Structure of precipitates and orientation relationships in Al,Ge, Mo-doped higher manganese silicide crystals

Abstract

The structure of Al, Ge, Mo-doped Higher Manganese Silicide (HMS) crystals with the general formulas Mn(Si_0.99Ge_0.01)_1.75, Mn(Si_0.995Ge_0.005)_1.75 and (Mn_0.98Mo_0.02)(Si_0.98Ge_0.02)_1.75]_0.99Al_0.01 was investigated by scanning and transmission electron microscopy, electron diffraction and X-ray energy dispersive spectrometry in a wide scale range from a few mm to several Å. Several secondary phases were identified in the Mn₄Si₇ matrix: Ge_1?xSi_x (0.1 < x < 0.9) solid solution precipitates with Ge concentration ranging from 5 at. % up to 93 at.%, MoSi₂ platelets, MnSi and Mn₅Si₃ precipitates. Their morphology, structure and crystallographic relationships with the HMS matrix were determined. Mostly local strains in the matrix and precipitates due to lattice misfits at interfaces derived from crystallographic relationships were found two orders of magnitude higher than deformation induced by thermal expansion mismatch. Only a few exceptions of specific relationships were found when the lattice misfit and thermal mismatch have close values. The largest misfit of about 22% was observed between MnSi and Mn₄Si₇ what led to big and numerous cracks in crystals. Therefore, doping can improve the material performance (1) by preventing the formation of MnSi precipitates with metallic properties and (2) by reduction of cracking and crack propagation because of larger MnSi /Mn₄Si₇ lattice misfit compared to Ge_1?xSi_x /Mn₄Si₇ or MoSi₂/Mn₄Si₇ misfits.