Bcc Type Research Articles

Phase decomposition of liquid-quenched β-type Ti-Cr alloys

Phase decomposition behaviour of liquid-quenched β (bcc) type Ti-Cr alloys was investigated by means of transmission electron microscopy and hardness measurements. It was found that decomposition of β to β1 (Ti-rich, bcc) + β2 (Ti-lean, bcc) takes place in the intermediate composition range of the Ti-Cr system. This experimental result proves the theoretical prediction made by Menon and Aaronson, but the observed β1 + β2 two-phase field expands towards higher temperatures than the predicted binodal line. The coherent β1 + β2 two-phase state exhibits the so-called 〈100〉 modulated structure and it was concluded that the formation of such a structure is a result of spinodal decomposition of the β-phase. We obtained time-temperature-transformation (TTT) diagrams of β-type Ti-30, 40 and 50 at % Cr alloys. A typical sequence of structural change is β → coherent β1 + β2 → incoherent β1 + β2 → incoherent β1 + β2 + grain boundary precipitates → stable state of β + TiCr2 or α + TiCr2. Not all the states in the above sequence appear, depending on alloy composition, liquid-quenching rate and ageing temperature.

Structure analysis of liquid Rb and Cs

The observed correlation functions $g(r)$ on liquid Rb and Cs have been compared with theoretical correlation functions which were calculated by means of three-dimensional convolution polynoms. These are given in terms of paracrystalline distorted body-centered tetragonal lattices with $\frac{c}{a}=1.23$. The lattice types of fcc and bcc can be excluded. The present method provides detailed statistical information about the short-range atomic arrangement in the liquid state. From the macroscopic density of the liquids and the nearest-neighbor average distance it can be concluded that about 4-7% of the atoms lie on interstices.

The enthalpy of solution for solid binary alloys of two 4d-transition metals

A demonstration is given how to derive the enthalpy of solid solution of alloys of two 4d transition metals for the three main crystallographic structures, fcc, hcp and bcc. Because of two additional terms the enthalpy of solution for solid phases differs from that in the corresponding liquid alloys. The first of these, accounting for elastic mismatch, can be estimated from Eshelby/Friedel-type elastic continuum theory. In our case we correct the atomic size mismatch for volume changes that accompany the formation of alloys. The second term accounts for the variation of structure dependent energies (i.e. the preference for either the fcc, bcc or hcp type of crystal structure) with the average number of electrons per atom. We derive an empirical curve for these structure dependent enthalpy terms, which is essentially based on theoretical results and gives a fair account of binary phase diagram characteristics.

Magnetic and X-Ray Studies on (Fe1-xVx)3Ge

The ternary system (Fe1-xVx)3Ge has three types of structure, hcp(D019), bcc(A2 or L21), and A15, which are stable 0≦x≦0.10, 0.17≦x≦0.28, and 0.55<x≦1.0 respectively. The magnetization of the ferromagnetic hcp and bcc alloys decreases linearly with x. The paramagnetism of the A15 alloy varies with substitution of Fe from the Pauli type to the Curie-Weiss type. The magnetic properties and the phase boundaries of the bcc alloy suggest that the structure is of the L21 type.

High temperature creep in a semi-coherent NiAl-Ni2AlTi alloy

Creep experiments have been made on a Ni-Ti-Al alloy, which has a microstructure consisting of a distribution of semi-coherent NiAl(β) precipitates with a Ni2AlTi(β′) Heusler phase matrix. The creep strength of this bcc type structure alloy is at least comparable with that of the nickel-base superailoy MARM-200 for values ofT/T m in the range 0.68 to 0.82. Quantitative electron microscope experiments show that both undissociated α0〈110〉 dislocations, and paired α0〈100〉 dislocations coupled by a sublattice A.P.B. exist within the β′ phase;α 0 is the lattice parameter of a bcc cell of which the large Ni2AlTi unit-cell is composed. The sublattice A.P.B. is a crystallographic fault created by wrong bonds between atoms on the Al-Ti sublattice. Theoretically the energy γ of a sublattice A.P.B. is shown to be minimum on {100}, and the experimental value for γ on {100} is ~40 mJ/m2.

Martensitic transformation in Zr−Ti alloys

Martensitic transformation in a series of Zr−Ti alloys has been studied by transmission electron microscopy. A transition in morphology and substructure was observed with increasing additions of titanium. The 5 wt pct Ti alloy was found to be predominantly dislocated lath martensite while the 10 wt pct Ti alloy showed mainly a twinned plate morphology. Twins within the martensite plates could be classified into two categories, namely, thick\(\{ 10\overline 1 1\} \) twins and thin twins mainly on\(\{ 10\overline 1 1\} \) planes and, to a lesser extent, on\(\{ 11\overline 2 2\} \) and\(\{ 11\overline 2 1\} \) planes. In the case of the thick\(\{ 10\overline 1 1\} \) twins, the specific variant of the twin plane was always found to correspond to a plane of the {110}bcc type, which is a mirror plane in the parent bcc crystal. When the specific variant of the composition plane was not parallel to a mirror plane, the twins were observed to be very thin. Evidence of slip within individual twin bands pointed to the operation of a multiple shear inhomogeneous deformation. Observations concerning\(\{ 10\overline 1 1\} \) deformation twins due to impingement effect are also discussed in this paper.

The Anderson-Grüneisen Parameter for Some Diatomic Crystals

The general equation for Anderson-Gruneisen parameter δ developed by Sharma and Tripathi (Phys. Letters A39 (1972) 281) has been tested theoretically by computing the δ values for bcc and fcc types of ionic crystals employing the central force rigid ion interaction model. Furthermore, the Anderson-Gruneisen parameter has also been calculated from two relations of Chang. The dilemma confronted by Chang, regarding the correctness of these two relations for δ has been resolved.

Elastic Anisotropy and Its Temperature Dependence of the Single Crystals of Fe-19.43% Cr Alloy

Rod-form single crystals of Fe-19.43% Cr alloy of the bcc type about 3 mm in diameter and 12 cm in length were prepared. The thermal expansion and Young’s modulus in the temperature range −150°C to 250°C and the rigidity modulus at 20°C of the alloy were measured by means of a dilatometer, a vibrator-controlled oscillator system and the torsion pendulum method, respectively. The results of calculations based on the measured values have shown that the Young’s moduli of the single crystals at 20°C in the 〈100〉, 〈110〉 and 〈111〉 directions are 14.22×105, 22.23×105 and 27.37×105 kg/cm2, and the rigidity moduli at 20°C are 11.36×105, 7.21×105 and 6.43×105 kg/cm2, respectively ; the elastic anisotropy is not very large in all cases. The Young’s modulus in the 〈111〉 shows a small minimum at 10°C and a small maximum at about 110°C with increasing temperature, while the Young’s moduli in the 〈100〉 and 〈110〉 directions monotonously decrease. The mean temperature coefficients of Young’s modulus at 10°∼110°C are −32.77×10−5, −9.85×10−5 and +4.71×10−5 in the 〈100〉, 〈110〉 and 〈111〉 directions, respectively. The calculated value of Young’s modulus for the polycrystal agrees fairly well with the measured value for the polycrystal specimen prepared by melting.