Persistence of 5:3 plates in RE₅(Si_xGe_1-x)₄ alloys

Abstract

Studies of RE 5 (Si x Ge 1-x ) 4 alloys, where RE equals rare earth, have revealed a second-phase having a thin-plate morphology in essentially every alloy examined, independent of exact composition and matrix crystal structure. Identified as having a composition approximating Gd 5 (Si x Ge 1-x ) 3 and a hexagonal crystal structure in the Gd-based system, it has been suggested that the observed thin-plate second phases seen in this family of rare earth alloys are all most likely of the form RE 5 (Si x Ge 1-x ) 3 . A number of interesting observations suggest that the formation of these second-phase plates is somewhat unusual. The purpose of this article is to investigate the stability of this second phase in Gd-and Er-based compounds. The stability was investigated as a function of thermal cycling and large-scale composition fluctuations. The results of scanning and transmission electron microscopy (SEM, TEM) studies indicate that the RE 5 (Si x Ge 1-x ) 3 phase is extremely stable once it forms in a RE 5 (Si x Ge 1-x ) 4 matrix. Studies of RE 5 (Si x Ge 1-x ) 4 alloys, where RE equals rare earth, have revealed a second-phase having a thin-plate morphology in essentially every alloy examined, independent of exact composition and matrix crystal structure. Identified as having a composition approximating Gd 5 (Si x Ge 1-x ) 3 and a hexagonal crystal structure in the Gd-based system, it has been suggested that the observed thin-plate second phases seen in this family of rare earth alloys are all most likely of the form RE 5 (Si x Ge 1-x ) 3 . A number of interesting observations suggest that the formation of these second-phase plates is somewhat unusual. The purpose of this article is to investigate the stability of this second phase in Gd-and Er-based compounds. The stability was investigated as a function of thermal cycling and large-scale composition fluctuations. The results of scanning and transmission electron microscopy (SEM, TEM) studies indicate that the RE 5 (Si x Ge 1-x ) 3 phase is extremely stable once it forms in a RE 5 (Si x Ge 1-x ) 4 matrix.

Microstructure study of the rare-earth intermetallic compounds R₅(Si_xGe_1-x)₄ and R₅(Si_xGe_1-x)₃

“…formation are yet unknown. The high degree of stability noted for the plates [85] suggests a strong kinetic driving force in much the same way martensitic transformations occur in opposition to equilibrium thermodynamic considerations. Further study of the 5:3 thin plates to define their formation temperature and range of stability is needed if one is to manipulate and control the structure.…”

“…The precipitation of 5:3 plates appears relatively independent of the initial crystal structure and composition of the parent matrix [79,82]. Somewhat surprising was that alloy compositions specifically chosen in an attempt to avoid 5:3 formation still possessed 5:3 plates whenever the 5:4 phase was present, pointing to an unusual stability of the 5:3 plates within a 5:4 matrix [85].…”

Thermal stability of RE5(SixGe1−x)3 plates in RE5(SixGe1−x)4 alloys, where RE = Gd and Ho

“…Extensive research on the magnetic properties of RE 5 (Si x Ge 1Àx ) 4 (hereafter referred to as "5:4" compounds), has resulted in the microstructures of this 5:4 system capturing the attention of researchers [5e11, 14,15]. The earliest studies [5] reported the existence of linear features visible on the surface of Gd 5 Si 4 , Gd 5 Si 2 Ge 2 , and Gd 5 Ge 4 samples.…”

“…Ugurlu et al [14] investigated the persistence of the 5:3 plates in Gd 5 Si 2 Ge 2 under thermal cycling and concluded that the 5:3 plates are stable in the temperature range À70 C to 850 C. Somewhat surprising was that alloy compositions specifically chosen in an attempt to avoid 5:3 formation still possessed 5:3 plates whenever the 5:4 phase was present, pointing to an unusual stability of the 5:3 plates within a 5:4 matrix. However, that study was limited in scope with an emphasis being on the cycling of the temperature, rather than on long-term stability at higher temperatures.…”

Microstructure study of the rare-earth intermetallic compounds R5(SixGe1-x)4 and R5(SixGe1-x)3

“…formation are yet unknown. The high degree of stability noted for the plates [85] For instance, magnetic ordering occurs simultaneously with the crystallographic change in Gd 5 Si 2 Ge 2 [13], ferromagnetic ordering accompanied by a crystal rearrangement can be triggered by a magnetic field in Gd 5 Ge 4 [14], and a pressure-induced magneto-structural coupling is present in Tb 5 Si 2 Ge 2 [15]. Conversely, the compound Er 5 Si 4 exhibits an interesting decoupling of the magnetic and structural transformations [16 -17].…”