The effect of Si additions on the thermal stability of the supercooled liquid before crystallization, glass-forming ability (GFA) and soft magnetic properties was examined for amorphous alloy series Fe
72−xAl
5Ga
2P
11C
6B
4Si
x, Fe
72Al
5−xGa
2P
11C
6B
4Si
x, Fe
72Al
5Ga
2P
11−xC
6B
4Si
x and Fe
72Al
5Ga
2P
11C
6−xB
4Si
x. The increases in the thermal stability and GFA and the improvement of soft magnetic properties were recognized in the replacements of P by 1 to 2 at%Si and of C by 1 at%Si. The supercooled liquid region (Δ
Tx) defined by the difference between crystallization temperature (
Tx) and glass transition temperature (
Tg) increases from 53 K for Fe
72Al
5Ga
2P
11C
6B
4 to 58 K for Fe
72Al
5Ga
2P
11C
5B
4Si
1. The maximum thickness for glass formation (
tmax) by copper mold casting increases from 1 mm for the Fe–Al–Ga–P–C–B alloy to 2 mm for the Fe
72Al
5Ga
2P
10C
6B
4Si
1 alloy. The increases in Δ
Tx and
tmax are presumably because of the increase in the degree of the satisfaction of the three empirical rules for the achievement of large glass-forming ability, i.e., (1) multicomponent alloy systems consisting of more than three elements, (2) significantly different atomic size ratios above about 12% and (3) negative heats of mixing. The soft magnetic properties are also improved by the replacement of 1 at%Si for P or C through the increase in the squareness ratio of
B-
H loop (
Br⁄
Bs) and the decrease in coercive force (
Hc). The best soft magnetic properties for the bulk amorphous alloys are obtained for the Fe
72Al
5Ga
2P
10C
6B
4Si
1 alloy and the saturation magnetization (
Bs),
Hc,
Br⁄
Bs, and Curie temperature are 1.14 T, 1.5 A/m, 0.45 and 594 K, respectively. The success of forming the Fe-based bulk amorphous alloys of 2 mm in thickness exhibiting the good soft magnetic properties is promising for future development as a new type of soft magnetic material.
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