Transus Temperature Research Articles

Globularization using heat treatment in additively manufactured Ti-6Al-4V for high strength and toughness

A bimodal globularized microstructure in contrast to martensitic laths is known to impart high strength and toughness in Ti-6Al-4V. Heat treatment for the phase transformation of the laths to the globularized microstructure must be preceded by plastic deformation. This work reports an innovative strategy to obtain the bimodal microstructure consisting of globular α in additively manufactured Ti-6Al-4V alloy by heat treatment alone. The heat treatment schedule involves repeated thermal cycling close to but below the β transus temperature to form globular α eliminating the need for plastic deformation prior to heat treatment. A new mechanism of globularization other than known in literature is proposed to explain the formation of globular α. The inherent dislocation sub-structure of the martensitic laths initiates globularization by thermal grooving and boundary splitting but is unable to completely globularize the microstructure. Mechanisms such as cylinderization and edge spheroidization also do not lead to globularization. The purposefully designed thermal cycling causes oscillations in the volume fractions of α and β phases that in synergism with the slow cooling segments of the cycle globularize the α phase by epitaxial growth. The bimodal microstructure thus produced led to a significant improvement in the ductility by 80% and the toughness by 66%, which are desirable for structural applications. Furthermore, beneficial compressive stresses were generated in the alloy because of cyclic heat treatment. It is envisaged that the exceptional combination of mechanical properties observed here will lead to the fabrication of SLM printed Ti-6Al-4V parts that could leverage the advantages of additive manufacturing with material properties that are comparable to those obtained by conventional fabrication routes.

Study of Interstitial Impurities Influence on Properties of Titanium Alloy Ti-5Al-5V-5Mo-3Cr-1Zr

The effect of different interstitial impurities content (oxygen, carbon) on β - transus temperature and complex of properties in the heat-strengthened condition of the Ti-5Al-5V-5Mo-3Cr-1Zr (VST55531) titanium alloy has been studied. The possibility of determining alloy β - transus temperature by the differential scanning calorimetry method with accuracy comparable with the test quenching method is shown in this paper. It is shown that under identical aging regimes an increase of oxygen content from 0.176 to 0.25 wt.% and carbon content from 0.009 to 0.025 wt.% in VST55531 alloy results in higher strength, plastic characteristics and lower ductility, toughness characteristics, especially those which related to the crack propagation.

Insights into Machining of a β Titanium Biomedical Alloy from Chip Microstructures

New metastable β titanium alloys are receiving increasing attention due to their excellent biomechanical properties and machinability is critical to their uptake. In this study, machining chip microstructure has been investigated to gain an understanding of strain and temperature fields during cutting. For higher cutting speeds, ≥60 m/min, the chips have segmented morphologies characterised by a serrated appearance. High levels of strain in the primary shear zone promote formation of expanded shear band regions between segments which exhibit intensive refinement of the β phase down to grain sizes below 100 nm. The presence of both α and β phases across the expanded shear band suggests that temperatures during cutting are in the range of 400–600 °C. For the secondary shear zone, very large strains at the cutting interface result in heavily refined and approximately equiaxed nanocrystalline β grains with sizes around 20–50 nm, while further from the interface the β grains become highly elongated in the shear direction. An absence of the α phase in the region immediately adjacent to the cutting interface indicates recrystallization during cutting and temperatures in excess of the 720 °C β transus temperature.

Temperature, Stress and Distortion of Ti–6Al–4V Alloy Low-Temperature Friction Stir Welding Assisted by Trailing Intensive Cooling

The low-temperature friction stir welding (FSW) in which peak temperature is lower than the transus temperature of β phase was achieved using rotational speed of 100 rpm and welding speed of 30 mm/min. Trailing intensive cooling with liquid nitrogen was successfully applied to FSW under the low-temperature welding conditions. Comparisons of the temperature field, plastic strain, residual stress and welding distortion between intensive and conventional cooling were investigated by experiment and simulation. Results reveal that trailing intensive cooling is attributed to shrink high-temperature area and reduce the value of peak temperature and plastic strain. Longitudinal residual stress presents M shape, and the reduction of maximum tensile residual stress reaches 4.8%. The welding distortion shows an anti-saddle shape, and the decrement of welding distortion in transverse direction is 34.5%.

The effect of heat treatment on the tensile strength and ductility of pure titanium grade 2

The commercially pure titanium (CP) and titanium alloy are widely used in medical, aerospace and automotive industries due to their attractive mechanical properties especially because of its strength-to-weight ratio that is the highest among all known metal. A thorough understanding of the mechanical properties of titanium is very important so that an optimize machining parameters can be selected. Pure titanium has a higher level of corrosion resistance compared to titanium alloy due to the presence of an oxide layer on its surface. In this paper, heat treatment especially the annealing process was used prior to machining in order to study on the variation of its tensile strength and ductility at two different annealing conditions. The three conditions that were used in this experiment are untreated (T1), heat-treated at 700°C for 1 hour (T2) and 900°C for 1 hour (T3). In this work, a higher level of oxidation was observed for T3 compared to T2 due to the exposure to a temperature that is above the beta transus temperature that triggered the formation of acicular martensite. It was also observed that there was a decrease in the tensile strength when treated at T2 and a recovery of the strength was noticed at T3. The ductility properties were more significant at T2.

Effect of Initial Microstructure on High-Temperature Dynamic Deformation of Ti-6Al-4V Alloy

One of the attractive properties of Ti-6Al-4V alloy is control of microstructure through heat treatment to vary the mechanical properties. In this study, three different microstructures, Lamellar, Widmanstatten, and Martensitic morphologies, were created through heat treatment at a post-β transus temperature followed by cooling at different rates. With faster cooling rates, the microstructures evolved finer lamellae, smaller colony sizes, and thinner grain boundary layers. High-temperature dynamic compression was conducted on these specimens at a strain rate of 1000 s−1 and temperatures in the range of 23 °C to 1045 °C. Flow stresses decreased linearly with colony size and grain boundary layer thickness, but increased with inverse square root of lamellar thickness. This strong correlation of flow stress to several microstructural feature sizes indicated multiple modes of deformation. All three microstructures showed identical thermal softening. The softening rate was intensified at elevated temperatures due to hcp → bcc allotropic phase transformation. Gangireddy modification to Johnson–Cook model could account for this augmented softening and the modified J–C model predicted the three microstructures to follow a similar thermal softening coefficient m = 0.8. The kinetics of phase transformation appear to be very rapid irrespective of the microstructural differences in the Ti-6Al-4V alloy.

A Review of Metastable Beta Titanium Alloys

In this article, we provide a broad and extensive review of beta titanium alloys. Beta titanium alloys are an important class of alloys that have found use in demanding applications such as aircraft structures and engines, and orthopedic and orthodontic implants. Their high strength, good corrosion resistance, excellent biocompatibility, and ease of fabrication provide significant advantages compared to other high performance alloys. The body-centered cubic (bcc) β-phase is metastable at temperatures below the beta transus temperature, providing these alloys with a wide range of microstructures and mechanical properties through processing and heat treatment. One attribute important for biomedical applications is the ability to adjust the modulus of elasticity through alloying and altering phase volume fractions. Furthermore, since these alloys are metastable, they experience stress-induced transformations in response to deformation. The attributes of these alloys make them the subject of many recent studies. In addition, researchers are pursuing development of new metastable and near-beta Ti alloys for advanced applications. In this article, we review several important topics of these alloys including phase stability, development history, thermo-mechanical processing and heat treatment, and stress-induced transformations. In addition, we address recent developments in new alloys, phase stability, superelasticity, and additive manufacturing.

Calculating Ti-6Al-4V β Transus Through a Chemistry-Based Equation Derived from Combined Element Binary Phase Diagrams

A mathematical model was developed to correlate the relationship between titanium alloying elements (Al, O, N, C, V, and Fe) in Ti-6Al-4V, and the resulting beta transus temperature (βt). The model was produced by analyzing the slope of the lines separating the alpha + beta (α + β) and beta (β) phase fields on combined element binary phase diagrams generated through the Pandat modeling software. Experimental results showed a strong agreement between the baseline data, obtained through differential thermal analysis (DTA), and the calculated βt, determined by measuring the chemistry at a location adjacent to the DTA sample. The difference between the calculated βt and the baseline DTA data was approximately ± 10 °F (± 5.56 °C). The thermodynamically ideal βt temperature was additionally modeled for each set of chemistries through the Pandat software. These results were compared to the βt temperatures determined through the chemistry-based equation and DTA methods.

Effect of Cold Rolling and Heat Treatment on Microstructure and Mechanical Properties of Ti-4Al-1Mn Titanium Alloy

Mechanical behavior of Ti-4Al-1Mn titanium alloy has been studied in annealed, cold-rolled and heat-treated conditions. Room temperature tensile strength as well as % elongation has been found to be low with increasing amount of cold rolling. Lowering of strength in cold worked condition is attributed to premature failure. However, the same has been mitigated after heat treatment. Significant effect of cooling media (air and water) from heat treatment temperature on microstructure was not found except for the degree of fineness of α plates. Optimum properties (strength as well as ductility) were exhibited by samples subjected to 15% cold rolling and heat treatment below β transus temperature, which can be attributed to presence of recrystallized microstructure. In cold worked condition, the microstructure shows fine fragmented α plates/Widmanstatten morphology with high dislocation density along with a large amount of strain fields and twinning, which gets transformed to recrystallized equiaxed microstructure and with plate-like morphology after near β heat treatment. Prior cold work is found to have a significant effect on mechanical properties supported by evolution of microstructure. Twinning is found to be assisting in deformation as well as in recrystallization through the formation of deformation and annealing twins during cold working and heat treatment. Fracture analysis of the tested sample with prior cold work and heat-treated condition revealed quasi-ductile failure as compared to only ductile failure features seen for samples heat treated without prior cold work.

Influence of oxygen content on the microstructure and mechanical properties of cold rolled Ti-32.5Nb-6.8Zr-2.7Sn-xO alloys after aging treatment

The influence of oxygen (O) content on the microstructure and mechanical properties of cold rolled Ti-32.5Nb-6.8Zr-2.7Sn-xO (TNZS-xO; x = 0, 0.3, 0.6; mass%) alloys after aging at temperature range from 350 to 600 °C for 24 h was investigated. Results showed that the cold rolled TNZS-xO alloys possess single β phase. During the aging process, O could not only effectively suppress the precipitation of ω phase but also retard the formation and decomposition of α phase. In addition, the corresponding temperatures of the maximal volume fraction of α phase precipitation and the (α+β)/β transus temperatures of the TNZS-xO alloys were all increased with the increasing of O content. For mechanical properties, it was found that the strength and Young's modulus of the TNZS-xO alloys increased owing to the ω phase and/or α phase precipitation and decreased owing to the α phase decomposition. However, the elongation showed the opposite change tendency with the above mentioned strength. The mechanical properties of TNZS-xO alloys can be controlled over a wide range by subjecting to aging treatment and/or changing their O content. When aged at or below 450 °C, the TNZS-xO alloys exhibit great potential to become a series of new candidates for biomedical applications since they possess high strength (870–1460 MPa), low Young's modulus (45.1–75.6 GPa), high strength-to-modulus ratio (0.018–0.02) and appropriate elongation (7.2%–14.9%), which are superior to those of Ti-6Al–4 V alloy and suitable for the use as bio-implant materials.

Effect of travel speed and sub-β transus post deposition heat treatments on thin Ti-6Al-4V laser wire deposits

Annealing or HIP below the β transus temperature was found to coarsen the as-built developed microstructure while preserving the developed macro morphology of Laser Wire Deposition (LWD) buildups. Following these thermal cycles, a general decrease in strength along with an increase in elongation have been observed. However, no trends in the tensile results could be extracted due to the scattering of the results. An additional aging cycle substantially stabilized the developed results. Samples processed at 1.4 mm/s exhibited low strength hardly meeting any of the ASTM and AMS minimum requirements and a strong anisotropy in elongation with lower values when tested along the Z direction. Samples processed at 7.2 mm/s exhibited good strength meeting the minimum wrought requirements as set by AMS4911 while elongation showed an isotropic behavior. Hardness has been found to be mostly independent of the experienced sub-β transus post deposition heat treatment. Moreover, the grain boundary strengthening mechanism with regards to the α platelets thicknesses was found to be travel speed dependent.

Dynamic transformation of Ti–6Al–4V during torsion in the two-phase region

Isothermal torsion tests were performed on a Ti–6Al–4V alloy in the two-phase region. The results show that straining leads to an increase in the beta phase fraction, which increases slightly with strain rate. Transformation took place at 880, 940, 960, 980 and 1000 °C. The extent of this type of dynamic transformation (alpha to beta) was increased when the temperature approached the transus temperature. The reverse transformation (beta to alpha) occurred during isothermal holding after torsion and the volume fraction retransformed increased with time. The driving forces promoting dynamic and reverse transformation together with the energy barriers opposing these transformations were derived and compared. The critical stresses required to initiate dynamic transformation are calculated from the flow curves. This analysis confirms that the peak stresses are always higher than the critical stresses at the temperatures employed in the present tests, which makes it possible for the transformation to occur.

Opposing and Driving Forces Associated with the Dynamic Transformation of Ti-6Al-4V

The opposing and driving forces associated with dynamic transformation in Ti-6Al-4V are evaluated. It is shown that the stored energy is less than the opposing forces inhibiting transformation and insufficient to initiate the phenomenon. By contrast, the driving force derived from the net softening is greater than the opposing force and is responsible for initiating the reaction. This approach indicates that deformation reduces the transus temperature and leads to the formation of a thermodynamically metastable state.

Microstructural evolution of ω assisted α precipitates in β-CEZ alloy during ageing process

The microstructural evolution in β-CEZ (Ti-5Al-2Sn-4Zr-4Mo-2Cr-1Fe wt%) alloy, after quenching and ageing, has been investigated by combining results from orientation imaging microscopy, transmission electron microscopy, high-resolution TEM (HRTEM). Ultrafine Athermal ω precipitates (about 1 nm) are formed in this alloy by quenching from above the β transus temperature. In the ageing process, these ω precipitates coarsen, accompanied by the form of needle-like α phase. The scale of ω assisted α increased from 5 to 10 nm in width and about 50 nm in length to 10 nm in width and 100–300 nm in length while extending ageing time from 2 h to 5 h at 350 °C. However, when going on extending the ageing time from 5 h to 20 h at 350 °C, the scale of α didn't grow dramatically but the density of α increased clearly. When increasing the ageing temperature from 350 °C to 450 °C, the scale of ω assisted α grow dramatically. Furthermore, the structural relationship of ω and α is observed by HRTEM images. It can be found that ω precipitates were formed by {111}β planes shift 0.5d222 along the 〈111〉 direction, and the superfine α particles are formed in close proximity of the ω/β interface.

Formation of Deformation-Induced Products in a Metastable-β Titanium Alloy during High Temperature Compression

A metastable-β titanium alloy, Ti-10V-3Fe-3Al (wt. %), was subjected to thermo-mechanical processing including the compression test at 725 °C, which is below the β transus temperature (780 °C), and at strain rate of 10−3 s−1. The presence of phases was determined using transmission electron microscopy and X-ray diffraction. Although the dynamic recovery took place together with slip, both deformation-induced α” martensite and ω were detected as other operating mechanisms for the first time in metastable-β titanium alloy deformed in α + β region. The volume fraction of stress-induced α” was higher than that of the same alloy deformed at room temperature due to a higher strain applied. Stress-induced twinning was not operational, which could be related to the priority of slip mechanism at high temperature resulted from thermally-assisted nucleation and lateral migration of kink-pairs.

Thermo-fluid-metallurgical modelling of the selective laser melting process chain

The entire process chain of selective laser melting of Ti-6Al-4V is analysed. First, a thermo-fluid dynamical model is used to investigate the temperature profile during the process and estimate the size and shape of the melt pool. The inclusion of the Marangoni effect improves upon previous work by showing the liquid velocity in the melt pool. Next, this information allows us to estimate the morphology of the grains of a part produced by selective laser melting. Finally, a cellular automata is used to model the microstructural evolution during a uniform heat treatment at the beta transus temperature. It is shown that the model shows good agreement with earlier experimental results.

Microstructural, mechanical and biological characterizations of the promising titanium-tantalum alloy for biomedical applications

In this study, the microstructural, mechanical and biological characterizations of the Ti-25Ta (wt.%) alloy were investigated by means of optical microscope, X-ray diffraction, electron microscope, microhardness test, contact angle goniometer and in vitro cytotoxicity assay. As the alloy underwent heat-treatment at temperatures between 700 °C and 1000 °C for 30 min, its microstructure was a mixture of β phase and needle-like α′′ martensite phase. The analytical results indicated that the volume fraction of the needle-like α′′ martensite phase decreased with increasing heat-treatment temperature. The β transus temperature of the investigated Ti-25Ta alloy was below 700 °C. Moreover, the alloy heat-treated at 900 °C exhibited the maximum hardness Hv 743 ± 12.93. For wettability evaluation, all investigated samples possessed surface hydrophilicity. The cytotoxicity assay results also demonstrated that the heat-treated Ti-25Ta samples have well cell proliferation and adhesion behaviors. Therefore, these results and features could be used to further understand the relationship between the high temperature microstructure, mechanical behavior and in vivo biocompatibility of the Ti-25Ta alloy, and develop as a potential biomedical alloy.

Effect of subtransus heat treatment on the microstructure and mechanical properties of additively manufactured Ti-6Al-4V alloy

As a powder-bed-based additive manufacturing technology, selective laser melting (SLM) offers high-level flexibility and enables efficient fabrication of complex parts. In connection with complex thermal events occurring during dynamic sequential layer-by-layer deposition, the as-built material is usually hierarchical at different length scales and possesses anisotropy at each level. As a result of a moderate heating temperature of the baseplate and high cooling rates involved in the process, the as-built Ti-6Al-4V alloy has an α′ martensite microstructure. Microstructure evolution occurring during post-SLM heat treatment is strongly affected by the stability of the initial acicular martensite. The present study was aimed at developing an optimum post-SLM heat treatment scheme at a temperature below the β transus temperature, based on the understanding of microstructure evolution occurring during subtransus treatment and the resultant mechanical properties of the alloy. It was observed that the growth of the α and β phases during the heat treatment was inhibited by the initial α′ phase. A higher heating temperature could effectively improve microstructure homogeneity on a micrometer-scale to some extent. Heating temperature affected the strength and fracture strain of the alloy far more than cooling rate. A post-SLM heat treatment at a temperature of 850 °C or higher could lead to an improvement of fracture strain to the level of the forged counterpart, accompanied by the losses in yield strength and ultimate compressive strength from the as-built values. Full annealing (i.e., subtransus treatment at a high temperature) was thus recommended to be an appropriate post-SLM heat treatment for Ti-6Al-4V.

Effect of quenching on the matrix microstructure of SiCf/Ti–6Al–4V composites

Heat treatment can change the matrix microstructure of metal based composites, which leads to the change of mechanical properties of the composites. In the present work, the variation of microstructure and hardness of the matrix of SiCf/Ti–6Al–4V composites prepared by foil-fiber-foil (FFF) method were investigated after quenching from 950, 980 and 990 °C. The results indicated that the β transus temperature of the matrix was close to 990 °C, which is slightly higher than that of the Ti–6Al–4V alloy, which is between 950 and 970 °C. This increase was attributed to the diffusion effect of carbon atoms. Some carbide particles were precipitated at the prior β grain boundaries when full martensite was obtained. Transmission electron microscopy examination observed three α′ variants that obeyed the Burgers orientation relationship with the β phase in the sample quenched from 950 °C. The hardness of the matrix increased with the increase in quenching temperature, and the highest hardness of 420 HV was obtained after quenching from 990 °C, which is much higher than that of the Ti–6Al–4V alloy. The high hardness was resulted from phase transformation strengthening by martensite, solid solution strengthening of carbon atoms and the dispersion strengthening of TiC precipitates.

Laser Additive Manufactured Ti–6Al–4V Alloy: Heat Treatment Studies

The effect of heat treatment on microstructure and mechanical behaviours of direct metal laser sintered Ti–6Al–4V samples have been studied. Rectangular parts were built in two different directions; vertical and horizontal and subjected to two different heat treatment cycles: above β transus and below β transus with air cooling. Surface characteristics, microstructural examination and mechanical properties have been investigated. Below β transus treatment creates a modification in the surface morphology with a fine dimple network. Above β transus treatment leads to extensive grain growth at the middle section of the vertically build component thereby increasing its microhardness. Both the selected heat treatment cycles significantly reduces the tensile strength and improves the elongation when compared to as-sintered material. However, below transus temperature treated vertical built specimen results in optimum combination of tensile strength (1124 MPa) and elongation (20%). Higher coefficient of friction has been recorded for specimens after heat treatment.