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Constitutive Equations for plastic behaviour of metals and alloys

The design of components, operating under demanding conditions of stress and temperature, requires the use of constitutive equations able to describe the mechanical behaviour of materials.
The development of such equations, based on physical modelling of damage and deformation of metals and alloys, is one of the main task in Milan branch of ICMATE.

A great amount of work has been dedicated to modelling the uniaxial creep behaviour of nickel based superalloys used for gas turbines. An accurate description of deformation and damage mechanisms operating in the alloy as a function of the test conditions, allows to extrapolate the mechanical behavior of the alloy at times, stresses and temperatures the components experience during the service, namely:

  • Longer times to rupture (lower stress and temperatures with respect to the ones experienced during a standard creep test);
  • Variable loads and temperature;
  • Triaxial stress state.

Part of the research activity is focused on plastic behaviour concerning instantaneous deformations at low and high temperatures in order to refine constitutive models based on dislocation density and microstructural parameters, such as grain size in polycrystalline metal alloys, width of lamellar channels in ferritic pearlitic ductile irons, and ferritic-austenitic phase volume fractions and characteristic dimensions in austempered ductile irons.

  • Creep strain and creep rates for a nickel based superalloys (experimental and modelled data)
  • Experimental and modelled stress relaxation data (Bolts steeel)

The plastic deformation modelling consists of kinetic equation of plastic flow and strain hardening. The hardening behaviour results from the balance between two opposite and concomitant micro-plastic phenomena:

  • multiplication and storage of dislocations that increases dislocation density and is sensitive to microstructure;
  • dynamic recovery that reduces the dislocation density.

The research on plastic deformation consists of identifying the kinetic equation and the contribution of the two micro-mechanisms ? in function of the alloy microstructure.

Experimental and modelled tensile data (stainless steel) at high temperatures and dislocation structure
  • KEYWORDS:
    modelling creep fatigue stress relaxation plastic deformation
  • RECENT PUBLICATIONS:
    • G. Angella
      Strain hardening analysis of an austenitic stainless steel at high temperatures based on the one-parameter model
      Materials Science & Engineering A, 532 (2012), pp. 381-391.
    • G. Angella, R. Donnini, D. Ripamonti, M. Maldini
      Combination between Voce formalism and improved Kocks-Mecking approach to model small strains of flow curves at high temperatures
      Materials Science & Engineering A 594 (2014) pp.381-388.
    • G. Angella, D. Della Torre, R. Donnini, M. Maldini, D. Ripamonti, F. Pero, E. Poggio, A. Riva, A. Sanguineti
      Stress relaxation modeling using creep data
      Proceedings of 10th Conference on Materials for Advanced Power Engineering (Eds: J. Lecomte-Beckers, O. Dedry, J. Oakey, B. Kuhn), Liege (BE), 09/2014, pp.121-130.
    • G. Angella, R. Donnini, D. Ripamonti, M. Maldini
      The role of particle ripening on the creep acceleration of Nimonic 263 superalloy
      Proceedings of Eurosuperalloys 2014 Conf., MATEC Web of Conferences 14-14001 (2014), pp.1-6.
    • A. Riva, M. Maldini
      Stress relaxation modelling
      Proceedings of ASME Turbo Expo 2015, Montreal (CA), 06/2015.