The first- and second-order isothermal phase transitions in Fe₃Ga-type compounds

Abstract

Structural features and kinetics of the transition between ordered metastable b.c.c.‐derived D03 and equilibrium f.c.c.‐derived L12 phases of Fe–xGa alloys (x = 27.2% and 28.0%) have been analyzed by in situ real‐time neutron diffraction during isothermal annealing in the temperature range 405–470°C. It has been revealed that the transition proceeds with alternation of the first‐ and second‐order phase transformations according to a D03 → A2 → A1 → L12 scheme, where A2 and A1 are disordered b.c.c. and f.c.c. structures. Deformations of the crystal lattice that arise due to these transitions are determined. The kinetics of the L12 phase nucleation and growth were analyzed in the frame of the Johnson–Mehl–Avrami–Kolmogorov (JMAK) model; however, only the early stage of the D03 → L12 transition is well described by the JMAK equation. The value of the Avrami exponent corresponds to the constant growth rate of the new L12 phase and decreasing nucleation rate in the Fe–27.2Ga alloy and indicates the presence of pre‐existing nucleation centres of the L12 phase in the Fe–28.0Ga alloy.

Structural and Elastic Properties of Fe-Ge Alloys: ab initio studies

“…The V a of the B2 structure is close to D0 3 . Under the transition from disordered to ordered state, the unit-cell parameters decrease slightly and, therefore, the atomic volume also decreases [25][26][27]. The obtained dependencies of V a on Ge content correspond to the sequence of phase transitions observed experimentally [5,13]: A2→B2→D0 3 (8≤ x ≤22 at.…”

Anelastic Effects in Fe–Ga and Fe–Ga-Based Alloys: A Review

“…Thus, there is no doubt left that the mechanism of the transient internal friction P Tr peak originates from the first-order transition from metastable D0 3 to equilibrium L1 2 phase. Independent of the details of the transition mechanism (discussed in more detail in [ 21 ]), the related anelastic strain comes from the lattice deformation when the material is transformed. The D0 3 → L1 2 transition orientation relations can be expressed as [ 136 , 137 ]: where the elastic deformation values are estimated to be ε c ∼0.265 and ε a ∼0.105.…”

“…To simplify figures, we did not show that the transition from one ordered phase to another one goes through disordering of the ordered phases. For example, the transition from D0 3 to L1 2 goes through disordering of the D0 3 phase to A2, then the transition A2 → A1 takes place, and, finally, the ordering of the A1 lattice leads to formation of the L1 2 phase [ 21 ]. Thus, from the viewpoint of anelasticity, the transition from D0 3 to L1 2 structure is practically the same as with bcc (A2) to fcc (A1) transition, and corresponding change in lattice parameter can be described by Equation (14).…”

“…In (Fe–25–28)Ga alloys, the recorded sequence of first-order phase transitions upon heating is D0 3 → L1 2 → D0 19 → B2/A2 (depending on the heating rate: A2 for 2 K/min or B2 for 1 K/min). A second-order A2 → D0 3 transition precedes the sequence of first-order transitions if only a surface of the directly solidified or quenched samples is analyzed by XRD [ 21 ] and position annihilation [ 22 ] studies. The range of the D0 3 → L1 2 transition depends greatly on the heating rate.…”

“…Atomic arrangement from A2 disordered phase to B2 and D0 3 (second-order transition, for both systems) or to L1 2 ordered (through two second-order and one first-order transition in Fe–Ga [ 21 ]) phases affects both the value and the sign of magnetostriction coefficients in Fe–Ga. In Fe–Ga alloys with Ga > 30%, another 3 phases were observed in addition to the B2, D0 3 , and L1 2 : Fe 13 Ga 9 [ 25 ] and α- and β-Fe 6 Ga 5 [ 26 , 27 ].…”