Microstructural Evolution and Mechanical Properties of an Advanced γ-TiAl Based Alloy Processed by Spark Plasma Sintering

Abstract

Intermetallic γ-TiAl based alloys are innovative lightweight structural high-temperature materials used in aerospace and automotive applications due to already established industrial-scale processing routes, like casting and hot-working, i.e., forging. A promising alternative method of production, regarding manufacturing of near net-shape components, goes over the powder metallurgy route, more precisely by densification of TiAl powder via spark plasma sintering. In this study, gas atomized powder from the 4th generation TNM alloy, Ti-43.5Al-4Nb-1Mo-0.1B (in at.%), was densified and the microstructure was investigated by means of electron microscopy and X-ray diffraction. The sintered microstructure exhibits lamellar α2-Ti3Al /γ-TiAl colonies surrounded by globular γ- and ordered βo-TiAl phase. The coarse lamellar spacing stems from the low cooling rate after densification at sintering temperature. Against this background, subsequent heat treatments were designed to decrease the lamellar widths by a factor of ten. Accompanying, tensile tests and creep experiments at different temperatures revealed that the modified almost fully lamellar microstructure is enhanced in strength and creep resistance, where a small volume fraction of globular γ-phase provides ductility at ambient temperatures.

Spark Plasma Sintering of Titanium Aluminides: A Progress Review on Processing, Structure-Property Relations, Alloy Development and Challenges

“…The fourth generation of TiAl, commonly referred to as TNM alloys, exhibit excellent oxidation resistance through Nb and or Ta additions and outstanding creep properties promoted by Mo additions. In addition, these alloys are said to have excellent workability making them universally applicable for various processing routes other than casting and hot forming such as additive manufacturing and SPS [141]. Some of the alloys developed to date include the β-solidifying Ti-43.5Al-4Nb-1Mo-0.1B [141], Ti-46Al-8Nb and Ti-46Al-8Ta [142].…”

“…In addition, these alloys are said to have excellent workability making them universally applicable for various processing routes other than casting and hot forming such as additive manufacturing and SPS [141]. Some of the alloys developed to date include the β-solidifying Ti-43.5Al-4Nb-1Mo-0.1B [141], Ti-46Al-8Nb and Ti-46Al-8Ta [142]. Wimler et al [141] studied the capabilities of SPS coupled with subsequent heat treatments to produce a Ti-43.5Al-4Nb-1Mo-0.1B alloy with strength and creep performance at 750 • C comparable to those produced by conventional processing techniques.…”

“…Some of the alloys developed to date include the β-solidifying Ti-43.5Al-4Nb-1Mo-0.1B [141], Ti-46Al-8Nb and Ti-46Al-8Ta [142]. Wimler et al [141] studied the capabilities of SPS coupled with subsequent heat treatments to produce a Ti-43.5Al-4Nb-1Mo-0.1B alloy with strength and creep performance at 750 • C comparable to those produced by conventional processing techniques. After SPS consolidation of the gas atomised powder alloy at 1300 • C, the microstructure produced consisted of a refined and homogeneous NL γ and β 0 .…”

“…SEM images showing (a) NL beta (β); (b) fully lamella (FL); and (c) NL gamma (γ)microstructures after two-step heat treatment[141].…”

“…Plots of creep stress and strain versus time of the heat-treated microstructures tested at 750 °C and 150 MPa[141].…”