Effect of high-temperature deformation on the texture of a two-phase titanium alloy

Changes in the texture of both the - and -phases of the two-phase alloy Ti-6AI-4V have been determined in order to clarify the mechanisms of high-temperature deformation. A strain of 1.5 was applied to the alloy at 928 °C, at strain rates representative of superplastic and nonsuperplastic conditions. The -phase texture changed very little with strain rate whereas that for the -phase was much more sensitive. The -phase texture was weakened at superplastic strain rates but developed a fibre texture at non-superplastic rates. It is postulated that under superplastic conditions the alloy deforms predominantly by grain boundary sliding of the soft -phase grains, with the hard primary -phase grains remaining in their original orientations and the measured loss in texture intensity being attributed to the loss in texture of the secondary -phase only. Under non-superplastic conditions there is a greater contribution from plastic deformation in the -phase which, in turn, can enhance the secondary -phase texture.     

Superplastic deformation of strongly textured Ti-6Al-4V

Changes in microstructure and texture during superplastic deformation of strongly textured Ti-6Al-4V bar have been determined in order to establish the cause of stress and strain anisotropy. The effect of strain on the microstructure of the alloy was to cause a progressive break-up, due to grain-boundary sliding, of an initially directional microstructure containing contiguous-phase. The texture of the-phase, however, changed very little with superplastic strain but that of the-phase was randomized. Shape changes predicted by permitted deformation modes in the-phase did not correlate with the observed shape changes, whereas the observed anisotropy could be explained by the break-up of the contiguous-phase. A model to account for this anisotropy is described briefly, together with a typical microstructure which should exhibit isotropic superplastic deformation.     

Influence of the martensitic transformation on the deformation behaviour of an AISI 316 stainless steel at low temperatures

The deformation behaviour of an AISI 316 stainless steel under uniaxial tension was examined at 25, – 70 and – 196° C. The flow curves exhibited peculiar shapes and the work hardening rates were found to increase with strain beyond certain values of plastic strain. X-ray diffraction analyses showed that the transformation to-martensite commenced at these values of plastic strain and thereafter the volume fraction of increased steadily with strain. On the other hand, the amount of the-martensite was found to increase with plastic strain initially, reach a maximum and then decrease gradually. The contribution of the-phase to the flow stress of the alloy was found to be directly proportional to the volume fraction of. It is shown that the analysis of the flow curves provides a simple method of detecting the onset of the strain-induced martensitic transformation as well as estimating the amount of this martensite during further deformation.     

Superplastic deformation in Ti-4%Al-4% Mo-2%Sn-0.5%Si (IMI550)

The superplastic deformation properties, i.e. flow scenes and stain rate sensitivities, of the commercial alloy Ti-4% Al-4% Mo-2% Sn-0.5% Si (IMI55O) have been assessed in the temperature range 805 to 915° C and for different initial microstructures. Fine grained + microstructures showed superplastic properties at temperatures above 850° C and grain coarsening, which led to increases in flow stresses and e reduction in superplastic properties, was only pronounced at the higher temperatures. In transformed microstructures the method of strain rate cycling led to a breakdown of the acicular platelets end the development of an equiaxed + microstructure. At the same time the flow stresses at low strain rates decreased and the strain rate sensitivities increased. Light and electron microscopy showed that the-phase became the continuous or matrix phase even at volume fractions below 50% (i.e. at the lower deformation temperatures) and that molybdenum segregation at the longitudinal interphase boundaries (parallel to the tensile axis) occurred. The results suggest that there is a strain distribution between the-phase and the -phase, the-phase effectively behaving as a deforming mantle around the grains.     

Solid state phase transformation of BaB2O4 during the isothermal annealing process

Solid-state phase transformation of BaB₂O₄ during the isothermal annealing process for both to and to were investigated using a platinum crucible. For the -phase crystal at the -phase stable temperature (> 925 °C), the phase transforms to the phase perfectly below the melting temperature of 1100 °C. Meanwhile, for the -phase crystal at the -phase stable temperature (< 925 °C), the phase transforms to the phase perfectly above 800 °C. There is some difference in phase transformation behaviour between bulk-shape crystals and the powder, caused by thermal stress.     

Effect of prior cold rolling and test temperature on stress-strain rate behaviour of a Zr-2.5Nb alloy

Zr-2.5 wt% Nb alloy sheet, obtained by unfolding and straightening a pressure tube, was further cold rolled upto 39% reduction in thickness to investigate the effect of cold working on the stress ()-strain rate () behaviour over a strain rate range of 2 × 10–5 to 5 × 10–3 s–1 and a temperature range of 625 to 700 °C. Irrespective of the amount of rolling, the log vs log plots exhibit superplastic behaviour with strain rate sensitivity index, m, as high as 0.8, which decreases to 0.2 at higher strain rates. On the other hand, the activation energy for deformation, Q, increases from 171.1 kJ/mol for superplastic deformation to 249 kJ/mol in Region III. The tendency for improved superplasticity (m) is seen upon cold working by 22% or more at the test temperatures 675 and 700 °C.     

TEM study of metastable β-phase decomposition in rapidly solidified Ti-6Al-4V alloy

A new rationale is presented for various decomposition products obtained from the metastable -phase found in Ti-6A1-4V alloy produced by hot isostatic pressing comminuted melt-spun fibres and cooled to room temperature by furnace cooling. This alloy has an -matrix with about 8 vol% retained -phase, which is supersaturated with -stabilizers to such an extent that the martensitic transformation has been suppressed. The metastable -phase decomposes by different modes during continuous cooling, depending on the actual composition of individual -grains. Less enrichment of vanadium and iron favours the direct formation of the equilibrium -phase from the -matrix, while greater enrichment of vanadium and iron leads to a spinodal decomposition of the metastable -phase, resulting in a + two-phase structure. During further continuous cooling, the -phase which is lean in -stabilizers will transform into isothermal -phase. In addition, an unknown phase has also been observed in the -phase, which is typified by the appearance of 1/2{112} reflections in the SAD patterns.     

Formation of submicrocrystalline structure in the titanium alloy VT8 and its influence on mechanical properties

It is shown that the formation of microstructure with grain size up to 0.06 m may occur during the course of plastic deformation of the Ti-6Al-3.2Mo (+)-alloy with the initial coarse-grained lamellar structure. The formation of submicrocrystalline structure results from the development of dynamic recrystallization concurrent with the process of spheroidization. The temperature of superplastic deformation significantly decreases while strength characteristics at room temperature sharply increase in the alloy with such a microstructure.     

Effect of phase proportions on deformation and cavitation of superplastic α/β brass

Studies have been made, using metallographic and precision density techniques, of the deformation and cavitation behaviour during superplastic tensile straining at 873 K of three microduplex/ brasses which, as a consequence of varying composition, contained varying proportions of and phases. It was observed that both strain-rate sensitivity and elongation-to-failure passed through a maximum when approximately equivolume proportions of the two phases were present. Cavitation, on the other hand, decreased rapidly as the volume fraction of phase was increased. The cavitation behaviour was attributed to the relative abilities of the phases to accommodate grain boundary sliding. When a high proportion of phase is present accommodation is minimal and cavity nucleation. occurs readily. Evidence is presented to show that grain-boundary sliding plays a predominant role in cavity growth. When a high proportion of phase is present accommodation is almost complete and cavity nucleation is minimal.     

Study of the paramagnetic-antiferromagnetic transition and the γ → ε martensitic transformation in Fe-Mn alloys

The paramagnetic-antiferromagnetic transition and the martensitic transformation of Fe-Mn (Mn 15–32 wt%) alloys have been investigated by resistivity, dilatometry and X-ray diffraction (XRD). The results show that paramagnetic-antiferromagnetic transition increases the resistivity and the volume of alloys, whereas the martensitic transformation reduces the resistivity and volume of alloys. The A _f that was determined by the dilatometric method is not the temperature that martensites in the Fe-Mn alloys have reverse transformed to austenites completely. Mn additions reduce M _s, increase T _N and the lattice parameter of austenite in the Fe-Mn alloys. Both the antiferromagnetic transition and the martensitic transformation lead to an increase in the lattice parameter of austenite. The lattice parameters both above T _N and below T _N decrease linearly with temperature. The lattice parameter below M _s increases first and then decreases. Moreover, the (110) and (002) atomic planes in the Fe-15Mn-0.15C alloy are separated into two peaks: 2 for (002) is 44.16°, 2 for (110) is 44.47°.     

On invariants of a convective drying process

The Lewis relation is proved to be valid in the absence of an analogy between heat and mass transfer. Invariance of temperature and equilibrium curves is established for some drying processes.Notation I enthalpy - c heat capacity - t, temperature of the gas and a material, respectively - u, u_s, u_cr.red, u_max.h, u_eq mean, surface, critical reduced, maximum hygroscopic, equilibrium moisture content of the material, respectively - X air moisture content - q_b binding energy of moisture with the material - r₀ heat of liquid evaporation at 0°C - D vapor diffusion coefficient in the gas calculated from the moisture content gradient - f specific contact surface of phases, m²/kg - l determining dimension - X, t moisture content and temperature of the gas in the boundary layer - y normal to the contact surface of phases - , interphase heat and mass transfer coefficients - gas thermal conductivity - density - time - Rb=c_m(–d/du)r Reminder number - Nu=l/, Nu_m=l/D heat and mass transfer Nusselt numbers. Indices - g, gas, m material - s surface - d.g dry gas - l liquid - v vapor - sat saturated - w.t wet-bulb thermometer - f final, index 0 stands for parameters at the dryer inlet Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 63, No. 4, pp. 442–448, October, 1992.     

γ ? α2 Phase transformation in fractured high temperature stress rupture Ti-48Al-2Nb(at.%)

The ₂ phase transformation in fractured high temperature stress rupture Ti-48Al-2Nb(at.%) alloy has been studied by analytical electron microscopy. ₂ and phases were found at grain boundaries. ₂ layers that suspended in layers and interfacial ledge higher than 2d ₍₁₁₁₎ at /₂ interfaces were observed in the lamellar grains. These facts indicated that ₂ phase transformation and dynamic recrystallization have occurred during high temperature stress rupture deformation. It can be concluded that deformation induced ₂ phase transformation and dynamic recrystallization resulted in the presence of particles at grain boundaries. A structural and compositional transition area between deformation-induced ₂(or ) and its adjacently original (or ₂) phases was found by HREM and EDS and is suggested as a way to transform between and ₂ phase during high temperature stress rupture deformation. The transition area was formed by slide of partial dislocations on close-packed planes and diffusion of atoms.     

The morphology of α′ martensite in a two-phase (α+γ) Fe-Cr-Ni stainless steel

Both lath-shaped and martensites are induced by tensile deformation within of a two-phase ( + ) Fe-Cr-Ni stainless steel, forms from the through the at an intersection of two crystals. These are observed both when is surrounded by and when borders . The amount of strain at which both and nucleate, increases with test temperatures in the range –196 to 50° C. Adjacent laths are either twin-related, or 5, 9, 15 or 19° off the twin relationship, as found by analysing electron diffraction patterns.     

Martensitic transformations in Ti-Mo alloys

A detailed investigation has been made of the structure of alloys of the Ti-Mo system containing up to 10wt% Mo, water-quenched from the-phase region. With increase in molybdenum content, the martensite structure changes from hexagonal () to orthorhombic () at 4 wt% Mo, and at 10 wt% Mo, the structure is completely retained . For alloy compositions <4 wt% Mo, there is a diffusional component in the transformation of the -phase at the quench rates employed. There is a transition, with increase in molybdenum content, in morphology (from massive to acicular) and in substructure (from dislocations to twins). However, the transitions in crystallography, morphology and sub-structure are not directly related to one another except for an abrupt loss of dislocation substructure at the/ transition. The to crystallographic transition has the characteristics of a second order transformation, and evidence has been obtained of the existence of a spinodal within the metastable orthorhombic system. The orthorhombic martensites of Ti-6 and 8 wt% Mo decompose during quenching producing a fine modulated structure within the martensite plates, consistent with a proposed spinodal mode of decomposition.     

The dielectric response of a commercial polybutylene terephthalate

Contour maps of complex relative permittivity within the range –170 to +175° C and 0.1 Hz to 3 MHz are presented for a commercial polybutylene terephthalate. The interdependence of and, as required by the Kramers-Krönig relations, is particularly apparent. Individual dispersion regions are attributed to- and-relaxation processes and to Maxwell-Wagner-Sillars polarizations. Possible explanations are proposed for the structure which is apparent in the-peak.     

Similarity solutions in axisymmetric mixed-convection boundary-layer flow

The similarity equations for mixed-convection axisymmetric boundary-layer flow are considered. The equations involve a buoyancy parameter and a curvature parameter . The equations are solved numerically and it is found that for large , and of O(1), an asymptotic solution is approached, the nature of which is discussed. When is also large, of O(^1/4), the problem, at leading order, becomes independent of the mainstream and the free-convection limit is obtained. This problem is also discussed, including the behaviour for large values of ₀, the free-convection curvature parameter. For < 0 we find that the solution can be continued past the point where the wall heat transfer becomes zero (where previous mixed-convection similarity solutions in plane geometry were terminated) with the solution ending as 0^–. The nature of this limit is also discussed. For < 0 it is also found that there are solutions only in _b = < 0 with two branches of solution bifurcating out of = _b , and values of _b are computed for a range of . The behaviour of the solution for large values of the curvature parameter , and of O(1), is discussed where it is shown that the solution proceeds in inverse powers of log .     

Activated slip systems during yielding of α-β brass two-phase bicrystals

- brass two-phase bicrystals, consisting of fcc () single crystals and bee () single crystals, which were made by the solid state diffusion couple technique, were tensile-tested at room temperature in order to clarify the role of phase-interface on the deformation. The two-phase bicrystals had small concentration gradients in the- and-phases and satisfied the Kurjumov-Sach's orientation relationships i.e. {1 1 1} {1 1 0} and [1 1 0] [1 1 1] at the interface. The slip traces observed in bicrystals deformed to about 3% plastic strain showed a striking contrast between the- and-phases; the slip traces in the-phase were clear and straight, while those in the-phase were fine and wavy. The slip systems in the bicrystals were attributed to those observed in and single crystals, and were explained by a plastic strain incompatibility mechanism. The slip systems, originating at the interface or propagating from another phase, were observed on matching planes.On leave from Mechanical Engineering Department, Faculty of Engineering, Al-Azhar University, Cairo, Egypt.     

The α- to β-Si3N4 transformation in the presence of liquid silicon

The compacts consisted of , -Si₃N₄ and free silicon are heat treated in the range 1650° C to 1750° C in an argon atmosphere in order to observe the following behaviours; the to phase transformation and variations of the microstructure during heat treatment in silicon nitride. For the microstructural observation of the heat treated specimens, the same grains in the polished surface were investigated before and after eliminating the retained silicon by etching. The to phase transformation, in this case, occurs via silicon melts irrespective of added -Si₃N₄. Both and phases are soluted and precipitated into molten silicon and their morphology are changed from an equiaxed shape to prismatic one. Although elongated grains are precipitated at low temperature or in the early stage of heat treatment, fine precipitated grains are mainly observed with increasing heat treating temperature.     

A high-resolution-electron-microscopy study of a γ/γ′-α directionally solidified eutectic alloy

The microstructure of a /- directionally solidified (DS) eutectic alloy with a nominal composition of Ni-30.26Mo-6.08Al-1.43V (wt%) was investigated by means of high-resolution electron microscopy (HREM) and analytical electron microscopy. The -fibres exhibited a typical morphology with a rectangular cross-section and they displayed the Bain orientation relationship (OR) with the / matrix; that is, [001][001] and (110)(010). Misfit dislocations and lattice strain fields existed at the / interface for different habit planes; that is, (110)(010) and (100)(110) were analysed. EDAX (Energy dispersive X-ray) analysis showed that the composition of the -phase was approximately Ni₄(Mo, Al, V); it contained 90° rotational domains of Ni₃(Mo, Al, V) with a DO₂₂ structure and Ni₂(Mo, Al, V) with a Pt₂Mo structure.     

Activation energies for superplastic tensile and compressive flow in microduplex α/ß copper alloys

Logarithmic stress against strain rate curves have been determined at various temperatures for a superplastic commercial/ nickel-silver alloy strained in tension, and a laboratory prepared microduplex alloy of nominally similar composition strained in compression. The shapes of the curves were found to be affected by grain growth at high temperatures and strain softening at low temperatures. After taking these factors into account, it was apparent that with decreasing strain rate in both alloys a change in deformation mechanism occurred giving rise to a Region I of low strain-rate sensitivity. By confining activation energy (Q) measurements to temperatures at which steady-state deformation occurred, it was found thatQ for Region II was very similar to that measured for grain boundary diffusion in the phase of a nickel-silver alloy of similar composition, whileQ for Region I was substantially higher than that for lattice diffusion. Values of strain-rate sensitivity andQ were found to be similar for each direction of applied stress.