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The effect of an unconventional thermal treatment method aimed to improve toughness behavior in Al alloys is reported. The method involves solution heat treating and an intermediate warm working step, before final ageing thermal treatment on a AA7050 high resistance aluminum alloy. Results show the possibility to increase fracture toughness behavior without tensile and conductivity (IACS) properties loss by adopting a warm deformation process instead of the standard cold deformation. Moreover, the adoption of an intermediate warm deformation instead of standard cold deformation, allows to reduce material microstructural grain-size heterogeneity.

Dislocation-density-related equations were proved to be promising tools to correlate tensile plastic behaviour and microstructure in ductile irons (DIs), revealing distinct relationships between equation parameters and microstructure features in austempered ductile irons (ADI) and isothermed ductile irons (IDI). These equations resulted to be successful also in the characterization of the austempering process through the plastic behaviour analysis of tensile tests of an ADI 1050 that was quenched at different times during austempering. The equation parameters could indeed be correlated to the time window for the best ausferrite, and could predict the precipitation of the deleterious ε' carbides for long austempering times. In the present work the results of the tensile plastic behaviour analysis of different DIs through dislocation-density-related equations are reported. The aim of the analysis was to test the capability of these equations to assess the microstructure quality of DIs and support their industrial production.

Laser (LBW) and Electron Beam (EBW) welding have been used to produce seams on 2 mm thick plates of directionally solidified (DS) IN792 superalloy. For each welding technique a grid of samples were prepared by varying the pass speed (v) and keeping constant the other process parameters. The experiments were carried out at room temperature and with pre-heating (PHT) at 200 °C and 300 °C to find the best process conditions. The microstructural changes in molten zone (MZ) and heat affected zone (HAZ) were investigated finding that EBW guarantee a better quality and efficiency of the process without any macro defects. About the microstructure, the amount of ordered γ’ phase in the MZ is similar (≈ 25 %) for both welding techniques and quite lower than the value (70 %) of the original alloy.

A component in service experiences stress conditions that change continuously with time. Since service conditions are usually difficult and expensive to reproduce in laboratory, the creep behaviour of alloys in service has to be extrapolated from a limited number of creep tests at constant loads and temperatures. Empirical rules have been proposed to forecast the effects of variable load and temperature both on the time to rupture, as the life fraction rule (LFR), and on the accumulation of creep strain with time, as the strain hardening rule (SHR). Two directionally solidified (DS) nickel based superalloys have been investigated with creep tests at constant and variable loads and constant temperature. Nickel based superalloys, for the typical stresses experienced in service, are often characterised by a small negligible primary, a minimum of strain rate with no secondary state, and a dominant accelerating creep caused by dislocation multiplication. The damage mechanisms causing the final rupture appear only in the very last percentage of life. In the present work, simulation results are reported to show that the physical-sounded model used to describe the accelerating creep due to dislocation multiplication can be employed to better predict the times to rupture and the creep curves of the two DS nickel based super-alloys with step-like variable stress than the empirical LF and SH rules.

Tungsten is a promising armour material for plasma facing components of nuclear fusion reactors. Two materials with different density and purity have been examined by optical microscopy, X-ray diffraction (XRD), instrumented indentation tests (FIMEC) and mechanical spectroscopy. For both the materials yield stress and elastic modulus strictly depend on the residual porosity. Moreover, the material with higher porosity (≈ 9%) is not stable and remarkable modulus variations are observed during heating. The IF spectrum exhibits a relaxation Q-1 peak superimposed to an exponentially increasing background. The peak is a single Debye peak with activation energy H = 74.86 kJ mol-1 and pre-exponential factor τ0 = 1.76 x 10-9 s that has been ascribed to dislocation interaction with intrinsic point defects (autointerstitial and substitutional).

Titanium-metal-matrix composites (Ti-MMC) are materials with very large specific resistance and potential operative temperature up to 800° C. At present these composites are produced by Hot Isostatic Pressing (HIP), a reliable but expensive manufacturing method. To cut production costs, Centro Sviluppo Materiali SpA (CSM) has developed and patented an experimental plant for co-rolling at high temperature sheets of titanium alloy and silicon carbide monofilaments fabrics. The experimental Roll Diffusion Bonding (RDB) pilot plant permits a reduction of process costs of about 40% with respect to the HIP process. This work reports the results of microstructural and mechanical examinations carried out on composites realized by RDB and HIP. The comparison shows that the fibre-matrix interface is stable in both the composites while the mechanical properties of RDB composite are better due to its smaller grain size and high dislocation density.

The aim of this paper is to investigate the behaviour in terms of drilling forces and roughness of Metal Matrix Composites (MMC) in hot drilling machining. In particular, Al2009/(SiC)w, Al6061/(SiC)w, and Al6061(Al2O3)p metal matrix composites were used, and the adopted temperature were in the range 20°C-160°C. A comparison with drilling at room temperature has been discussed. The results have shown the sensible influence of the working temperature on drilling forces and on surface material properties. In the case of Al2009/(SiC)w a minimum in the drilling forces has been found, making possible the dry machining and improving the cutting conditions. Instead, for Al6061/(SiC)W and Al6061(Al2O3)p in the used temperature range no minimum appears.

The paper reports the results of an extensive characterization of the Ti6Al4V-SiCf composite produced by hot isostatic pressing (HIP) to assess its capability to withstand the in-service conditions of turbine blades operating at middle temperatures in aeronautical engines. The microstructure of composite, in as-fabricated condition and after long-term heat treatments (up to 1,000 hours) in the temperature range 673-873 K, has been investigated by means of different techniques. Particular attention was paid to the micro-chemical evolution of fibre-matrix interface which is scarcely affected also by the most severe heat treatments examined here. This leads to stable mechanical properties as evidenced by hardness, tensile and FIMEC instrumented indentation tests. Therefore, the composite can operate at the maximum temperature (873 K) foreseen for its aeronautical applications without remarkable modifications of its microstructure and degradation of mechanical properties. The mechanical characterization has been completed by internal friction and dynamic modulus measurements carried out both at constant and increasing temperature, from 80 to 1173 K.

The composite, consisting of Ti6Al4V matrix reinforced by unidirectional SiC fibres (SCS-6), has been investigated by mechanical spectroscopy at temperatures up to 1,173 K. For comparison, the same experiments have been performed on the corresponding monolithic alloy. The internal friction (IF) spectrum of the composite exhibits a new relaxation peak superimposed to an exponentially increasing background. This peak, which is not present in the monolithic alloy, has an activation energy H = 186 kJ mol-1 and a relaxation time 0 = 2.3 x 10-15 s. The phenomenon has been attributed to a reorientation of interstitial-substitutional pairs in the  phase of Ti6Al4V matrix around the fibres. This explanation is supported by the results of micro-chemical characterization carried out by X-ray photoelectron spectroscopy (XPS) combined with Ar ion sputtering.

Roll Diffusion Bonding (RDB) is a new process, developed at C.S.M., for producing Ti composites reinforced by long fibres. The prototypal “diffusion bonding” plant permits to co-roll at high temperature in superplastic rolling field (under temperature and strain rate control) foils of titanium alloy and fabrics made of SiC monofilaments. This study evidenced that the Ti6Al4V-SiCf composite produced by roll-bonding exhibits superior mechanical properties with respect the same material prepared by Hot Isostatic Pressing (HIP) owing to the smaller grain size and the higher dislocation density.