Papers by Keyword: Fully Lamellar

Abstract: Effects of holding temperature and time at (β+γ) two phase region on the microstructure of fully lamellar Ti-46Al-7Nb-0.7Cr-0.2Ni-0.1Si (mol%) intermetallic compounds are studies. Fully lamellar microstructure is observed after homogenization heat treatment for 3.6 ks at 1643 K (α single phase state). Fine β phased grains precipitate at fully lamellar structure after heat treatment of homogenized material at 1373 K. Holding the homogenized material for 72 ks at 1373 K decompose partially the lamellar structure. Heat treatment of homogenized material at 1273 K also precipitates the fine β phased grains in fully lamellar structure. In this temperature range, decomposition of lamellar structure is not observed up to 72 ks heat treatment. The toughness of homogenized material is ~ 15 MPa√m. Heat treatment of homogenized material at 1373 K and 1273 K for 3.6 ks indicates maximum fracture toughness in each temperature range. This may due to the precipitation of fine β phased grains. The fracture toughness decreases with the increase in heat treatment time up to 18 ks and/or 36 ks. Then, the value of fracture toughness became constant. Specimens heat treated at 1373 K for 36 ks and 72 ks indicate lower toughness than homogenized material. However, when the specimens are heat treated at 1273 K for 36 ks and 72 ks, the toughness is higher than that of homogenized material. This change is due to the decomposition of the lamellar structure.

Abstract: Crept microstructures in g-TiAl based alloys reveal a preponderance of 1/2[110]-type jogged-screw dislocations, suggesting that the rate of creep deformation is controlled by the glide of such dislocations. A creep model based on these microstructural observations has been recently developed. This leads to an excellent prediction of creep rates and stress exponents. In this paper, the framework of this model including the verification and validation of the functional dependencies of various microstructural model parameters is reviewed. It has also been observed that creep phenomenology is extremely sensitive to microstructure – fully lamellar g-based alloys exhibit lower creep rates and higher stress exponents even though the deformation microstructure is similar to that in equiaxed alloys. The modifications made to the model that account for the constrained nature of deformation in lamellar alloys are discussed. The applicability of the model is explored in materials systems, including a-Ti and a+b Ti alloys where similar creep exponents and deformation structures have been observed. Finally, the relevance, applicability and shortcomings of the model are critically analyzed.