The Ti-Mn system revisited: experimental investigation and thermodynamic modelling

Abstract

As the Ti-Mn phase diagram is part of numerous ternary and higher order systems of technological importance, the present paper defines phase relations which have been experimentally established throughout this work from 800 1C to the melting range based on Differential Thermal Analyses (DTA), X-ray powder diffraction, metallography and Electron Probe Micro Analysis (EPMA) techniques on B50 alloys, which were prepared by arc melting or high frequency melting under high purity argon starting from freshly cleaned metal ingots. Novel compounds were identified and reaction isotherms were redefined accordingly. In the Ti-rich region a novel compound TiMn was detected, sandwiched between the known phases: TiMn1 x (B45 at% Mn) and TiMn1+x (B55 at% Mn). In the Mn-rich region the hitherto unknown crystal structure of TiMnB3 was solved from X-ray single crystal diffraction data and found to be of a unique structure type Ti6(Ti1 xMnx)6Mn25 (x = 0.462; space group Pbam (#55); a = 0.79081(3) nm, b = 2.58557(9) nm, c = 0.47931(2) nm), which consists of two consecutive layers of the hexagonal MgZn2-type Laves phase (TiMn2) and a combined layer of alternate structure blocks of MgZn2 type and Zr4Al3 type. Whereas TiMn can be considered as a line compound (solubility range oB1 at%), the homogeneity regions of the Ti-Mn compounds are significant (determined by EPMA): TiMn1 x (44.0 to 46.6 at% Mn), TiMn1+x (54.6 to 56.3 at% Mn), Ti1+xMn2 x (MgZn2-type, 59 to 69 at% Mn at 1000 1C: 0.08 o x o 0.23), TiMnB3 (unique type; 74 to 76.5 at% Mn) and TiMnB4 (R-phase: Ti8(TixMn1 x)6Mn39, 80 to 84 at% Ti). Supported by ab initio calculations of the ground state energy for the Laves phase, the new experimental results enabled thermodynamic modelling of the entire Ti-Mn phase diagram providing a complete and novel set of thermodynamic data thus providing a sound basis for future thermodynamic predictions of higher order Ti-Mn-X-Y systems.