Two-phase Titanium Alloy Research Articles

Study of structural changes for a two-phase titanium alloy with shock-wave loading

1. Original fcc interlayers are disrupted with high-speed shock-wave loading for lamellar two-phase alloy VT8. 2. Within the limits of a single colony, slip and twinning propagate unhindered across all of the α-platelets, and they are stopped at high-angle boundaries (colony boundaries). 3. With an increase in shock loading rate from 60 to 600 m/sec intensification of twinning is observed. 4. Formation of unstable packing defects, decorated mainly with hydrogen, along planes {1010}α and hydrides in β-phase with a shock loading rate of 600m/sec have been noted. 5. Electron-microscopic studies have not revealed formation of ω-phase with shock loading for the test alloy.

Rules of failure of two-phase titanium alloys with different microstructures

Rules of failure of two-phase titanium alloys with different microstructures

Control of the structure of two-phase titanium alloys in thermomechanical working

The embryos of α- and ω-phases obtained in α+β-titanium alloys in hardening and aging are preserved in cold working and become centers of growth of these phases in subsequent aging. As a result it is possible to obtain ω-phase or more uniformly distributed α-phase in cold-worked and aged alloys.

Mechanisms of the isothermal decomposition of ?-solid solution in two-phase martensitic titanium alloys

1. The kinetics of isothermal decomposition of high-temperature β-phase for two-phase martensitic titanium alloys after undercooling from different temperatures are described by typical C-shaped curves; this decomposition may have one or two stages above the MS point. The type of isothermal decomposition diagram for this phase is governed by the alloy heating temperature and the temperature for the start of the martensite β→α″-transformation. 2. An increase in alloy heating temperature into the two-phase region leads to a shift in C-shaped curves in the direction of higher temperatures and shorter exposures, i.e., the decomposition process is accelerated. 3. With isothermal exposure for undercooled alloys in the martensite range it is possible for the martensitic β→α″-transformation to occur. With undercooling to a temperature above MS high-temperature β-solid solution is stabilized and the martensitic β→α″-transformation does not occur during subsequent cooling. 4. Mechanical properties of the alloys are goverved by the mechanism of high-temperature β-phase decomposition. Decomposition by an intermediate mechanism provides high alloy strength, although the ductility is lower. As a result of β-solid solution decomposition by a diffusion mechanism, an alloy has higher ductility with quite high strength. The optimum combination of strength and ductility as well as high property stability is obtained as a result of isothermal exposure for an alloy undercooled from the two-phase region to a temperature corresponding to the maximum rate for β-solid solution decomposition in the first stage.

Rate sensitivity of mechanical properties and fracture characteristics of two-phase titanium alloys subjected to thermal cycles of diffusion welding

The effect of strain rate on mechanical properties and fracture characteristics of two dual-phase titanium alloys was studied using standard test equipment. The alloys had been subjected to various thermal cycles, simulating diffusion welding. It is shown that diffusion welding is possible within the alloys β phase region without serious loss of service characteristics provided service temperature and rate of stress application are accounted for. A service temperature (which also depends on loading rate) of between 60 and 100°C is suggested for titanium articles — commensurable in size with that studied — to be diffusion welded in the β region.

Fatigue crack initiation from notches in two titanium alloys

Fatigue crack initiation is arbitrarily, but practically defined as the first deviation indicated by the potential drop technique [1]. The number of cycles to produce such initiation is correlated with the empirical parameter ΔK/ϱ 1 2 where ΔK is the stress intensity factor range as if the notch were a sharp crack, and ϱ is an inscribed radius at the notch root. It is shown that the above correlation affords a practical method of design against initiation of fatigue in a single phase and in an α-β two-phase titanium alloy.

Fatigue failure of a two-phase titanium alloy in vacuum

Fatigue failure of a two-phase titanium alloy in vacuum

Cracking of titanium alloy VT22 during aging

1. Spontaneous brittle fracture during isothermal aging at 250–500°C is a natural phenomenon in precipitation-hardening metastable two-phase titanium alloys. 2. The increase in strength and sharp reduction of the ductility at 250–500° are accompanied by a reduction in volume (increase of density) and the appearance of relief on the surface due to structural transformations in the β solid solution in the early stages of decomposition long before the formation of α phase. 3. The processes occurring before precipitation of α phase are associated with extremely high structural stresses capable not only of inducing microdiscontinuities but also macrocracks and even complete fracture of the piece. 4. To prevent brittle fracture during heat treatment of this alloy it must be heated from 250 to 500° at a high rate and the aging temperature should be at least 500° to ensure high strength.

Topography of fatigue fracture of two-phase titanium alloy

1. Plastic deformation during cyclic loading of two-phase α+β titanium alloy, unlike the single-phase alloy, results from slip without twinning. 2. During fatigue failure of the α+β titanium alloy subjected to cyclic torsion the fatigue crack propagates in three stages — the initial stage consitsts of cleavage along slip planes under the influence of normal stresses; the intermediate stage consists of cleavage accompanied by considerable plastic deformation under the influence of shearing stresses; the final stage consists of quasibrittle fracture as the result of normal stresses. 3. Raising the temperature from −140 to +150°C changes the relationship between the areas of these zones — the area of the intermediate zone decreases and the area of the final zone increases substantially. The character of the zones themselves remains unchanged. 4. The increase in the fatigue life of the samples at low temperatures is affected by the initial stages of the process before the formation of the fatigue crack.

Creation of residual stresses in titanium alloys on cutting and subjecting to surface plastic deformation

1. When machining two-phase titanium alloys, or hardening these by surface plastic deformation, the value of the residual stresses is affected by the structure and phase composition of the alloys, as well as by the force factor. 2. In the course of hydraulic shot hardening and vibrational tumbling, the hardening associated with plastic deformation is accompanied by hydrogen absorption in the hardened surface.

Metallographic preparation of two-phase titanium alloys for replica electron microscopy

Metallographic preparation of two-phase titanium alloys for replica electron microscopy

Heat treatment of welded two-phase titanium alloys

1. Neither quenching followed by aging nor annealing affects the mechanical properties of weld seams of the low-alloyed OT4 alloy. 2. Weld seams of the VT6 alloy are strengthened by heat treatment. The optimum heat treatment conditions ensuring equality of the strengths of the weld seam and the base metal (up to 120 kg/mm2) and a satisfactory ductility are: quenching from 850–900°C and aging at 500–550°C for about 10 h. 3. The weld seams of medium-alloyed alloys are embrittled by heat treatment because of the formation of the ω-phase (alloys No. 1 and No. 2).