Figure 4.- Fracture Toughness, KI results on Fe78-xCrxSi10B12 ribbons. HT steel wire was characteristic of this type of high strength steel. Fatigue The fatigue performance of these wires is shown in figure 6. It can be seen that, in general, increasing the Cr content leads to improved fatigue performance, especially under high cycle conditions. In particular, there is a noticeable difference in performance between the 0 and 3.5 at% Cr samples and the 6.25 and 8 at% Cr samples; corrosion pits, that could act as crack nucleating sites were observed for both the 0 and 3.5 at% Cr alloys. This is in line with previously reported enhancements in the fatigue performance of Fe77.5-xCrxSi7.5B15 and Fe75-xCrxSi10B15 amorphous alloys with increasing Cr content [3-5]. Previous studies [3-5] have shown that the initiation of the fatigue crack is environmentally dependent. The presence of corrosion pits at fracture initiation sites observed in the present study lends support to this conclusion. In addition, wire smoothness is expected to play an important role; alloys with 0 and 3.5 at% Cr showed periodic shallow necking along their lengths, leading to stress concentrations and premature fatigue failure. Olofinjana and Davies [3] suggested that the enhancement of fatigue strength due to Cr addition could not be attributed to the effect of the Cr on the intrinsic mechanical properties alone, since the fatigue life does not scale with the static strength, whose increase with Cr content is relatively modest. It was assumed [3,4] that fatigue life enhancement is probably largely due to a surface phenomenon and that fatigue crack initiation is suppressed by a Cr rich film. XPS studies [3,5] demonstrated that the spontaneous formation of a passive thin film formed of a Cr/Si hydrated oxide on the outside surface of the wires appeared to accelerate the improvement of fatigue strength through the enhancement of corrosion resistance. No change occurred in microhardness for any of the wires tested, which suggests that there was no softening or hardening during the process of fatigue. Fatigue experiments with σm = 200 and 900 and 1550 MPa were carried out on the DP machine only since tensioning load stresses of 900 MPa or lower could not be used in the SP machine. In contrast, the DP (a) (b) Figure 5.- Tensile fracture morphologies for (a) wire and (b) ribbon amorphous alloys. 0 2 0 4 0 6 0 8 0 0 2 4 6 8 X at % Cr K I (MPa m 1/2 )
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