Volume 93, Issue 1

ATi-50.3 at.% Ni alloy was deformed in the range of stable austenite between 400 and 900 °C and subsequently quenched. The analysis of microstructure and rolling texture is in agreement with an alternating <100>-slip and twinning mechanism, and the absence of recrystallisation. Defects introduced by ausforming were related to strengthening of the austenite and the consequent shift to lower temperatures of the transformation cycle.

The anisotropic high cycle fatigue behaviour has been investigated for a duplex stainless steel in as-received and prestrained condition. It was found that the anisotropy of the fatigue limit is different from the anisotropy of yield strength. The main reason for this is the influence of microstresses on fatigue crack initiation. Fatigue cracks are seen to nucleate preferably in the austenite phase, which has tensile microstresses. Prestraining alters the microstresses from being higher in the transverse direction to being higher in the rolling direction. At the same time, the fatigue limit is changed from being higher in the rolling direction to being higher in the transverse direction.

In this paper, the behavior of liquid and solid inclusions at advancing planar and cellular solid fronts during the solidification of steels was investigated through in situ visualization with a confocal scanning laser microscope. In the case of the liquid spherical Al 2 O 3 –CaO–MgO inclusions, theoretical prediction models agree with the experimental observations with respect to the critical front velocity required for engulfment. For the irregular solid Al 2 O 3 inclusions, the prediction models underestimate the critical velocities. At very low growth velocities, the inclusions were found to slide along the front and get entrapped in the intercellular regions. Once entrapped, it was found that liquid inclusions appeared to be more readily engulfed at the intercellular regions than at the planar fronts. This was not observed for the large solid inclusion clusters.

Initial permeability measurements on nanocrystalline soft magnetic Fe 73.5 Si 13.5 B 9 Nb 3 Cu 1 and Fe 86 Zr 7 B 6 Cu 1 alloys revealed that, in agreement with previous studies, in the optimum annealed state, the Fe –Si–B–Nb–Cu alloy exhibits much better soft magnetic behaviour than the Fe –Zr – B–Cu alloy. On the other hand, the density remained almost unchanged upon the amorphous– nanocrystalline transformation in Fe – Zr –B–Cu whereas a significant increase (~ 2%) occurred in Fe – Si–B–Nb – Cu. From X-ray diffraction and transmission electron microscopy studies, the amount of the residual amorphous phase, the lattice parameter, the internal stresses, and the grain morphology of the crystallized phase were found to evolve differently with increasing annealing temperature for the two alloys whereas their grain sizes were nearly the same for a given annealing temperature. It is pointed out that all these observed differences may also contribute to the differences in their soft magnetic behaviour. Furthermore, an analysis of the phases in the fully annealed state gave some hints to explain the different behaviour of density upon crystallization.

In this work, the effect of the cooling rate after homogenisation on the critical deformation ratio, maximum grain size and recrystallisation grain size has been studied. Experimental results have revealed that hot deformation – restoration properties and recrystallised grain structures are affected to a great extend by the cooling rate. The values of the critical deformation ratio and maximum grain size of a hot rolled specimen cooled at a rate of 0.9 K/min were found to be 23% and 1.3 mm while those for a specimen cooled at 6 K/min were measured as 37% and 0.8 mm, respectively.

The residual stress state in an AISI 347 welded pipe for nuclear applications has been investigated by means of neutron time-of-flight diffraction. Two kinds of welds were compared: a ‘bare’ manual and a dressed tungsten inert gas (m-TIG with and without last pass heat sink, respectively). Thorough investigations along a line transverse to the weld seam and through-thickness have been carried out with very fine-step scans. Complementary microstructural investigations have been carried out by means of optical microscopy. Results show that the grain size varies from the parent material to the heat affected zone and weld pool. Some martensite is present in the parent material because of the cold working following each welding pass. In the weld pool, the heat flow seems to be redistributed effectively by the cap pass of the LPHSW. Although the microstructural heat affected zone is rather small, the effect of the weld on the stress state seems to extend further (the so-called strain affected zone). In the as-welded conditions, large compressive stresses are found in the inner layers both in the axial and hoop directions and the radial stress is relatively low (< 100 MPa) everywhere, suggesting a nearly ‘plane stress’ state. Stresses have different signs in the m-TIG and LPHSW. An overall relaxation is encountered in the aged conditions: the stress range shrinks in all directions and the beneficial compressive stresses in the inner layers are almost removed.

The plasticity of high Si steels is improved remarkably by adding Cr in high purity steels. The factors which contribute to this phenomenon were investigated. At the normal level of purity, the plasticity of Fe –Si alloys is deteriorated by Cr addition. However, in high purity steels, the plasticity drastically improves when Cr is added. When the influence of microstructure was removed, the improvement of plasticity by adding Cr to high purity Fe –Si alloys was attributed to the suppression of deformation twinning formation. The microstructural improvement was explained by grain refinement and by formation of subgrains during hot rolling. The properties of these steels are expected to be applied to new functional steels.

The V–Sn system was investigated with regard to the phase formation in the Sn-rich region and the reaction between vanadium and liquid tin. Phase equilibria were studied by isothermal heating of powder alloy samples in the temperature range 200 – 600 °C for compositions 60 – 80 at.% Sn up to 680 h. Phase identification was done using X-ray diffraction (XRD), scanning electron microscopy (SEM) and energy-dispersive X-ray (EDX) methods. High-temperature XRD up to 800 °C was used to determine the stability range of V 2 Sn 3 . A thermodynamic modeling of the V–Sn system was done and the phase diagram was calculated. The reaction kinetics between solid Vand liquid Sn was investigated using differently prepared bulk diffusion couples heated at 450 – 600 °C for up to 100 h. Reaction layers after isothermal heating of diffusion couples were analysed with SEM/ EDX and XRD. The fastest forming intermetallic phase if the liquid alloy is reacting with V is V 2 Sn 3 and it is the predominant phase in tempered powder and bulk diffusion couples. The “V 3 Sn” phase is virtually not formed under these conditions. However, V 2 Sn 3 phase formation is too slow for practical applications in transient liquid-phase bonding.

The influence of heat treatment and cold deformation on microstructure and mechanical properties of Cu–Ag alloys with more than 20 wt.% Ag has been investigated. Due to the embedding of the Cu-rich dendrites by the Ag–Cu eutectic high-angle grain boundaries are absent in the as-cast state of these compositions. Hence, the continuous precipitation of Ag particles is favored in comparison with discontinuous precipitation reactions observed in Cu alloys containing less than 8 wt.% Ag. The tensile strength after heavy cold deformation is enhanced by a beneficial superposition of several effects: (i) The solid solution hardening of the Cu-rich dendrites, (ii) the age hardening of silver precipitates inside the dendrites and the high work hardening rate of the peak-aged state, (iii) the phase boundary hardening of the Ag-rich eutectic, and (iv) the increase of cooling velocity after solidification of continuously cast billets which determines the extent of the Ag supersaturation in the copper matrix. As a consequence, with a continuously cast starting billet of 12 mm diameter an optimum value for the ultimate tensile strength of 1.3 GPa is obtained after a total drawing strain of only η = 4.3.

A novel solid – liquid reaction synthesis and simultaneous densification process was developed to fabricate Ti 2 AlC material using the stoichiometric mixture of Ti, Al and graphite powders as starting materials. This method is a one-step process for the preparation of dense poycrystalline Ti 2 AlC. Moreover, it provides the advantages of low synthesis temperature, short reaction time and simultaneous synthesis and densification. The microstructure evolution during the high temperature processing was described and the properties of polycrystalline Ti 2 AlC prepared by this method at 1400 °C were investigated. The results showed that Ti 2 AlC exhibited metallic electrical conductivity and high mechanical properties. The electrical conductivity of Ti 2 AlC at room temperature was 4.42⨯10 6 Ω – 1 ⨯ m – 1 and increased with decreasing temperature. The Vickers hardness of Ti 2 AlC was 2.8 GPa. No indentation cracks were observed and the microdamage is confined in the immediate vicinity of the indent indicating that Ti 2 AlC is damage tolerant. The flexural strength and the fracture toughness were 275 MPa and 6.5 MPa ⨯ m 1/2 , respectively.

In copper, a dislocation glide on non-compact slip planes {100}, {110}, and also {11 X }, X = 2, 3, . . . can locally appear in the deformation process at high temperatures. The present work gives a summary of recent investigations of microstructures formed at temperatures of 773– 973 K in the case of compressive and tensile creep and in single shear deformation realized on double-notch specimens, first of all the detection of traces of this process in the slip pattern and dislocation structure. A discussion leads to the conclusion that favorable conditions for the non-compact glide are given by a double (multiple) slip on symmetrically oriented <110> {111} slip systems, operating on different planes, and a stress concentration. The last condition can be justified, e. g., by the inhomogeneous bi-axial stress field.

X-ray stress analyses on 7 sintered Cu–Fe –TiC alloys and on the pure phases Cu and Fe were performed. The samples of different compositions distinguish in the temperature of sintering, the content of pores, and the material characteristics. The macro and micro residual stresses were determined after grinding and after shot peening. The residual stresses versus penetration depth profiles were evaluated with X-rays of different wavelengths and at different peaks. The portions of macro and micro residual stresses are discussed.