Deformation Behavior Of Ti Research Articles

Hot deformation, processing maps, and microstructural evolution of Ti/Ni/Ti layered composites

Ti/Ni/Ti layered composites, which combine the advantages of Ti and Ni, are widely used in sensor and biomedical applications due to their light weight, high strength, high conductivity, good stability and excellent corrosion resistance. However, it is difficult to control the structural parameters, microstructure, and mechanical properties of layered composites during co-deformation processing due to the distinct differences in the crystal structure, mechanical properties, and deformation behavior of Ti and Ni layers. In this paper, the hot deformation, processing maps, and microstructural evolution of Ti/Ni/Ti layered composites during hot deformation processing are investigated through hot compression tests at 550–850 °C/0.001–1.0 s−1, with a reduction of 65%. It is shown that the composites exhibit mainly dynamic recovery or similar dynamic recrystallization characteristics. Combined with the interface structure, processing maps and microstructure, the optimal parameters for Ti/Ni/Ti layered composites are 680–730 °C/0.007–0.015 s−1. During co-deformation, the microstructural evolution of the constituent layers is very complex. The deformation mechanism is mainly dynamic recovery (DRV), or DRV and dynamic recrystallization (DRX) in Ti layer, and work hardening (WH) or DRV in Ni layer.

Effect of hydrogen addition on compression deformation behaviour of Ti–0.3Mo–0.8Ni alloy argon-arc welded joints

The effect of hydrogen addition on compression deformation behaviour of Ti–0.3Mo–0.8Ni alloy argon-arc welded joint has been investigated. Evolution mechanism of hydrogen-induced flow stress was discussed in detail. The results show that with increasing hydrogen content, the stretching and bending extent of fully lamellar microstructures including α lamellas and acicular hydride continued to increase, the morphology of dynamic recrystallization (DRX) grains tended to change from approximately equiaxed to large lamellar shape, and the quantity of DRX grains and recrystallization degree of grains increased obviously. A large number of dislocations concentrated in the vicinity of the hydride. Steady stress was decreased continuously with increasing hydrogen content, while peak stress of the hydrogenated 0.12 wt.% H weld zone was decreased to the minimum value and then increased slowly. A slight decrease in flow stress of the hydrogenated 0.05 wt.% H weld zone was caused by limited increase in the volume fraction of softer β phase. Hydrogen-induced DRX of α phase and improved dislocation movement by strong interaction between the hydride and dislocation directly resulted in a sharp drop in flow stress of the hydrogenated 0.12 and 0.21 wt.% H weld zone. Solute hydrogen also finitely contributed to a sharp drop in flow stress of the hydrogenated 0.12 and 0.21 wt.% H weld zone by promoted local softening, which induced continuous DRX and more movable dislocations to participate in slipping or climbing. The reinforcement effect and plastic deformation of the hydride and solution strengthening of β phase induced by solute hydrogen finally led to the increase in flow stress of the hydrogenated 0.21 wt.% H weld zone in its true strain range from 0 to 0.36.

Mechanical and high-temperature deformation properties of Ti–43.5Al–3V–2Nb alloy

We studied mechanical properties and deformation behavior of Ti–43.5Al–3V–2Nb alloy at high-temperatures. Stress–strain curves demonstrated standard work-hardening and flow-softening characteristics,and the data obtained from these curves were used to obtain a corresponding formula to calculate activation energy (which was qual to 565 kJ mol−1). Dynamic materials modeling was used to develop a processing map with a true strain equal to 0.6. Ti–43.5Al–3V–2Nb alloy with an equiaxed structure has a wide processing window. Instability zones were in the 1000 °C–1050 °C and >0.5 s−1. Flow-soft mechanism in the unstable region is related to stress-relaxation due to the incompatible deformation of each phase. Flow-soft mechanism in the safe region is related to the dynamic recrystallization and dynamic recovery of γ, α and β polymorphic modifications. Heat treatment at 1235 °C for 60 min/AC + 900 °C/6 h/FC improved room-temperature tensile properties of the forged alloy significantly: tensile strength was 920MPa and tensile ductility was 2.5%. Thus, the alloy demonstrated excellent combination of high strength and ductility after this heat-treatment. Heat-treated Ti–43.5Al–3V–2Nb alloy also maintained tensile strength higher >820 MPa up to 750 °C.

A Study on Yield Function for Ti–6Al–4V Titanium Alloy Sheets at Elevated Temperatures

Currently, the finite element (FE) method simulation technique is being widely applied to optimize the forming process. To achieve high-accuracy FE simulation results, the identification of material properties and deformation characteristics such as yield criteria is important. In this study, characterisation of plastic deformation behavior of Ti–6Al–4V titanium alloy using anisotropic sheets were carried out using, biaxial tensile tests at 600 °C with a cruciform specimen. The test was performed until the specimen was broken down. The experimental results for the yield locus were first compared with other theoretical models to show the mismatch of the conventional function. A new yield function was then proposed in this study to describe not only the anisotropic but also the asymmetric behavior between the tension and compression of Ti–6Al–4V titanium alloy sheets using a fitting equation to determine the material constants of the yield function. Finally, yield loci at other elevated temperatures were obtained based on the relationship between tension yield stress and temperature.

Effect of annealing on the microstructures and deformation behaviour of Ti–50.7at.%Ni shape memory alloy

In this paper Ti–50.7at.%Ni as a commercial material was examined to analyse the effect of annealing temperature on the microstructure and deformation behaviour. It was found that the microstructure and deformation behaviour of as-received material was affected by the existence of dislocations that was introduced during manufacturing process. The presence of martensite variant and the B2 austenite broadening in XRD spectra of as-received material verified this conclusion. However, annealing treatment has gradually eliminated this effect. The plateau region on stress–strain curve which was absent in the as-received material started to appear and get longer with increasing annealing temperature. Maximum tensile strength was observed in the sample annealed at 400 ℃ for 1 h. Annealing treatment above 500 ℃ transformed the B19′ martensite to B2 austenite, which leads to a decrease in the tensile strength of the material. Nearly perfect superelasticity was found in the as-received material and sample annealed at 300 ℃.

High temperature deformation behaviour of Ti–5Al–5Mo–5V–3Cr during thermomechanical processing

The flow stress behaviour of Ti-5553 titanium alloy during hot compression deformation was studied by thermal simulation test at the deformation temperature of 750–900°C and the strain rate of 0·001–0·1 s−1 on the Gleeble-3500 thermal and mechanical simulator. The results show that the flow stress was controlled by both strain rate and deformation temperature. The flow stress decreased with the increasing deformation temperature, and increased with the increasing strain rate until the flow stress reached the peak value, then the flow stress remained constant. The flow behaviours were described by hyperbolic sine constitutive equation, and the average activation energy was calculated. Meanwhile the dynamic recrystallisation phenomenon has been observed during hot deformation.

On Temperature and Strain Rate Dependent Strain Localization Behavior in Ti–6.5Al–3.5Mo–1.5Zr–0.3Si Alloy

Deformation behaviors of Ti–6.5Al–3.5Mo–1.5Zr–0.3Si alloys with α/β lamellar structure were investigated systematically at different temperatures from room temperature to 950 °C and different strain rates. Results reveal that when the deformation temperature is higher than a critical temperature of 600 °C, an evident transition of deformation behavior from localized shear banding to α/β lamella kinking, flow softening and temperature/strain rate-dependent peak flow stress occurred in the alloy. The critical conditions for the occurrence of internal cracking and strain localization behavior associated with temperature and strain rate were determined.

Study of the hot deformation behaviour in Ti–5Al–5Mo–5V–3Cr–1Zr

Structural applications of near beta titanium alloys are gradually increasing in the aerospace industry because of their high specific mechanical properties and good corrosion resistance. Furthermore, a wide range of microstructures can be obtained by thermomechanical processes. This work determines by the use of EBSD technique the mechanism of restoration active in the near beta titanium alloy Ti–5Al–5Mo–5V–3Cr–1Zr for deformations in both α + β and β field near to the β transus temperature (Tβ = 803 °C). Hot compression tests are carried out up to 0.7 true strain by means of a Gleeble® 1500 machine at strain rates of 0.01, 0.1 and 1 s−1. Dynamic recovery of β phase and rotation of the α grains take place predominantly in the α + β field. Further deformation produces continuous dynamic recrystallization of the β phase influenced by the strain rate. Dynamic recovery is observed during deformation above the Tβ, where the misorientation is increasing towards the grain boundaries forming new small grains with a substructure at high strain rates and larger deformation. The stress exponent and the apparent activation energy for the sinh constitutive equations are determined and the microstructural features are correlated with the Zener–Hollomon parameter.

Mechanical behaviors of Ti–V–(Al, Sn) alloys with α′ martensite microstructure

The mechanical properties and deformation behavior of Ti–V–(Al, Sn) alloys with a α′ martensite microstructure are examined in this work. In as-quenched Ti–V–Al alloys consisting of α′ martensite microstructure with compositions near α − (α + β), the cold-rolling ability drastically decreases from a reduction of more than 75% to less than 40% with an increase in Al content. While, excellent cold-rolling ability (more than 75% reduction) is seen in the constituent phases of α′ in the compositional region near β − (α + β), and α″ and β. On the other hand, in as-quenched Ti–V–Sn alloys, excellent cold-rolling ability with a reduction of more than 80% that is independent of phase constituents is seen. In α′ martensite Ti–V–Al–Sn alloy with compositions in α − (α + β) that is tensile-deformed at a strain of 5%, a homogenous deformation substructure with straight dislocations is seen in alloy that with a low Al content. In contrast, the dislocation substructure begins to exhibit a planar configuration with increasing Al content. This change in deformation substructure due to increasing Al content is considered to cause the low ductility in α′ martensite Ti alloy that contains high Al. On the other hand, in α′ martensite Ti–V–Al alloy with a composition near β − (α + β), plastic deformation is found to be proceeded via the activation of the basal 〈 a〉 slip and the { 10 1 ¯ 1 } twin with a homogenous deformation substructure. As a consequence, this deformation mode is considered to cause the high ductility in the α′ phase as compared with the equilibrium (α + β) phase.

Identification of the optimal (α + β) forging process parameters of Ti–6.5Al–3.5Mo–1.5Zr–0.3Si based on processing-maps

The deformation behavior of Ti–6.5Al–3.5Mo–1.5Zr–0.3Si at the deformation temperature of 780–990 °C and the strain rate between 0.001 and 70.0 s −1 was systematically investigated for the identification of the optimal process parameters and the control of microstructure in (α + β) forging process. The processing-maps (P-maps) of the Ti-alloy in the above-mentioned deformation conditions were first constructed based on the experimental data of isothermal compression test. The experimental results show that the flow stress of the alloy is affected by the deformation temperature, strain rate and strain in the deformation process. The flow stress curves generated based on the experiments exhibit a steady-state characteristic and the flow softening behavior at the higher temperature and the lower strain rate. The suitable process parameters in (α + β) forging region are identified to be 855–920 °C, 0.001–0.005 s −1 for the strain less than 0.7. Under this process parameter configuration, the optimum deformation condition is 890 °C, 0.001 s −1. When the strain is greater than 0.7, the process parameter configurations for superplasticity deformation identified based on P-maps, microstructure observation and verified by the physical experiment of superplasticity deformation are 785–810 °C, 0.001–0.002 s −1; 875–925 °C, 0.001–0.005 s −1 and 945–990 °C, 0.001–0.016 s −1. The optimum (α + β) forging process parameters are 810 °C, 0.001 s −1; 900 °C, 0.001 s −1 and 960 °C, 0.001 s −1.

Plastic deformation of single crystals of Ti5Si3 with the hexagonal D88 structure

The deformation behavior of Ti 5 Si 3 single crystals with the hexagonal D8 8 structure has been investigated in compression as a function of crystal orientation in a temperature range from 1200 to 1500 °C. Three different types of deformation modes – { 1 1 ¯ 0 0 }[0 0 0 1] prismatic slip, { 2 1 ¯ 1 ¯ 2 }1/3〈 2 1 ¯ 1 ¯ 3 ¯ 〉 pyramidal slip and { 2 1 ¯ 1 ¯ 8 }〈 8 4 ¯ 4 ¯ 3 ¯ 〉 twinning – were identified for the first time as being operative in Ti 5 Si 3 at temperatures above 1300 °C, depending on the loading axis orientation. The critical resolved shear stresses (CRSSs) decrease steeply with increasing temperature for all deformation modes. The values of the CRSSs for { 1 1 ¯ 0 0 }[0 0 0 1] prismatic slip are considerably lower than those for { 2 1 ¯ 1 ¯ 2 }1/3〈 2 1 ¯ 1 ¯ 3 ¯ 〉 pyramidal slip, but are comparable to those for { 2 1 ¯ 1 ¯ 8 }〈 8 4 ¯ 4 ¯ 3 ¯ 〉 twinning. The favored deformation modes are discussed on the basis of anisotropic elasticity theory of dislocations.

The optimal determination of forging process parameters for Ti–6.5Al–3.5Mo–1.5Zr–0.3Si alloy with thick lamellar microstructure in two phase field based on P-map

The deformation behavior of Ti–6.5Al–3.5Mo–1.5Zr–0.3Si alloy with thick lamellar α microstructure is investigated by using the Processing-map (P-map). The results show that the P-map can predict the regime of flow instability and reveal deformation mechanisms well. Through analyzing P-maps and observing the microstructure evolution of Ti–6.5Al–3.5Mo–1.5Zr–0.3Si alloy in forging process, the phenomena of flow instability are found to occur at the temperature and strain rate ranges of (750–880 °C, 0.005–10.0 s −1) and (880–950 °C, 0.17–10.0 s −1), which include macrocracks, adiabatic shear bands and prior β boundary cavities. The preferable temperature and strain rate for hot working of the Ti-alloy are (790–900 °C, 0.001–0.003 s −1) and (900–950 °C, 0.001–0.017 s −1). In these two deformation domains, the globularization of α lamellae occurs, and the combination of the globularization of α lamellae and α + β → β phase transformation happen, respectively. For forging of Ti–6.5Al–3.5Mo–1.5Zr–0.3Si alloy in α + β phase field, the optimum temperature can be selected from the temperature range of 850–950 °C and the optimum stain rate is 0.001 s −1 based on the volume fraction of α phase for obtaining the needed properties of forgings in design of forging processes.

Effect of ageing treatment on the deformation behaviour of Ti–50.9 at.% Ni

This study investigated the effect of ageing on mechanical behaviour of a Ti–50.9 at.% Ni alloy. The findings enable the construction of a deformation diagram of stress-induced martensitic transformations, depicting the regions and limitations of pseudoelasticity and the shape memory effect of this alloy. It was found that the alloy exhibits pseudoelasticity within a certain temperature and stress window. This window narrows with increasing ageing temperature between 673 and 833 K, with the sample aged at 673 K for 3.6 ks exhibiting the maximum temperature window of ∼40 K for pseudoelasticity. This diagram enables the design of ageing treatment for different thermomechanical behaviour of the alloy. The experimental evidence also indicates that the mechanically induced transformation sequence is different from the thermally induced transformation sequence, implying that the mechanical behaviour of the alloy cannot be predicted from the knowledge of the thermally induced transformations based on the Clausius–Clapeyron correlation between stress and temperature. This study is a sequel to a previous work published in Acta Materialia (2008;56:736). The experimental evidence and the discussions presented in this work are complex. Reading the two articles together is strongly recommended.

Deformation Behavior of Ti–Zr–Ni–Cu–Be Metallic Glass and Composite in the Supercooled Liquid Region

Deformation Behavior of Ti–Zr–Ni–Cu–Be Metallic Glass and Composite in the Supercooled Liquid Region

Deformation Behavior of Ti–Zr–Ni–Cu–Be Metallic Glass and Composite in the Supercooled Liquid Region

The deformation behavior of Ti-based bulk metallic glass (BMG) and metallic glass matrix composite (MGMC), both having a multistep crystallization behavior upon heating, has been investigated in the supercooled liquid region. The BMG deforms homogeneously and exhibits moderate elongation to failure due to its multistep crystallization behavior, but shows a significant variation of the flow stress during deformation. For the MGMC containing an in situ β-phase, a stress-overshoot characteristic, observed in the BMG, is not presented, but elongation to failure is rather limited because the crystalline β-phase prevents the viscous flow of the amorphous phase. The different presence of the crystalline phases in the metallic glasses can differently affect the flow behavior of metallic glass in the supercooled liquid region.

Effect of omega phase on deformation behavior of Ti–7.5Mo– xFe alloys

In an earlier study, it was found that Ti–7.5Mo–1Fe and Ti–7.5Mo–2Fe alloys exhibited markedly different mechanical properties. While Ti–7.5Mo–2Fe alloy exhibited excellent mechanical properties, Ti–7.5Mo–1Fe alloy showed an extremely brittle behavior. The present study is a continual research on these two particular alloys with a focus on the effect of ω phase on the deformation behavior of the alloys. The results indicate that 2Fe alloy exhibited a ductile type fracture surface covered with normal deformation dimples. The fracture surface of 1Fe alloy exhibited a terrace-like morphology with numerous sub-micron-sized dimples. Throughout the entire plastically deformed region of 1Fe alloy, {1 1 2}〈1 1 1〉 type twins were observed, but few twins formed in 2Fe alloy. It is suggested that the numerous, larger-sized ω particles in 1Fe alloy have effectively restricted the slip of dislocations that largely limited plastic deformation capability of the alloy. The less and smaller ω particles could not do the same to 2Fe that led to the observed ductile behavior.

Plastic deformation behaviour and deformation substructure in Al-rich TiAl single crystals deformed at high temperatures

Plastic deformation behaviour in Ti–54.7 at.%Al and Ti–58.0 at.%Al single crystals was examined around and above the anomalous strengthening peak temperature (Tp) focusing on the effect of Al5Ti3 superstructure. The Al5Ti3 superstructure developed in the L10 matrix of Ti–58.0at.%Al, and the size of the Al5Ti3 phase once increased during annealing at 8008C and then decreased withincreasing temperature, while no significant evidence of the Al5Ti3 particles was obtained in Ti–54.7 at.%Al from TEM observation although diffuse scattering corresponding to the spots for the Al5Ti3 superstructure was observed. The transition of slip plane for ½<110]; ordinary dislocations from {111} to {110) and/or (001) occurred at and above Tp due to anisotropy of anti-phase boundary energies on {111}, {110) and(001) in the Al5Ti3 superstructure. Anomalous strengthening is related to the development of this superstructure which may assist the cross-slip of some parts of ½<110] ordinary dislocations onto {110) and/or (001) resulting in the formation of dragging points to the motion of the dislocations.

Interfacial dislocation arrays in twin boundaries of deformed titanium

Mechanical deformation of hexagonal close-packed (hcp) metals presents great interest. At low temperatures, the dominant deformation mechanism in these metals is mechanical twinning. The crystallography and core structure of twinning dislocations for the principal twin systems of hcp metals have been analyzed by Serra, Bacon and Pond. The properties of these twinning dislocations have been investigated by atomic scale computer simulation. These models can be used as a reference in order to investigate the properties of hcp metals during deformation. From a geometric point of view, different behavior is expected among distinct hcp metals. This could be attributed to the deviation of the individual c/a ratios from the ideal close-packed ratio (c/a = (8/3)[sup 1/2]), which also results in the different shear moduli of hcp metals. A study of these metals is performed in order to clarify the dominance of mechanical twinning in the mechanical deformation of hcp metals, at low temperatures and various deformation rates. In this work the deformation behavior of Ti is studied, by means of Transmission Electron Microscopy (TEM). The observations are focused in the range of 20% to 40% deformation, because under 20% mechanical twinning is negligible, whereas over 40% the vast quantity of dislocationmore » prohibits any clear observation. For 25% deformation, a mechanism of slip transmission of dislocations through the twin is observed.« less

Deformation Mechanisms in TiAl-Based Alloys Containing Low Oxygen

ABSTRACTThe effects of oxygen on the deformation behavior of Ti-(48-52)Al alloys is reported. Two types of studies were conducted. In the first, high purity alloy buttons containing low oxygen (~250 ppm) were prepared, whereas in the second, alloys with additions of 1 at.% Er to scavenge the oxygen from the matrix were prepared. The alloys were heat treated to produce large grains and the microstructures characterized by analytical electron microscopy. Samples prepared from the heat treated alloys were electropolished and deformed in compression to a plastic strain of 1.0-1.5% at temperatures between 25 and 800°C and the yield stress measured. The morphology of deformation, that is, slip lines and the presence of twinning, was studied by optical microscopy and the dislocation structures were characterized by weak-beam imaging in the transmission electron microscope. The results of these various studies are presented and discussed in terms of recent developments regarding the factors that appear to control the dislocation structure and the mobility of dislocations.

An analysis of the low temperature, low and high strain-rate deformation of Ti−6Al−4V

The deformation behavior of Ti−6Al−4V at temperatures between 76 and 495 K, strain rates between 0.001 and 3000 s−1, and compressive strains to 0.3 has been investigated. Measurements of yield stress as a function of test temperature, strain rate, and prestrain history are analyzed according to the model proposed by Kocks and Mecking. The mechanical threshold stress (flow stress at 0 K) is used as an internal state variable, and the contributions to the mechanical threshold stress from the various strengthening mechanisms present in this alloy are analyzed. Transmission electron microscopy (TEM) is used to correlate deformation substructure evolution with the constitutive behavior. The deformation substructure of Ti-6-4 is observed to consist of planar slip in the α grains at quasistatic strain rates. At high strain rates, deformation twinning is observed in addition to planar slip. Increasing the temperature to 495 K is seen to alter the deformation mode to more random slip; the effect of this on the proposed deformation model is discussed.