Bengt Loberg

The Role of Alloy Composition on the Stability of Nitrides in Ti-Microalloyed Steels during Weld Thermal Cycles

The effect of weld thermal cycling on titanium nitride stability in a range of normalized Timicroalloyed steels containing various alloying additions of V, Al, and N has been investigated. Nitride dispersions and the chemical analysis of individual particles are studied using transmission electron microscopy and a quantitative 200 kV STEM-EDX-EELS microanalysis system. It is found that whereas the normalized material contains various nitrides of mixed compositions, only nitrides based on TiN survive high energy simulated weld cycles. Grain growth in weld cycled material is highly dependent upon the stability of nitrides during the weld cycle, and this stability depends on the original composition of the nitrides and hence of the steel itself. The presence of aluminum in the particles is particularly detrimental in this respect. The best grain growth control and highest toughness are found in steels based on optimum ratios of Ti/V/N and on low Al levels. The possible way in which the steel’s composition affects particle stability and hence grain growth during weld thermal cycles is discussed.

Microstructure and flux pinning in superconducting Bi-Pb-Sr-Ca-Cu-O wires

Abstract The critical current density ( J c ) of Ag-clad Bi-Pb-Sr-Ca-Cu-O wire has been measured to be 1.2×10 4 A/cm 2 at 77 K in zero field. The high J c is attributed to a combination of elimination of the poisoning effect of Ag on superconductivity, grain alignment, and enhancement of flux pinning. J c - H dependence was significantly improved in the Ag-clad tape, which has a J c of 1.0×10 3 A /cm 2 at 77 K and 4000 Oe, while the J c of the sintered pellet drops two orders of magnitude at only 100 Oe. A pronounced anisotropy in J c under high magnetic field is attributed to the grain alignment. Planar defects, such as heavy stacking faults parallel to the a - b plane in the rolled tape, are considered to be effective pinning centres.

Stress-relief heat treatment of submerged-arcvvelded microalloyed steels

A wide range of microalloyed steels containing various combinations of Al, Ti, V, Nb, and submerged-arc welded using wires of either high or low Mo additions, were studied in both the as-welded and ...

Transition from elastic to dissipative motion in a magnet–high‐Tc superconductor system

Using quasistatic harmonic excitations, we have measured the lateral force between a permanent magnet and a Bi‐based ceramic high‐Tc superconductor (HTSC) for lateral displacement amplitudes down to 1 μm. We find clear evidence for a transition from elastic (nondissipative) to inelastic interaction, and attribute the effect to flux pinning. The crossover amplitude can easily reach several micrometers, with the consequence that the lateral disturbance of a levitated magnet will decay to this amplitude, whereas further damping will be extremely slow. For applications of HTSCs in magnetic bearing systems this can be a very relevant aspect of the interaction, and it can set the limit for precision positioning of levitated objects.

Formation of the 124-phase superconductor YBa2Cu4O8 by retaining oxygen in a reaction HIP sintering process

The ceramic superconductor YBa2Cu4O8 has been produced by high temperature sintering of a mixture of CuO and YBa2Cu3O7 in a glass capsule under high hydrostatic argon pressure. The resulting highly dense material is investigated by X-ray diffraction, optical and electron microscopy, resistance measurements and hardness measurements, and shown to be a homogeneous High transition temperature superconductor.

Heat treatments of the low-expansion superalloy incoloy 909 for application in ceramic/metal joints and in metal matrix composites

Abstract Increasing efficiency demands on gas turbines have promoted the development of superalloys with low thermal expansion up to intermediate temperatures, to reduce the need of cooling for preservation of efficient clearances between rotating and stationary parts. These materials are also of prime interest for joins to engineering ceramics such as silicon nitride and silicon carbide, and as a matrix with W or SiC fibres in metal matrix composites (MMC). The paper deals with the thermomechanical and microstructural stability of Incoloy 909 during possible joining and densification treatments. Thermal expansion behaviour and joining/densification temperatures suitable for retaining the desired structure of the superalloy are presented.

Electrical transport properties of dense bulk YBa2Cu4O8 produced by hot isostatic pressing

Abstract Highly dense sintered YBa 2 Cu 4 O 8 has been produced by hot isostatic pressing (HIP). The electrical resistivity ϱ of this material has been measured as a function of temperature T and pressure ϱ in the range 40–650 K and 0–0.7 GPa. Both the temperature dependence and the pressure dependence of ϱ are found to be well described by a model based on the standard Bloch-Gruneisen theory. It is pointed out that ϱ is liner in T only under isobaric conditions, while ϱ is strongly nonlinear in all high- T c superconductors under isochoric (constant volume) conditions. The critical current density of the material is 900 A/cm 2 at 4 K, while the resistivity is 630 μΩ cm at 294 K.

High-pressure properties of high-Tc superconductor samples produced by hot isostatic pressing

The electrical resistance of dense YBa2Cu3Ox and YBa2Cu4Oy produced by hot isostatic pressing has been measured vs. T and p. At 295 K we find d (ln R)/dp ≃ -0.12 and -0.09 GPa-1, respectively, with no systematic dependence on initial density. For 1-2-4, dTC/dp ≃ 5.1 K/GPa, which is ten times that of 1-2-3.

AEM investigation of ceramic/lncoloy 909 diffusional reactions after joining by HIP

Diffusion bonding by hot isostatic pressing (HIP) was performed between Incoloy 909 and five different ceramics. Two of the ceramics were composites made from powder mixtures of Si3N4 and either 60 vol% TiN or 50 vol% TiB2, while three were monolithic materials, namely Si3N4 with 2.5 wt% Y2O3 as a sintering additive, Si3N4 without additives, and Si2 N2O without additives. A diffusion couple geometry was developed to facilitate the preparation of thin-foil specimens for examination by analytical electron microscopy (AEM). Diffusion bonding was performed by HIP at 927°C (1200K) and 200 MPa for 4 h. The formation of reaction layers was very limited, being less than 1 μm in total layer thickness. Two reaction products were found by AEM; a continuous, very thin, (⩽100 nm) layer of fine TiN crystals at the initial ceramic/metal interface, and larger grains extending about 100–500 nm into the superalloy and forming a semi-continuous layer of a G-phase suicide containing mainly nickel, silicon and niobium.

High critical currents in Bi-2223 phase bars☆

Abstract The critical current I c and the critical current density J c of Bi-2223 phase bars have been found to depend on the mass density of the bars. Bars densified using a cold isostatic pressing (CIPing) process have been obtained with transport critical currents of > 150 A with J c levels up to ≈ 10 3 A cm −2 at 77 K in the absence of an applied magnetic field. Bending or kinking of the grains may promote densification during CIPing of a sintered bar.