Materials Science Forum Vols. 604-605

Abstract: Alloying high-chromium steels with Nitrogen leads to increase in strength, fatigue life and corrosion resistance, but reduce ductility and could induce cracks formation during forging. In order to address these problems, the hot working response of a high Nitrogen martensitic stainless steel (Fe-16.2%Cr-1.1%Mo-0.33%N-0.34%C) has been investigated by means of hot torsion tests up to rupture, in the temperature and strain rate ranges of 900-1200°C and 0.005-5 s-1 respectively. The peak stresses of the flow curves were related to strain rate (e&) and temperature (T) by the well known sinh equation. The ductility and the safe working conditions were presented in terms of processing map. The microstructure of the steel in the quenched state after deformation was analyzed by means of optical microscopy; the differences in term of morphology and distribution of the various constituents were discussed.

Abstract: Inherent failure mechanisms at elevated temperature are primarily wedge crack growth at triple junctions arising from differential grain boundary (GB) sliding at higher stresses and pore formation on sliding boundaries due to vacancy diffusion at lower stresses and higher T. The behaviours of these mechanisms have been ascertained in creep where they can be studied over long periods. They continue to operate in hot working although their effects per unit strain have been reduced by decrease in fractional contribution of GB sliding. Dynamic recovery (DRV) significantly develops a stable substructure that strongly mitigates stress concentration. In alloys of austenitic steel, Ni and Cu, dynamic recrystallization (DRX) aids DRV in reducing stress concentrations and the migrating GB isolate fissures so they cannot propagate. Solutes and precipitates generally reduce ductility by diminishing DRV and DRX. Large particles and inclusions, notably on GB, introduce new sources of fissure nucleation, lowering ductility; solidification segregation and low melting constituents, especially if they spread along the GB, create severe problems.

Abstract: The microstructure of Ti6Al4V-SiCf composite, in as-fabricated condition and after long-term heat treatments (up to 1,000 hours) in the temperature range 400 - 600 °C, has been investigated by means of high-temperature X-ray diffraction (HT-XRD), energy dispersion spectrometry (EDS), X-ray photoelectron spectroscopy (XPS) and Auger electron spectroscopy (AES). Particular attention was paid to the strains, arising during heating, and to the micro-chemical evolution of fibre-matrix interface. Micro-chemical examinations evidenced that a thin TiC layer has formed between the fibre carbon coating and the matrix during the fabrication process. TiC slows down further diffusion of carbon towards the matrix and guarantees the interface stability also for the most severe treatments examined here.

Abstract: Ti6Al4V-SiCf composite, manufactured by Hot Isostatic Pressing (HIP) at Centro Sviluppo Materiali, has been submitted to long-term heat treatments (up to 1000 hours) at 400 and 600°C. The mechanical properties of the material, in as-fabricated condition and after heat treatments, have been investigated by instrumented indentation (FIMEC), dynamic modulus, tensile and fatigue tests. For comparison some experiments have been carried out also on the monolithic Ti6Al4V alloy. Results show that heat treatments, also the most severe examined here, do not produce remarkable variations of mechanical characteristics. In agreement with the microstructure examinations presented in part I, this behaviour, quite promising for future aeronautical applications, can be primarily ascribed to the stability of fibre-matrix interface.

Abstract: Primarystageofcreepprocessisaperiodwhenthecreeprateisdecreasingduetomi-crostructuredevelopment.Thedislocationdensityevolvestothedynamicequilibriumvaluerelatedtotheappliedstress.Inlow-strainratecreepexperiments,similarstageofdecreasingstrainrateisobservedalongwithapparenttransitiontosteadystate.Nevertheless,deeperanalysisofthevariousparametersofthestageshowsthatthesubstanceoftheprocessmustbedifferent.Themainreasonsare:i)thestrainistoosmallfordislocationsubstructurerearrangement,ii)stressandtemperaturedependenciesofthetransitionstrainandrelaxationtimeoftheprocessdoesnotcorrespondtoanyoftheknownprimarycreepprocesses,iii)thestrainreachedduringthestageisalmostfullyrecoverable,sothatthedeformationisratheranelasticthanplasticone.Thetransitionstrainobservedinlowstrainrateexperimentsthusshouldbetreatedasaspecial“pre-primary”stage,whiletheapparentsecondary(steadystate)stageisinfactpartoftheprimarycreepstage.

Abstract: The creep behaviour of the AE44 magnesium alloy was described by means of a variation of the composite model, already successfully used to analyse the behaviour of die-cast Mg-Al alloys. Since the microstructure of these alloys is constituted by primary Mg globules surrounded by a divorced eutectic formed by supersaturated a-Mg and precipitates, these materials were compared to a composite with soft (globules) and hard (divorced eutectic) regions, both deforming with a similar strain rate during creep. The strengthening role of the precipitates formed in the hard region during cooling from injection temperature and during creep was expressed by introducing a threshold stress. The agreement between experimental data and model was very good.

Abstract: Significant developments have been made in recent years in the description of microstructure evolution and its effects on the creep behaviour in advanced 9-12%Cr steels. However, data available for assessing the predictability of the creep behaviour are somewhat scarce since creep testing has generally been for constant temperature and load conditions. The present investigation was conducted on three advanced 9-12%Cr martensitic/ferritic steels (P91, P92 and E911) in an effort to obtain more complete description and understanding of the role of degradation processes in high temperature creep during intermittent heating. A comparison between the creep characteristics of non-steady and monotonously loaded creep specimens has revealed no significant deterioration of the creep strength and fracture resistance of the steels P91, P92 and E911 under non-steady loading in power-law (dislocation) creep. The final part of this paper deals with detailed experimental microstructural and fractographic investigations of crept specimens to explain the observed creep behaviour. Special attention is paid to the thermodynamic calculations using the software package Thermo-Calc, that have been used to predict precipitation reactions during intermittent heating of 9-12% chromium steels. The results of the thermodynamic calculations are in a good agreement with experimental data.

Abstract: The effect of the process parameters on the sheet formability of AZ31 magnesium alloy has been investigated by means of uniaxial tensile and hemispherical punch tests at different temperatures and strain rates. The results of the uniaxial tensile tests were analysed in terms of flow curves, ductility and microstructural evolution; the constitutive parameters were evaluated and related to the forming limit curves obtained by the hemispherical punch tests carried out at different temperatures and punch speeds.

Abstract: Plastic deformation of crystalline materials is not controlled by interaction among free dislocations only, but the interaction of free dislocations with internal boundaries. i) Low-angle boundaries: Modeling of deformation of pure materials with conventional grain size on the basis of structure evolution indicates that low-angle boundaries act as obstacles of free dislocations. The migration of the low-angle boundaries constitutes an essential recovery process determining the deformation resistance in the steady state. ii) High-angle boundaries: Severe plastic deformation transforms low-angle boundaries into high-angle ones. They differ in obstacle and recovery characteristics from low-angle boundaries, which explains the special properties of ultrafine-grained and nanocrystalline materials with regard to strength, strain rate sensitivity and ductility. iii) Phase boundaries in Ni-base superalloys enhance the strengthening by hard phases with strengthening by dense dislocation networks serving to reduce coherency stresses. It is concluded that internal boundaries play a crucial role in controlling the evolution of structure and strength in crystalline materials.

Abstract: The results of intercept and profile counts are commonly interpreted as a suitable estimates of the mean grain size as represented e.g. by the grain density V. The term grain size is not explicitly defined even when some relation to grain volume and/or mean grain breadth (the mean Ferret diameter) is tacitly assumed. However, the intercept count L is directly related to the mean value of grain boundary area per unit volume SV and the profile count A is, under relatively general assumptions, directly related to the mean value of grain junctions per unit volume LV. Their relation to V can be generally written as V = c¢(A)3/2 = c²(L)3, but the coefficients c¢ and c² strongly depend on the structural characteristics like grain size dispersion, anisotropy etc. and their evaluation is far from being simple. Consequently, whereas the reliable estimates of SV and LV result from intercept and profile counts, the estimate of grain density based on them requires a careful consideration.