Effect of Boron Content and Cooling Rate on the Microstructure and Boride Formation of β-Solidifying γ-TiAl TNM Alloy

Daniel Bernal,Iñaki Madariaga,Xabier Chamorro,Iñaki Hurtado

doi:10.3390/met10050698

Abstract

Boron is a unique and popular grain refiner element in cast titanium aluminide (TiAl) alloys, as it helps to improve mechanical properties if properly alloyed. However, the formation mechanism of different types of borides in cast TiAl alloys is not yet clearly understood. This study seeks to correlate the chemical composition and cooling rate during solidification of cast TiAl alloys, with the type of boride precipitated and the resulting microstructure. Several β-solidifying γ-TiAl alloys of the TNM family were cast, alloying boron to a starting Ti-44.5Al-4Nb-1Mo-0.1B (at.%) alloy. The alloys were manufactured with an induction skull melting furnace and poured into a stepped 2, 4, 8 and 16 mm thickness mold to achieve different cooling rates. On one hand, the results reveal that boron contents below 0.5 at.% and cooling rates during solidification above 10 K/s promote the formation of detrimental ribbon borides. On the other hand, boron contents above 0.5 at.% and cooling rates during solidification below 10 K/s promote the formation of a refined microstructure with blocky borides. Finally, the formation mechanisms of both ribbon and blocky borides are proposed.

Highlights

Titanium aluminide (TiAl) alloys are promising lightweight materials because of their singular mechanical properties, such as high specific strength and good resistance against oxidation and corrosion.As a result, these materials are suitable for aircraft engines and gas-burning power-generation plants [1].Of the available titanium aluminides, this work focuses on the β-solidifying γ-TiAl TNM (Ti-Nb-Mo) alloys, which are used in the low-pressure turbine blades of the new geared turbofan [2] to reduce fuel burn, pollutants, noise emissions and operating costs [3].1.1
The results reveal that boron contents below 0.5 at.% and cooling rates during solidification above 10 K/s promote the formation of detrimental ribbon borides
The TNM-0.1B master alloy was melted within an induction skull melting (ISM) furnace in an argon atmosphere, a process that allows for melting of reactive alloys with almost no variation in chemical composition

Summary

Introduction

Titanium aluminide (TiAl) alloys are promising lightweight materials because of their singular mechanical properties, such as high specific strength and good resistance against oxidation and corrosion.As a result, these materials are suitable for aircraft engines and gas-burning power-generation plants [1].Of the available titanium aluminides, this work focuses on the β-solidifying γ-TiAl TNM (Ti-Nb-Mo) alloys, which are used in the low-pressure turbine blades of the new geared turbofan [2] to reduce fuel burn, pollutants, noise emissions and operating costs [3].1.1. Titanium aluminide (TiAl) alloys are promising lightweight materials because of their singular mechanical properties, such as high specific strength and good resistance against oxidation and corrosion. As a result, these materials are suitable for aircraft engines and gas-burning power-generation plants [1]. ), which leads to an isotropic structure, equiaxial grain, microstructure without texture and little micro-segregation. ) can happen, leading to an anisotropic microstructure, segregation and texture [4]. Whenever a peritectic reaction happens, borides are no longer effective as grain refiners because the α-phase can nucleate on the β-phase dendrites invading the microstructure

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