Abstract
Samples of Hot Isostatically Pressed (HIPped) powder of TIMETAL 6-4 (Ti-6Al-4V, compositions in wt pct unless indicated), which was HIPped at 1203 K (930°C), and of forged bar stock, which was slowly cooled from above the beta transus, were both subsequently held at 773 K (500°C) for times up to 5 weeks and analyzed using scanning and transmission electron microscopy and atom probe analysis. It has been shown that in the samples aged for 5 weeks at 773 K (500°C), there is a high density of alpha2 (a 2 , an ordered phase based on the composition Ti 3 Al) precipitates, which are typically 5 nm in size, and a significantly smaller density was present in the slowly cooled samples. The fatigue and tensile properties of samples aged for 5 weeks at 773 K (500°C) have been compared with those of the HIPped powder and of the forged samples which were slowly cooled from just above the transus, and although no significant difference was found between the fatigue properties, the tensile strength of the aged samples was 5 pct higher than that of the as-HIPped and slowly cooled forged samples. The ductility of the forged samples did not decrease after aging at 773 K (500°C) despite the strength increase. Transmission electron microscopy has been used to assess the nature of dislocations generated during tensile and fatigue deformation and it has been found that not just is planar slip observed, but dislocation pairs are not uncommon in samples aged at 773 K (500°C) and some are seen in slowly cooled Ti6Al4V.

Samples of Hot Isostatically Pressed (HIPped) powder of TIMETAL 6-4 (Ti-6Al-4V, compositions in wt pct unless indicated), which was HIPped at 1203 K (930°C), and of forged bar stock, which was slowly cooled from above the beta transus, were both subsequently held at 773 K (500°C) for times up to 5 weeks and analyzed using scanning and transmission electron microscopy and atom probe analysis. It has been shown that in the samples aged for 5 weeks at 773 K (500°C), there is a high density of alpha2 (a 2 , an ordered phase based on the composition Ti 3 Al) precipitates, which are typically 5 nm in size, and a significantly smaller density was present in the slowly cooled samples. The fatigue and tensile properties of samples aged for 5 weeks at 773 K (500°C) have been compared with those of the HIPped powder and of the forged samples which were slowly cooled from just above the transus, and although no significant difference was found between the fatigue properties, the tensile strength of the aged samples was 5 pct higher than that of the as-HIPped and slowly cooled forged samples. The ductility of the forged samples did not decrease after aging at 773 K (500°C) despite the strength increase. Transmission electron microscopy has been used to assess the nature of dislocations generated during tensile and fatigue deformation and it has been found that not just is planar slip observed, but dislocation pairs are not uncommon in samples aged at 773 K (500°C) and some are seen in slowly cooled Ti6Al4V. THE alloy Ti-6Al-4V is commonly referred to as the workhorse Ti alloy because it is the most widely used Ti alloy. It was developed over 50 years ago and a great deal of research has been carried out aimed at understanding how to optimize the microstructure for specific properties. But, despite that, there are some areas of uncertainty and this paper is focussed mainly on just one aspect-the development of order and/or precipitation of ordered a 2 in the alpha phase and its influence on properties. Earlier experimental work [1, 2] has shown that the alpha phase in a-stabilized alloys is able to develop short-range order (SRO); ordering is expected from phase diagram modeling. [3] Other work on deformed Ti6Al [2] had shown that weak fringes were observed in TEM images within the planar slip bands and these were interpreted as being due to the destruction of SRO by the movement of dislocations. In fact, there have been many examples [e.g., References 4 through 8] of the observation of planar slip in Ti alloys with more than 4 pct Al and of fringes in other alloys which could well have been due to the destruction of order. The work presented in this paper has been carried out with the aim of establishing whether ordering or precipitation of a 2 occurs in the alloy Ti-6Al-4V, and if so, if there is an influence on the tensile and fatigue properties.
II. EXPERIMENTAL
The Ti-6Al-4V used in most of this work was prepared from PREP powder that was HIPped (Hot Isostatically Pressed) at 1203 K (930°C) for 4 h at a pressure of 150 Mpa and cooled in the HIP at 5 K/min, and its analysis is shown in Table I . In addition, a limited amount of work was carried out on forged and heat-treated Ti6Al4V. Cylindrical samples of Ti6Al4V, which were large enough to prepare standard tensile and fatigue samples, were cut from these HIPped samples and some of these were used to define the properties and microstructure in this standard condition and some of these samples were aged at 773 K (500°C) for times up to 5 weeks in sealed tubes; the analyses of the aged samples are shown in Table I . Samples made from forged bar stock were also aged for 5 weeks at 773 K (500°C) to assess the response of this starting material to this treatment. The samples were analyzed after aging to check on the extent of oxygen pick-up; the aged samples were assessed microstructurally and their tensile and fatigue properties compared with those of the initially slowly cooled samples.
Scanning electron microscopy was carried out at 20 kV on a Philips XL30 to examine the fracture surfaces on samples tested in tension and in fatigue. Transmission electron microscopy was carried out on a JEOL 2100 operated at 200 kV on the variously heat-treated samples in order to assess whether a 2 was detectable and on samples tested in tension and in fatigue to assess whether any a 2 , which may be present, influenced the dislocation behavior. Atom probe work was carried out on a Cameca LEAP 3000 X HR on heat-treated samples. The tensile tests were carried out at an initial strain rate of 1 9 10 À4 s À1 on a Zwick machine using samples with a diameter of 4 mm and a gauge length of 20 mm. The fatigue tests were carried out on an Amsler machine under load control at R = 0.1 (R is the ratio of the minimum load to the maximum load), using a frequency of 52 Hz. The samples had a diameter of 3.6 mm and gauge length of 20 mm. Applied peak loads were selected in order to produce SN curves covering the range from 10 5 to 10 8 cycles. Samples which survived after more than 10 7 cycles are termed run-out. Phase equilibrium calculations were carried out using the Pandat software package.
III. RESULTS

A. Microstructural Characterization

TEM observations
Areas, corresponding to the a-Ti phase, of thin foils taken from samples that have been HIPped and then slowly cooled to room temperature have been studied in the TEM. At first, the diffraction patterns appeared to contain only intensity maxima corresponding to fundamental reflections expected from the disordered a-Ti matrix. However, very weak superlattice maxima could be observed by using Kikuchi lines from fundamental reflections to tilt the sample so as to maximize the intensity of such a reflection, were it present, and double exposing a diffraction pattern, initially with the objective aperture in the position to select the superlattice maximum (although it cannot be seen on the screen) and then with the aperture removed. These maxima are just visible on suitably exposed negatives as shown in Figure 1 (a). It was not possible to obtain dark field images of precipitates from these reflections because the signal to noise ratio was too small. To confirm that an intensity maximum is present, as indicated in Figure 1 (a), an intensity profile was taken across this pattern where the background intensity has been subtracted, and this is shown in Figure 1(b) ; the presence of a superlattice reflection was confirmed. Figure 2 shows a typical electron diffraction pattern taken from a sample of HIPped Ti-6Al-4V which was aged 5 weeks at 773 K (500°C), and it is clear that the sample contains a 2 since obvious superlattice maxima are visible. A dark field micrograph taken using one of the superlattice maxima visible in Figure 2 (a) is shown in Figure 2 (b); small particles of a 2 distributed throughout the whole volume are visible. These are typically about 5 nm in diameter, but some are at least 10 nm.
Atom probe tomography studies
The sample which had been aged for 5 weeks at 773 K (500°C) was analyzed in greater detail using atom probe tomography. The results of this analysis have been summarized in Figures 3 and 4 . Figure 3 (a) shows an atomic (or ionic) reconstruction of the Al ions (volume of 90 9 90 9 80 nm) within an alpha region of this sample. Using the IVASä three-dimensional reconstruction and analysis software, the Al-rich clusters have been clearly identified and delineated by ellipsoids in the same figure. These clusters can be better visualized in Figure 3(b) where the surrounding ions have not been plotted. Postprocessing of the reconstructed data using IVASä permits 
have an Al content greater than 15 at. pct. The composition profiles for V and Al, plotted in Figure 4 , represent an average compositional variation of different elements as a function of distance from the core of these Al-rich clusters. This plot clearly shows that within these Al-rich clusters, the Al concentration varies from the core (~20 at. pct) to the edge of the cluster (~10 at. pct) at a distance~1 nm from the core.
Based on the atom probe analysis, it can be conjectured that the Al-rich clusters identified within the alpha region of this sample correspond to the a 2 phase observed in the electron diffraction evidence presented in Figure 2 . This indicates that the a 2 composition at the core of the precipitate is~20 at. pct Al, which is very close to the predicted value. [3] The outer regions of these Al-rich clusters contain only~10 at. pct Al, suggesting that these regions are far below the stoichiometric composition for a 2 formation. However, in contrast, the measured V content within these Al-rich clusters of about 2 at. pct is far higher than the calculated value which is expected to be below 0.1 at. pct, suggesting that the slower diffusion of V precludes equilibrium being established. [9] Additionally, there appears to be a marginal enrichment of V right at the center of the cluster, the reason for which is not presently understood. While these atom probe results are promising and indicate the presence of Al-rich clusters possibly corresponding to the a 2 precipitates, it should be noted that further detailed investigations are required in order to conclusively establish the actual composition and spatial extent of the a 2 precipitates within these aged Ti-6Al-4V samples.
B. Tensile and Fatigue Properties of HIPped and of Heat-Treated Samples
Typical stress strain data for slowly cooled HIPped samples and for aged samples are shown in Figure 5 and there is a significant difference between the samples, with the aged sample having a higher yield strength of over 1000 MPa. The top three curves, which are superimposed and appear virtually as a single curve, are from the samples aged at 773 K (500°C) for 5 weeks and the bottom three are from the as-HIPped samples.
It should be noted that analysis carried out on these samples (see Table I ) shows no difference in the level of oxygen. Additional control experiments were also carried out on samples of forged and heat-treated Ti-6Al-4V and it was found that the tensile strength also increased after 5 weeks at 773 K (500°C), but there was no reduction in ductility-it remained at about 20 pct, as shown in Table II . Five samples were used for each condition and the scatter was very small. The lower tensile strength of the forged material (955 Mpa), as compared to the powder, is presumably associated with slight differences in composition and the scale of the microstructures.
A detailed examination of the fracture surfaces (see below) of the HIPped samples shows that fracture is sometimes initiated in regions where inclusions in the powder samples were present, but that the sample which failed after about 20 pct ductility showed a conventional ductile failure. In view of the fact that with forged samples, the strength increases on aging for 5 weeks at 773 K (500°C) with no decrease in ductility, it is inferred that the scatter between the samples shown in Figure 5 is not associated with the heat treatment, but is a problem with the powder cleanliness, as shown below.
Regarding the fatigue response, the SN data obtained from the slowly cooled and aged samples are shown in Figure 6 , and it appears that there is a large scatter in both groups of samples and it is not possible to determine from this limited data if there is any difference in fatigue properties. Again, this is consistent with issues regarding powder cleanliness.
C. SEM Observations of Samples Tested in Tension and Fatigue
A micrograph of the fracture surface of a sample tested to failure in tension is shown in Figure 7 . Clearly, the particular HIPped sample fractured in a ductile manner, but all other samples, whether they were as-HIPped or HIPped plus aged at 773 K (500°C) for 5 weeks, commonly showed obvious fracture initiation sites associated with inclusions as illustrated in Figure 8 .
D. TEM of Samples Deformed in Tension and Fatigue
Typical images of a sample which had been aged at 773 K (500°C) for 5 weeks and deformed in tension are shown in Figure 9 . In Figure 9 (a), planar slip is obvious and some weak fringes can be seen on the slip plane, whereas only irregular dislocations can be seen in (b), which was taken from the same sample.
A typical image of a slowly cooled sample deformed in fatigue is shown in Figure 10 . Again, it is clear that the slip is planar, but interestingly there is an indication of some pairing of dislocations (arrowed) of the same sign within the slip band. A series of images taken from a sample which had been aged at 773 K (500°C) for 5 weeks before being fatigued to failure is shown in Figure 11 . In this case, it is apparent that dislocations are paired and because the images were taken using ±g pairs and the spacing between the images of pairs of dislocations did not change upon reversal of the sign of the diffracting vector, it is clear that they are pairs of like sign dislocations. It is noted that the spacing between pairs is not constant.
IV. DISCUSSION
In the HIPped samples which had been aged at 773 K (500°C), both the diffraction evidence and the dark field imaging (Figure 2) are consistent with the presence of particles of a 2 . Interestingly, and somewhat surprisingly, the diffraction evidence (Figure 1 ) recorded from the asHIPped samples also indicates the presence of this ordered phase, although the scattering in the case of this latter sample is considerably more diffuse and weaker than that of the superlattice reflections in the aged samples. The atom probe observations on the aged Ti6Al-4V have shown that the Al content throughout much of the diameter of the ordered domains is well below the predicted value of about 20 at. pct and presumably even longer heat treatments would be required to reach this predicted Al composition at 773 K (500°C). The data may also be consistent with the presence of a compositionally and structurally diffuse interface, similar to that recently reported for c¢ particles in a Ni-base superalloy. [10] Similarly, the measured V content, of about 2 at. pct, is an order of magnitude higher than calculations predict. Presumably, the slower diffusion of V [9] contributes to this and the fact that diffusion would have to take place through an ordered matrix within the a 2 domains would further decrease the rate of diffusion.
The tensile strength of the ordered Ti6Al4V is significantly higher than that of the as-HIPped samples and as noted earlier, a similar increase of about 5 pct is found in forged samples which were aged at 773 K (500°C) for 5 weeks. This increase in strength in forged samples is not associated with any decrease in ductility. This relatively small increase in strength associated with such a high density of small ordered domains is somewhat surprising, but since ordered domains are present in the slowly cooled samples, only a small increase may be expected because a comparison is being made between two samples with different extents of order rather than between an ordered and a disordered one. In addition, the formation of domains with Al contents up to 20 at. pct would lead to depletion of Al in the disordered alpha matrix, therefore reducing the strength of this phase. This would offset somewhat the strengthening expected from the presence of ordered particles.
The scatter in ductility in the HIPped powder samples has been shown to be related to the presence of inclusions, although the minimum elongation observed was still above 10 pct. A similar scatter in the fatigue behavior is probably also associated with inclusions and either forged samples or powder samples prepared under more controlled conditions would need to be used if the influence of aging at 773 K (500°C) on fatigue life was to be investigated.
The higher strengths observed in samples that had been aged at 773 K (500°C) for 5 weeks tested in tension are presumably associated with the obvious influence of the microstructure on the behavior of dislocations. Thus, in many cases, planar slip is observed, consistent with slip localization due to dislocation cutting of particles. Furthermore, paired dislocations are commonly seen. The images shown of paired dislocations were all obtained using g = 0002 and the dislocations are therefore of the type b = 1/3h11À26i, but other images have shown that dislocations with b along h11À20i are also paired. The present work was done mainly with g = 0002 close to a h10À10i beam direction, so that it was straightforward to image the same area with a 1-210-type diffracting vector and thus insure that all dislocations in the field of view would be in contrast and any pairing or other unexpected shape was not influenced by out of contrast dislocations. It is also noted that the spacing between pairs is not constant and this would suggest that the extent of order varies from region to region, as would be inevitable since there are regions between domains that are not ordered and the domains are randomly distributed. In addition to planar slip and pairing of dislocations, weak fringes are also observed in the sample, aged at 773 K (500°C) for 5 weeks, deformed in tension. Presumably, these are due to the presence of SRO in addition to the refined distribution of ordered particles. Interestingly, no such fringes are observed in samples deformed in fatigue, and it is tempting to speculate that the absence of fringes is due to the cyclic stresses and hence changing motion of dislocations in response to those stresses. This is the subject of continuing research.
The observation of regions of the HIPped sample tested in tension where planar slip is not observed (Figure 9(b) ) is of interest. This would be consistent with the absence of precipitation of particles of a 2 , presumably due to local compositional differences. Although there does not appear to be a compositional difference in terms of the principal alloying elements, local variations of oxygen may be occurring.
V. CONCLUSIONS
1. In Ti-6Al-4V, superlattice maxima are strongly visible after 5 weeks of exposure at 773 K (500°C), and particles of a 2 can be imaged using these reflections. The maxima formed in slowly cooled samples are much weaker, but have been detected for the first time. 2. This aging treatment increases the tensile strength of Ti-6Al-4V. 3. Planar slip bands which contain paired dislocations are most obvious in fatigued samples which were aged for 5 weeks at 773 K (500°C) and are linked to the presence of the ordered domains of a 2 .
VI. ACKNOWLEDGMENTS
Experimental assistance from J. Shurvinton and C. Yang is gratefully acknowledged, as are discussions with Dr S Fox (Timet). Financial support from the School of Metallurgy and Materials, University of Birmingham, is acknowledged.

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