Electron beam powder bed fusion of Y2O3/γ-TiAl nanocomposite with balanced strength and toughness

This paper reports the use of multi-spot melt strategy coupled with smaller layer thickness to additively manufacture Y2O3/& gamma;-TiAl nanocomposite. In contrast to the hatch melt, the multi-spot melt strategy results in a lower fraction of & gamma; and B2-phase, a smaller lamellar spacing of 208 & PLUSMN; 72 nm with straight interface between & alpha;2/& gamma;, and a uniformly distributed nanoparticles with a finer size of 90 & PLUSMN; 38 nm. Twins can form in the equiaxed & gamma; grains and within & gamma; lamellae; this applies to both the multi-spot and hatch melt samples. Twins within the & gamma; lamellae can propagate across the twin interface but terminate at the & gamma;/& alpha;2 interface. A good combination of 556 & PLUSMN; 11 MPa (tensile strength) and 17.0 & PLUSMN; 3.1 % (ductility) at 800 & DEG;C with 16.5 & PLUSMN; 0.3 MPa & RADIC;m (room-temperature fracture toughness) is achieved in the as-built condition. Quantitative microscopy confirms a homogeneous microstructure within the x-y plane for the multi-spot sample, whilst the use of smaller layer thickness helps to reduce the microstructure degradation due to thermal cycling. In terms of the Y2O3 nanoparticles, both the rodlike Y2O3 with monoclinic and the near spherical Y2O3 with cubic crystal system are identified using transmission electron microscopy (TEM). High-resolution TEM reveals that the Y2O3/TiAl interface is clean, free of interfacial reactions, and with a semi-coherent or coherent type, suggesting a strong bonding.