Alpha Titanium Alloy Research Articles

Evolution of equiaxed and lamellar α during hot compression in a near alpha titanium alloy with bimodal microstructure

Microstructural evolution of equiaxed α and lamellar α during hot compression in a bimodal-structure near alpha Ti-6Al-3Nb-2Zr-1Mo alloy were investigated. Focus was concentrated on their own dynamic restoration mechanism via electron backscatter diffraction (EBSD) technique. Based on analysis of grain boundary misorientation distribution and misorientation gradient within α grain, it was revealed that limited continuous dynamic recrystallization (CDRX) as well as adequate dynamic recovery (DRV) took place within equiaxed α, whereas CDRX occurred as the main restoration mechanism in lamellar α. Further, the deformation heterogeneous among equiaxed α grains was found to be relevant to the difference in slip systems. Under the same nominal strain of 75% height reduction, process of weak DRV, strong DRV and CDRX coexisted in distinct equiaxed α grains owning to small, medium and high Schmid Factor of prismatic slip. Besides, globularization heterogeneity induced by CDRX in lamellar α was considered to be closely related with its initial orientation to compression axis and the imposed strain. For flat colony α, the microstructure was relatively stable during straining showing a thinning process and a weak dynamic globularization, while the kinked colony α with large strain was globularized sufficiently.

A Hierarchical Multiscale Modeling Investigation on the Behavior of Microtextured Regions in Ti-6242 α/β Processing

Ti-6242 is a near alpha titanium alloy, which has excellent high-temperature creep resistance and is widely used in jet engine compressors. This alloy is susceptible to creep fatigue failure under dwell loading below 473 K. The existence of microtextured regions (MTRs) contributes significantly to this fast crack propagation. Mechanical processing in the alpha + beta region has been employed to eliminate MTRs, but the efficiency depends significantly on the applied strain path. Previous investigations based on crystal plasticity finite element (CPFE) simulations have demonstrated the relationship between breakdown efficiency and loading direction. Therein, MTRs with regular geometry and pure initial orientation were used to isolate the effect of loading direction from initial microstructure. In this paper, the behavior of MTRs with realistic initial microstructure was investigated using a hierarchical multiscale modeling framework, and the microscale results were analyzed in detail to understand the behavior of MTRs under different loading conditions. It was shown that a hierarchical multiscale model with realistic initial microstructure at the microscale can reflect the influences from different strain paths, initial orientation distributions, and positions of the region simultaneously. The combined effect of initial orientation distribution and loading direction on the MTR breakdown efficiency is discussed in detail.

Creep Features of Ti-600 Alloy at the Temperature of 650°C

Creep tests were carried out on one kind of near alpha titanium alloy named after Ti-600 alloy at the temperature of 650°C, and with the stresses of 150MPa, 200MPa, 250 MPa, 300 MPa and 350 MPa, respectively. The alloy ingot was conventionally forged and rolled to diameter 18mm bars. The creep samples were cut from the rolling bars and were solutioned at 1020°C for 1 h, air cooling, then aged at 650°C for 8 h, air cooling (STA). Steady state creep rate and the stress exponent n at different stresses were calculated for the alloy. Threshold stress σ0 was introduced to get the true stress exponent p. Creep deformation mechanism was also investigated. The results indicated that the steady state creep rate will increase with the rise of stress, and the creep time will also be shortened at the same time. At 650°C, the threshold stress is 83.8MPa. The value of n and p is 7.7 and 3.3 respectively for the alloy crept at lower stress region (150-200MPa); and which is 2.1 and 4.7 respectively for the alloy crept at relatively higher stress region (200-350MPa). Constitutive equations of steady state creep rate were also established for the alloy crept at 650°C. The creep deformation for the alloy is controlled by dislocation slipping at lower stress region, and which is mainly controlled by dislocation climbing and subordinately controlled by dislocation slipping at higher stress region.

Additive manufacturing of a novel alpha titanium alloy from commercially pure titanium with minor addition of Mo2C

In this study, a fully dense and crack-free alpha titanium alloy was fabricated using selective laser melting (SLM) of commercially pure titanium (CP Ti) blended with 1 vol.% micrometre-sized Mo2C powder. The microstructure and mechanical properties of the fabricated Ti alloy were systematically investigated. It was found that the fabricated Ti alloy consisted of both alpha (α) and beta (β) phases and exhibited a nest-like inhomogeneous microstructure with graded local phase formation, composition and crystallographic texture. This novel microstructure was formed through melting and rapid solidification of CP Ti and Mo2C during SLM, assisted by the high laser absorptivity and resultant high temperature rise of the Mo2C powder. The fabricated Ti alloy exhibited higher tensile strength (∼1250 ± 50 MPa) and improved ductility (elongation ∼17 ± 1%) when compared to CP Ti fabricated using the same SLM process. The underlying reasons were mainly attributed to (i) the nest-like inhomogeneous microstructure consisting of fine grained α-Ti and β-Ti, (ii) the overall solid solution strengthening of the Ti matrix by interstitial C, and (iii) the high dislocation density in the Ti matrix arising from the mismatch between α-Ti and β-Ti phases. This study not only provides a fundamental knowledge about the modification of CP Ti with minor addition of secondary particles through SLM but also provides insight into the fabrication of inhomogeneous Ti alloys with graded microstructure and local composition through SLM for enhanced mechanical properties.

On the microstructure evolution during isothermal low cycle fatigue of β-annealed Ti-6242S titanium alloy: Internal damage mechanism, substructure development and early globularization

The present work deals with the room and high temperature microstructure evolutions of a near alpha titanium alloy (Ti-6242S) during isothermal low cycle fatigue. The initial fully lamellar microstructure has been purposefully annealed to form α/β interface phase layers. The shearing and displacement of the β layers is identified as the main governing internal damage mechanism. This is phenomenal owning to the very low imposed strain; and is justified considering: (i) the large colony size and thin thickness of the β layers within the initial lamellar microstructure, (ii) the presence of the interface phase, and (iii) the intensified substructure development within the α and β phases. The shearing and displacement of the β layers lead to the segmentation and this is followed by initial stage of dynamic globularization even at room temperature condition. The globularization kinetic is intensified at higher temperature cyclic loading. In this regard, the sub-boundary induced boundary splitting/grooving is characterized as an involved mechanism. Interestingly, the early dynamic globularization of the α lamellae is also traced through the high temperature fatigued microstructures. The β phase penetration along the α phase sub-boundary is realized as the main globularization mechanisms. The interface phase layer stimulates the possibility of globularization via providing the high-energy non-coherent interfaces in structure. In addition, the deformation twinning within the interface phase, so called as “twinning induced micro-plasticity effect”, appears to have a significant effect on the fatigue damage behavior via compensation of incompatibility between α and β phases and deformation twin formation.

Origins of different tensile behaviors induced by cooling rate in a near alpha titanium alloy Ti65

The influence of cooling rate after solution treatment on tensile behaviors of a near α titanium alloy Ti65 at test temperature from room temperature (RT) to 650 °C was investigated. The results show that tensile elongation and area reduction for water quenched (WQ) microstructure are 55% and 89% higher than those of air cooling microstructure tested at 650 °C, while yielding/ultimate strength are 613/743 MPa, around 8% higher than those of air cooling (AC1) microstructure. With the increase of test temperature, strength and ductility were both dramatically improved in water quenched (WQ) microstructure compared with air cooling (AC1) microstructure. This phenomenon was primarily attributed to the distinction of microstructure morphologies induced by cooling rates. The origin of differences of tensile behaviors between air cooling (AC1) and water quenched (WQ) microstructures is discussed in terms of microstructure morphologies, grain orientation dependency, tensile fracture surfaces, and the role of twining. The schematics of tensile fracture modes are proposed for air cooling and water quenched microstructures tested at room temperature and 650 °C.

Crystal plasticity modeling of a near alpha titanium alloy under dynamic compression

The purpose of this study is to describe the dynamic behavior of near alpha titanium alloy at elevated temperatures using Crystal plasticity and other models. Three constitutive models: Johnson-Cook (J-C), Zerilli–Armstrong (Z-A), Cowper-Symonds (C–S) and J-C fracture models are established. Experimental results observed that material displayed strain rate sensitivity at various strain rates. The generated model constants have been incorporated into finite element code. Finite Element simulations of tensile tests under different strain rates have been performed using Autodyn software. A crystal plasticity model is established and shows that it can predict the stress strain response under dynamic loading conditions. The developed crystal plasticity model was implemented in the ABAQUS code to simulate high strain rate compression and showed good agreement. Subsequently, a Charpy impact test on specimen has also been simulated. The fracture surfaces of specimens tested under quasi static state at various temperatures were studied under scanning electron microscopy (SEM).

An experimental research on the machinability of a high temperature titanium alloy BTi-6431S in turning process

Titanium alloys are extensively applied in the aircraft manufacturing due to their excellent mechanical and physical properties. At present, the α + β alloy Ti6Al4V is the most commonly used titanium alloy in the industry. However, the highest temperature that it can be used only up to 300 °C. BTi-6431S is one of the latest developed high temperature titanium alloys, which belongs to the near-α alloy group and has considerably high tensile strength at 650 °C. This paper investigates the machinability of BTi-6431S in the terms of cutting forces, chip formation and tool wear. The experiments are carried out in a range of cutting parameters and the results had been investigated and analyzed. The investigation shows that: (1) the specific cutting forces in the machining of BTi-6431S alloy are higher than in the machining of Ti6Al4V alloy; (2) the regular saw-tooth chips more easily formed and the shear bands are narrower in the machining of BTi-6431S; (3) SEM and EDS observations of the worn tools indicate that more cobalt elements diffuse into the workpiece from tool inserts during machining of BTi-6431S alloy, which significantly aggravates tool wear rate. The experimental results indicate that the machinability of BTi-6431S near alpha titanium alloy is significantly lower than Ti-6Al-4V alloy.

Coarsening kinetics of primary alpha in a near alpha titanium alloy

In this study, the coarsening behavior of primary alpha after hot deformation of a near alpha titanium alloy (Ti-1100) was investigated in the context of heat treatment parameters. Ti-1100 slab with initial martensitic microstructure was rolled to a moderate level of strain (ε ˜ 0.8) at 850 °C revealing the small number of the spheroidized alpha particles. After rolling, only 10 min of heat treatment at 1000 °C was needed to reach a uniform bimodal microstructure. Decreasing the heat treatment temperature to 930 °C resulted in complete spheroidization of the alpha laths after elapsing 5 h. Evaluation of the microstructure heat treated at 930 °C, 980 °C, and 1000 °C up to 50 h showed that the primary alpha particles were coarsened with heat treatment time. The linear relation of the cube of average particles radius with the heat treatment time and the scaling behavior of the particle size distribution proved that the coarsening was a volume diffusion controlled process. The Lifshitz-Slyozov-Wagner (LSW) model could describe the coarsening process in this alloy from low (∼20%) to high (∼85%) volume fraction of the primary alpha without considering any further modification for the changes in the volume fractions. The rate limiting level for the primary alpha coarsening was found to be the outward diffusion of Mo.

Evolution of equiaxed alpha phase during heat treatment in a near alpha titanium alloy

The evolution of equiaxed primary α phase (αp) during post-deformation heat treatment of near-α titanium alloy Ti-5.6Al-3.8Sn-3.2Zr-1.0Ta-0.5Mo-0.4Nb-0.35Si was studied. This alloy was isothermally compressed at 1019 °C and subsequently heat treatment at 1024 °C for times ranging from 0.5 to 24 h. The recrystallization behavior and boundary splitting within equiaxed αp were analyzed by crystallographic orientation and microstructure observations. The results showed that the formation of internal (sub)boundaries within the αp was by the strong recovery. In most cases, it is difficult to obtain a refinement of big equiaxed αp by boundary splitting and αp grains were still contiguous with “peanut” shape at relatively short heat treatment time (≤24 h, 1024 °C). The groove in the “peanut” structure was indicative of boundary of α/α. Only in a few cases, when an isolated β grain can form within the interior αp along the (sub)boundary, refinement of αp grain may be achieved due to the obvious reduction of boundary splitting distance. These observations were rationalized on the basis of the classical Mullins grooving analysis.

Development of a trimodal microstructure with superior combined strength, ductility and creep-rupture properties in a near alpha titanium alloy

In the present study a new microstructure was introduced for a Ti-6242S alloy that has an appropriate combination of mechanical properties such as strength, ductility and creep-rupture. This trimodal microstructure was developed through thermomechanical processing consisted of a near beta rolling followed by solution annealing in alpha/beta region. In the following, the mechanical properties of this microstructure was investigated and compared to the conventional microstructures i.e., widmanstätten and bimodal microstructures to demonstrate its superiority. The results indicated that the yield strength, ductility and toughness in trimodal microstructure lies between those of the two other microstructures. However, the ductility and toughness of trimodal microstructure is significantly higher than that of the widmanstätten microstructure and almost close to the bimodal microstructure. The steady state creep rate in trimodal microstructure at 540°C and under an applied stress of 540MPa approached closely to that of widmanstätten microstructure and was considerably lower than the value in bimodal microstructure. Furthermore, time to rupture for trimodal microstructure is 1.3 and 3.5 times higher than the widmanstätten and bimodal microstructures, respectively. These results clearly confirm a more optimized combination of mechanical properties in the trimodal microstructure compared to the widmanstätten and bimodal microstructures.

Experimental investigation of surface roughness in high-speed micro end milling of near alpha titanium alloy

Super alloys are widely used in many applications such as aerospace, automotive, biomedical, marine, mining and oil industries. In high-speed micro end milling (HSMEM) of super alloys, the proper selection of cutting tool is necessary to achieve good surface quality without burrs. Surface roughness characterisation is crucial on the work materials to achieve the desired production rate in limited time and at low cost. In this research work, surface roughness characterisation was investigated in order to opt for the better possible tool for machining Near Alpha Titanium Alloy Grade-12 under feasible conditions. The influence of machining parameters i.e. cutting speed, feed and depth of cut on surface roughness was investigated in the present work. Experiments were conducted under dry cutting conditions using Uncoated, PVD Coated TiAlN & AlTiN tungsten carbide tools. It was observed that machining with uncoated tungsten carbide tools generates less surface roughness on near alpha titanium alloy.

Experimental investigation of surface roughness in high-speed micro end milling of near alpha titanium alloy

Experimental investigation of surface roughness in high-speed micro end milling of near alpha titanium alloy

Peak stress studies of hot compressed TiHy 600 alloy

TiHy 600 is a near alpha titanium alloy similar to IMI 834 Ti alloy, widely used for gas turbine engine applications such as disc and blades for high pressure compressors. Flow behavior of TiHy 600 alloy is investigated by conducting hot compression tests at temperatures ranging from 900°C to 1050°C with in the strain rate range of (0.001/s) respectively up to 50% deformation. Constitute modeling of work hardening is established, using Cingara equation to verify the stress up to peak stress. It was found that experimental true stress-true strain curves are in good agreement with Cingara equation curves for all temperatures at strain rate of (0.001/s)up to their peak stress values. The average activation energy (Q) is calculated from the peak stress of the flow curves using the hyperbolic-sine law equation (Arrhenius equation) i.e. 851.506KJ/mol. The microstructures of 50% deformed samples are correlated with the post-peak stress flow curve, performed mainly to compare the various softening processes (DRV or DRX).

Crystal Plasticity Model Validation Using Combined High-Energy Diffraction Microscopy Data for a Ti-7Al Specimen

High-Energy Diffraction Microscopy (HEDM) is a 3-d X-ray characterization method that is uniquely suited to measuring the evolving micro-mechanical state and microstructure of polycrystalline materials during in situ processing. The near-field and far-field configurations provide complementary information; orientation maps computed from the near-field measurements provide grain morphologies, while the high angular resolution of the far-field measurements provides intergranular strain tensors. The ability to measure these data during deformation in situ makes HEDM an ideal tool for validating micro-mechanical deformation models that make their predictions at the scale of individual grains. Crystal Plasticity Finite Element Models (CPFEM) are one such class of micro-mechanical models. While there have been extensive studies validating homogenized CPFEM response at a macroscopic level, a lack of detailed data measured at the level of the microstructure has hindered more stringent model validation efforts. We utilize an HEDM dataset from an alpha-titanium alloy (Ti-7Al), collected at the Advanced Photon Source, Argonne National Laboratory, under in situ tensile deformation. The initial microstructure of the central slab of the gage section, measured via near-field HEDM, is used to inform a CPFEM model. The predicted intergranular stresses for 39 internal grains are then directly compared to data from 4 far-field measurements taken between ~4 and ~80 pct of the macroscopic yield strength. The evolution of the elastic strain state from the CPFEM model and far-field HEDM measurements up to incipient yield are shown to be in good agreement, while residual stress at the individual grain level is found to influence the intergranular stress state even upon loading. Implications for application of such an integrated computational/experimental approach to phenomena such as fatigue are discussed.

A comparative study of pulsed laser and pulsed TIG welding of Ti-5Al-2.5Sn titanium alloy sheet

Pulsed Nd:YAG laser beam welding (P-LBW) and pulsed tungsten inert gas (P-TIG) welding were used to prepare full penetration bead-on-plate weldments of 1.6mm thick Ti-5Al-2.5Sn alpha titanium alloy sheet. The influence of welding phenomenon on the microstructure, micro-hardness, tensile properties, surface and sub-surface residual stress distribution and deformation and distortion of both the weldments were studied. Higher cooling rate in P-LBW resulted in complete α' martensitic transformation in fusion zone whereas in P-TIG weldment α' and acicular α was formed within equiaxed β matrix due to lower cooling rate. Hardness in fusion zone of P-LBW was higher than that of the fusion zone of P-TIG weldment due to faster cooling rate in P-LBW. The welded zone in both the weldments showed higher hardness and strength than that of the parent metal since a ductile fracture occurred in the un-welded section during tensile testing. Residual stresses in both P-LBW and P-TIG weldments showed similar trend but the distribution was much narrower in P-LBW due to less width of heat affected zone. P-LBW resulted in more nonuniformity in through thickness stress profile because of greater top to bottom width ratio. Less residual stresses, deformation and distortion and superior mechanical properties in P-LBW made the process more feasible than P-TIG for the welding of Ti-5Al-2.5Sn alloy sheet.

Effect of Silicide on Dynamic Recrystallization in Near Alpha Titanium Alloy

Dynamic recrystallization in Ti-1100 was investigated. Ti-1100 is one of near α titanium alloys and contains Si for improving high temperature mechanical properties. Ti-1100 exhibits martensitic transformation by quenching into iced brine after solid solution treatment. Hereafter specimens subjected to quenching into iced brine and to cooling in air after solid solution treatment are called IBQ specimen and AC specimen, respectively. After tensile test at high temperature, IBQ specimen exhibits morphological change from lath structure to equiaxed structure, but AC specimen does not. It is indicated that dynamic recrystallization occurs during the tensile test of IBQ specimen. Effect of silicide on the dynamic recrystallization was investigated using two specimens: one included more silicide precipitates and the other less. The former specimen shows smaller recrystallized grains than the latter. It is indicated that the specimen including more silicides exhibits smaller recrystallized grains.

A study of epitaxial growth behaviors of equiaxed alpha phase at different cooling rates in near alpha titanium alloy

The epitaxial growth behaviors of equiaxed primary α phase (αp) at different cooling rates (150–0.15 °C/s) in a near α titanium alloy Ti60 were studied by optical micrograph, back scattered electron (BSE) images, high-resolution electron backscatter diffraction technique (EBSD) and electron probe microanalysis (EPMA). Microstructural observations indicated that the size of αp significantly increased with decreasing cooling rate. The rim-α phase observed by BSE image, which formed at the periphery of αp during cooling and has an identical crystallographic orientation to the interior region of αp analyzed by Kikuchi diffraction patterns, is considered to be evidence of epitaxial growth of αp. EBSD analysis also showed that αp preferentially grew extending for a distance along the β/β boundary resulting in extension-α phase from αp. The EPMA confirmed that contrast difference in BSE image within αp is caused by the difference in composition. The further microanalysis of local composition indicated that epitaxial growth during continuous cooling is mainly controlled by the diffusional redistribution of aluminum and molybdenum atoms between αp and β matrix. On this basis, the sizes of αp were theoretically calculated after continuous cooling based on a diffusion-controlled model, and model predictions showed good agreement with experimental measurements.

Effects of α + β phase deformation on microstructure, fatigue and dwell fatigue behavior of a near alpha titanium alloy

In the present study, a near α titanium alloy, similar to IMI 834 alloy, was melted and thermo-mechanically processed. Different amount of deformation was imparted to the alloy in the α–β region and thereafter tensile, fatigue, and dwell fatigue properties were evaluated in a standard heat treatment condition. Higher α–β deformation resulted into a more refined microstructure. Further, based on EBSD analysis, it was found that higher α–β deformation led to a higher fatigue and dwell fatigue life, along with dwell fatigue life debit as compared to relatively lesser deformed specimen mainly due to hard (0001) basal pole orientations, approximately, parallel to the loading axis.

Flow and fracture characteristics of near alpha titanium alloy

To understand the flow and fracture behavior of near-α titanium alloy, the influence of stress triaxiality and strain rate on the failure behavior of this alloy was considered. The combined effect of strain rate, temperature and stress triaxiality on the behavior was studied by testing both smooth and notched specimens. Johnson-Cook (J-C) constitutive and fracture models were established based on high strain rate tensile data obtained from Split hopkinson tension bar (SHTB) and quasi-static tests. The Johnson Cook constitutive and fracture models have been calibrated. A modified Johnson–Cook model was established and proved to have high accuracy. The calibrated model has been validated by simulating the tension tests at various triaxialities using ANSYS autodyn software. Finite element simulations were used to predict the effective plastic strain versus triaxiality history within the deforming specimens. The fracture surfaces of specimens tested under various strain rates and temperatures were studied under scanning electron microscopy (SEM).