Phase TiAl Alloys Research Articles

{1[formula omitted]01} tension twins and {1[formula omitted]01}-{2[formula omitted]01}/{2[formula omitted]01}-{1[formula omitted]01} double twins in the D019 ordered hexagonal α2-Ti3Al phase

Compared to the disordered HCP metals, the α2-Ti3Al phase is reluctant to twin due to its D019 ordered structure. Recently, abundant {202¯1} compression twins were observed in this phase. However, other type of twin has not been reported besides the {202¯1} ones. In this study, we observed the {11¯01} tension twins in the ordered α2-Ti3Al phase for the first time in the α2+γ two phase TiAl alloy. In addition, the {11¯01}<1¯102> tensile mode occurs together with {202¯1}<1¯014> compression twins already known for the α2 phase, and these two modes are often twinned internally. Two types of double twins ({11¯01}-{22¯01} and {22¯01}-{11¯01}) were observed. The structure of the various interfaces was examined with high-resolution electron microscopy. The factors governing the evolution of the observed twin architecture are discussed with particular emphasis on deformation induced internal stresses and the interchange shuffles necessary to restore the D019 ordering of the α2 phase in its twinned counterpart. The transformation matrix was calculated for the observed two types of double twin structures, which explains the reorientation of the grains and the shear continuity, further confirming the double twin mechanism. The variant selection mechanism of the secondary twin is also discussed. These results indicate that abundant twinning activity can be initiated in the ordered α2 phase under severe deformation.

Evaluating the phase stability of binary titanium alloy Ti-X (X = Mo, Nb, Al, and Zr) using first-principles calculations and a Debye model

To realize bottom-up design of alloys based on theoretical calculations, the thermodynamic stabilities of phases in Ti binary alloys were estimated by a combination of density functional theory calculations for the internal enthalpy energy, the Bragg-Williams approximation for the mixing entropy contribution, the Debye model for the vibrational free energy, and the Sommerfeld model for the electronic excitation entropy. The special quasirandom structure (SQS) model was used to describe the disordered distribution of the alloying element in the solid solution state. We focused on Ti–Mo, Ti–Nb, Ti–Al, and Ti–Zr binary alloys, which have different phases, such as the α phase in the hexagonal close-packed (hcp) structure and the β phase in the body-centered cubic (bcc) structure, depending on the temperature and alloying element fraction. The elastic constants, bulk modulus, and Poisson's ratios were calculated using a strain energy method. Excitations from the vibrational contribution to the quasi-harmonic Debye approximation were added to the 0 K free energy originally derived from ab initio calculations. The effect of temperature up to 1000 K on phase stability was analyzed. Furthermore, to compare phase stabilities, the free energies of formation were calculated using the ground states of the constituent phases as references. The calculated elastic property indicated the mechanical instability of most bcc Ti–Al and bcc Ti–Zr alloys, hcp Ti–Mo and hcp Ti–Nb at high fraction range. The SQS supercell models showed good agreement in elastic constant, bulk modulus, and Poisson's ratio compared to the previous experimental and theoretical results. Free energy results showed that Mo and Nb are β-phase stabilizers, Al is an α-phase stabilizer, and Zr is a neutral element. As the fraction of the alloying element changed, stabilizing or destabilizing effects were observed under different temperatures. Moreover, the linear relationship between the filling of the d band and phase stability was identified in low temperature range. For the β phase, Mo had a stronger stabilizing effect than Nb; both Mo and Nb destabilized the α phase at low temperatures, whereas high temperatures increased the stability of the α phase and the temperature effect became more significant than the element effect. In the examined temperature range, the α phase Ti–Al alloys were stable at all Al fractions, where the thermal effect was negligible. All the α Ti–Zr alloys in this study had similar stabilities to their constituent phases (hcp Ti and hcp Zr) over a wide temperature range.

Lamellar orientation control in a Ti–46Al–5Nb alloy by directional solidification

Lamellar microstructures of the Ti–46Al–5Nb alloy solidifying through the primary β phase were successfully aligned parallel to the growth direction by using the double directional solidification (DS) technique. The same growth conditions in both DS processes and the completion of the peritectic transformation in the dendritic regions are necessary for the primary β phase TiAl alloys to control the lamellar orientation.

Cyclic Response and Fatigue Life of TiAl Alloys at High Temperatures

Fatigue properties of the new generation of TiAl alloys with 8at.% of Nb were studied and compared with those reported in the literature for previous generation with 2at.% of Nb. The high Nb content improves substantially the fatigue life time in the Basquin representation. Nevertheless, the fracture behaviour is still rather brittle even at 750°C. TEM study of the material after the fatigue testing was performed. The dislocation activity is concentrated in the  phase. The properties and behaviour of individual dislocations seem to be similar as those observed in single -phase TiAl alloys.

Study on Fracture Behavior of γ-TiAl Based Alloys

Fracture behavior of fully lamellar (FL) and duplex phase (DP) TiAl alloys is reported in this paper. It was found that the inverse behavior of coarse FL TiAl alloy showing inferior tensile properties but superior fracture toughness resulted from the different fracture mechanisms of these two types’ tests. In tensile specimens, the final fracture happened at a section that was most heavily damaged by the accumulation of large interlamellar microcracks and arbitrarily located within the gauge-limited volume. In 3PB notch specimens, the propagation of the main crack was constrained within a narrow strip nearby the centerline where the normal stress was the highest. Large lamellar grains caused serious damage in tensile tested specimens. However multi-oriented large lamellar grains formed seriously bifurcated crack tips, which made the crack propagation more difficult in 3PB notched specimen. The main mechanisms of toughening in FL specimens were the deflection of main crack, bifurcation and blunting of crack tip and formation of a diffuse zone of microcracks. These phenomena reduced the driving force for crack extending and then increased the fracture toughness.

Effects of loading rate on damage and fracture behavior of TiAl alloys

Based on the results of tensile tests and notch 3PB tests with various loading rates and the observation of fracture surfaces, the effects of loading mode and rate on damage and fracture mechanisms of fully lamellar (FL) and duplex phase (DP) TiAl alloys are indicated: (1) For the FL specimen fractured in tensile test, a number of interlamellar cracks occur before final fracture, which is produced by the cracking of the area remained between the existing cracks on a most weakened cross-section. However the DP specimen in tensile test is fractured by the propagation of a crack with a critical length acting as a Griffiths crack in brittle materials. In the 3PB tests of notched specimens the fracture mechanism is different with that in the tensile tests. Crack initiates at the notch root and propagates along a strip around the center line where the normal stress is highest. For FL specimen a more tortured path through low-resistance-interlamellar cracks can be taken at a low loading rate. Because of the low resistance and the rate-dependence of the interlamellar cracking, the loading rate affects significantly the fracture mode. (2) Based on the variation of the fracture mechanisms, the reason why the FL alloy shows inferior tensile properties but matching even superior fracture toughness to the DP alloy is explained further incorporating the effects of loading rate. (3) The tensile strength and fracture toughness show a decreasing trend with lowering loading rate and it is associated with rate-dependent interlamellar cracks involving in the fracture process.

Fracture behaviour of precracked specimens of a TiAl alloy

Fracture behaviour of a two phase TiAl alloy was investigated based on observations of fracture surfaces of three point bending (3PB) precracked specimens and detailed observations of configuration changes at precrack tips, based on which new findings are presented. The higher toughness of coarse, near fully lamellar microstructure as compared to that of finer duplex microstructure is attributed to the path of crack propagation. In coarser NFL microstructure, the crack should cross more large grains with unfavourable orientation or bypass them by much more tortuous path, which causes a much rougher fracture surface than that observed in the finer duplex microstructure. The inverse relationships, showing a low tensile ductility but a higher fracture toughness of the coarser NFL microstructure, are explained by the difference in sampling volume between tensile testing and toughness testing. In the precracked 3PB specimens, the sampling volume is restricted to the vicinity along the precrack direction. The coarser NFL microstructure shows the higher toughness. However, in tensile tests, where a much higher sampling volume includes the entire region within the gauge, the cleavage fracture of the coarser NFL microstructure is able to take the weakest route via the high number of interlamellar cracks. The toughening mechanisms, which make the main crack difficult to propagate or cause it to be stopped, are attributed to reducing the tensile stress perpendicular to the interface between lamellae.

The Fatigue Mechanism of Dual Phase TiAl Alloy Coated with TiAlN Film by PVD Process at an Elevated Temperature.

The influence of dual phase microstructure and TiAlN films on the fatigue mechanism of TiAl intermetallic alloy coated with TiAlN film by the physical vapor deposition (PVD) was studied in vacuum at 1073 K from the view point of mesoscopic size which is 1 μm-1 mm size range. The crack initiation and crack growth was observed and photographed under high resolution condition using elevated-temperature loading cyclic stage for the scanning electron microscope. The results indicated that the fatigue strength of coated TiAl alloy was higher than that of uncoated TiAl alloy at an elevated temperature. TiAlN films reduced plastic deformation and crack initiation in γ phase. Therefore the fatigue process of coated TiAl alloy was dominated by initiation and growth mechanism of some mesocracks, which is 200-300 mm length. The fatigue property of coated TiAl was controlled by meso-structure, which was composed of films and substrate.

Effect of 1 at.% nitrogen addition on the creep resistance of two phase TiAl alloy

Abstract The effect of the 1 at.% nitrogen addition on the lamellar Ti–48.5Al–1.5Mo (at.%) alloy was investigated at the creep test conditions of temperature range from 750 to 850°C under constant stress ranging from 150 to 250 MPa in air. Effect of N addition on the minimum creep rate depends on stress level. In 200 and 250 MPa, N reduced the minimum creep rate significantly, however, gave no effect in 150 MPa. It was also observed that the N addition significantly enhanced the creep resistance in primary regime. Precipitate hardening of p-phase (Ti 3 AlN) and solute hardening of nitrogen atoms may be responsible for this beneficial effect. While creep fracture of mother alloy occurred by secondary crack, N addition alloy fractured by cavity formation. We considered the controlling process of the minimum creep rate at 150 MPa stress level to be identical for the mother alloy and N addition alloy, and the nitrides and nitrogen atoms give little effect on the rate controlling process.

In-situ micro-twinning of TiAl in the field ion microscope

Stress-induced transformations were observed in the γ phase of a two phase TiAl alloy and were manifested by the sudden appearance of newly formed regions in the field ion image having significant contrast difference from the matrix. This in-situ transformation was often accompanied by a slight shift of all or part of the atoms in the field ion image. The TiAl alloy investigated was found to be particularly prone to specimen failure on the application of the electric field, which suggests that the formation of these twin regions can lead to an unstable specimen geometry. Different types of contrast were observed depending on the position and orientation of the twinned region with respect to the previously untransformed γ matrix. Twinned regions may appear with dark or bright contrast in the field ion image and ultrathin microtwins can be generated that are similar in appearance to thin α2 laths.

Fractal Model and Remaining Life Assessment on the Fatigue Cracks in Dual Phase TiAl Alloy.

Mesocracks controlling fatigue fracture at 1073 K in dual phase TiAl alloy were observed by SEM-servo. In this study, the quantitative analysis of the mesocracks and the remaining life assessment was carried out by applying fractal concept. The main results obtained were follows : (1) The feature of mesocracks growth behavior has fractal characteristics. (2) Fractal dimension is raised with increasing the number of cyclic loadings. Thus, this characteristic of fractal dimension makes possible remaninig life assessment. (3) We design comlex system algolrithm which consists of damage process analysis, fractal modeling and simulating. Furthermore, this algorithm connects material design, structual design and prevenance maintemance.

Interfacial energies in two-phase TiAl-Ti 3Al alloy

The intrinsic values of interfacial energies based on first-principles calculations, including atomic relaxation, were obtained for the three types of γ/γ interfaces and the α 2/γ lamellar boundary in two- phase TiAl alloy. The pseudo-twin boundary energy is highest, Γ P = 270 mJ/m 2, and the true-twin boundary energy is lowest, Γ P = 60 mJ/m 2. Planar fault energies at pseudo-twin and 120°-rotational interfaces are markedly different from those in the bulk of the γ-phase, i.e., approximately, E APB and E CSF decreases by more than 30% and E SISF increases by threefold. Enhanced mobility of ordinary dislocations along the γ/γ and α 2/α interfaces (except true twin boundaries) is predicted based on the reduced E CSF values at the interfaces.

Temperature dependence of the mechanical properties of a TiAlCr alloy

TiAl based intermetallics are considered as potential high-temperature structural materials due to their attractive properties at high temperatures. Previous studies of the mechanical behavior of single phase and dual phase TiAl alloys revealed that these types of alloys have good ductility at high temperatures, but poor ductility at ambient and lower temperatures. Recently some triple phase TiAl alloys were developed by the addition of [beta] stabilizing elements, and the [beta] phase was found not to improve the room temperature ductility. However, few reports have been made about the high temperature behavior of the triple phase TiAl alloys. This research is concentrated on the temperature dependence of the mechanical properties of a triple phase Ti-Al-Cr alloy.

Active deformation mechanisms responsible for ductility and flow stress dependence on temperature in TiAl alloys

The active dislocation mechanisms responsible for the deformation process of two phase TiAl alloys have been analysed and their influence on ductility and strength have been interpreted as a function of deformation temperature. The measured values of misfit strain between the α 2 and the γ phases have been shown to be responsible for the accumulation of dislocations that occurs at their interfaces. The stress concentrations that build up at these sites lead to microcrack formation and determine the lack of ductility of the materials. The anomalous strengthening effect that is observed with increasing temperature has been shown to be determined by the activity of ordinary 1/2<110] dislocations that are generated from twin intersections. The increased jog density produced by the increased number of emitted dislocations up to the peak temperature leads to the strengthening mechanism observed

Tensile properties and fracture behavior of Ti 52Al 48 and Ti 50Al 48Cr 2 prepared from elemental powders

Titanium aluminide alloys, based on gamma TiAl, are currently of interest because of potential applications in high performance airframe and gas turbines. Their low densities, high melting temperatures, good elevated temperature strength and modulus retention, and environmental resistance favors them for these applications. However, their practical use are largely limited by their poor workability and ductility at temperatures lower than 700 C. Although the ductility has been improved in two phase TiAl alloys by adding alloying elements such as Cr, Mn, Nb and V, and by microstructural control in recent years, the ability to manufacture them still remains a problem. The reactive powder processing method offers a promising alternative to overcome this problem. This method involves cold-extrusion of an elemental powder mixture and reactive sintering. The as-extruded material can easily be machined or reformed into different shapes, since titanium aluminides are not present at this stage. The reactive sintering is conducted as the last step to form the desired titanium aluminides in the finished products. By this route, the poor workability of titanium aluminides can be avoided. In the present paper, a binary alloy Ti[sub 52]Al[sub 48](TiAl) and a ternary alloy Ti[sub 50]Al[sub 48]Cr[sub 2](TiAlCr), prepared in the same waymore » from elemental powders, are investigated. The tensile tests were carried out at room temperature to 900 C in air. The influence of 2 at.% Cr addition on the tensile properties and fracture behavior of the alloys are reported. An emphasis is placed on the correlation between microstructure and deformation, and fracture behavior of the alloys.« less

Dislocation configurations in two phase TiAl alloys. II. Structures after compression

Abstract Deformation microstructures produced by compression on the alloys Ti-48Al and Ti-47.5Al 2.5Cr have been analysed and compared to those obtained by hardness indentations of the same materials in Part I of this study. It is shown that while most of the strain in the indented specimens was achieved by glide of 1/2<110] dislocations, an important part of the deformation process occurs by twinning during compression. Also, other contributions from different active systems are observed that depend on the orientation relationships between the different grains. Both alloys contain single 1/2<110] dislocations, however the superdislocations present in the binary alloy are of the <101] type while the chromium containing alloy deforms by 1/2<112] segments that trail faulted dipoles. The dissociations of the <101] segments observed in the binary alloy seem to depend on the orientation of the line direction. The slightly better ductility of the chromium containing alloy has been attributed to the increased ca...

Dislocation configurations in two phase TiAl alloys. I. Annealed and indented structures

Abstract Two alloys of nominal composition Ti-48 Al and Ti-47.5 Al-2.5 Cr with small grain sizes and fully lamellar structures have been compared in terms of mechanical properties and of the dislocation configurations present before and after deformation. The first part of the study shows the effect of hardness indentations on the slip steps, crack initiation and propagation and dislocation activity in zones where different amounts of strain have taken place. It appears that the action of local stresses produced by hardness indentations is that of promoting glide of the slip systems already present in the annealed materials. Although large densities of ordinary 1/2 dislocations are responsible for the strains achieved locally in the γ phase, these do not promote large ductilities of the alloys. Since propagation of deformation cannot occur across the α2/γ interface, microcracks form and the material as a whole behaves in a brittle manner.

Superplastic behaviour and mechanical properties of two phase TiAl alloys

High temperature plastic flow properties of two phase TiAl alloys containing 45 to 49 at.%Al have been investigated in thermo-mechanically grain refined materials in order to clarify the favourable microstructure and chemical composition for TiAl based superplastic materials. Grain sizes of thermomechanical treated materials and their grain size stability during subsequent high temperature deformation strongly depend on chemical composition. It was found that Ti-46at%Al offers the best α 2 /γ ratio which produces a fine and stable microstructure, whilst exhibiting superior superplasticity at temperatures exceedig 1100 o C and a strain rate of around 1×10 -4 s -1 (with m-value of 0.44 at 1100 o C and 0.64 at 1150 o C) as well as preferred mechanical properties at temperature of up to 1000 o C. This alloy was proposed as a baseline alloy for superplastic materials to be later modified by third elements effective for the formation of metallic phase. Correspondingly, effects of heat treatment on changes in microstructure and tensile properties have been studied in fine grain TiAl alloys in order to estimate the possibility of improving high temperature strength after superplastic forming. A new kind of microstructure consisting of coarse lamellar colonies and fine colony boundary grains of γ and lamella (to be refereed to as partially-transformed structure) was found to be obtained by heat-treatment of just above the α-transus. It was also found that the partially-transformed structures exhibit a better combination of room temperature ductility and high temperature strength than any other microstructure previously observed

Sputtering and secondary ion yields of TiAl alloys subjected to oxygen ion bombardment

Total sputtering and secondary ion yields of single phase TiAl alloys containing 0–35 at% Al under oxygen ion bombardment were measured for thin film targets with known thicknesses and compositions. It was found that the total sputtering yield initially decreased with increasing Al content and became constant beyond 20 at% Al. On the other hand, the secondary ion yield of Al roughly increases with increasing Al content, while that of Ti shows a complementary decrease. Their variations, however, show a large flucutation with Al content. Detailed observations revealed that the secondary O 2 ion yield also exhibited the same pattern of fluctuation as that of Al and Ti. The modified degrees of ionization of both Al and Ti in the consideration for the secondary oxygen ion yield indicated an exponential dependence upon the alloy composition. The trends of the modified degrees of ionization revealed that the matrix effect due to the alloy composition affected more strongly on Ti than on Al.