Microstructural control of Ti-46Al-7Nb-0.7Cr-0.2Ni-0.1Si alloy by heat treatment

Abstract

The effects of holding temperature, time and cooling rate on the microstructure of Ti-46Al-7Nb-0.7Cr-0.2Ni-0.1Si (at.%) alloys are studied. Three kinds of segregations are found in the as-cast material. In back scattered electron images these segregations are observed as dark regions formed by the solidification process, bright regions with irregular shaped blocks and imaged regions of lighter contrast formed by the cooling process from β phase to α phase and from α phase to (β + γ) two phase or (α + β + γ) three phase, respectively. Addition of small amounts of Cr, Ni and Si to the Ti-45Al-7Nb alloy shifts the (β + γ) two phase state and (α + γ + β) three phase state to a lower Nb concentration range. While cooling from the α single phase state to the (β + γ) two phase or (α + β + γ) three phase states, sequential type phase transformation occurs. The amounts of Cr, Ni and Si are too small to induce the pearlitic mode of transformation. Therefore, the sequential mode of the ternary alloy containing Nb occurs. The microstructures change depending on the cooling rate from α single phase region. Massive transformation occurs in the range of 300 K s−1 to 50 K s−1. However, the α phase is partially retained at the cooling rate of 300 K s−1. A fully lamellar structure appears at cooling rates lower than 10 K s−1.

Fracture Toughness of Massively Transformed and Subsequently Heat Treated TiAl Intermetallic Compound

“…The specimens with 7 x 7 x 12 mm or 10 x 10 x 20 mm were prepared by mechanical cutting and electric discharge machining. Heat treatment temperature is determined by Ti-Al-Nb ternary system in Ti-45 mol% Al [8,9]. All the specimens were homogenized in α single phase state (1643 K) for 86.4 ks.…”

“…On the other hand, the method which uses massive transformation and subsequent heating can control the lamellar colony size without using thermo-mechanical processing [5][6][7][8]. Reheating of massively transformed microstructure into (α+γ) two phase region may form the fine convoluted microstructure which is the mixture of lamellar colony, α single phased grains and γ single phased grains [7,8]. However, the process to control the lamellar colony size in fully lamellar structure is not well known.…”

Oxidation performance of cold spray Ti–Al barrier coated γ-TiAl intermetallic substrates

“…1). According to EBSD studies performed by Hasegawa et al [19,20], the grains are considered as β phase (BCC; as such, the alloy ranks as "near-γ"). In the lamellar γ structure, the EDX analysis revealed a higher Nb content in the the bright-appearing lamellae (preferably oriented Nb-rich lamellae in Al-richer matrix).…”