Anomalously Ductile Intermetallic Compounds
Personnel: K. A. Gschneidner,
Jr.a, A. M. Russella, T.
A. Lograssoa, J.
R. Morrisa, A. O. Pecharskya,
B. N. Harmonb, D. K. Hsuc and
C. H. C. Loc
aMaterials and Engineering Physics
Program, Ames Laboratory, Iowa State University
bCondensed Matter Physics Program,
Ames Laboratory, Iowa State University
cCenter for Nondestructive Evaluation,
Institute for Physical Research and Technology, Iowa State University
Abstract:
Recently we observed extraordinarily high ductility in some stoichiometric
CsCl-type RM intermetallic compounds at room temperature. Ductilities of >20%
have been found, which is unprecedented for intermetallic compounds. The
few measurements made to date are described.
Recent Results:
Initial quantitative mechanical tests involved a series of compressive tests
on ~4 mm diameter x 12 mm high samples of a stoichiometric, fully ordered
CsCl-type structure RM intermetallic compound. Three specimens were subjected
to true strains of 10 to 20% at a strain rate of 2.8 x 10-4s-1. The deformation
was terminated at ~20% true strain since beyond this point the constraints
at the specimens end could negatively affect the deformation behavior. Figure
1 shows the comparison of the deformed specimens at 14.1% (b) and 20.5% (c)
true strains with the undeformed specimen (a). The most remarkable feature
is the fact that although RM was deformed to >20% true strain, no macroscopic
cracks were generated. This is rather unusual for a stoichiometric, fully
ordered intermetallic compound with B2 crystal structure as compared to similar
alloys such as FeAl, CoTi and NiAl
Figure
1 Figure
2
Fig. 1. Comparison of as received and deformed specimens: a) Undeformed, and b) and c) deformed specimens at 14.1 % and 20.5 % true strain, respectively.
Fig. 2. Tensile stress strain properties of a RM compound at 20°C (red curve) and a commercial aluminum alloy, 3105-H24 (blue curve).
Recently we measured the tensile stress-strain relationship in a RM phase. As shown in Fig. 2 this compound has an elongation of 20% at the onset of fracture and 27% at the final fracture at 20°C. Such large tensile ductility is unprecedented for a completely stoichiometric and fully ordered intermetallic compound. The tensile properties are comparable to those of a commercial aluminum alloy.
Another CsCl-type RM single crystal material was subjected to tensile tests. Specimens with [100], [111], and [112] tensile axes all failed in fracture without any noticeable elongation (i.e. brittle failure) at maximum fractures stresses of 129, 190 and 226 MPa, respectively. These are high stress levels for a single crystal. These results show that not all of the RM CsCl-type B2 phases are ductile compounds.
To further understand these new materials we have formed a team of scientists from the Materials and Engineering Physics and Condensed Matter Physics Programs of the Ames Laboratory, and from the Center for Nondestructive Evaluation, Institute for Physical Research and Technology (IPRT) to carry out both experimental and theoretical studies on a selected number of RM materials. A small amount of seed money from IPRT has enabled the team to make some of the preliminary measurements and first principle calculations.
Significance:
Stoichiometric intermetallic compounds are generally brittle at room temperature,
especially if they are completely ordered. They cannot be fabricated into wire,
thin sheets, etc. by standard metallurgical processes at room temperature;
they can be deformed only at high temperature. Furthermore, intermetallic compounds
are usually superior to conventional metal alloys (such as solid solution and
precipitation hardening alloys) in that they are typically stronger and stiffer
at elevated temperature and often provide better resistance to oxidation/corrosion
than conventional alloys. Since such high ductility is unheard of for a stoichiometric
intermetallic compound at room temperature, there must be a fundamental difference
in the electronic structure/bonding in these compounds compared to the typical
intermetallic compound.
Future Work:
Pending funding, future work would include: (1) mechanical property measurements
combined with optical metallography and electron microscopy; (2) low temperature
heat capacity and the elastic constants measurements; and (3) theoretical calculations
of the electronic structure and phonon modes of the selected materials used
in the experimental portion of this study. Total energy calculations will establish
the energy landscape and likely slip plane dynamics. Possible explanations
of the anomalously high ductility in these materials include:
- The RM compounds may possess larger numbers of active slip systems than are typical in other B2 intermetallic compounds. Room temperature slip is typically observed on just one of the following slip systems in other B2 compounds ({100}<110>, {100}<100>, {110}<111>, {211}<111>); two or more of these slip systems may be active in the RM compounds.
- The a/2<111> antiphase boundary energy may be unusually low in the RM compounds, allowing them to display the polycrystalline ductility of ordinary BCC metals.
- The room temperature brittleness of other B2 materials is partially attributable to the segregation of H at grain boundaries. The RM compounds may not suffer from this grain boundary embrittlement problem, allowing slip rather than intergranular cleavage to predominate at 20°C.
- The RM materials may possess novel twinning behavior that re-orients the active slip system(s) to more favorable Schmid factors during plastic deformation.
Interactions:
To determine whether one or more of these possible explanations account for
the high RM ductility, it will be necessary to compare theoretical predictions
of behavior with experimental measurements, including microscopic characterization
of these materials.
A critical part of this research is the preparation of well characterized polycrystalline
and single crystal specimens.
_____________________
Some ductility (~2%) has been observed
in transition metal 1:1 (B2) and 1:3 (Ll2,AuCu3) phases if they are non-stoichiometric
(i.e. an excess of one component and/or vacancies) or if they are atomically
disordered phases.
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Last update: February 19, 2008