702 research outputs found
Work hardening behavior in a steel with multiple TRIP mechanisms
Transformation induced plasticity (TRIP) behavior was studied in steel with
composition Fe-0.07C-2.85Si-15.3Mn-2.4Al-0.017N that exhibited two TRIP
mechanisms. The initial microstructure consisted of both {\epsilon}- and
{\alpha}-martensites with 27% retained austenite. TRIP behavior in the first 5%
strain was predominately austenite transforming to {\epsilon}-martensite (Stage
I), but upon saturation of Stage I, the {\epsilon}-martensite transformed to
{\alpha}-martensite (Stage II). Alloy segregation also affected the TRIP
behavior with alloy rich regions producing TRIP just prior to necking. This
behavior was explained by first principle calculations that revealed aluminum
significantly affected the stacking fault energy in Fe-Mn-Al-C steels by
decreasing the unstable stacking fault energy and promoting easy nucleation of
{\epsilon}-martensite. The addition of aluminum also raised the intrinsic
stacking fault energy and caused the {\epsilon}-martensite to be unstable and
transform to {\alpha}-martensite under further deformation. The two stage TRIP
behavior produced a high strain hardening exponent of 1.4 and led to ultimate
tensile strength of 1165 MPa and elongation to failure of 35%.Comment: submitted to Met. Mater. Trans. A manuscript E-TP-12-953-
The effect of microstructural scale on hardness of MoSi2-Mo5Si3 eutectics
Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/31005/1/0000680.pd
Processing, microstructure, and elevated temperature mechanical properties of MoSi2 containing Er2Mo3Si4 and Er2O3 particles
Powders of MoSi2 containing Er2Mo3Si4 and Er2O3 particles were produced by abll milling rice-melted buttons of MoSi2 containing 20 vol.% Er2Mo3Si4. Two composites with grain diameters of 9 and 16 [mu]m were produced by hot pressing the powders to 98% of theoretical density at 1565 [deg]C and 1650 [deg]C respectively. Some evidence of mechanical alloying was observed, but the majority of the Er2Mo3Si4 and Er2O3 particles were situated on grain boundaries. Compressive decremental step-strain rate tests were performed in the homologous temperature range of 0.54 Tm to 0.7 Tm (1100-1400 [deg]C) for strain rates ranging from 5 x 10-4 s-1 to 1 x 10-6 s-1. Nominal values for the stress exponent, n, and the activation energy for creep, Q, were determined using a constitutive equation for power-law creep. Below 1200 [deg]C, creep was controlled by dislocation climb and glide mechanisms with n [approximate] 4.5 and Q 425 +/- 15 kJ mol-1. At 1300 [deg]C and above, the creep resistance was shown to be grain size dependent with creep resistance increasing with larger grain size.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/31129/1/0000026.pd
Damping behavior of incramute modified by the addition of erbium to eliminate room temperature aging
Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/27678/1/0000061.pd
Effect of microstructure on the cyclic response and fatigue behavior of an XDTM aluminum metal matrix composite
Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/29121/1/0000160.pd
Atomic self-interaction correction for molecules and solids
We present an atomic orbital based approximate scheme for self-interaction
correction (SIC) to the local density approximation of density functional
theory. The method, based on the idea of Filippetti and Spaldin [Phys. Rev. B
67, 125109 (2003)], is implemented in a code using localized numerical atomic
orbital basis sets and is now suitable for both molecules and extended solids.
After deriving the fundamental equations as a non-variational approximation of
the self-consistent SIC theory, we present results for a wide range of
molecules and insulators. In particular, we investigate the effect of
re-scaling the self-interaction correction and we establish a link with the
existing atomic-like corrective scheme LDA+U. We find that when no re-scaling
is applied, i.e. when we consider the entire atomic correction, the Kohn-Sham
HOMO eigenvalue is a rather good approximation to the experimental ionization
potential for molecules. Similarly the HOMO eigenvalues of negatively charged
molecules reproduce closely the molecular affinities. In contrast a re-scaling
of about 50% is necessary to reproduce insulator bandgaps in solids, which
otherwise are largely overestimated. The method therefore represents a
Kohn-Sham based single-particle theory and offers good prospects for
applications where the actual position of the Kohn-Sham eigenvalues is
important, such as quantum transport.Comment: 16 pages, 7 figure
Evidence for orbital ordering in LaCoO3
We present powder and single crystal X-ray diffraction data as evidence for a
monoclinic distortion in the low spin (S=0) and intermediate spin state (S=1)
of LaCoO3. The alternation of short and long bonds in the ab plane indicates
the presence of eg orbital ordering induced by a cooperative Jahn-Teller
distortion. We observe an increase of the Jahn-Teller distortion with
temperature in agreement with a thermally activated behavior of the Co3+ ions
from a low-spin ground state to an intermediate-spin excited state.Comment: Accepted to Phys. Rev.
A study of the static yield stress in a binary Lennard-Jones glass
The stress-strain relations and the yield behavior of model glass (a 80:20
binary Lennard-Jones mixture) is studied by means of MD simulations. First, a
thorough analysis of the static yield stress is presented via simulations under
imposed stress. Furthermore, using steady shear simulations, the effect of
physical aging, shear rate and temperature on the stress-strain relation is
investigated. In particular, we find that the stress at the yield point (the
``peak''-value of the stress-strain curve) exhibits a logarithmic dependence
both on the imposed shear rate and on the ``age'' of the system in qualitative
agreement with experiments on amorphous polymers and on metallic glasses. In
addition to the very observation of the yield stress which is an important
feature seen in experiments on complex systems like pastes, dense colloidal
suspensions and foams, further links between our model and soft glassy
materials are found. An example are hysteresis loops in the system response to
a varying imposed stress. Finally, we measure the static yield stress for our
model and study its dependence on temperature. We find that for temperatures
far below the mode coupling critical temperature of the model (),
\sigmay decreases slowly upon heating followed by a stronger decrease as
\Tc is approached. We discuss the reliability of results on the static yield
stress and give a criterion for its validity in terms of the time scales
relevant to the problem.Comment: 14 pages, 18 figure
First principle study of cobalt impurity in bcc Fe with Cu precipitates
Abstract The addition of cobalt was experimentally observed to increase the strength and impact toughness of Cu precipitation hardened steel. In order to understand the mechanism of this strengthening, we studied the effect of cobalt in the bulks and surfaces of bcc Fe and bcc Cu, as well as at the Fe/Cu interface by ab initio density-functional approach. We investigated the cobalt distribution between the Fe matrix and Cu precipitate, and found that cobalt is rejected from the core of the Cu particle. The calculated elastic constants and stacking fault energies show that cobalt does not produce any solid solution softening or hardening in bcc Fe. However, cobalt segregated in the interfacial region increases the cleavage fracture energies and cleavage stress of the Fe/Co/Cu interface. The compressive stress, which arises near the interface due to strong Fe-Co bonds, may serve as a barrier for dislocation motion through the interface resulting in additional hardening
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