stress-free transformation strains and the stress field created by the dislocation arrays. It is shown that the interaction is different for each twin-related variant in the γ′ martensite. The energy necessary to maintain the right volume relationship of the twinned γ′ variants to produce an undistorted β/γ′ habit plane is defined as the inhibition energy. A value of around 12 J mol⁻¹ was obtained, which is in reasonable agreement with experimental results.     

The role carbon plays in the martensitic phase transformation of an Fe–Mn–Al alloy

We observed direct evidence that 18R martensite is induced by carbon atoms in the BCC grains of an Fe–27.0wt.%Mn–5.3wt.%Al–0.1wt.%C alloy via high-temperature quenching. A single BCC phase structure formed 18R martensite in the present study. The lowest carbon content found for the formation of 18R martensite is 0.035 wt.% in Fe–Mn–Al alloys.     

Creep- and coarsening properties of Al–0.06 at.% Sc–0.06 at.% Ti at 300–450 °C

Upon aging at 300–450 °C, nanosize, coherent Al₃(Sc_1−xTi_x) precipitates are formed in pure aluminum micro-alloyed with 0.06 at.% Sc and 0.06 at.% Ti. The outstanding coarsening resistance of these precipitates at these elevated temperatures (61–77% of the melting temperature of aluminum) is explained by the significantly smaller diffusivity of Ti in Al when compared to that of Sc in Al. Furthermore, this coarse-grained alloy exhibits good compressive creep resistance for a castable, heat-treatable aluminum alloy: the creep threshold stress varies from 17 MPa at 300 °C to 7 MPa at 425 °C, as expected if the climb bypass by dislocations of the mismatching precipitates is hindered by their elastic stress fields.     

Model of the B2 → R martensitic transformation in alloys with a B2 superstructure

Various B2 ?? R structural transformations that can occur through an intermediate R ₀ structure belonging to the space group D _3d ³ have been considered. Subgroups with a hexagonal lattice (group D _3d ³ ) related to the symmetry breakdown of the R ₀ structure due to atomic displacements with wave vectors belonging to the two-arm star $K_{13}^H = \left\{ {k_1^H = \frac{1} {3}\left( {b_1^H + b_2^H } \right),k_2^H = - k_1^H } \right\}$ , where b ₁ ^H and b ₂ ^H are the reciprocal-lattice vectors of the hexagonal system, have been calculated. A crystallographic model of the possible transformations of the B2 structure into the R martensite, which has a single (common for all transformations) order parameter, has been suggested.     

Phase-field simulations of α → γ precipitations and transition to massive transformation in the Ti–Al alloy

A phase-field model for the solid–solid α → γ transition of Ti–Al binary alloys is presented based on analytical Gibbs free energies and couplings to the thermodynamical database ThermoCalc. The equilibrium values recover the α + γ phase boundaries. Morphological transitions from diffusive to massive (partitionless) growth are observed on increasing the initial mole fraction of aluminum. Temporal evolution of the interface shows a $\sqrt{t}$ behavior for diffusive and a linear behavior for massive growth, which is in accordance with theoretical predictions. An estimate of the interfacial mobility of Ti–Al based on the Burke–Turnbull equation is calculated. The expression of the mobility follows an Arrhenius law. Using the derived interfacial mobility, the calculated interfacial velocities of the massive transformation are in quantitative agreement with those observed in experiments.     

In situ synchrotron X-ray diffraction study of stress-induced martensitic transformation in a metastable β-type Ti-33Nb-4Sn alloy

In this study, the stress-induced martensitic (SIM) transformation of a recently developed metastable β-type Ti-33Nb-4Sn alloy consisting of a mixture of β and α″ phases are investigated by in situ synchrotron X-ray diffraction (SXRD). It is shown that though the SIM transformation covers a wide strain range, some remaining β phase is still observed after loading, indicating that the SIM transformation is incomplete. During SIM, the parameter of b_α″ increases with macroscopic strain within the strain range of 1.5–4.7%, while the parameters of a_α″ ([100]_α″) and c_α″ ([002]_α″) remain constant or decrease with increasing strain, respectively. This provides a plausible explanation for why the (020)_α″ peak intensifies, but the (002)_α″ peak decreases and even eliminates in the loading direction during loading. Additionally, the activation sequence of different deformation mechanisms is clarified unambiguously.     

Effect of grain boundary orientation on radiation-induced segregation in a Fe–15.2 at.% Cr alloy

Ferritic chromium steels are important structural materials for future nuclear fission and fusion reactors due to their advantages over traditional austenitic steels, such as higher thermal fatigue resistance, lower thermal expansion coefficients and reduced swelling. However radiation-induced segregation or depletion (RIS/RID) of solute atoms at grain boundaries in these materials is a concern because these phenomena could adversely affect their mechanical properties. In an effort to develop a full mechanistic understanding of RIS/RID, a systematic approach combining orientation imaging, site-specific specimen preparation and three-dimensional atomic-scale analysis has been developed to characterize the behaviour of Cr and C at grain boundaries during irradiation. This methodology has been applied to a Fe–15.2 at.% Cr alloy to investigate the effects of grain boundary misorientation, irradiation depth and impurities. Systematic differences in Cr segregation are reported as a function of grain boundary character and irradiation conditions. The similar properties demonstrated by grain boundaries of similar type means that it should be possible to apply relatively simple models to predict the long-term behaviour of these materials under irradiation conditions.     

Non-steady state carburisation of martensitic 9–12%Cr steels in CO2 rich gases at 550 °C

Two martensitic steels were reacted with Ar–50%CO₂ at 550 °C for exposure times up to 150 h. Microstructural analyses and glow discharge optical emission spectroscopy revealed that internal carburisation of both steels beneath external oxide scales occurred, in spite of the very low equilibrium carbon activity of the atmosphere. The carbon concentration at the metal–scale interface increased throughout the reaction and the carbon uptake varied approximately linear with time. A model is proposed to describe the non steady-state carburisation kinetics whereby the usual diffusion equation describing carbon movement into the alloy is modified to account for loss of carbon as precipitated carbides.     

The type of order in Cu–10 at.% Au—evidence from the diffuse scattering of X-rays

In a recent publication, Kuhlmann et al. (Acta mater., 1997, 45, 3319) claimed Cu–10 at.% Au to be long-range ordered at temperatures above 473 K and as high as 723 K. This is in contrast to the generally accepted phase diagram where this region is attributed to the range of solid solutions. To clarify this problem, a three-dimensional diffuse X-ray scattering experiment was performed at room temperature on a Cu–10 at.% Au single crystal aged at 573 K. The Warren–Cowley short-range order parameters derived from this experiment show conclusively that only short-range order is present. Effective pair interaction parameters determined by the inverse Monte-Carlo method are somewhat different from those obtained for Cu–25 at.% Au in the short-range ordered state.     

New modelling of the B2 phase and its associated martensitic transformation in the Ti–Ni system

A symmetric two-sublattice model (Ni, Ti, Va)_0.5(Ni, Ti, Va)_0.5 is applied to describe the intermediate B2 compound in order to cope with the order–disorder transition in the Ti–Ni system. Using this model, the ordered B2 and the disordered Ti-rich b.c.c. are described by a single Gibbs free energy function. The B2 phase is the parent phase of the martensitic transformation in the TiNi shape memory alloys (SMAs), and its thermodynamic properties are then reassessed with emphasis on its composition range that is critical for SMAs. The low temperature B19′ phase is also evaluated on the basis of the selected experimental data from the martensitic transformation. Properties related to the transformation are studied in comparison with experimental data. The magnetic contribution is examined for the martensitic transformation. All calculations are in satisfactory agreement with experimental phenomena.