Decomposition diagram and regime of heat treatment of double-phase titanium alloys

Abstract

A substantiated choice of an optimum regime of heat treatment of titanium alloys is possible only on the basis of a complex investigation of the regular features of the structural and phase transformations and their relation with the mechanical properties after annealing conducted for different times and in a wide temperature range. The present work concerns low-temperature transformations in double-phase titanium alloys and their effect on the processes of embrittlement in a temperature range of 300 -800~ with holds lasting from 0.25 to 500 h.Structural and phase transformations and their relation with the properties of titanium alloys have been described in many monographs [1][2][3][4][5], which shows the complexity of these problems. Primarily, this is due to the great number of metastabte phases formed in titanium alloys in the processes of mechanical and heat treamaent. In accordance with the growth of the content of alloying 13-stabilizing elements, these phases can be arranged into the following series: a m , ct', a", co, 13m" Many questions related with the mechanism and kinetics of the ct ~ 13 transformation and the formation and decomposition of metastable phases, especially in titanium alloys after plastic deformation, are still being discussed.We studied flat deformed preforms up to 150 mm thick from an industrial titanium alloy containing Ti-6% A1-2% V-6% Zr. The preforms were fabricated by the method of hot deformation in the 13-(Gt + 13) region. The temperatures of the transition of the alloy into (cL + 13)-and 13-regions corresponded to 850 and 1010~Metallographic and electron microscopic studies have shown that the structure of deformed preforms over the thickness is represented by macrostrips flattened and extended in the direction of the rolling. Each strip consists of several colonies of plates of an a-phase (with a hexagonal closepacked lattice) separated by interphase layers sometimes of a composite "sandwich" structure but based on the crystal structure of a 13-phase (with a body-centered cubic lattice) (Fig. la). The mean width of a-plates is 3.0 ~-n, the thickness of the interphase layers is 0.3 gm. The initial hot-deformed state of the alloy is characterized by the presence of a 310great number of bending extinction contours in its structure, which indicates that the substructure is in a stress state (Fig. la ).By the data of x-ray diffraction analysis, we see that the structure of the alloy in the initial state consists of ct(ct')-, 13-, and ct"-phases.The results of an x-ray spectral analysis (Fig. 2a ) show that the hot deformed alloy is characterized by a considerable concentration inhomogeneity in the distribution of the alloying elements both within a-colonies and in individual a-plates and 13-layers. When scanning the surface of polished specimens by an electron probe, we identified 13-layers with the help of the profiles of the currents of absorbed electrons. It has been established that the interphase layers (13r~t) contain an elevated amount of not only 13-stabilizing elements...

Annealing of cold worked two-phase Zr-2.5 Nb—Associated microstructural developments

“…The impressive range of literature on the subject is, however, concentrated mostly on annealing of singlephase metallic materials -including alloys containing hard nonshearable particles. Studies on annealing in common two-phase (with shearable or semi-shearable 2nd phase) alloys are less frequent [39][40][41][42][43]; while such studies in two-phase Zr alloys are rare [44,45]. This was the motivation behind the present study.…”

Electron-Beam Welding - Structural-Phase State and Microhardnes in the Weld Zone in a Submicrocrystalline Titanium Alloy Grade2

“…The obtained data of X-ray diffraction studies reflect the ability of titanium alloys to form metastable phases as a result of the mechanical and thermal treatment [8]. According to the published data, the weld zone and the heat-affected zone in titanium alloys have a different number of characteristic sections that reflect different limiting heating temperatures and different cooling rates [9].…”

“…This process can be step-like, and each stage is complex. Thus, formation of stable and metastable phases takes place at the first stage [8]: formation of a stable α-Ti phase and formation of a metastable β-Ti phase. At the second stage, the metastable phase β-Ti can breakdown into a number of metastable phases ω-Ti, α′′-Ti and α2-Ti [8,11].…”

Effect of Severe Plastic Deformation on the Structure and Properties of the Aluminum Alloy System Al-Cu-Mg

“…Heating of supersaturated solid solutions of alloy D16 in the lower part αareas, in the temperature range 100...180°С and exposure times to 10 h (artificial aging), where the processes of structural relaxation developed by the relaxation mechanism, led to the decomposition of nonequilibrium solid solutions. Such microregions do not have an interface with the matrix, but have a different etching [22,23], were observed long before the appearance of particles Θ(CuAl2)-phase and were preallocations (Θ / -phase), that is, microvolumes in which phase transformations took place (restructuring of the crystal lattice) preceding the separation of the secondary Θ(CuAl2)phase, and accompanied by the occurrence of maximum structural stresses as a result of size mismatch at the interface "pre-release of the emerging Θ(CuAl2)-phase − α-matrix". α-solid solutions decay goes through the stages of microbundling, the formation of coherent predispositions (Θ / ) and isolation of particles of a new Θphase.…”

“…α-solid solutions decay goes through the stages of microbundling, the formation of coherent predispositions (Θ / ) and isolation of particles of a new Θphase. In the works [22][23][24][25] it is shown that a decrease in the relative elongation and contraction took place at the stage of completion of the microbinding of solid solutions and the formation of microblasts enriched in those impurity atoms that prevailed in the newly formed phase.…”