Johan Börjesson

Improved cationic stoichiometry and insulating behavior at the interface of LaAlO3/SrTiO3 formed at high oxygen pressure during pulsed-laser deposition

Medium-energy ion spectroscopy, MEIS, and scanning transmission electron microscopy, STEM, were used to correlate the atomic structure of LaAlO3/SrTiO3 interfaces with their electrical properties. Interfaces were prepared at high (5x10(-2) mbar) and low (10(-4) mbar) oxygen pressure by pulsed-laser deposition. The high-oxygen-pressure heterostructures were insulating for all thicknesses while the low-oxygen-pressure ones became metallic for thicknesses above 4 unit cells. MEIS data show enhancement of the Sr surface peak and suppression of the La one in interfaces prepared at low oxygen pressure, which is interpreted as a La-Sr intermixing. The effect was considerably smaller in high-oxygen-pressure samples. Analysis of high-angle annular-dark-field STEM images of the LAO films also indicates intermixing between La and Sr in low-oxygen-pressure samples, supporting MEIS data. Our results reveal the important role of oxygen pressure on the formation of the interface electron gas. Copyright (C) EPLA, 2011

Orientation relationships of intragranular austenite in duplex stainless steel weld metals

Abstract Formation and characteristics of fine intragranular austenite were studied for low energy input duplex stainless steel welds. Microstructures were largely ferritic with some allotriomorphic grain boundary austenite, Widmanstätten type austenite, fine intragranular austenite and nitrides. Electron backscattered diffraction analysis revealed that grain boundary austenite had a random orientation relationship (OR) with one of the adjacent ferrite grains and was close to Kurdjumov–Sachs (KS) with the other, whereas Widmanstätten austenite always showed an OR near KS. The finest intragranular austenite was mainly randomly oriented, whereas coarser austenite more often was close to KS. It is argued that the OR of intragranular austenite with the ferritic matrix is governed by a combination of composition, determining driving force for nucleation at temperature, cooling rate and the availability of nitrides acting as nucleation sites. A random OR is most likely for higher cooling rates and compositions promoting nucleation at lower temperatures.

Role of inclusions in formation of high strength steel weld metal microstructures

Abstract A series of high strength weld metals with varying Al content are studied. The inclusions are characterised using energy dispersive X-ray analysis and electron diffraction. The tendency for alignment of the microstructure is characterised quantitatively using electron backscatter diffraction and a recently developed post-processing technique. Correlation is found between the inclusion phases present and the amount of aligned neighbouring grains in the microstructure. It is shown that amorphous Si–Al oxides form at low Al weld metal contents and an Mg–Al spinel at higher contents. The former is associated with less alignment of the microstructure and therefore higher impact toughness. The effect of these inclusions on the formation of the microstructure is discussed.

A new EBSD based methodology for the quantitative characterisation of microstructures formed by displacive fcc-bcc transformations.

This work is concerned with a new methodology that can be used to quantify the degree to which grains in the microstructure are aligned in the form of packets. The methodology is based on a crystallographic definition of the term packet which is used to deduce the theoretically ideal misorientations of intra-packet grain boundaries. A misorientation distribution obtained from extensive EBSD mapping can thus be split into intra- and inter-packet misorientations and the corresponding fractions can be determined by integration. The theoretical framework of the methodology is explained and a step-by-step description of the procedure is given. Results from a trace analysis are provided to justify the assumptions made regarding habit plane and examples are included showing how the grain boundary network can be split into two separate parts, one for lath boundaries and the other for packet boundaries. Moreover, example weld metal microstructures along with the corresponding misorientation distributions as well as quantitative values of the microstructures are presented.

Effects of Alloying Concepts on Ferrite Morphology and Toughness of Lean Duplex Stainless Steel Weld Metals

Alloying concepts for lean duplex weld metals have been explored. Focus was on compositions with 21.5–24 % Cr with different levels of Ni and Mn in combination with N producing a minimum PREn value of 26. Microstructures and properties of weld metals produced with experimental covered electrodes or metal-cored wires were evaluated. The electron backscattered diffraction (EBSD) technique combined with scanning electron microscopy (SEM) was used to explore relations between weld metal morphology and crystallographic orientation relationships between ferrite and austenite. A duplex microstructure with Widmanstätten type ferrite characteristic of a ferritic solidification was found throughout most weld metals. Regions with a morphology suggesting a mixed ferritic-austenitic solidification occurred particularly for higher Ni- and N-contents. EBSD studies showed that most of the austenite formed following a ferritic solidification was near either the Nishiyama-Wasserman (NW) or Kurjdumov-Sachs (KS) relationships with adjacent ferrite. More phase boundaries in the mixed mode solidification regions had a random orientation relationship. Strength, ductility and PREn requirements were readily matched but the combined Mn, Ni and N alloying levels and the ferrite morphology affected toughness significantly. Both a fully ferritic solidification and a sufficient Ni-content were necessary to produce acceptable and reproducible impact toughness at room and sub-zero temperatures. It was concluded that a composition of 23–24 % Cr, 7–8 % Ni and 0.12–0.16 % N was well suited to fulfil requirements.

Atomic structure of functional interfaces in Sr2RuO4/Sr3Ru2O7 eutectic crystals

Sr2RuO4/Sr3Ru2O7 eutectic system is investigated by transmission electron microscopy (TEM) and high angle annular dark field scanning TEM (STEM). The Sr2RuO4/Sr3Ru2O7 lamellar structure allows studying tunneling and proximity effects between spin-triplet superconductor Sr2RuO4 and metamagnetic normal metal Sr3Ru2O7. Our analyses reveal two typologies of interfaces within the eutectic: interfaces parallel to the growth direction are sharp and defect-free whereas interfaces perpendicular to the growth direction appear wavy and decorated with Ru precipitates. These results indicate that interfaces parallel to the growth direction are the best candidates for Sr2RuO4/Sr3Ru2O7 natural junctions to study the unusual superconductivity of Sr2RuO4

Effect of various deposition conditions on the electrical properties of LAO/STO hetero interfaces

We have examined the effects of partial oxygen pressure and laser energy density on the electrical transport properties of thin LAO films grown on (100) TiO2-terminated SrTiO3 substrates. Films were grown by pulsed laser deposition monitored by in-situ reflection high-energy electron diffraction (RHEED). Layer-by-layer growth, as indicated by clear RHEED oscillations, can be obtained in a wide range of oxygen partial pressures from 10−6 to 5×10−2 mbar. Transmission electron microscopy (TEM) analysis shows that the interface is coherent and atomically sharp for all deposition conditions. The STO substrate is oxygen self reduced at an oxygen pressure of 10−6 mbar and the electrical properties of the interface are dominated by the presence of oxygen vacancies. By increasing the oxygen pressure above 10−4 mbar, the substrate itself is insulating but the interface still shows metallic conductivity. However, the interface becomes insulating at an oxygen pressure of 5×10−2 mbar. We also found that the interface exhibits insulator-to-metal transition by changing the laser fluence during the deposition of the film. The interface prepared at 5×10−2 mbar shows metallic conductivity at high fluence, above 3.5 J/cm2.

Inhomogeneous Microstructure and Electrical Transport Properties at the LaAlO3/SrTiO3 Interface

Medium-energy ion spectroscopy (MEIS), scanning transmission electron microscopy (STEM) and X-ray photoemission spectroscopy (XPS) were used to investigate the composition and microstructure of LaAlO3/SrTiO3 (LAO/STO) interfaces grown by pulsed laser deposition of LAO on TiO2-terminated STO substrates under different oxidizing conditions. MEIS and XPS indicated Sr/La and Al/Ti intermixing within several atomic layers at all studied interfaces. XPS and STEM revealed that La diffuses deeper than Al. Analysis of the MEIS data suggests inhomogeneous lateral distribution of the diffused elements. This is further supported by the observation of a large positive magneto-resistance at low temperatures. We discuss the role of lateral inhomogeneities on the formation of the electron gas at the LAO/STO interface.