The B2 To R Transformation in Ti50Ni47Fe3 and Ti49.5Ni50.5 alloys

In near equiatomic NiTi alloys, selected area electron diffraction patterns, obtained by high-angle-tilting, cold-stage experiments, have been used to determine the unit cell dimensions of the R phase and the orientation relationship between the parent B2 and product R phase. Convergent beam electron diffraction patterns establish the point group of the R phase to be 3̄m and hence of rhombohedral symmetry as has been postulated by other investigators. Differential scanning calorimetry shows the B2 to R transformation to be a first-order phase transformation with an enthalpy of transformation of 3.75 J/g.     

Damping characteristics of TiNi shape memory alloys

The damping characteristics of TiNi SMAs have been systematically studied by using techniques of resonant-bar and low-frequency inverted torsion pendulum. Experimental results show that both the martensite phase (M) and R phase (R) have high damping due to the movement of twin boundaries. Because the B2 parent phase (B2) has smaller damping, it is suggested that this may come from the dynamic ordering process of lattice defects. In the transformation re-gions of B2 ↔ M, B2 ↔ R, and R ↔ M, there are maxima of the damping capacity which are attributed to two contributions. One arises from the plastic strain and twin-interface move-ment during the thermal transformation, which obeys a linear variation of peak heightsQ ⁻¹ _max vst att ≥ 1 °C/min. The other originates from the stress-induced transformation formed by the applied external stress which dominates atT < 1 °C/min. The elastic modulusE of martensite and the R phase is lower than that of the B2 phase, and a modulus minimum appears in the transformation region.     

Mechanical twinning in Ti50Ni47Fe3 and Ti49Ni51 alloys

Pseudo-twinning and mechanical twinning have been observed in a transmission electron microscopy study of Ti₅₀Ni₄₇Fe₃, and Ti₄₉Ni₅₁ alloys which have the B2(CsCl) structure. Observation of twinning in ordered alloys is rare and this is the first observation of twinning reported in a B2 structure. The twin planes are the {112} and {114} planes. For {112} pseudo-twins, the composition plane is not the twin plane and the pseudo-twin does not have the B2 structure. For {114} mechanical twins, the composition plane is the twin plane and the twin does have the B2 structure. It is shown that a shear on the {114} plane plus a shuffle of the atoms results in the ordered B2 structure in the twinned region.     

Synthesis,phase study and magnetic characterisation of Co50Fe40Cu10 ternary alloy nanopowders prepared by mechanochemical alloying process

Abstract

Mechanical milling and hydrogen reduction of pure oxide mixture and magnetic characterisation of Co–Fe–Cu ternary alloy nanopowders were investigated. A powder mixture of Co₃O₄, CuO and Fe₂O₃ with Co₅₀Fe₄₀Cu₁₀ stoichiometry was first milled by a high energy planetary ball mill and then reduced in a hydrogen reduction system.

The optimum condition of the reduction under the hydrogen atmosphere was 650°C and 1 h. The X-ray diffraction patterns exhibit that the powder has ordered bcc structure with b2–bcc space group and 2·87 Å lattice parameter. Mean crystallite sizes calculated from X-ray diffraction results and mean particle size observed from electron microscopes were over 75 nm. Magnetic evaluation of ternary alloy nanopowders showed a saturation magnetisation value about 143 Am² kg^–1 and a low coercivity value of 0·93 Am^–1.     

Microstructural study of Ti50Pd50 and Ti50Ni50−xPdx alloys produced by combustion synthesis

The microstructure of the Ti-Ni-Pd ternary shape memory alloys (SMAs) produced by combustion synthesis was studied by scanning electron microscopy/energy-dispersive X-ray analysis (SEM/EDAX) and X-ray diffraction methods. It was found that, apart from the parent phase, Ti₅₀Ni_50-xPd_x, there also exists a second phase with the stoichiometry of (Ni_1−y, Pd_y,)Ti₂. Higher palladium and lower nickel contents were found in the parent Ti-Ni-Pd phase compared with the theoretical, ideal solution values. Lower palladium and higher nickel contents were found in the second (Ni_1−y,Pd_y)Ti₂ phase. These discrepancies were explained in that palladium has a greater affinity for titanium than for nickel. The X-ray diffraction data show that the structure of the parent phase is orthorhombic (B19), and the orthorhombic unit cell contracts with in-creasing nickel content. This paper is based on a presentation made in the symposium “Reaction Synthesis of Materials” presented during the TMS Annual Meeting, New Orleans, LA, February 17-21, 1991, under the auspices of the TMS Powder Metallurgy Committee.     

Wear characteristics of TiNi shape memory alloys

The wear characteristics of TiNi shape memory alloys against Cr-steel have been studied. Experimental results indicate that the Ti₄₉Ni₅₁ alloy can exhibit a better wear resistance than Ti₅₀Ni₅₀ alloy due to their higher hardness and pseudoelastic behaviors. Four main mechanisms, adhesion, abrasion, surface fatigue, and brinelling, are found to have important contributions to the wear characteristics of TiNi alloys. The weight loss increases with increasing wear load and sliding distance but decreases with increasing sliding speed. The contact area during sliding wear will be increased due to the variant accommodation and/or pseudoelasticity and, hence, will reduce the average compressive stress and wear damage. Variant accommodation and/or pseudoelasticity can also stabilize the crack tips and hinder crack propagation, hence improving the wear characteristics of TiNi alloys.     

Quasi-static constitutive behavior of Zr41.25Ti13.75Ni10Cu12.5Be22.5 bulk amorphous alloys

Mechanical tests have been performed on bulk amorphous metal alloys to determine their constitutive behavior. Based on the experimental results, it appears that amorphous metal alloys obey a Von Mises yield criterion. This result has implications in determining the micromechanisms of plastic deformation in these materials.     

Deformation,twinning and thermo-mechanical strengthening of Ti50Ni47Fe3

The dislocation Burgers vectors for the B2 intermetallic compound, Ti₅₀Ni₄₇Fe₃, are shown to be the 〈010〉 type, which provide this alloy with only three independent slip systems. However, concurrent dislocation slip and mechanical twinning on {114} planes affords the polycrystalline material with greater than 50% room temperature ductility. Individual twins do not grow greater than 50–150 nm in width, and the twinning density increases with the extent of cold working. Annealing a cold worked structure, comprised of mechanical twins and dislocations, results in the formation of subgrains with a size limited by the width of the twins. Greater cold working produces a finer twin spacing and, subsequently after annealing, a refined subgrain size which in turn brings about an improved combination of yield strength and ductility.     

Crystallization of the amorphous alloys Fe50Ni30B20and Fe80oB20

Metallurgical and Materials Transactions A - The structure and growth characteristics of crystals growing in the amorphous alloys Fe50Ni30B20 and Fe8oB20 were studied by transmission electron...     

Influence of aging on transformation characteristics in shape memory CuZnAl alloys

Shape memory alloys exhibit superelasticity when they are deformed in a temperature range where the thermoelastic martensite forms on application of a strain. The martensite persists upon removal of the applied strain, and the alloy recovers the original shape on heating over the reverse-transformation temperature after removing the strain. The β-phase CuZnAl alloys have β-type superlattice in the parent case, and M9R or M18R martensites occur on quenching the alloys from the homogenization temperature. The basal plane of martensite is exposed to hexagonal distortion with martensitic transformation as well as the monoclinic distortion in the crystal structure, and splittings are observed in some selected diffraction-peak pairs due to the differences in atom sizes in lattice points. These pairs have a great importance as ordering criteria and satisfy a special relation between Miller indices. The present text reports the variation of the differences in interplane spacings (Δd) between some selected planes upon the further aging at room temperature at which alloys are fully martensitic. The decrease of Δd during the aging implies that the monoclinic distortion decreases. The mass increases are caused by the oxidation upon heating the alloys at high temperatures close to the betatizing temperature at free atmosphere.     

Cu基形状记忆合金的阻尼及记忆效应的应用

综合回顾了Cu基形状记忆合金的研究发展现状,并概述了其阻尼机制及分类,分析了Cu-Al、Cu-Zn系合金的阻尼能力和影响因素,并介绍了Cu基形状记忆合金的应用,提出了其应用推广前要解决的问题.最后给出了一些应用实例.     

Diffusivity of hydrogen and deuterium at low concentration in Fe0.5Ti0.5

The diffusion coefficients of hydrogen and deuterium were measured in the solid solution phase Fe_o.5Ti_o.5Hv/Dv (x < 0.03) in an absorption/desorption experiment with palladium coated bulk samples. The pressure and temperature ranges covered were 1 to 120 bar and 370 to 800 K, respectively. Forx → 0, the diffusion coefficients obey Arrhenius laws. They are (in cm₂ per second withT in K): 10.1 10^?4 exp(-5890/T) and 5.5 10^?4 exp(?5710/T) for hydrogen, 8.2 10^?4 exp(?6080/T) and 5.5 10^?4 exp(?5980/T) for deuterium, as measured during absorption and desorption, respectively. The pick-up and release of hydrogen by uncoated samples was controlled predominantly by surface effects.