Grain boundary precipitation strengthening in high temperature creep of Fe15Cr25Ni alloys

Grain boundary carbide precipitation gives rise to a dramatic decrease in creep rates compared with those of a single phase alloy. The high stress exponent (n=7) and creep activation energy (Q_C > Q_sd) have been found due to these intergranular carbides, indicating that they both increase creep resistance and change creep characteristics of alloys. The model present here rationalizes the observed behavior in single phase and two phase alloys with grain boundary carbides by a unified power law equation involving boundary obstacle stress. The predictions of this model are in close agreement with experimental results.     

Hillock growth on PbInAu alloy films

This paper describes the hillock growth mechanism of PbInAu-alloy films during heat treatment at 80°C. The samples examined have a similar structure to those used in Josephson integrated circuit fabrication technology. The number of hillocks in an SiO isolation layer window is influenced by several experimental parameters. These parameters are the ration of the SiO window area to the lead-alloy film area, the SiO film stress and the lead alloy film thickness. The hillock growth mechanism in the SiO window and methods to suppress hillocks are discussed. These results could provide a guideline to improve the reliability of lead alloy Josephson large-scale integrated circuits.     

Effects of silver doping on the superconducting YBaCu oxide

The doping effects of Ag on the ceramic body of YBaCu oxide have been studied. Two kinds of specimens were investigated, nominally, YBa₂Cu₃O_7−δ +Ag (Ag-added) and YBa₂Cu_3−xAg_xO_7−δ (Ag-substituted), for their structural and super-conducting properties. It was found that Cu cations were not completely substituted by Ag ions even in the Ag-substituted specimens, and the excess metallic Ag filled the pores of the bulks in all cases. The excess Ag in the pores promoted a liquid phase sintering process that encouraged the formation of grains in shape of stacked large flat rectangular platelets and, hence, resulted in C-axis preferred orientation. The Tc was almost unchanged in the Ag-added specimens but decreased in the Ag-substituted specimens with x values larger than 0.1 mole fraction due to Cu-deficiency. In both kinds of specimens, the Jc were improved by the Ag-doping in the YBCO ceramics up to a certain amount of Ag, above which the appearance of semiconducting phases degraded the superconducting properties of the material.     

Influence of preparation parameters on the tunnelling characteristics of PbBiIn/oxide/PbBiIn junctions

The quasiparticle millimetre wave mixer requires high quality tunnel junctions with a large gap voltage, a normal resistance R_n of several tens of ohms and areas A of only a few square micrometres or less. We show that water vapour at the upper surface of the PbBiIn alloy junction can increase the product R_nA by more than two orders of magnitude. However, indium at the upper surface can drastically decrease R_nA but does so mostly at the expense of the quality of the junction. Appropriate annealing allows the normal resistance to be increased or decreased to close to the desired value.     

Effect of cyclic loading on subsequent creep behaviour and its implications in creep–fatigue life assessment

Abstract

Evaluation of creep–fatigue failure is essential in design and fitness evaluation of high-temperature components in power generation plants. Cyclic deformation may alter the creep properties of the material and taking cyclic effects into account may improve the accuracy of creep–fatigue failure life prediction. To evaluate such a possibility, creep tests were conducted on 316FR and modified 9Cr–1Mo steel specimens subjected to prior cyclic loading; their creep deformation and rupture behaviours were compared with those of as-received materials. It was found that creep rupture life and elongation generally decreased following cyclic loading in both materials. In particular, the rupture elongation of 316FR in long-term creep conditions drastically decreases as a result of being cyclically deformed at a large strain range. Use of creep rupture properties after cyclic deformation, instead of those of as-received material, in strain-based and energy-based life estimation approaches brought about a clear improvement of creep–fatigue life prediction.     

Effects of phosphorus on creep behaviour of nickel and Ni–20Cr and on creep and strength of Nimonic 80A

Abstract

The influence of P on the creep behaviour of Ni, Ni–20Cr (wt-%), and Nimonic 80A was investigated by carrying out creep tests under various loads and at different temperatures. After creep fracture the samples were investigated using optical, scanning electron, and transmission electron microscopy. The grain boundary segregation was examined using Auger electron spectroscopy (AES). It was found that P segregates to the grain boundaries in all the materials investigated. The creep rate of Ni–20Cr and Nimonic 80A is decreased by the addition of P. Grain boundary segregation of P and its influence on strength was also investigated using AES for specimens aged between 600 and 700°C after fracture by a tensile test inside an ultrahigh vacuum chamber. Maxima of tensile strength are observed to be time dependent as a result of carbide precipitation, which is affected by the P segregation.

MST/1679     

Effect of primary α content on creep and creep crack growth behaviour of near α-Ti alloy

Abstract

The effect of primary α content on creep and creep crack growth behaviour of a near α-Ti alloy has been investigated at 600°C. The alloy was heat treated at different temperatures so as to obtain different volume fractions of equiaxed primary α in the range from 5 to 40%. Constant load creep tests were carried out at 600°C in the stress range 250–400 MPa until rupture of the specimens. Creep crack growth tests were carried out at 600°C and at an initial stress intensity level of 25 MPa m^1/2. Creep data reveal that minimum creep rate increases and time to rupture decreases with increase in primary α content indicating that higher primary α leads to creep weakening. On similar lines, maximum creep crack growth resistance is associated with the alloy with lowest primary α content (i.e. 5%). Microstructural and fractographic examination has revealed that creep fracture occurs by nucleation, growth and coalescence of microvoids nucleated at primary α/transformed β (matrix) interfaces. On the other hand, creep crack growth occurs by surface cracks nucleated by fracture of primary α particles as well as by growth and coalescence of microvoids nucleated at primary α/transformed β (matrix) interfaces in the interior of the specimen ahead of the crack tip.