Phase Separation and Lattice Misfit in NiAl(β1)–Ni2AlTi(H)–NiTi(β2) System

Origin of the phase separation into B2 and L21 ordered phases in X-Al-Ti (X: Fe, Co, and Ni) alloys based on the first-principles cluster variation method

The phase separation behaviors from a single B2 ordered phase into two separate B2 and L2₁ ordered phases in X-Al-Ti (X: Fe, Co, and Ni) alloys are analyzed utilizing the cluster variation method (CVM), based on interaction energies derived from electronic band structure calculations. A cubic approximation of the CVM is adopted for X₂Al_2-x Ti _x ([Formula: see text]) alloys limiting the interchange between Al and Ti atoms on the [Formula: see text]- and [Formula: see text]-sublattices of an L2₁ ordered structure with X atoms fixed on the [Formula: see text]-sublattice. The phase stabilities of the B2 and L2₁ structures are examined, and phase diagrams at the pseudo-binary section, XAl-XTi, are determined. The two-phase regions of the B2 and L2₁ phases (i.e. phase separation behavior) are successfully produced in Co- and Ni-Al-Ti alloy systems, and no phase separation is observed in the Fe-Al-Ti alloy. The origins of phase separation in the Co- and Ni-Al-Ti alloys are mechanical instability and a combination of mechanical instability and chemical repulsions of unlike pairs, respectively.

Structural characterization of semi-heusler/light metal composites prepared by spark plasma sintering

A composite of powders of semi-Heusler ferromagnetic shape memory and pure titanium was successfully prepared by spark plasma sintering at the temperature of 950 °C. Sintering resulted in the formation of small precipitates and intermetallic phases at the heterogeneous interfaces. Various complementary experimental methods were used to fully characterize the microstructure. Imaging methods including transmission and scanning electron microscopy with energy dispersive X-ray spectroscopy revealed a position and chemical composition of individual intermetallic phases and precipitates. The crystalline structure of the phases was examined by a joint refinement of X-ray and neutron diffraction patterns. It was found that Co38Ni33Al29 decomposes into the B2-(Co,Ni)Al matrix and A1-(Co,Ni,Al) particles during sintering, while Al, Co and Ni diffuse into Ti forming an eutectic two phase structure with C9-Ti2(Co,Ni) precipitates. Complicated interface intermetallic structure containing C9-Ti2(Co,Ni), B2-(Co,Ni)Ti and L21-(Co,Ni)(Al,Ti) was completely revealed. In addition, C9-Ti2(Co,Ni) and A1-(Co,Ni,Al) precipitates were investigated by an advanced method of small angle neutron scattering. This study proves that powder metallurgy followed by spark plasma sintering is an appropriate technique to prepare bulk composites from very dissimilar materials.