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 ($Tc = 0.435$), \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