Study of the fatigue behaviour of some Fe-based metallic glass wires.

Fe78_C.,r xSiioBj2 and Fe77S. xCr-,S i7S. Bjs (with x values ranging from 0 to 8) glassy
metallic alloys have been produced in the form of both wires and ribbons using the
rotating water bath melt spinning process and chill block melt spinning process,
respectively. It was found that the Fe77.5C.,r,, ,S ii. 5B15s eries had better amorphousw ire
forming ability than the Fe7g_XCrXSiloBs1e2ri es. However, in ribbon form, both series
showed good castability as amorphous alloys. Selected mechanical properties of the
glassy samples have been compared with those of high tensile steel wire.
Both thermal and mechanical properties were found to be composition-dependent,
especially on the Cr content, with the Fe77.X5.C rrSi7.5B1s5e ries wires being thermally
more stable and strongert han their Fe7gX. CrrSiloB12c ounterparts.
A new bend type fatigue testing machine that uses two pulleys (DP) has been
developed to perform tests on the basis of tension-compression loading cycles. The
results have been compared with those obtained by using an existing single pulley (SP)
machine that performs only tensile-tensile loading cycles. Although the DP machine can
impose compressive to tensile bend stresses at different constant mean stress, it was
observed that, for low stress ranges and large constant mean stresses the machine
performed equivalent bend tests to those using the SP machine. The fatigue
performance of amorphous wires was improved on substituting Fe by Cr and on
decreasing the constant mean stress. The high tensile (HT) steel wire apparently had
better fatigue performance than amorphous wires in terms of stress range; however,
when the results were expressed in terms of strain range, the fatigue performance of
amorphous wires was slightly better than HT steel wires, as had been demonstrated in
previous studies.
It was observed that, when the DP machine performed compressive to tensile
bending stress cycles, the fracture surfaces differed from those observed in the SP
machine. Under tensile to tensile conditions, the fracture surfaces observed for samples
tested in the DP machine were similar to those tested in the SP machine. Fracture
mechanics analysis of the results suggests that embrittlement is occurring ahead of
growing fatigue cracks in these amorphous wires.