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-

Ground state and constrained domain walls in Gd/Fe multilayers

The magnetic ground state of antiferromagnetically coupled Gd/Fe multilayers and the evolution of in-plane domain walls is modelled with micromagnetics. The twisted state is characterised by a rapid decrease of the interface angle with increasing magnetic field. We found that for certain ratios M(Fe):M(Gd), the twisted state is already present at low fields. However, the magnetic ground state is not only determined by the ratio M(Fe):M(Gd) but also by the thicknesses of the layers, that is the total moments of the layer. The dependence of the magnetic ground state is explained by the amount of overlap of the domain walls at the interface. Thicker layers suppress the Fe aligned and the Gd aligned state in favour of the twisted state. Whereas ultrathin layers exclude the twisted state, since wider domain walls can not form in these ultrathin layers

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

Theory of the thermoelectricity of intermetallic compounds with Ce or Yb ions

The thermoelectric properties of intermetallic compounds with Ce or Yb ions are explained by the single-impurity Anderson model which takes into account the crystal-field splitting of the 4{\it f} ground-state multiplet, and assumes a strong Coulomb repulsion which restricts the number of {\it f} electrons or {\it f} holes to $n_f\leq 1$ for Ce and $n_f^{hole}\leq 1$ for Yb ions. Using the non-crossing approximation and imposing the charge neutrality constraint on the local scattering problem at each temperature and pressure, the excitation spectrum and the transport coefficients of the model are obtained. The thermopower calculated in such a way exhibits all the characteristic features observed in Ce and Yb intermetallics. Calculating the effect of pressure on various characteristic energy scales of the model, we obtain the $(T,p)$ phase diagram which agrees with the experimental data on CeRu$_{2}$Si$_2$, CeCu$_{2}$Si$_2$, CePd$_{2}$Si$_2$, and similar compounds. The evolution of the thermopower and the electrical resistance as a function of temperature, pressure or doping is explained in terms of the crossovers between various fixed points of the model and the redistribution of the single-particle spectral weight within the Fermi window.Comment: 13 pages, 11 figure

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

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