Single step heat treatment for the development of beta titanium composites with in-situ TiB and TiC reinforcement

Highlights

• Additions of B₄C in Beta 21S form in-situ TiB and TiC increasing stability of β-Ti.

• TiB and TiC reinforcements cause grain refinement.

• TiB and TiC precipitates affect α-Ti morphology.

• Increased compressive strength with little decrease in ductility if found for composite with 3% of B₄C addition.

Abstract

Titanium matrix composites have been attracting great interest from aerospace industry due to favorable properties like thermal stability, high specific strength, corrosion, and wear resistance. Optimal relation between mechanical properties demands complex processing routes and, in this context, the effects of a single-step processing route on microstructure and mechanical properties of β titanium matrix composites was placed on focus in this study. The commercial TIMETAL Beta 21S alloy and its modification with the addition of B₄C were developed, allowing in-situ formation of TiB and TiC particles in a β matrix. The composites presented highest values of mechanical strength and hardness, and the addition of 3% of B₄C provided a significant reduction in grain size, and compressive yield strength and ultimate compressive strength values of 1205 MPa and 1636 MPa, respectively, with a maximum deformation of 20.5%. An orientation relationship investigation provided information about some unconventional relation between the present phases.

Introduction

Titanium matrix composites with discontinuous in-situ reinforcements have recently attracted great attention due to a wide range of possible properties, allowing a great diversity of applications, especially in the aerospace industry, where strict requirements of thermal stability, high specific strength, corrosion, and wear resistance may be present [[1], [2], [3]]. Beta 21S (Ti-15Mo-3Nb-3Al-0.2Si, wt%) is a titanium alloy developed as a suitable matrix material for composites, being resistant to highly oxidizing and corrosive environments at elevated temperatures, as in sections of an aircraft engines, such as the plug-and-nozzle [4]. Additions of B and C, separately, in Beta 21S at low concentrations, have been subject of previous researches, which demonstrated the effects on grain refinement, α precipitation, and mechanical properties changes [[5], [6], [7]], due to TiB or TiC formation. Other β metastable alloys, such as Ti-15Mo, Ti-15-3, Ti-5533, with B, C, or B₄C additions have also been previously investigated [[8], [9], [10], [11]], which shows the increasing interest in developing β Ti matrix composites with these types of reinforcements.

The synthesis of thermodynamically stable in-situ reinforcements, such as TiB and TiC in β titanium matrix, increases strength and hardness, but ductility decreases depending on the volume of precipitates. The ductility decrement can be affected on a smaller degree with heat treatments and formation of fine α phase, where the cooling rate from temperatures above β transus plays an important role on the final morphology of α precipitates, as equiaxial shape has been reported after slow cooling of titanium matrix composites based on Ti-6Al-4V, needle-like shape [12,13], and Widmanstätten colonies have also been reported after aging [14].

On one hand, the presence of substoichiometric carbides seems to refine α precipitates due to gettering of oxygen from the matrix, decreasing formation of detrimental grain boundary α phase [15]. Borides, on the other hand, have shown to act as nucleation sites for α, improving homogeneous distribution [16].

Mechanical properties are also modified by β grain refinement, and TiB and TiC have been widely studied as effective restraining particles for grain boundaries mobility in high temperature treatments, through the mechanism of Zener pinning [[17], [18], [19]]. However, conventional processing routes of cast β titanium alloys involve homogenization, hot or cold deformation, annealing, and aging [20], while a single-step heat treatment yielding good relation between strength and ductility would be very desirable due to decreases in production cost and time. Thus, from the development of a β-type titanium metal matrix composites through the incorporation of TiB and TiC in Beta 21S by in-situ reactions, the present study aims to propose a new processing route, from the as-cast condition followed by annealing solution in a single-step. Potential benefits for the aerospace industry would include reductions in manufacturing costs of a newly developed composite with improved mechanical properties in comparison to the alloy, allowing a range of new applications in more mechanically demanding sections of aircraft engines.

Complementarily, the orientation relationship (OR) between β, α, TiB and TiC is generally studied in materials with either the boride or the carbide, however, investigating the OR of these phases in a hybrid context, through EBSD and TEM-ASTAR images helps to extend the understanding of the predominant morphology of α and its nucleation sites, which impacts mechanical properties. EDS line scans show an interesting pattern of distribution of elements in the composites. In this research, the morphology and orientation relationship between each phase in materials heat treated above β transus followed by slow cooling were investigated. Effects on microstructure refinement due to the reinforcements and mechanical properties are discussed as well.