Toughening mechanism in the lamellar and duplex TiAl-based alloys at ambient temperature: Microcrack analysis

Abstract

The Ti48Al2Wo.5Si alloy used in this study was supplied by ABB Power Generation in HIP`ed and heat treated condition as follows: the FL bar in 25.4 mm diameter, HIP`ed at 1185 C/172MPa/3h and HT at 1302 C/20h(Ar)/GFC+913 C/4h(Ar)/GFC; the ND bar in 15 mm diameter, HIP`ed at 1185 C/150MPa/4h and HT at 1300/20h/GFC+900 C/4h/GFC. The samples were cut from the rim of the FL bar and the center of the ND bar along the longitudinal direction, which exhibited a fully lamellar microstructure (FL) and a duplex microstructure (ND), respectively. Compression tests were conducted on a MTS testing machine at initial strain rate of 1{times}10{sup {minus}2}s{sup {minus}1} at room temperature using cylindrical specimens with a diameter of 5mm and a length of 10mm. Conclusions were as follows: (1) The lamellar interfaces were preferred sites for crack nucleation in both lamellar nd duplex microstructures, attributed to the anisotropic deformability of the lamellar colonies. (2) The compatible flow prior to cracking was nearly one third of the fracture strain in lamellar specimens and about half of the maximum strain in duplex one. The suppression of microcracking in the duplex specimens was attributed to their fine microstructure and to the plastic accommodation of gammamore » phase on the colony boundaries. (3) The average length and specific ratio (L/W) of cracks in lamellar specimens were much greater than that of duplex ones. (4) The crack development in lamellar microstructure was predominantly by the crack extension, corresponding to a large crack profile area density, while it was essentially by the nucleation of new cracks in duplex microstructure, responsible for a high number density cracks. (5) The good ductility of duplex structure was originated from its good plastic accommodation ability. The excellent fracture toughness of lamellar structure was dictated by the shear ligament toughening.« less