Papers by Author: Giuliano Angella

Abstract: The tensile curves of AISI 316L deformed at temperatures between 700 and 1000 °C in the strain rates range 10^-5-10^-2 s^-1 are modelled with the Voce equation, starting from strain hardening analysis. The parameters, needed to draw the Voce equation, are the saturation stress σ_V, the critical strain ε_C and the stress σ_o, that respectively define the height of the flow curve, the velocity to achieve σ_V and namely the back-extrapolated flow stress to zero strain. A two-parameter model of strain hardening recently proposed [ is used to analyze the strain hardening rate, dσ/dε, vs. the flow stress, σ. Through this analysis, σ_V, ε_C and the thermal activation of plastic flow s are obtained. In fact, the two-parameter model assumes that s and the total dislocation density ρ are the only two parameters needed to describe strain hardening. It has been reported [ that the parameter s can be parameterised in terms of strain rate and temperature and, furthermore, relationships between σ_V, σ_o, ε_C and s can be established. At this stage, the Voce equation can reproduce the experimental tensile curves at the explored temperatures and strain rates. However, the obtained Voce equations can well describe the tensile curves at large strains, while significant discrepancy occurs at small strains [. Preliminary results of an improved model based on two coupled differential equations with physical meaning are reported to correct this discrepancy at low strains.

Abstract: The present contribution is aimed at investigating the microstructure evolution of commercially pure silver under severe plastic deformation conditions. ECAP billets have been produced by using a die with channels intersecting at 90° and straining the samples at room temperature. The evolution of the microstructure as a function of imparted strain was evaluated by scanning electron microscopy as well as X-ray diffractometry. Furthermore, tensile properties were measured from ECAP billets in order to evaluate the strengthening and work hardening behaviour of silver as a function of structure evolution. Comparison in terms of grain structure and corresponding properties are also drawn by considering published data about Al-Mg-Si alloy samples ECAP-processed by identical routes and parameters.

Abstract: The flow curves of an austenitic stainless steel deformed at temperatures 700-1000°C with strain rates 10^-5-10^-2 s^-1 were modelled with the Voce equation. The parameters needed to draw the Voce equation, are the saturation stress σ_V that defines the height of the flow curve, the critical strain ε_C that defines the velocity to achieve σ_V, and the stress σ_o, namely the back-extrapolated flow stress to zero strain. A modified strain hardening analysis based on the one-parameter model was used to analyze the strain hardening rate dσ/dε vs. the flow stress σ in order to obtain σ_V and ε_C. The modified approach was based on the assumption that the dislocation multiplication component of strain hardening was temperature and strain rate dependent through the thermal activation term s of flow stress. A parameter s’ proportional to s was obtained from the strain hardening analysis and a relationship between s’ and temperature and strain rate was found. Relationships between σ_V, σ_o, ε_C and s’ were finally established and at this stage the Voce equation could reproduce the experimental flow curves at any imposed deformation conditions of temperature and strain rate.

Abstract: The objective of this paper is to critically analyse the effect of the ’ morphology evolution on the creep strain rate behaviour in the temperature range 900 - 1100°C where rafts form in superalloys for single crystal turbine blade and vane applications. A close examination of the experimental results has shown different regimes of strain accumulation depending on the value of the applied stress and temperature. The experimental results have been rationalised in terms of the ’ shape evolution during creep.

Abstract: The high temperature creep and fatigue properties of two  -TiAl base intermetallic alloys, for gas turbine components, have been investigated within the Integrated European project IMPRESS. The alloys contain 8% at. of Ta or Nb, respectively. The microstructure of both alloys was cross convoluted lamellar rather than the well known conventional lamellar, typical of the usual -TiAl. The microstructure of the Ta containing alloy was homogeneous in all the analyzed batches whilst that of the Nb alloy appeared significantly spread out from specimen to specimen. The creep properties of the alloys were investigated in the temperature range 700-850°C with applied stresses in order to have times to rupture up to about 3,000 h. The creep behaviour presented no steady state regimes, but only minima of the creep rates between significant decelerating and accelerating regimes. The minimum creep rates of the Ta alloy resulted to be significantly slower than the Niobium alloy at the same creep conditions. In low cycle fatigue at 650 and 700°C the Ta  -TiAl showed longer lives than the Nb alloy, whilst the fatigue crack propagation rate in the same temperature range did not show any significant difference. Threshold values of stress intensity factor range were accurately measured at different R ratio. The microstructures of the two alloys were analysed by scanning microscopy in order to rationalise the different mechanical behaviour.

Abstract: The work hardening and tensile behaviour of AISI 316L austenitic stainless steel deformed at temperatures between 600 and 1000°C has been investigated. The alloy has been heat-treated according two different roots: the first solutioning treatment was imposed at 1100°C and the second was designed at 875°C to reduce the dynamic ageing that occurs at temperatures up to 650°C in AISI 316L. In the solutionised material the work hardening data presented two linear regions: at high stresses the linearity has been described as the conventional Stage III of work hardening, whilst at low stresses the linearity has not been rationalised in any conventional stage of work hardening. In the second heat treatment alloy the work hardening data showed a single linear region at high stresses, whilst no linear stage occurred at low stresses. Therefore, the work hardening and tensile behaviour of AISI 316L has resulted to be significantly affected by the two different heat treatments and dynamic aging has been proved to influence work hardening behaviour well beyond the range of temperatures in which serrated yielding occurs.

Abstract: The creep behaviour of the solid solution strengthened nickel-based superalloy Haynes 230 has been investigated under constant load and temperature conditions on as received, after conventional solution treatment, and on overaged conditions. The experimental results have shown a very strong dependence of the creep curve shape with the applied stress/temperature: in the tests performed at high stresses/low temperatures, the primary/decelerating stage takes an important portion of the creep curve. At these test conditions, the accelerating creep is mainly caused by the increase of the applied stress with the strain as it happens in constant load creep tests. In the tests performed at low stresses/high temperatures, the primary stage is very small and the following accelerating creep is characterized by different accelerating creep stages. The analysis of the creep curves on the as received and overaged alloys, has shown that a large portion of the accelerating creep at low stresses/high temperatures is caused by microstructural instability.

Abstract: The possibility of refining the grain structure of commercial Al-Mg-Si and Al-Mg-Mn alloys was evaluated using the asymmetric rolling technique in the severe plastic deformation regime. The study demonstrated that asymmetric rolling can readily promote the achievement of an ultrafine grained structure in the alloys investigated. Comparative analyses showed that by increasing the asymmetry ratio and by alternating the shear direction during rolling, higher hardness values and reduced crystallite size could be obtained in the alloys.

Abstract: A study was carried out on a ECAP processed Sc-containing Al-Mg-Si alloy and on a reference 6082 alloy to investigated grain structure evolution during severe plastic deformation and post-ECAP aging behaviour. The results showed that the mechanism of ultrafine structure development was substantially unchanged with respect to a reference Sc-free alloy. Also the aging sequence and precipitation kinetics of the two alloys revealed to be comparable. The ECAP processed samples of the 6082 reference alloy showed a clear recrystallization peak at temperatures in the range 315-360°C, depending on the amount of strain experienced, whereas the Sc-containing alloy retained its ultrafine structure up to temperatures well exceeding 450°C, under the conditions reproduced in a DSC temperature scan.