Structure and Anelasticity of Fe-Ge Alloys

Abstract

Several ternary Fe – Ge - C alloys with Ge contents ranging between 3 and 27 at. % have been studied. The structure, anelastic, thermodynamic and kinetic phenomena in Fe - 3, - 12, - 19/21 and – 27 Ge have been examined by X-ray diffraction (XRD), heat flow (DSC), vibrating sample magnetometry (VSM), optical-light and scanning electron microscopy, and internal friction (IF) methods. The Fe - 3Ge and Fe - 12Ge alloys form b.c.c. solid solutions. A Snoek-type internal friction (P1) peak is recorded in the Fe - 3Ge alloy with parameters similar to those for α-Fe: Н = 0.86 eV, Δ = 0.015, β = 0.72 and τ0 = 2 × 10-15 s, showing that Ge atoms have little influence on the diffusivity of carbon in iron. The Fe - 12Ge alloy, with a Curie point around 1008 K, has several IF peaks: a broad Snoek-type (P1 and P2), the P3 peak caused by structural changes in as quenched specimens during annealing, and a P4 (Zener) peak at higher temperature (Tm ≈ 773 K at f = 2 Hz, β ≈ 0.7). The Fe - 21Ge alloy has bcc or bcc plus hexagonal structure depending on heat treatment. The structure of the Fe3Ge-type alloy (Fe - 27Ge) consists mainly of hexagonal phases, i.e. hexagonal ε (D019), β (B81), and cubic ε′ (L12), and exhibits corresponding magnetic ordering transitions below 873 K which are not well-reflected in the common Fe - Ge phase diagrams. In particular a high stability of the hexagonal ε phase at room temperature is noted. A broad internal friction relaxation peak with Δ = 0.0036, H ≈ 1.8 eV and τ 0 = 2 ⋅ 10-17 s is found in Fe – 27 Ge and is classified as a double Zener peak in the ε and β two-phase mixture.

Structure and anelasticity of ordered and disordered Fe–Ge alloys

“…As this composition is close to the boundary to disordered bcc alloys, ordering is weak. A phase transition interval from B2 to A2 was detected at 1300-1350 K [21], while DSC and magnetometry yield a Curie point T C w 1000 K and a solidus temperature T S ¼ 1620 K (Table 1); these points are in reasonable agreement with literature data. After homogenizing for 24 h at 1273 K and air cooling to room temperature, weak ordering in the bcc solution (both D0 3 and B2) was detected by XRD in one specimen, in contrast to two others where only the disordered A2 phase was recorded [21].…”

“…In summary, these findings exclude P3 to result from a thermally activated relaxation process (as argued for Fe-8Ge above). Most probably it represents an effect linked to vacancy annealing, which is additionally supported by the fact that P3 is more pronounced in specimens quenched from higher temperatures [21]; this should be checked by positron annihilation experiments. Ordering in Fe-12Ge Fig.…”

“…These parameters suggest the Zener relaxation [1,2] to operate, i.e. stress-assisted thermally activated reorientations of solute atom pairs in the present alloy with some low degree of B2 order [21]. A slight indication of another peak, around 670 K for f ¼ 1.5 Hz, can be observed only during first heating after quenching from 1000 K, but nearly completely disappears in the second run after heating to 873 K. It should be noted that the interpretation of P4 as a Zener peak is supported by recent Fe 59 tracer diffusion results in Fe-Ge [29] where an activation enthalpy of about 2.5 eV has been found.…”

“…1). After most heat treatments the alloy consists of a single ordered a phase (a 1 or a 2 ), except after quenching from 1223 K where it is characterized by a two-phase structure with bcc (a) and hexagonal (3) phases [21,23] as shown in Fig. 7 (a).…”

“…Very few results on anelasticity in Fe-Ge alloys can be found in the literature [18][19][20][21], mainly because of brittleness (as already mentioned in [18]) hindering proper specimen preparation, and in none of these papers the whole range of the iron side of Fe-Ge diagram has been considered. Several different IF effects were observed in this previous work, calling for a deeper study to clarify their mechanisms.…”