7.3.1 Sintering
The green densities of Ti-0.4Er and Ti6Al4V-0.4Er are 74.99% and 70.56%
respectively, which is in the normal compaction range for Ti and Ti6Al4V samples. After sintering, the densities increased to 95.56% and 91.91%
respectively, which is a little higher (about 1%) than the sintered density of Er-free compacts. Two worm-like-voids were found in the sintered sample, which can be clearly seen by the naked eye, as shown in Fig. 7.1(a). Compared with the erbium powder shown in Fig. 7.1(b), the shape of the voids and their size are nearly identical to that of the Er particles.
Fig. 7.1 Images of 11mm diameter Ti and Ti6Al4V sintered samples with erbium: (a)
Ti-0.4Er and (b) Ti6Al4V-Ti-0.4Er with some erbium powder particles visible in the plastic
container
Fig. 7.2 shows back scattered electron (BSE) images of voids present in Ti-0.4Er and Ti6Al4V-0.4Er alloys. A two phase structure with a white and grey
appearance is observed. The voids are located in the centre of the phase with a white appearance. EDS results indicate that this phase is Er-rich. Columnar grains, which are a product of solidification of the liquid phase, are found on the surface
11mm
(a) (b)
0.4Er, the penetration distance is about 200μm and for Ti6Al4V-0.4Er it is a little higher, with a penetration distance of 400μm. Samples cut from the sintered compacts with/without the segregation and void structure have quite different properties. Thus they cannot be used for mechanical testing.
Fig. 7.2 Back Scattered Electron (BSE) images of as-sintered Ti-0.4Er and Ti6Al4V-0.4Er:
(a) and (b), Ti-0.4Er; (c) and (d), Ti64-0.4Er
Fig. 7.3 shows the elemental distribution in the matrix and in the white area in a Ti6Al4V-0.4Er alloy. The matrix is almost Er-free. The liquid phase is Er based because it has Er in abundance with a very low Ti content. V content is lower than found in the matrix whereas Al, C and O contents are higher.
The formation of a liquid phase can be explained by reference to the Ti-Er binary phase diagram. As shown in Fig. 7.4, the Ti-Er system is a typical eutectic phase diagram, with liquid formed from 1300-1340℃. Therefore the liquid phase is
(a)
generated due to the higher isothermal sintering temperature of 1350℃. The formation of Er segregation and voids will be explained in the discussion section.
Fig. 7.3 Elemental distribution along a line in the microstructure near the voids in a Ti6Al4V-0.4Er alloy.
Fig. 7.4 Ti-Er binary phase diagram
7.3.2 Open die forging
Fig. 7.5 shows the Er distribution in an ODFed sample. Fig. 7.5 (a) depicts many white spots ranging in size from 1-5μm in the transverse face, which is
perpendicular to the press direction. Fig. 7.5 (b), which is taken from the longitudinal face, indicates that the Er segregation has been elongated to an acicular form with an aspect ratio of 8-10. At the centre, the segregation is almost gone, but at the edge of an as-forged sample, large amounts of Er segregation still exist, as shown in Fig. 7.5(c). The diffusion distance of the Er has increased to 600-1000μm. Thus ODF leads to homogenisation of the Er distribution. Fig.
7.5(d) shows that big voids have been closed.
Fig. 7.5 SEM images of as-forged Ti-0.4Er samples: (a) radial direction in the centre region;
(b) longitudinal direction in the centre region; (c) edge region showing the existence of Er
segregation; (d) a higher magnification image of (c) showing the closure of voids
Table 7.2 gives the mechanical properties of as-forged Ti-0.4Er alloy. Test-piece no.1, which is from the top surface on which the applied pressure acts, has
(a)
excellent UTS and ductility. But test-piece no.3, which was cut from the bottom surface, has lower properties. This result is acceptable because it lies in the range of values for an as-forged pure Ti sample (700-800MPa and 6-14% strain).
However, there is no obvious improvement as a result of Er additions is.
Fig. 7.6 shows the fracture surfaces of test pieces No.1 and 3. Cracks can be found in the fracture surface, which must be sites of fracture initiation. In sample No.1, shown in Fig. 7.6(a), the crack is not very clear and its length is only 300μm. Er rich segregation was found along the two sides of the crack. In sample No.3, shown in Fig. 7.6(c), T-shaped long cracks (~1mm) are found. Er segregation was also found to be associated with the crack. Thus Er segregation still exists in the test pieces which cause deterioration in mechanical properties.
Table 7.2 Mechanical properties of as-forged Ti-0.4Er samples
No. UTS (MPa) Strain to break (%)
1 764 10.50
2 818 7.77
3 729 1.16
Fig. 7.6 Fracture surfaces of test pieces cut from as-forged Ti-0.4Er alloys (a) No.1; (b) the
crack in Fig. 7.6 (a), indicating the existence of ER segregation. (c) No.3; (d) the crack in Fig.
7.6 (c)
Fig. 7.7 shows an Er platelet observed in a Ti6Al4V-0.4Er alloy. A near spherical plate with a white appearance is located in the matrix. The diameter of the plate is about 40μm and the thickness is at the micron level, indicating that it was forged from a bulk texture to a thin plate. The boundary between the plate and the matrix is cracked. Furthermore, the plate has many cracks. Therefore the strength of material with Er segregation is far more inferior than the Ti alloy matrix. It also depicts that the segregation is rich in Al and O but low in V.
(a)
Fig. 7.7 An Er platelet in a Ti6Al4V-0.4Er alloy and associated elemental mapping
The heat treat microstructure of a Ti6Al4V-0.4Er alloy will be introduced in the next section, together with results for the heat treated Ti (Ti6Al4V)-0.5Y alloy.
20μm