Volume 94, Issue 1

The paper discusses the instability of the strain ratio and crystallographic orientation in face-centred cubic (fcc) real single crystals exposed to uniaxial straining. This behaviour can be described by the r ( ε ) function defining the variation of plastic anisotropy with strain, even in nominally stable crystals with [100], [111] or [110] orientation. The instability is due to the error in the orientation of the sample axis ( α is the angle between the sample and the [ uvw ] nominal direction) and/or to microscatter of the crystallographic orientation in the whole volume of a single crystal (described by the half-width of the rocking curve ρ ). The instability and, consequently, texture in single crystals have been shown to depend on the stacking fault energy ( γ ). A quantitative evaluation of the instability of the crystallographic orientation in deformed single crystals has been proposed and its physical meaning revealed in experiments on low (Cu–Zn alloys, Ag)-, medium (Cu)- and high (Ni, Al)-stacking fault energy metals. Two different mechanisms should be assumed when investigating the influence of stacking fault energy on the instability of the crystallographic orientation in deformed fcc single crystals: changing from one mechanism to another, when γ varies from γ Ag to γ Cu .

A modelisation of the Pb–Zn system is performed taking into account available thermodynamic and phase diagram data. The liquid phase and the two solid solutions are described by a Redlich–Kister Equation. The optimized coefficients obtained and the calculated phase diagram are presented.

The temperature dependence of the γ – γ′ lattice misfit (d) between precipitates and matrix in three modified CMSX4-type nickel-base superalloys was investigated. The effect of the so-called standard heat treatment and of the addition of elements like Nb, Ti and Re on d was also monitored. The misfit was measured by in situ high-resolution X-ray diffraction (up to 1375 K), and the microstructure was investigated by means of optical and transmission electron microscopy. It was found that with the addition of Nb the microstructure becomes more sensitive to thermal treatments, while the presence of Ti strongly influences the lattice misfit δ . Re also influences the value of δ , but in combination with other elements. A standard CMSX4 alloy shows relatively less influence of the thermal treatments on the misfit values and the microstructural properties. For all samples investigated, the misfit is negative at all temperatures.

The paper presents results from thermochemical surface gas nitriding of Ti-6Al-4V and Ti-6Al-2Sn-4Zr-2Mo-0.08Si alloys using differential scanning calorimetry equipment. The experiments were carried out in nitrogen atmosphere at temperatures of 950 and 1050 °C for 1, 3 and 5 h. Results for the surface hardness, the microhardness profiles and the thickness of the nitrided layers were obtained. Microstructure analyses were made by optical microscopy. X-ray diffraction analyses indicated new phases formed (TiN, TiN 0.3 , Ti 2 N and TiO 2 ), depending on the time and the temperature of nitriding. The obtained thickness of the nitrided layers was in the range 60–320 μm depending on the process parameters. The Vickers microhardness along the cross-section depth varied between 610 and 1700.

Creep of three chill cast alloys based on the composition Mg-2.5RE-0.35Zn-0.3Mn-0.03Zr:(wt.%) with partial or complete substitution of Nd for Ce mischmetal has been investigated for the stress range 50–90 MPa and temperature range 150-240°C. The Nd substitution reduces the creep rate by factors as high as 8 for the conditions studied and stress to produce 0.1 % strain in 100 h matches reported results for A380 aluminium alloy at temperatures near 200 °C. The possible origins of this improvement in performance are discussed.

Incomplete internal oxidation of rolling bearing steel SAE 52100 in Rhines pack consisting of Fe 2 O 3 and Fe powder was applied to determine the permeability for oxygen, at the dissociation pressure of wüstite, between 1073 and 1373 K. Simultaneous rim decarburization permitted the examination of ferritic and austenitic base material and the ascertainment of the influence of the α – γ phase transformation on the oxidation rate. The narrow transition range at around 1170 K is characterized by oxygen partly diffusing in ferrite and in austenite. Bulk diffusion of oxygen was found to be the main transport mechanism. The slight preceding of grain boundary oxidation diminishes with increasing temperature in both investigated iron phases. The permeation constants of ferrite and austenite for oxygen, in equilibrium with wüstite, can be described by Arrhenius-type equations. From these expressions, the solid solubilities of oxygen in ferrite and austenite were derived by inserting literature data on the concerning diffusivities. These results are comparable with literature data published previously. Incomplete oxidation of alloying elements due to the formation of protective oxide layers around precipitated particles and accelerated oxygen diffusion along metal/oxide phase boundaries may raise the deduced permeability for oxygen.

The oxidation behavior of TiNi, TiNiPd and the Ce-containing TiNiPd shape memory alloys are investigated using thermogravimetric analysis, X-ray analysis, scanning electron microscopy and electron probe microanalysis. It has been found that the oxidation scale of the alloys exhibits different sub-layers. Generally, the outmost layer is mainly composed of rutile TiO 2 . The TiNiO 3 phase exists for all the alloys, and the Ti 4 Pd 2 O phase for TiNiPd alloys as well. These oxides act as important media during the ion exchanging process during the oxidation and cause different oxidation resistance. The addition of Ce in the TiNiPd alloy effectively impedes the Ti ions outward diffusion, causes a wide distribution of Ni and Pd over the scale depth, and obviously improves the oxidation resistance.

The recrytallization behaviour of Al—Si—Mg matrix composites reinforced with SiC particles of 10 and 20 vol.% and unreinforced matrix alloy have been studied. Also, the effect of annealing temperature and time on grain growth and particle coarsening has been evaluated. Cast ingots of the matrix alloy and composites were extruded at 500 °C at an extrusion ratio of 10: 1. The microstructures and hardness of the composite samples have been investigated. It has been found that particle volume fraction and size affect the recrystallization temperature of the materials.

The microstructural stability of a peak-aged (24 h at 160°C) Al–Ge–Si alloy, containing incoherent GeSi precipitates with high interfacial energy, has been examined under different creep conditions, with temperatures varying from 120 to 140 °C and initial stresses varying from 65 to 85 MPa. A combination of transmission electron microscopy (TEM) and X-ray diffraction revealed that under all creep conditions, fresh nucleation of second-phase precipitates occurs in the alloy. TEM and quantitative image analysis demonstrated that exposure at 120 °C with and without stress has no observable effect on the precipitate size, while increasing the creep temperature to 140 °C coarsens the precipitates. Furthermore, prolonging the isothermal aging time at 160 °C coarsens the second-phase precipitates considerably. These results are discussed in terms of the known factors responsible for the coarsening of precipitates in age-hardenable alloy systems.

Tensile flow curves of interstitial-free (IF) and C- and Padded (CP) steels were measured at temperatures from 77 to 773 K with a strain rate of 3.3 × 10 −3 s −1 . Flow curves at 2.0 × 10 −3 s −1 were estimated by using the Kocks-Mecking model and compared with the results obtained by a Hopkinson pressure bar method. Flow stresses at 2.0 × 10 3 s −1 were satisfactorily estimated when the parameters of the model were determined from the data obtained at low strain rates both for IF and CP steels. The temperature dependence of the deformation microstructure evolution hardly affected the flow stress when a temperature change test was performed. The flow stress ratio or difference between 2.0 × 10 3 s −1 and 3.3×10 −3 s −1 at 295 K was larger for IF steel than for CP steel although a thermal component of flow stress at 0 K was estimated to increase by solid solution hardening. These results are reasonably explained by a model calculation.

The aim of the study is to investigate the transient temperature, structure and internal stress evolution and distribution of oil-quenched centric and eccentric cylindrical tubes by a finite element method. Thermal analysis is first performed to obtain the cooling curves for the core and surface of the tubes, and then followed by a structural analysis. The study finds that at the initial stages of the quenching process, the residual axial stresses are tensile at the surface and compressive in the core for both geometries. However, toward the end of quenching, the interactions of thermal and transformation stresses make the surface and core axial stresses of the centric tube reverse their sign several times. Under the same quenching process, it has been found that the residual stress distribution of the eccentric tube is quite different from the centric one.