الفهرس 
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Abstract
The influence of α-phase morphology (equiaxed and lamellar) on the mechanical characteristics,
cycle oxidation, and hot corrosion behavior of Ti-6Al-3Mo-2Nb-2Zr-2Sn-1.5Cr (TC21) alloy was
investigated. The morphology and distribution of microstructure constituents were characterized
by optical microscopy and scanning electron microscopy (SEM). The thermal oxidation process
was applied on two microstructures (equiaxed and lamellar), where the mechanical properties
(hardness and wear) and hot corrosion of the formed protective oxide layers were investigated.
The oxidation process was applied at temperatures 600, 700, 800, and 900 °C for 5, 20, and 50 h.
NaCl-induced hot corrosion testing was carried out on raw (un-oxidized) and oxidized samples at
600 °C & 800°C for 50 h each. The cyclic testing was performed at 600 °C for durations of 5, 10,
20, 30, 40, and 50 h.
In addition, constituent phase compositions and elements map concentration were determined with
energy-dispersive spectroscopy (EDS). The phases in the oxide layer and corrosion products after
different treatments were identified by the X-ray diffraction (XRD) technique. The initial
microstructure consists of equiaxed α-phase and intergranular β-grains. Result of heat treatment
reveal that the microstructure changed to lamellar, consisting of large grains of β-phase containing
colonies of different orientations. The results also showed that enhanced wear resistance and
microhardness is increased due to the microstructure change from equiaxed to lamellar. Surface
characterization demonstrates that the β-phase is more resistant to corrosion than the α-phase
owing to the higher concentration of Nb, Mo, and Zr.
During cycle oxidation at 600℃ for 50 h, the formed oxide layer was increased thick with time.
The weight gain of equiaxed and lamellar microstructures samples was 0.3 and 0.28 mg/cm2 after
oxidation, where the difference in weight change was tiny. After NaCl hot corrosion test, the
weight loss of equiaxed and lamellar microstructures samples was 13.2 and 6.4 mg/cm2
. Also, after
NaCl + Na2SO4 hot corrosion test, the weight change of equiaxed and lamellar microstructures
samples was -0.88 and 0.07 mg/cm2
, respectively. The main corrosion products were rutile TiO2,
Na4Ti5O12, NaTiS2, and Ti6O, as well as a small amount of NaCl, as identified by XRD on the
corroded surfaces depending on the sprayed salt type.
The layer’s average thickness formed during thermal oxidation is increased with increased
oxidation time and temperature. A thin oxide layer (average 0.16 µm) was generated by oxidation
at a temperature of 600 °C for a duration of 5 h, and a large oxide layer of 9.7 µm thickness was
formed at 800 °C for 50 h duration. The most significant surface hardness was 1000 ± 151 HV0.05,
produced for the layer oxidized at 900 °C. On the other hand, the lowest hardness of 342 ± 20
HV0.05 and 360 ± 15 HV0.05 was recorded for the equiaxed and lamellar samples. Best wear
resistance had been achieved for samples oxidized at 800 °C.
During NaCl hot corrosion test, the weight loss of the raw (unoxidized) sample with lamellar
structure was 6.4 mg/cm2 due to the flaking of the corrosion product. However, for samples
oxidized at 600 °C for 50 h, weight loss after corrosion testing was 1.7 mg/cm2
, less than that for
the sample before corrosion. Oxidized samples at 800 °C exhibited the best mechanical
characteristics and corrosion resistance. NaCl and NaCl + Na2SO4 hot corrosion salt were applied
on raw (un-oxidized) and oxidized samples at 600 and 800 °C for 50 h. The hot corrosion testing
was carried out at 600 °C for 5 cycles with 10 h steps. In the best oxide layer, compact and adherent
layers are observed at 800 °C. The surface hardness layer oxide at 800 °C was 900 ± 60 HV0.05
due to the formation of hard oxides as TiO2 and Al2O3. During NaCl hot corrosion test, the weight
loss was significant for all samples except the oxidized at 800 °C for 5 h due to the flaking off of
the corrosion product and oxide layer. In the NaCl + Na2SO4 hot corrosion test, weight gain was
observed at 600 and 800°C for 5h. The weight loss was observed at raw (lamellar) and oxidized at
800°C for 20 and 50 h, where the oxide layer was flaked off. The surface hardness was changed
under a hot corrosion test due to the formation of brittle scale as TiO2 and Na4Ti5O12. The samples
oxidized at 800 °C for 5 h gave a combination of the best mechanical characteristics and corrosion
resistance.
Keywords: TC21 Ti-alloy, Thermal oxidation, NaCl + Na2SO4 corrosion, NaCl corrosion,
Hardness, Wear