Search

The presence of substantial concentrations of multiple principal elements in high entropy alloys or complex concentrated alloys, inevitably results in competing for clustering (or phase separation) and ordering tendencies between the constituent elemental pairs. Such competition in turn can result in interesting phase transformation pathways involving spinodal decomposition coupled with chemical ordering. Recently, a microstructure resembling that of Ni-base superalloys, with discrete BCC precipitates in a B2 matrix, in a checkered fashion, has been reported in multiple refractory high entropy alloys. This paper examines the microstructural evolution and the phase transformation pathways involved in two such candidate alloys, Al0.5NbTa0.8Ti1.5V0.2Zr and Al0.5Mo0.5NbTa0.5TiZr. The formation of this checkered pattern B2 + BCC microstructure is rationalized using schematic free energy curves and phase modulus criteria. Further, the effect of such phase distribution on mechanical properties is investigated via room-temperature compression.

ABSTRACTProcess efficiency is a crucial parameter for inertia friction welding (IFW). A new method is developed to determine the efficiency by comparing the workpiece torque used to heat and deform the joint to the total torque. Particularly, the former is measured by torque load cell attached to the non-rotating workpiece, while the latter is determined from the deceleration rate of flywheel. The efficiency measured for IFW of AISI 1018 steel is inputted into analytical heat balance calculation of the upset length and finite element thermo-mechanical modelling of the extruded flash profile. The calculated results are consistent with the respective experiment data. The transient behaviour of torque and efficiency is discussed based on the energy loss and the bond formation.

Process efficiency is a crucial parameter for inertia friction welding (IFW). A new method is developed to determine the efficiency by comparing the workpiece torque used to heat and deform the joint to the total torque. Particularly, the former is measured by torque load cell attached to the non-rotating workpiece, while the latter is determined from the deceleration rate of flywheel. The efficiency measured for IFW of AISI 1018 steel is inputted into analytical heat balance calculation of the upset length and finite element thermo-mechanical modelling of the extruded flash profile. The calculated results are consistent with the respective experiment data. The transient behaviour of torque and efficiency is discussed based on the energy loss and the bond formation.

Extruded and T6 tempered plates of 7042, an Sc modified Al–Zn–Mg–Cu alloy, were joined by friction stir welding (FSW) at a constant weld velocity and various pin rotation speeds (PRSs). At a low PRS, hardness decreased from each edge of the weld to a local minimum at the weld centre, but at a high PRS, the hardness initially decreased from each edge of the weld, but then rose towards the weld centre. The transition in the hardness profile was related to different heating and cooling conditions during FSW. Differential scanning calorimetry of baseline and welded samples revealed that the volume fraction of strengthening particles within the weld regions strongly depended on the weld conditions. For FSW heating rates, secondary phase dissolution/precipitation temperatures are in proximity to the welding temperatures. The maximum welding temperature during FSW increased from ∼250 to 450°C as the PRS increased from 175 to 400 rev min−1.

Specific criteria for selection of solute atoms and composition are formulated and used to prepare a series of Ce–Al–(Cu,Ni) amorphous alloys by melt‐spinning. The effect of alloy compositions on the thermal behaviour of the amorphous ribbons is characterized in order to evaluate their glass forming ability.

Abstract: TiAl alloy specimens with microcrystalline (MC, grain size is 5 µm) and submicrocrystalline (SMC, grain size is 0.4 µm) structures were successfully pack-rolled to sheet with a thickness down to 0.4 mm in the temperature ranges of 800°C to 1000°C and 900°C to 1000°C, respectively. An 18/10 stainless steel was used as a rather inexpensive can material for pack-rolling. Unidirectional rolling and bidirectional cross-rolling were used. Because of a wider temperature range for pack rolling and a lower cost for production of the alloy preforms, the microcrystalline structure was found to be a better microstructural condition for the TiAl sheet rolling. The sheet produced by unidirectional rolling had an anisotropy of mechanical properties, i.e., strength was smaller and elongation larger in the rolling direction than in a transverse direction. The anisotropy decreased when the rolling temperature increased. The bidirectional rolling led to in-plane isotropic properties of the sheet. The produced sheet showed elongation of about 3 pct at room temperature, brittle-to-ductile transition in the temperature range of 750°C to 850°C, and superplastic behavior in the temperature range 900°C to 1000°C.

The objectives of this work were to study the microstructural changes and tensile properties that resulted from controlled cooling of a Ti-6Al-4V alloy that had been pancake forged. The alloy was forged at 954°C and specimens cut from the forging were annealed at 975°C for 30 min and cooled down to room temperature at different cooling rates ranging from 20 K min-1to 490 K min-1. The specimens had a duplex microstructure consisting of elongated primary a particles and secondary finer a lamellae with some residual ß phase layers between them. The average length and width of primary a particles decreased and the aspect ratio and volume fraction of particles increased when the cooling rate was increased. The average lamellar spacing decreased from about 1 µm to 0.3 µm when the cooling rate increased from 25 K min-1to 490 K min-1. Lamellar a contained 3.0 - 5.2 wt-%Al and 1.4 - 2.1 wt-%V, while the ß layers contained 1.4 - 2.8 wt-%Al, 12 - 18 wt-%V and 1.5 - 2.4 wt-%Fe. In the a lamellae, the concentration of aluminium increased and the concentration of vanadium went through a minimum with an increase in cooling rate. In the ß layers, the concentration of aluminium increased, while the concentration of vanadium decreased with increasing cooling rate. The volume fraction of the ß phase decreased from 10 - 12% to 4 -5% as the cooling rate increased from 25 to 490 K min-1. The tensile strength increased by approximately 7% when increasing the cooling rate from 25 to 370 K min-1, while the reduction in area achieved a 29% peak increase when the cooling rate was increased from 25 to approximately 325 K min-1. A discussion of the effects of cooling rate on microstructure and properties is also given.

Abstract: Thermohydrogen processing, a technique in which hydrogen is used as a temporary alloying element in titanium, allows enhanced microstructural control and improved mechanical properties. This paper provides an improved definition of the phase transformations taking place in the Ti-6Al-4V-hydrogen system, which should lead to increased application of the technique.

Abstract: Microstructures, phases, and phase transformations in Ti-6Al-4V alloy specimens containing 0, 10, 20, and 30 at. pct hydrogen were investigated using optical microscopy (OM), transmission electron microscopy (TEM), X-ray diffraction (XRD), and microhardness testing. Alloying with hydrogen was achieved by holding the specimens in a pure hydrogen atmosphere of different pressures at 780 C for 24 hours. The phases present in the temperature range of 20 C to 1000 C were determined by microstructural characterization of the specimens quenched from different temperatures. Increasing the hydrogen addition from 0 to 30 at. pct lowered the beta-transus temperature of the alloy from 1005 C to 815 C, significantly slowed down the kinetics of the beta-to-alpha transformation, and led to formation of an orthorhombic martensite instead of the hexagonal martensite found in quenched specimens containing 0 pct H. A hydride phase was detected in specimens containing 20 and 30 at. pct hydrogen. The time-temperature-transformation (TTT) diagrams for beta-phase decomposition were determined at different hydrogen concentrations. The nose temperature for the beginning of the transformation decreased from 725 C to 580 C, and the nose time increased from 12 seconds to 42 minutes when the hydrogen concentration was increased from 0 to 30 at. pct.

Abstract: Prealloyed, gas-atomized (GA) Ti-47Al-3Cr alloy powder, containing about 70 pct of the α 2 (Ti3Al) phase and 30 pct of the γ (TiAl) phase, was fully amorphized by mechanical alloying. The amorphous phase was stable during heating to 600 C, but decomposed at higher temperatures, with an exothermic reaction peak at 624 C as the material transformed to a mixture of α 2 and γ and then to a fully γ structure at 722 C. A nanocrystalline compact with a mean grain size of 42 nm was obtained by hot isostatic pressing (HIP’ing) of the amorphous powder at 725 C. Isothermal annealing experiments were conducted in the two-phase α+γ field, at 1200 C, using holding times of 5, 10, 25, and 35 hours, followed by air cooling. The X-ray diffractometry and analytical transmission electron microscopy investigations carried out on annealed and air-cooled specimens revealed only the presence of the γ grains, which coarsened on annealing. Initially, the grains grew, followed by a saturation stage after annealing for 25 hours, with a saturation grain size of about 1 m. This grain growth and saturation behavior can be described with a normal grain growth mechanism in which a permanent pinning force is taken into account. Twins formed in the γ grains as a result of annealing and air cooling and exhibited a common twinning plane of (111) with the matrix phase. The minimum γ grain size in which twinning occurred in the annealed specimens was determined to be 0.25 m, which suggests that twinning is energetically unfavorable in the nanometer-sized grains.

The postjunctional potentials and chronoinotropic reactions of the heart evoked by activation of multimodal neurons and/or left palliai nerve were investigated in three species of land pulmonates: Achatina fulicaFerrussac, Helix lucorumL., Arianta arbustorumL. Both spikes of giant homologous neurons (d-VLN, d-RPLN by A. fulica, command neurons of pneumostoma LPa3, RPa3 by H. lucorum)and stimulation of the peripheral end of the left palliai nerve evoked the similar biphasic inhibitory-excitatory junction potentials in the heart, in mantle muscles and in different parts of visceral complex. The positive chronoinotropic effects of this input in the hearts of whole-mount preparations were modified due to interaction with well-known neural cardioregulating network of the system of intestinal nerve.

Prealloyed, gas-atomized (GA) Ti−47Al−3Cr alloy powder, containing about 70 pct of the α2(Ti3Al) phase and 30 pct of the γ (TiAl) phase, was fully amorphized by mechanical alloying. The amorphous phase was stable during heating to 600°C, but decomposed at higher temperatures, with an exothermic reaction peak at 624°C as the material transformed to a mixture of α2and γ and then to a fully γ structure at 722°C. A nanocrystalline compact with a mean grain size of 42 nm was obtained by hot isostatic pressing (HIP'ing) of the amorphous powder at 725°C. Isothermal annealing experiments were conducted in the two-phase α+γ field, at 1200°C, using holding times of 5, 10, 25, and 35 hours, followed by air cooling. The X-ray diffractometry and analytical transmission electron microscopy investigations carried out on annealed and air-cooled specimens revealed only the presence of the γ grains, which coarsened on annealing. Initially, the grains grew, followed by a saturation stage after annealing for 25 hours, with a saturation grain size of about 1 μm. This grain growth and saturation behavior can be described with a normal grain growth mechanism in which a permanent pinning force is taken into account. Twins formed in the γ grains as a result of annealing and air cooling and exhibited a common twinning plane of (111) with the matrix phase. The minimum γ grain size in which twinning occurred in the annealed specimens was determined to be 0.25 μm, which suggests that twinning is energetically unfavorable in the nanometer-sized grains.

Abstract: Compression tests were carried out on seven titanium-hydrogen alloys containing hydrogen concen-trations up to 31 at. pct. All the experiments were performed within the β-phase field at strain rates of 0.001 to 1.0s ~1 The dependences of the steady-state flow stress on strain rate, temperature, and hydrogen concentration were determined. The strain rate sensitivity increases with temperature but decreases with hydrogen concentration. The experimental activation energy of deformation decreases when the flow stress or strain rate is increased. At a fixed strain rate, it decreases when the hydrogen concentration is increased. However, when measured at a fixed steady-state stress, the activation energies are nearly the same for all the alloys. The steady-state flow stress increases with hydrogen concentration as can be expressed by both linear and quadratic dependences. The flow behavior of the alloys can also be described in terms of thermally activated glide and the relation

Abstract: The elastic properties of a series of polycrystalline titanium-hydrogen alloys (containing up to 25 at. pct H) were measured over the temperature range 20 C to 1100 C. The latter limits permitted investigation of adjacent parts of the α+δ, α, and β phase fields. A laser ultrasonic technique was employed to measure the temperature and hydrogen-concentration dependencies of the elastic constants. The room-temperature elastic properties of the alloys depended only slightly on hydrogen concentration, remaining almost independent of the volume fraction of the δ hydride phase. In the α phase field, the addition of hydrogen decreased the shear and Young’s moduli and increased the bulk modulus, Lam constant, and Poisson’s ratio. The Young’s and shear moduli decreased more rapidly with increasing temperature than in cubic phases. By contrast, Poisson’s ratio increased with temperature. In the β phase field, the temperature dependence of the elastic constants was weak. However, alloying with hydrogen increased the shear and Young’s moduli, decreased Poisson’s ratio, but did not appreciably affect the bulk modulus and Lam constant. The different effects of hydrogen on the elastic constants of alpha and beta titanium are interpreted in terms of the influence of dissolved hydrogen on the stability of the hexagonal close-packed (hcp) and body-centered cubic (bcc) lattices in the vicinity of the α-to-β transformation. The present results are also used to help account for the effect of hydrogen concentration on the mechanical properties of Ti-H alloys.

Compression tests were carried out on samples of commercial-purity titanium charged with up to 4.7 at. pct hydrogen. Strain rates of 10−3to 1 s−1were employed and testing was limited to the α phase field at temperatures of 773 to 973 K. The dependences of the flow stress on strain, strain rate, and temperature were determined. A plateau or bulge appeared in the temperature and strain-rate dependences of the flow stress, and the work-hardening rate also showed peaks. Serrations were observed on some of the stress-strain curves. All these features indicated that dynamic strain aging (DSA) was occurring. Analysis of the results (together with data from other authors) indicates that there are three ranges of DSA behavior in this material within the experimentally investigated temperature range; these appear to be associated with the diffusion of iron, carbon, and oxygen, respectively. Alloying with hydrogen decreases the magnitude of the DSA attributable to these elements and displaces the phenomenon to higher temperatures and/or to lower strain rates. The dependence on strain rate and temperature of the relative softening attributable to hydrogen addition was determined. The results indicate that hydrogen-induced softening is related to the occurrence of DSA in this temperature range. Possible explanations for this relationship are discussed.

Compression tests were carried out on a series of titanium-hydrogen alloys (containing up to 31 at. pct H) over the temperature range 500 °C to 1000 °C within the α, α + β, and β phase fields. The effect of hydrogen content was studied on the flow stress, rate of work hardening, and mechanical anisotropy. Hydrogen in solid solution produces significant softening of the α phase and (α + β) phase mixture, while it induces hardening of the β phase. The occurrence of strain softening was detected in the (α + β) phase field as well, the extent of which depends on temperature, strain rate, and hydrogen concentration. The work required to deform the Ti-H alloys is sensitive to the hydrogen concentration and has the lowest value at interstitial levels that correspond to the (α + β)/β phase boundary at the temperature of interest.

Abstract: A study was initiated to understand the effect of grain refinement on mechanical properties of TiAl-based alloys at room and elevated temperatures. The following was found in that study. Utilizing a processing approach that includes homogenizing annealing or deformation in the α-phase field, followed by multi-step isothermal forging in the γ+ α2 phase range at relatively low strain rates and a continuously decreasing temperature, homogeneous submicro-crystalline structures were produced in titanium aluminides, facilitating further processing such as superplastic forming at temperatures below 1,000°C. Also described in this paper is a fully lamellar structure with a colony size as small as 15 µm, produced in gamma TiAl-based alloys containing borides by the fast-heating of an initially submicro-crystalline structure in the alpha-phase range followed by air cooling.

One way of reducing the cost of titanium components is to use near-net shape powder-metallurgy techniques. This article describes a number of new approaches to producing components using the powder-metallurgy method for the aerospace, industrial, and consumer marketplaces.