O.N. Senkov

Thermohydrogen processing of titanium alloys

Abstract Use of hydrogen as a temporary alloying element in titanium alloys is an attractive approach for enhancing processability including working, machining, sintering, compaction, etc., and also for controlling the microstructure and thereby improving final mechanical properties. In this article, the status of the methods and applications of thermohydrogen processing (THP) to titanium alloys is reviewed. Effects of hydrogen alloying on the phases present, their composition, and the kinetics of phase reactions are considered. The effect of hydrogen on the hot workability, composite- and powder-metallurgy-product processing, and microstructure modification of wrought and cast conventional alloys and intermetallics, including production of nanocrystalline structures is discussed. Two recently discovered effects, i.e. hydrogen-induced softening of α titanium and hydrogen-induced hardening of β titanium are also discussed. Thermohydrogen processing has clear advantages in the development of improved microstructures and mechanical properties. In the case of near net shapes it is the only method for significant microstructural modification. It allows energy savings in processing to final products by improving the workability.

Synthesis of nanocrystalline materials: an overview

Abstract This paper reviews research work at the Institute for Materials and Advanced Processes (IMAP), University of Idaho, on the synthesis of nanocrystalline materials and their consolidation. Nanocrystalline materials have been synthesized by a number of ‘far from equilibrium processes’, including mechanical alloying (MA), mechanochemical processing (MCP), supercritical fluid processing (SCFP), and severe plastic deformation (SPD). Examples of the materials include the Ti–Al based intermetallic compounds and composites produced by MA and SPD, Ti base alloys and metal carbides synthesized by MCP, thin film Cu produced by SCFP, and Al–Fe alloys produced by SPD. Details of the processes used and the enhancement of properties due to the nanoscale structures in consolidated material will be presented. The potential of these processes to substitute for conventional methods of production will also be discussed.

Kinetics of martensite decomposition in Ti-6Al-4V-xH alloys

Abstract The kinetics of martensite decomposition in hydrogenated Ti–6Al–4V alloy samples was studied. To produce the martensite structure, the samples containing 0, 10, 20 and 30 at.% H were annealed in the β phase field and water quenched. Optical microscopy, transmission electron microscopy, X-ray diffraction analysis and microhardness testing techniques were utilized to study the phases and phase transformations and to obtain time–temperature-transformation (TTT) diagrams for the martensite decomposition. The martensite structure of the hydrogenated samples consisted of a mixture of hexagonal close packed (hcp) α′ and orthorhombic α″ martensites. The amount of the orthorhombic α″ martensite increased from 0 to ∼80 vol.% when the hydrogen content of the alloy was increased from 0 to 30 at.%. During aging at temperatures below the β transus temperature and above the martensite start temperature (Ms), the martensite structure transformed into a mixture of hexagonal close-packed (hcp) α and body-centered-cubic (bcc) β phases. At aging temperatures below the Ms, on the other hand, the martensite first transformed partially into a metastable β phase, and then equilibrium α and β phases were formed. On quenching after aging, in both these cases, the β transformed into martensite plus residual β, with the amount of the latter increasing with an increase in the hydrogen concentration and a decrease in the aging temperature. Hydrogen additions lowered the Ms of the Ti–6Al–4V alloy, and for samples containing 30 at.% H the Ms was below 500xa0°C. In the alloys containing 20 and 30 at.% H, a hydride phase was also detected. Complete decomposition of the martensite structure in the samples containing 30 at.% H and aged at 530xa0°C resulted in a fine and homogenous equiaxed microstructure consisting of a mixture of α, β and hydride phases. The nose temperature for the start of the martensite decomposition decreased from 800 to 625xa0°C when the hydrogen concentration increased from 0 to 30 at.%. The nose time for the start of the martensite decomposition increased from 6 s to 10 min when the hydrogen concentration increased from 0 to 10 at.% and did not change significantly with further increase in the hydrogen concentration.

Recent advances in the thermohydrogen processing of titanium alloys

In this article, the status of the methods and applications of the thermohydrogen processing of Titanium alloys is reviewed. Increased understanding of the mechanisms by which such processing is enhanced and the microstructure refined should lead to industrial applications.

Formation of a submicrocrystalline structure in TiAl and Ti3Al intermetallics by hot working

A method based on initiation of dynamic recrystallization (DRX) during hot working has been developed to produce a submicrocrystalline (SMC) structure (d < 1 µm) in massive work-pieces of hard-to-deform materials, like titanium aluminides, The method involves continuous grain refinement due to dynamic recrystallization at a decreasing temperature. A microstructure with a grain size of 0.1 to 0.4 µm and no porosity was produced in different TiAl and Ti3Al based alloys. Partial disordering was detected in a Ti3Al alloy with the SMC structure. The grain refinement hardened the intermetallic alloys at room temperature (RT). In a fully ordered Ti3Al alloy RT ductility increased when the grain size decreased, while the ductility of a partially disordered SMC Ti3Al and TiAl alloys was close to zero.

Low-temperature superplasticity of titanium aluminides

Data on superplastic behavior of intermetallic alloys with high ordering energy, such as stoichiometric TiAl and Ti3Al and a number of alloys based on TiAl, with submicron grain size are summarized. The small grain size resulted in a temperature range for superplasticity of 600–900°C that is 200–400°C lower than that for material with micron-sized grains. This paper reports on the effects of grain size, composition and superlattice type on the low-temperature and high temperature superplastic properties of titanium aluminides. Low temperature and high-temperature superplastic properties of the titanium aluminides are compared.

Synthesis and characterization of a TiAl/Ti5Si3 composite with a submicrocrystalline structure

Abstract A Ti–47Al–3Cr/Ti 5 Si 3 composite with a submicrocrystalline structure was produced by mechanical alloying (MA) and hot isostatic pressing (HIP). Blends of elemental and pre-alloyed powders were used for the MA. Intimate mixing of the elements and powder amorphization occurred during MA. HIP of the MA powder resulted in a fully dense material with a submicron grain size and a homogeneously distributed mixture of gamma-TiAl and Ti 5 Si 3 grains. The average grain size increased from 140 to 540 nm when the HIP temperature was increased from 850 to 1050°C. Grain growth in the composite in the temperature range 850–1100°C was described by the equation D 4 −D o 4 =K o xa0exp(− Q / RT ) t where Q =330 kJ mol −1 and K o =3.0×10 −18 m 4 s −1 . The kinetic behavior was similar to that for a nanocrystalline Ti–47Al–3Cr alloy annealed in the same temperature range, i.e. the Ti 5 Si 3 phase had little effect on grain growth.

High temperature mechanical properties of a submicrocrystalline Ti-47Al-3Cr alloy produced by mechanical alloying and hot isostatic pressing

Abstract High temperature tensile properties of a TiAl-based alloy with a submicrocrystalline structure were studied in the temperature range of 800 to 1200°C and the strain rate range of 10 −4 to 3×10 −1 s −1 . The alloy was produced by hot isostatic pressing of an amorphous mechanically alloyed powder and contained two phases, TiAl and Ti 2 AlN; the latter resulted from contamination of the powder with nitrogen during mechanical alloying. Two temperature ranges with different behavior were distinguished. In the temperature range of 800 to 950°C, the stress exponent, n =6, and the activation energy of deformation, Q =497 kJxa0mol −1 , were typical to a climb controlled deformation mechanism. In the temperature range 1000 to 1200°C, the stress exponent, n =3, and the activation energy, Q =347 kJxa0mol −1 , were determined together with high elongation indicating superplastic behavior.

Synthesis of a low-density Ti–Mg–Si alloy

Abstract A low-density titanium alloy was synthesized from blended elemental powders of TiH 2 , Mg, and Si by mechanical alloying and/or heat treatment. The titanium hydride was used in place of titanium. Phase transformations occurring in the system during heating at a constant rate were studied with the use of DTA and XRD. During heating of the blended elemental powders decomposition of titanium hydride occurred in the temperature range 550–750°C and some silicon went into solid solution in titanium while the majority of the silicon reacted exothermically with magnesium at about 500°C producing an intermetallic phase Mg 2 Si. This phase was stable on heating up to 950°C, where a eutectic component of this phase began to melt leading to formation of a liquid solution of magnesium in silicon, followed by a reaction of the liquid silicon with titanium and formation of a Ti 5 Si 3 phase. A third reaction in the system was detected at about 1100°C due to formation of MgO, so that after annealing at 1150°C three stable phases, Ti(Si), Ti 5 Si 3 , and MgO, were present in the alloy. No decomposition of the Ti 5 Si 3 phase or formation of Mg 2 Si were detected either during subsequent cooling or a second heating of the alloy. Completely different kinetics of the phase reactions occurred in the mechanically alloyed powders. Magnesium and silicon dissolved in the titanium hydride during mechanical alloying. Decomposition of the titanium hydride occurred at 320–600°C, the Mg 2 Si phase was formed after heating to 450°C, and the Ti 5 Si 3 phase was detected after heating to 570°C. The Mg 2 Si decomposed completely at a temperature of 650°C with the formation of MgO and Ti 5 Si 3 . After heating to 1150°C, three stable phases, TiN 0.3 , Ti 5 Si 3 , and MgO, were present in the alloy. A discussion of the results is given.

Phase Transformations in the Ti-6Al-4V-H System

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.