Defect and Diffusion Forum Vols. 289-292

Abstract: Two High Alloyed Cast Irons (HACI) were studied, both belonging to the Fe-C-Cr-Si-X system where X represented a strong carbide forming element. One of these alloys was obtained after adding Nb, V and Ti to the chemical composition of the other alloy. Raw materials originated from spun cast rolls for hot strip mill were submitted to different heat treatments routes, in order to study the influence of alloying elements on the microstructure. Both HACI grades contained a mixture of martensite and retained austenite matrix in the as-cast conditions and after quenching. Differential Thermal Analysis was carried out on the heat treated samples in order to determine the phase transformations occurring during re-melting and subsequent solidification sequence. Diffusionless transformations leading to various types of martensite were found in the matrix. Bulky NbC carbides precipitating at the beginning of the solidification process strongly influence the nature and the rate of the subsequent diffusional phase transformations, particularly for HACI grade with Nb, V and Ti additions. Quantitative metallography was done to determine graphite, NbC carbides, cementite and matrix volume fraction in HACI studied grades.

Abstract: The prediction of the applicability range of beta titanium alloys in hydrogen containing environments and the systematic study of hydrogen effects on the microstructure during heat treatment require reliable information about the hydrogen diffusion coefficient DH in the respective titanium alloy. Up to now the little information available on hydrogen diffusivity in commercial titanium alloys indicates a higher hydrogen diffusion coefficient in beta titanium alloys as compared to alpha and alpha + beta titanium alloys. In the present study, the hydrogen diffusion coefficients were determined systematically by means of electrochemically charging the half length of thin titanium rods and subsequent annealing, thereby enabling hydrogen diffusion. The Matano technique was applied in order to identify any effect of hydrogen concentration on DH. The hydrogen diffusion coefficients determined were correlated with results from microstructure examination applying optical and electron microscopy. Since molybdenum and vanadium are the most important beta-stabilizing alloying elements, binary titanium alloys of the Ti–Mo and the Ti–V systems at various contents of the respective alloying element were systematically studied in addition to commerical beta titanium alloys. The results of the experiments revealed the strong effect of beta stability and phase composition on hydrogen diffusion.

Abstract: Kinetic characteristics of internal interfaces in ultrafine grained (UFG) materials represent an aspect that is central to their potential applications. Interface diffusion is strongly affected by ultrafine crystallinity and incorporated defects. In the present study the radiotracer technique was applied for measuring grain boundary (GB) diffusion of Ni in UFG Cu-1wt.%Pb alloy. The UFG specimens were prepared by equal channel angular pressing at room temperature. GB diffusion was investigated at room temperature in Harrison’s C kinetic regime under conditions in which Ni diffusion - even along general high-angle GBs in pure Cu - would hardly be measurable by the mechanical sectioning (as the corresponding penetration depths would be less than 100 nanometers). The present study highlights the existence of ultra-fast diffusion paths in severely deformed material. An atypical time dependence of the room-temperature diffusion data indicates a quite involved nature of these “fast” diffusion paths in the Cu-1wt.%Pb alloy.

Abstract: The addition of rhenium and ruthenium to single crystal nickel-base superalloys improves the high-temperature properties of the alloys. In this work the applicability of the database TTNi7 (ThermoTech Ltd, UK) for developing 4th generation single crystal superalloys containing rhenium (Re) and ruthenium (Ru) was investigated. We systematically compared experimentally determined alloy properties to the predictions of ThermoCalc with the database TTNi7. The investigated properties were liquidus, solidus and ´ solvus temperature as well as incipient melting point and segregation. Calculations were based on thermodynamic principles with the assumption of either equilibrium or Scheil-Gulliver conditions, i.e. no diffusion in the solid and complete diffusion in the liquid. Furthermore the composition of the  and the  phase of a Re- and Ru-containing superalloy was measured and compared to calculations. Our results show that the database is capable of simulating general trends of 4th generation superalloys up to 6 weight percent (wt.-%) Re and 6 wt.-% Ru. The present work shows that Scheil-Gulliver calculations can only be used as a first approximation for nickel-base superalloys.

Abstract: The effect of austenite grain size on the austenite to ferrite transformation temperature and different ferrite morphologies and growth behaviour in one Nb-microalloyed (HSLA) steel has been investigated. Three different austenite grain sizes were selected and cooled for obtaining austenite to ferrite and growth behaviour of ferrite. Moreover, samples with specific austenite grain size have been quenched, partially, for investigation of the microstructural evolution. The optical microscopy observation suggested that the nucleation site of ferrite is on edge and inside of austenite grains in Nb- microalloyed steels. Micrographs of different ferrite morphologies show that at high temperatures, where diffusion rates are higher, grain boundary ferrite nucleates both at the edge and corner of austenite grains and grows into both austenite grains. As the temperature is lowered and the driving force for ferrite formation increases, intragranular sites inside the austenite grains become operative as nucleation sites and suppress the grain boundary ferrite growth. With more undercooling,intragranular ferrites are seen to nucleate and grow more extensively , indicating the beginning of displacive transformation. Furthermore, growth rate of intragranular ferrite shows that by increasing of austenite grain size, the growth rate of intragranular ferrite increases extensively and growth rate of grain boundary ferrite decreases. The growth kinetics of grain boundary ferrite shows that this transformation is controlled by the diffusion of carbon in the austenite ahead of the interface.

Abstract: Electrical discharge machining (EDM) is one of the most widely used non-conventional machining processes for the production of moulds/dies, cutting tools and aero-engine parts, such as turbine/compressor blades. The paper details experimental research on the surface alloying/modification of chromium martensitic hot-work tool steel components (AISI H13, 55HRC) during EDM die-sinking operations using powder metallurgy (PM) tool electrodes, as a means of achieving enhanced workpiece wear resistance without resorting to a subsequent coating operation. Tool electrode performance of partially sintered WC/Co electrodes operating in a common hydrocarbon oil dielectric was assessed and subsequently compared with that of conventional electrode materials, such as Cu and Graphite. Surface/subsurface observations by optical and scanning electron microscopy (SEM) showed a recast solidified layer of ~ 8 µm when using WC/Co electrodes. Performed XRD and SEM-EDX analysis indicated that WC and Co contained in the PM tool electrodes, together with C decomposed from the hydrocarbon oil during sparking, were transferred and alloyed to the steel substrate surfaces. EDM surface alloyed layers were hardened over 1200 HK0.025. This hardening is related both to the formation of tungsten carbides with different stoichiometries and to the non-equilibrium microstructure evolution. Thickness of the hardened zone was shown to be dependent on EDM operating parameters, in particular peak-current (A) and pulse on-time (s).

Abstract: The thermal stability and creep behaviour of MgNi2Y1CeMM1 and MgNi3Y1.5CeMM1.5 alloys have been investigated at 523 and 623 K. Both alloys were processed by a powder metallurgy route involving rapid solidification of powders, cold isostatic pressing and extrusion at 673 K. The microstructure of both alloys was studied in the as-extrusion condition and after thermal treatments at 523 and 623 K up to 500 h. Mechanical characterization was carried out by hardness measurements in samples annealed at 523 and 623 K and stepwise stress compressive creep tests at these temperatures. Results have shown long-term stability of the microstructure after annealing at 523 K in both alloys and hardness values remained constant along all treatment. After treatment at 623 K, however, the microstructure of both alloys remained relatively stable and a decrease in hardness was noticed after 6 h. Creep behaviour of both alloys was very similar. The stress dependences of the creep rate can be well described by modified Garofalo sinh relationship. Activation energies for creep, practically twice of the activation enthalpy of lattice diffusion for pure magnesium (135 kJ mol-1), have been obtained. Therefore, no direct evidence of the controlling role of diffusion processes can be obtained from these experiments.

Abstract: Studying the corrosion of the alloy 600, under water pressure, is of high importance to understand the ageing process of pressurized water reactors. Today, the impact of the oxide growth on the mechanical properties of nickel alloys is a challenge. The surface analysis and the quantification of the local deformation are key factors to deduce the surface damage of the substrate produced by corrosion. Here, we introduce a new methodology to determine the deformation distribution of the alloy 600 by using polycrystalline samples. The method is based on nanopads disposed on the surface samples, which allow a mapping, at the microscopic scale, of the spatial deformation. We applied to the samples a tensile loading in the plastic domain from 0 to 4%. The obtained asymmetric deformations distribution reflects the polycrystallinity of the system.

Abstract: Diffusion of 65Zn in two commercial Mg-based alloys AZ91 and QE22 with short Saffil fibers was studied. Experiments were carried out in the temperature interval 648 – 728 K by serial sectioning method. The effective diffusion coefficients Deff were compared with 65Zn diffusion coefficients Dv obtained with the same alloys without Saffil fibers. The evaluation of the influence of the interface between the matrix and the fibers upon Deff was done and the zinc diffusion coefficient Di in the interface boundary matrix/Saffil was estimated. Unlike the Arrhenius-like behavior of volume diffusion in both alloys, it was observed that the temperature dependence of both Deff and Di was significantly concave in the measured temperature interval. This behavior was attributed to relaxation of thermo-elastic stresses in the composite induced by a large difference between coefficients of thermal expansion (CTE) of Saffil fibers and metal matrix. The maximum values of Deff and Di, respectively, lie close to 693 K, where CTE has a minimum.

Abstract: Extensive research work has been devoted to Mg-based alloys strengthened by precipitation hardening. Increasing the aging time leads to the appearance of zones depleted of precipitates near grain boundaries. The formation of precipitate depleted zones (PDZ's) is explained by near-grain boundary (NGB) coarsening. The evolution of PDZ's was considered on the basis of the model taking into account diffusional fluxes between adjacent precipitates. The set of equations was solved numerically by using a fourth-order Runge-Kutta method for different initial sizes of precipitates and densities of precipitate layers near grain boundaries. The dissolution of precipitates in the NGB-zones is initially provided by diffusion from them to large precipitates at the grain boundary, and then also by diffusion from these decreased precipitates to the larger precipitates at the outer border of the PDZ. As a result, the outer borders of the depleted zones are adjoined by bands of enlarged precipitates forming a PDZ "crust". Being a diffusion controlled process, the depleted zones are widened with temperature and aging time. Experimental investigation of PDZ evolution was conducted by SEM and TEM on Mg-Zn-Sn-alloys aged at different temperatures for different times. Comparison of the calculated results with experimental data allowed the evaluation of the model parameters and physical parameters of the system (diffusion coefficients and interface energy of the precipitated phases).