A novel B19′ martensite in nickel titanium shape memory alloys

A new type of the B19′ (monoclinic) Ni–Ti martensite—one which is internally twinned by the (0 0 1)_m compound twinning mode, has been found in thermally cycled Ni–Ti shape memory alloys. This is an unexpected and remarkable martensite type on account of the fact that the (0 0 1)_m compound twinning mode does not qualify to occur as the fine structure in the B2–B19′ martensite transformation. On the basis of the good concurrence of the observed crystallographic parameters with those predicted by the phenomenological theory of martensite transformations, it has been determined that this martensite is a product of the R phase-B19′ martensite transformation. However, the (0 0 1)_m compound twinning mode can qualify to occur as the lattice invariant shear (LIS) only when the rhombohedral angle of the R phase is less than the critical value of 86.2°. The preference of this twinning mode as the LIS to the [0 1 1]_m Type II twinning mode, which is the normally observed substructure of the Ni–Ti martensites, has been rationalized to be due to the closer proximity of the orientation relationships to the lattice correspondence and the lower magnitudes of twinning shear, shape strain shear, twin interface energy and nucleation strain energy.     

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.     

Two-stage reverse transformation in hyperstabilized β1′ martensite

A Cu–Al–Be alloy is hyperstabilized by quenching into β₁^′ martensite: only a fraction of martensite is retransformable to bulk β-phase at temperatures approximately 100 K above the nominal transformation temperatures. The rest of the hyperstabilized martensite undergoes the second stage of reverse transformation at much higher temperatures by means of the nucleation of lamellar β-phase.     

NiTi合金B2-B19′马氏体相变晶体学的拓扑模拟研究

运用拓扑模型研究了等原子比NiTi合金B2-B19'马氏体相变晶体学,根据最优扭转角(wo)准则计算得到wo=-0.969°,并获得了马氏体惯习面指数及母相-马氏体相界面位错结构特征,计算结果与实验测量值非常接近。NiTi合金马氏体相变所产生的相变应变包含一个平行于惯习面的剪切应变和一个垂直于惯习面的轴向应变,轴向应变量表示B2-B19'相变所引起的宏观体积变化为εHP33=6.6879×10-3,表明NiTi合金的负热膨胀现象来源于合金中马氏体相变所产生的相变应变。     

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.     

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.     

Crystal structure and thermal stability of martensite in Cu-24Al-3Mn alloy

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Crystal structure and thermal stability of martensite in Cu-25Al-3Mn alloy

The martensite structure in Cu-25Al-3Mn alloy and its thermal cycling and aging behavior are studied. It is convinced that the M2H martensite can be obtained by water-quenched, and the atoms distribution on the basal plane of the mart ensite is: Ⅰ(corner)-Al; Ⅱ(center of the plane)-Cu; Ⅲ(middle of b- side)-22/25Cu+3/25Mn. The lattice parameters are determined to be a=0.445 9 nm, b=0.527 9 nm, c=0.424 1 nm, β=88.64°. The triangle and other complicated configurations consisting of the variant group in the martensite are discovered. It is showed that the tested alloy has a high thermal stability when aging at average temperature in the parent phase, and the thermoelastic martensite amount is up to 90% af ter aging for 96 hat 400 ℃. The thermal cycling has a little influence on the transform ation temperature (Ms). When the number of thermal cycles is up to 1000, the increasing of Ms is only 8 ℃.     

On the instantaneous stabilization in Cu–Al–Be β1′ martensite

Cu–Al–Be β₁^′ martensite exhibits, upon various heat treatments, stabilization which is independent of ageing time in the martensitic state. The registered instantaneous stabilization effect is supposed to occur during the martensitic transformation itself. Two mechanisms are suggested:

• •“kinetic” stabilization during the martensitic transformation due to sweeping-up of quenched-in defects by moving interfaces;
• •non-elastic accommodation of variants during first direct transformation under combined action of transformation and thermal stresses.

     

The structure and mobility of the intervariant boundaries in 18R martensite in a Cu–Zn–Al alloy

Detailed crystallographic analysis was carried out on the martensitic transformation and the various variant combinations in 18R martensite in a Cu–Zn–Al alloy. The self-accommodation of martensitic shear strain is quite perfect within a variant group, but not effective or even does not exist for variant combinations which belong to different groups. Twenty-three unique variant combinations between 24 martensite variants can be divided into four groups, i.e. reflection twin, 180° rotation twin, 120° rotation twin and 90° rotation twin. TEM and HREM observations show that the A/C boundary is straight, well-defined and perfectly coherent, the A/B boundary is irrational, coherent and gradually curved, and the A/D boundary is stepped. The A/C and A/B boundaries have obvious mobility, and the mobility is not effective for A/D boundary. The interplate group boundaries are curved, blurred and immobile. The morphology, structure and mobility of interplate boundary are all related to the degree of self-accommodation and the misorientation of twin boundary.     

Study of precipitation and growth of γ′ and dislocation structure in Inconel X-750

The precipitation of the γ′ (Ll₂) phase in Inconel X-750 has been studied in the 704–871°C ageing temperature range using transmission electron microscopy. The morphology of γ′ was observed to be cuboidal for all ageing temperatures, deviating from this only at long ageing times. The loss of coherency of γ′ precipitate occurred by nucleation of dislocation loops at and around the precipitate, interaction of dislocations at the interface, and the attraction of dislocations toward the particle/matrix interface.     

Stability of B2 phase in Ti–Ni–Fe alloys against MeV electron-irradiation-induced solid-state amorphization and martensite transformation

The amorphization tendency of a B2 phase in Ti₅₀Ni_50?xFe_x (x = 2 to 40 at%) intermetallic compound was investigated in order to determine the relationship between solid-state amorphization (SSA) and martensite transformation. SSA was observed in the B2 phase at 103 K and 298 K by high-voltage electron microscopy (HVEM). The total dose required for the amorphization increased with the Fe content. The replacement of Ni by Fe increased the phase stability of the B2 phase in Ti–Ni–Fe intermetallic compound and suppressed SSA as well as martensite transformation.