Superalloys based on platinum-group metals (PGMs) are being developed for high
temperature applications. Currently, the optimum alloy is Pt84:Al11:Ru2:Cr3, and work is
ongoing. Niobium is a possible addition to increase the melting point, but limited phase
diagram data are available. Although work has been done on the Pt-Al-Nb system, there are
no reported data for Pt-Cr-Nb.
This is a study of the Pt-Cr-Nb system. As-cast samples of the Pt-Cr-Nb ternary system were
investigated using scanning electron microscopy with energy-dispersive X-ray spectroscopy,
and X-ray diffraction. The results were used to plot a solidification projection and all binary
phases extended into the ternary, except for α´Pt, which was not identified in this
investigation. The extensions of the binary phases were: (Cr) ~2 at.% Nb; (Pt) ~24 at.% Nb
and 65 at.% Cr; (Nb): ~12 at.% Pt and ~17 at.% Cr; ~NbPt2: ~20 at.% Cr; ~NbCr2: ~18
at.% Pt; ~Cr3Pt: ~10 at.% Nb; ~βNbPt3: ~4 at.% Cr and ~Nb1-XPt1+X: ~13 at.% Cr. Five
ternary phases were identified: τ1:~Nb17:Cr64:Pt19; τ2:~Nb28:Cr55:Pt17; τ3:~Nb30:Cr30:Pt40;
τ4:~Nb45:Cr27:Pt28 and τ5:~Nb40:Cr18:Pt42. The liquidus surface was derived using the binary
phase diagrams, identified primary phases, solidification sequences and the solidification
projection, together with the eutectic compositions. Nineteen ternary invariant reactions were
identified.
Hardness measurements were made on all samples, and also toughness and fracture
toughness were deduced. Alloys with (Pt) and ~NbPt2 were ductile with reasonable hardness.
Alloys containing (Cr), (Nb), ~Cr3Pt, ~NbCr2, ~Nb3Pt and ~Nb2Pt were extremely brittle with
cracks, whereas alloys containing ~βNbPt3 and ~Nb1-XPt1+X phases were slightly brittle.
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:wits/oai:wiredspace.wits.ac.za:10539/7422 |
| Date | 06 November 2009 |
| Creators | Mulaudzi, Fulufhelo Marandela Lloyd |
| Source Sets | South African National ETD Portal |
| Language | English |
| Detected Language | English |
| Type | Thesis |
| Format | application/pdf, application/pdf, application/pdf |
Page generated in 0.0157 seconds