Nanoscale austenite reversion through partitioning, segregation, and kinetic freezing: Example of a ductile 2 GPa Fe-Cr-C steel

Austenite reversion during tempering of a Fe-13.6Cr-0.44C (wt.%) martensite results in an ultrahigh strength ferritic stainless steel with excellent ductility. The austenite reversion mechanism is coupled to the kinetic freezing of carbon during low-temperature partitioning at the interfaces between martensite and retained austenite and to carbon segregation at martensite-martensite grain boundaries. An advantage of austenite reversion is its scalability, i.e., changing tempering time and temperature tailors the desired strength-ductility profiles (e.g. tempering at 400{\deg}C for 1 min. produces a 2 GPa ultimate tensile strength (UTS) and 14% elongation while 30 min. at 400{\deg}C results in a UTS of ~ 1.75 GPa with an elongation of 23%). The austenite reversion process, carbide precipitation, and carbon segregation have been characterized by XRD, EBSD, TEM, and atom probe tomography (APT) in order to develop the structure-property relationships that control the material's strength and ductility.Comment: in press Acta Materialia 201

The Shape of Covariantly Smeared Sources in Lattice QCD

Covariantly smeared sources are commonly used in lattice QCD to enhance the projection onto the ground state. Here we investigate the dependence of their shape on the gauge field background and find that the presence of localized concentrations of magnetic field can lead to strong distortions which reduce the smearing radii achievable by iterative smearing prescriptions. In particular, as $a\to 0$, iterative procedures like Jacobi smearing require increasingly large iteration counts in order to reach physically-sized smearing radii $r_{sm}\sim$ 0.5 fm, and the resulting sources are strongly distorted. To bypass this issue, we propose a covariant smearing procedure (``free-form smearing'') that allows us to create arbitrarily shaped sources, including in particular Gaussians of arbitrary radius.Comment: 1+15 pages, 7 figures (24 pdf images

Improved interpolating fields for hadrons at non-vanishing momentum

We demonstrate that a reduction in the noise-to-signal ratio may be obtained for hadrons at non-zero momenta whilst maintaining a good overlap with the ground state through a generalisation of Gaussian/Wuppertal smearing. The use of an anisotropic smearing wavefunction is motivated by the physical picture of a boosted hadron.Comment: 7 pages, 6 figures, poster presented at the 30th International Symposium on Lattice Field Theory (Lattice 2012), Cairns, Australia, June 24-29, 201

Nucleon axial form factors from two-flavour Lattice QCD

We present preliminary results on the axial form factor $G_A(Q^2)$ and the induced pseudoscalar form factor $G_P(Q^2)$ of the nucleon. A systematic analysis of the excited-state contributions to form factors is performed on the CLS ensemble `N6' with $m_\pi = 340 \ \text{MeV}$ and lattice spacing $a \sim 0.05 \ \text{fm}$. The relevant three-point functions were computed with source-sink separations ranging from $t_s \sim 0.6 \ \text{fm}$ to $t_s \sim \ 1.4 \ \text{fm}$. We observe that the form factors suffer from non-trivial excited-state contributions at the source-sink separations available to us. It is noted that naive plateau fits underestimate the excited-state contributions and that the method of summed operator insertions correctly accounts for these effects.Comment: 7 pages, 12 figures; talk presented at Lattice 2014 -- 32nd International Symposium on Lattice Field Theory, 23-28 June, 2014, Columbia University New York, N

Nucleon electromagnetic form factors in two-flavour QCD

We present results for the nucleon electromagnetic form factors, including the momentum transfer dependence and derived quantities (charge radii and magnetic moment). The analysis is performed using O(a) improved Wilson fermions in Nf=2 QCD measured on the CLS ensembles. Particular focus is placed on a systematic evaluation of the influence of excited states in three-point correlation functions, which lead to a biased evaluation, if not accounted for correctly. We argue that the use of summed operator insertions and fit ans\"atze including excited states allow us to suppress and control this effect. We employ a novel method to perform joint chiral and continuum extrapolations, by fitting the form factors directly to the expressions of covariant baryonic chiral effective field theory. The final results for the charge radii and magnetic moment from our lattice calculations include, for the first time, a full error budget. We find that our estimates are compatible with experimental results within their overall uncertainties.Comment: 22 pages, 10 figures, citations modifie