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No description available.

No description available.

No description available.

No description available.

There are numerous reports of attempts to deposit thick coatings of chromium-nickel and chromium-nickel-iron and although high quality deposits have been obtained these have been limited to very thin coatings only. The purpose of the present work was to identify and understand the controlling factors preventing continuous deposition and the build-up of thick coatings, and to develop a suitable process for such thick coatings. An amide electrolyte system was chosen asworking model for its relative simplicity and the high quality chromium-nickel deposits obtained from this system. This bath was investigated initially using conventional operating techniques (Constant Current) to establish its general characteristics. High quality deposits were obtained for thin coatings only. The need for a major change of operating philosophy was identified as essential. A number of published hypotheses explaining the failure of continuous deposition were experimentally tested and shown not to apply. Electrolytically generated chromium (II) has been identified to be an important controlling factor on the quality and composition of chromium-nickel alloys codeposited from amide electrolytes. The control of the low transient chromium (II) level, necessary for the maintenance of high quality codeposition with constant composition, can be achieved as follows. A controlled potential is required, in combination with circulation and agitation of the electrolyte and the generation of the chlorine gas as the preferred anodic product. The combination of the three techniques is necessary for the control of chromium (II) and if any of the three techniques are not employed sustained deposition fails. Chromium (II) has further been identified to catalyse an electrochemical ageing of the bath. Reproducible high quality true metallic deposits of constant composition of chromium-nickel and chromium-nickel-iron can be obtained to a thickness of several hundred microns. These have shown to display a laminar structure on etching and possible causes have been considered. Mossbauer spectroscopy and x-ray diffraction have shown that chromium-nickel-iron deposits contain a significant quantity of micro-crystalline austenite phase ~, fcc) with a dominant ferrite (e<, bcc) phase.

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.

published_or_final_version / Chemistry / Doctoral / Doctor of Philosophy

published_or_final_version / Mechanical Engineering / Doctoral / Doctor of Philosophy

published_or_final_version / Physics / Master / Master of Philosophy