Microstructure investigation on magnetostrictive Fe_100-xGa_x and (Fe_100-xGa_x)_99.8Tb_0.2 alloys for 19 ≤ x ≤ 29

The microstructure and, in particular, the modified-D0₃ (m-D0₃) nanoinclusions play an important role in the magnetostriction of binary Fe₈₃Ga₁₇ and Fe₈₃Ga₁₇Tb_0.2 alloys. However, the structure evolution of nanoinclusions as well as their influence on the magnetostriction with the Ga contents larger that 19 at. % remain unclear. Here, the microstructure of Fe_100-xGa_x and (Fe_100-xGa_x)_99.8Tb_0.2 as-cast alloys has been systematically investigated by X-ray diffraction and transmission electron microscopy for 19 ≤ x ≤ 29. Tetragonal m-D0₃, cubic D0₃ and B2 nanoinclusions with a size of ~2–8 nm are found to be dispersed in the A2 matrix in both Fe_100-xGa_x and (Fe_100-xGa_x)_99.8Tb_0.2 alloys. The structure evolution of nanoinclusions in Fe_100-xGa_x alloys with the increase of x is revealed: m-D0₃ (x = 19) → m-D0₃+D0₃ (x = 23) → D0₃ + B2 (x = 27). While for (Fe_100-xGa_x)_99.8Tb_0.2 alloys the presence of m-D0₃ nanoinclusions extends to higher Ga contents (x = 27). The density of m-D0₃ nanoinclusions in Fe_100-xGa_x and (Fe_100-xGa_x)_99.8Tb_0.2 alloys decreases with the increase of x, which leads to the lower magnetostriction in the alloys with x > 19. The relatively larger density of the m-D0₃ nanoinclusions can well explain the improvement in magnetostriction of Fe–Ga as-cast alloys for 19 ≤ x ≤ 29 with trace Tb doping.