Transformation and mechanical behavior of thermoelastic martensite in nickel-aluminum alloys
Kim, Do Young
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Permalink
https://hdl.handle.net/2142/21070
Description
Title
Transformation and mechanical behavior of thermoelastic martensite in nickel-aluminum alloys
Author(s)
Kim, Do Young
Issue Date
1990
Doctoral Committee Chair(s)
Wayman, C. Marvin
Department of Study
Materials Science and Engineering
Discipline
Materials Engineering
Degree Granting Institution
University of Illinois at Urbana-Champaign
Degree Name
Ph.D.
Degree Level
Dissertation
Keyword(s)
Engineering, Metallurgy
Engineering, Materials Science
Language
eng
Abstract
$\beta$-NiAl alloys represent potential structural materials for use at elevated temperatures because of their high melting point, low density, and resistance to oxidation and high temperature deformation. Also, nickel-rich $\beta$-NiAl alloys undergo high temperature martensitic transformation. This investigation focused on studying the transformation and the mechanical behavior of martensite in Ni-Al alloys for possible elevated temperature shape memory applications.
NiAl alloys were prepared by the power metallurgy method, using hot pressing and hot isostatic pressing techniques, and by vacuum arc melting. X-ray diffraction was used to determine the structure of NiAl martensite. The crystal structure of NiAl martensite is ordered face-centered tetragonal (L1$\sb0$). The transformation behavior was investigated by electrical resistivity and dilatometer measurements, and found to be thermoelastic. The M$\sb{\rm S}$ temperatures of the powder prepared NiAl alloys are, within experimental error, the same as those for cast and wrought alloys of the same chemical compositions. There is good agreement between the transformation temperatures determined by the dilatometer and electrical resistivity methods.
The 62.5 at. % Ni alloy could be deformed up to 5.7% by compression tests and 0.22% shape recovery was observed. The shape recovery begins at A$\sb{\rm S}$ and ends near the A$\sb{\rm f}$ temperature. Detwinning is the main deformation mode for the NiAl martensite. Applied stress also induces martensitic transformation above M$\sb{\rm S}$.
The fracture surfaces were examined by scanning electron microscopy and NiAl alloys exhibited brittle fracture behavior with intergranular and transgranular fracture surfaces.
The morphology of NiAl martensite was studied by transmission electron microscopy. The martensite was internal twins and the twin plane is determined as the $\{$111$\}$. The magnitude of the lattice invariant shear by twinning is determined by measuring the thickness of twin pairs and is 0.128. The structure and morphology of NiAl alloys with vanadium additions were also studied by transmission electron microscopy. The crystal structure if the L1$\sb0$ type and the c/a ratio decreases with increasing vanadium content. In addition to $\{$111$\}$ type twins, $\{$111$\}$ type stacking faults have been found in 0.5 at. % vanadium doped samples. For 5 at. % vanadium doped specimens, a high density of dislocations was observed. Analyzed dislocations are partials with Burgers vector b = 1/6($\overline{11}$2). By increasing the vanadium content, a modulated microstructure dominates while the twinned martensite disappears.
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