Titanium aluminide alloys based on the intermetallic phases gamma-TiAl and alpha2-Ti3Al are
widely recognized as having the potential to meet the requirements for moving structures. The
intention of these applications is to substitute the heavier super alloys in certain of stress and
temperature [1]. Gamma-TiAl alloys are good bulk properties, but are limited ductility and poor
surface properties [2-5]. The alloys, Ti-47at%Al-2at%Nb-2at%Cr (MJ12) and Ti-47at%Al-2at%Nb-
2at%Mn+0.8at%TiB2 (MJ47), are provided by Howmet Corp., Whitehall, MI, U.S.A. Following
similar plasma nitridation [6], the alloys were carburized using acetylene at - 3 kV bias voltage for 1-
4 h. By using 1 gf load, Knoop microhardness (HK) was measured for 10 times. Average and
standard deviation were calculated. The alloys were analyzed using XRD in combination with
JCPDS software [7] and SEM equipped with EDX.
HK of MJ12 and MJ47 (Fig 1) is increased with the increase of deposition time. Hardness of
untreated MJ12 and MJ47 is 234.9 19.6 kg.mm-2 and 289.8 66.0 kg.mm -2, respectively. For 4 h
deposition, HK of MJ12 and MJ47 is 1.88 and 1.57 times of the corresponding untreated alloys,
respectively. During plasma deposition, acetylene decomposed into atoms and ions. Only positive
ions were accelerated and attached on the alloy surfaces. The number of ions attached and embedded
on the alloy surfaces are increased with the increase in the deposition time. By using XRD, phases of
Al, AlTi, Al2Ti, TiC and C (Fig 2) were detected due to the phase transformation of TiAl into Al2Ti,
TiC formation with Al release and C condensation. EDX analysis revealed the presence of Ti, Al and
Nb on MJ12 and MJ47 (Fig 3). Additional Cr and Mn were detected on MJ12 and MJ47,
respectively. B is very light element and was not detected on MJ47. After carburizing, C was
detected as well. SEM micrographs (Fig 4) show morphologies of the alloy surfaces after deposition.
Their surfaces were very rough. Particle size of the deposition products is less than 100 nm. They
are arranged densely and have almost circular shape