# Synergistic Effects of Temperature and Cooling Rate on Lamellar Microstructure Evolution and Mechanical Performance in Ti-44.9Al-4.1Nb-1.0Mo-0.1B-0.05Y-0.05Si Alloy

## TL;DR

This paper explores how temperature and cooling rate affect the microstructure and strength of a titanium aluminide alloy used in aero-engines.

## Contribution

The study reveals how DDRX and cooling rate influence lamellar colony formation and mechanical performance in TiAl alloys.

## Key findings

- Higher heat treatment temperatures increase lamellar colony volume but do not affect interlamellar spacing.
- Faster cooling increases nucleation sites and refines interlamellar spacing by altering grain boundary morphology.
- Fine lamellar colonies and interlocking boundaries improve tensile strength and plasticity.

## Abstract

TiAl alloys are ideal candidates to replace nickel-based superalloys in aero-engines due to their low density and high specific strength, yet their industrial application is hindered by narrow heat treatment windows and unbalanced mechanical performance. To address this, this study investigates the microstructure and mechanical properties of Ti-44.9Al-4.1Nb-1.0Mo-0.1B-0.05Y-0.05Si (TNM-derived) alloys hot-rolled in the (α2 + γ) two-phase region. The research employs varying heat treatment temperatures (1150–1280 °C) and cooling rates (0.1–2.5 °C/s), combined with XRD, SEM, EBSD characterization, and 800 °C high-temperature tensile tests. Key findings: Discontinuous dynamic recrystallization (DDRX) of γ grains is the primary mechanism refining lamellar colonies during deformation. Higher heat treatment temperatures reduce γ/β phases (which constrain colony growth), increasing the volume fraction of lamellar colonies but exerting minimal impact on interlamellar spacing. Faster cooling shifts γ lamella nucleation from confined to grain boundaries to multi-sites (grain boundaries, γ lamella peripheries, α grains) and changes grain boundaries from jagged and interlocking to smooth and straight, which boosts nucleation sites and refines interlamellar spacing. Fine lamellar colonies and narrow interlamellar spacing enhance tensile strength, while eliminating brittle βo phases and promoting interlocking boundaries with uniform equiaxed γ grains improve plasticity.