| Summary: | Cavitation erosion is a major problem encountered in hydraulic machinery
components. Presently, solution to the problem is sought through a two-fold
approach of machine design and application of cavitation resistant materials.
Stainless steels are commonly used for cavitation protection. Cobalt based alloys
(stellites) and NiTi intermetallic compound are the best materials at present
known to resist cavitation erosion. Physical and mechanical properties of
various tested materials do not seem to correlate well with the cavitation erosion
resistance of these materials.
The present study was done to investigate the cavitation erosion resistance of the
intermetallic nickel aluminides Ni₃Al and NiAl. An ultrasonic (20 KHz)
vibratory test apparatus was used to conduct tests with a modification to the
ASTM G-32-92 method. The cavitation intensity and the surface finish were
varied, from the specification, to investigate their effects on the cavitation erosion
resistance of NiAl. Scanning electron microscopy and X-ray diffraction methods
were used as aids in analyzing the results.
Unalloyed stoichiometric Ni₃Al was found to exhibit a higher cavitation erosion
resistance that boron (0.2 wt. %) doped Ni₃Al. Alloying with boron imparts
room temperature ductility to the otherwise brittle Ni₃Al compound.
Investigations of deviation from stoichiometry were conducted on NiAl in the compositional range of 47 - 62.2 at. % Ni in this work. The erosion rate was
found to decrease monotomically with increasing nickel content, in the range
tested. Strain induced martensitic transformation was found to be associated
with the compositions which showed the low cavitation erosion rates. Thermally
induced martensite which was present in these compositions, was found to be
detrimental to the cavitation erosion resistance of NiAl.
NiAl compositions of 58 at. % Ni and 62.2 at. % Ni showed cavitation erosion
resistance comparable to that of the best known intermetallic NiTi. However, in
terms of materials processing NiAl offers advantages over NiTi. Hence NiAl
could find a niche as an industrially important material for cavitation protection
of hydraulic machinery. The most desirable NiAl compositions exhibit unusual
but desirable variations of erosion rate with cavitation intensity. Further studies
are needed for a better understanding of this phenomenon. === Applied Science, Faculty of === Materials Engineering, Department of === Graduate
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