Abstract
Literature on the processing, microstructure and properties of NbSi2 and other disilicides have been briefly reviewed, along with earlier work on the alloying behaviour of NbSi2 and the oxidation of both niobium and niobium silicides. Solidification microstructures and phase selection in ingots and melt spun ribbons of binary and ternary NbSi2 based alloys with additions of Cr and Fe have been studied. Low levels of interstitial contamination were achieved in all alloys (< 200wppmO2, < 10wppmH2) using a cold hearth non-consumable tungsten arc melting technique. The following alloys were prepared (at%): Nb-67Si, Nb-58Si, Nb-67Si-8Cr, Nb-60Si-20Cr, Nb-67Si-8Fe and Nb-60Si-20Fe. Evaluation of the microstructure and properties was performed using optical microscopy, SEM, EPMA,TEM,XRD, DSC/TG, isothermal oxidation, room temperature hardness, microhardness, high temperature hardness and nano-indentation. Alloys containing increased levels of Nb or Fe additions exhibited less porosity and cracking in their as-cast structures and lower melt viscosities in comparison to the other alloys. Extensive solubility of Cr in the NbSi2 matrix and of Nb in the CrSi2 matrix was noted and upon heat treatment widescale partitioning between the two phases was evident. Increased Cr additions led to the formation of the ternary phase Nb2Cr3Si6. In contrast the solubility of Nb in both FeSi and FeSi2 was extremely limited along with that of Fe in NbSi2. Rapid solidification refined the microstructures of the alloys by two orders of magnitude. Suppression of Nb5Si3 and NbFeSi2 took place under RS conditions. Segregation free microstructures were formed in zone A of Nb-67Si ribbons. The addition of either Cr or Fe destabilised the S/L front in the ternary alloys preventing the formation of zone A in the ribbons of these alloys. RS processing suppressed the eutectoidal decomposition of betaNb5Si3 to alphaNb5Si3 and the peretectoidal transformation of alphaFeSi2 to betaFeSi2. Three oxidation regimes were identified in binary Nb-67Si, which were associated with the oxidation kinetics of the various niobium oxide forms (NbO2, alphaNb2O5 and betaNb2O5). Pesting of the alloy occurred between 823 and 1123K, but was reduced in the ternary alloys and fully suppressed in Nb-60Si-20Cr. Duplex oxide structures were formed on the ternary alloys consisting of SiO2 and Cr2O3 layers in the Nb-Si-Cr alloys and two silicon oxide layers containing iron oxide particles in the Nb-Si-Fe alloys. Additions of Cr and Fe increased the hardness of the alloys. This was attributed to the volume fraction of the Fe and Cr silicide phases with FeSi and CrSi2 being harder than NbSi2. The elastic moduli of NbSi2 and Nb5Si3 were also measured using the nano-indentation technique. The hardness of Nb-67Si and Nb-67Si-8Cr were retained up to 1173K whilst that of the other ternary alloys decreased rapidly above 950K.