Papers by Author: Maurizio Maldini

Abstract: 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.

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 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 creep degraded nickel base single crystal superalloy CMSX-4 of two axial orientations [001] and [111] was investigated with aim to assess the structure degradation. Constant load creep tests were conducted in the stress/temperature ranges of 250–780 MPa/750 – 50°C resulting in rupture time variation from 50 to 4000 hours. A combination of scanning electron microscopy (SEM) and non-destructive small-angle neutron scattering method (SANS) was used to investigate the directional coarsening (rafting) of the gamma prime (γ') precipitates in relation to the stress and temperature applied as well as to the initial crystallographic orientation of the specimens. The SANS results are discussed in terms of the correlation with the raft development, the axial orientation of specimen, the creep parameters and the mechanical properties.

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.