Date of Award

2013-01-01

Degree Name

Doctor of Philosophy

Department

Material Science and Engineering

Advisor(s)

Lawrence E. Murr

Abstract

Gamma TiAl has been considered to be an ideal replacement for Ni-base superalloy aerospace applications, particularly in weight-critical applications. While bulk, fully dense gamma-TiAl is 50% less dense than Ni-base superalloys, it also has excellent mechanical properties at high temperature (up to ~800°C). Pre alloyed (Ti-48Al-2Cr-2Nb) powder was used to fabricate solid, fully dense (3.85 g/cm3) components and foam components with densities ranging from 0.33 g/cm3 to 0.46 g/cm3 by electron beam melting (EBM). A 10:1 blend of 2-phase TiAl (Ti-48Al-2Cr-2Nb in a/o) : Inconel 625 (Ni-22Cr-6Mo-2Nb in a/o) pre-alloyed powders produced a complex alloy having the composition 44Ti-39Al-7Ni-4Nb-4Cr-2Mo (in a/o). Solid, reticulated mesh samples and stochastic foam samples were fabricated by electron beam melting (EBM) using this blended alloy.

The microstructures of the solid and open-cellular components were observed by optical metallography (OM) and transmission electron microscopy (TEM), and confirmed by XRD analysis. The microstructure and residual hardness for solid components of 2-phase TiAl and Inconel 625 fabricated by EBM were compared to that of the blended alloy. The 2-phase TiAl alloy exhibited a duplex, equiaxed γ (TiAl) grain structure (15 μm) with lamellar colonies characterized by thin α2 (Ti3Al) plates ~20 nm thick having an orientation relationship: (111)γ||(0001)α2. The hardness (HV) of the blended alloy reached 7.5 GPa compared to 1.4 GPa for the Alloy 625 and 4.0 for the 2-phase TiAl alloy. Relative stiffness versus relative density values were plotted on a log-log basis for the reticulated mesh and stochastic foam samples for the blended alloy and were consistent with other alloys fitted to a straight line with a slope n=2 for ideal open cellular materials.

Language

en

Provenance

Received from ProQuest

File Size

98 pages

File Format

application/pdf

Rights Holder

Jennifer Hernandez

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