On the high creep strength of the W containing IRIS-TiAl alloy at 850 °C

Abstract

This paper presents a study of the creep at 850°C under 150 MPa of the IRIS alloy (Ti-Al48-W2-B0.1) densified by spark plasma sintering. The dislocation microstructure in a sample strained up to 1.5% was studied by post-mortem transmission electron microscopy. The deformation is mainly due to ordinary dislocations. Several populations of dislocations are evidenced. Their Burgers vectors, the plane in which they are moving and the corresponding deformation mechanisms are determined. In the discussion section, the deformation mechanisms, the factors controlling their activation and the role of tungsten as hardening element are examined.

Effect of ageing on the properties of the W-containing IRIS-TiAl alloy

“…In the author's opinion, the interpretation of these ageing effects requires deep understanding of the microstructure evolutions versus ageing time and of the deformation mechanisms activated under the various applied solicitations. The deformation mechanisms activated at room and high temperatures in the SPS-IRIS alloy have been investigated elsewhere by the present authors [9,35]. Indeed, it has been shown that, at high temperatures, the interaction between dislocations moving by glide or climb processes and atoms of tungsten is the controlling phenomenon for the alloy strength.…”

“…A significant part of the ductility loss measured in the aged alloy (0.22 %) could be attributed to the increase of the mean free path between obstacles for twins in the  phase with the reduced size of lamellar areas: the longer this distance, the larger the number of piledup twin dislocations at given stress, the higher the associated internal stress leading to the sample failure. At 800°C under high strain rate, the deformation results from the glide of ordinary dislocations controlled by a frictional force enhanced by the interaction between dislocations and tungsten atoms and from the crossing of / and /2 interfaces by the mobile dislocations [35]. Here again, the diminution of the interface number through that of the lamellar zones should slightly reduce the alloy strength.…”

“…However, the effect of W atoms on the intrinsic mobility of gliding dislocations [37], is still present and preserve the IRIS alloy of a drastic loss of strength. During creep at 800°C under low stress, the dislocation propagation occurs by climb of [001] and ordinary 1/2<110] dislocations in the  phase which is controlled by the low diffusivity of tungsten atoms [35,38]. The creep properties of the aged alloy are therefore moderately affected because a suitable amount of tungsten is maintained in this  phase.…”

Processing and Microstructure of As-Cast Ti-45Al-2W-xC Alloys

“…Refractory metals such as W with a low diffusivity in TiAl alloys can significantly increase mechanical properties and corrosion resistance at elevated temperatures [ 7 ]. Alloying with W reduces dislocation mobility and increases the thermal stability of lamellar microstructure, which leads to an increase in creep resistance [ 8 , 9 , 10 , 11 , 12 , 13 ]. The positive influence of carbon on the creep strength of TiAl-based alloys has been published by several authors [ 14 , 15 , 16 , 17 ].…”

Chemical heterogeneities in tungsten containing TiAl alloys processed by powder metallurgy

“…Thus, the presence of 0 phase in the final microstructure, which is unfavourable at high temperatures and brittle at room temperature, is avoided while the presence of  phase is promoted. The challenge is to have a homogeneous distribution of W atoms in the  phase to ensure a homogeneous structural hardening of that phase and the alloy via the interaction of W atoms with dislocations moving by various deformation mechanisms [6].…”