Cecilia Poletti

Characterisation and quantification of cavities in 9Cr martensitic steel for power plants

Abstract This work focuses on the characterisation of cavities evolution in a P91 steel pipe in three conditions: as received, and after creep at 650°C and 60 MPa for 7000 and 9000 h. A field emission gun scanning electron microscope (FEG-SEM) equipped with focused ion beam (FIB) gun, a conventional scanning electron microscope (SEM) and a light optical microscope (LOM) have been employed for the investigation. This study reveals two types of cavities: the pre-existing cavities, which are rare in this type of heat resistant steels, with a mean diameter of 2·56 μm and the cavities produced during creep with diameters smaller than 0·6 μm. Lath boundaries and precipitates are found to be preferential sites for cavity nucleation. Furthermore, the number density and volume fraction of these small cavities are calculated from 2D measurements and compared to 3D results obtained by FIB serial sectioning.

Investigation of the Precipitation Kinetics of a New Al-Mg-Sc-Zr Alloy

This study investigates the precipitation behavior of two similar cast Al-Mg-Sc-Zr alloys with different solidification and cooling rates. Microstructural analyses and hardness testing were performed after casting and ageing. In addition, the precipitation and hardness evolution after rapid quenching using electron beam re-solidification were studied. It was shown that the amount of Al3(Sc,Zr) phases present after casting or re-solidification increases with decreasing solidification and cooling rate. Consequently, the degree of supersaturation in Sc and Zr at room temperature and thus the hardening potential in a subsequent ageing process increases with the solidification and cooling rate. Therefore, the electron beam re-solidified samples revealed the most pronounced hardening.

Investigation of the Hot Deformation Behavior of an Al-Mg-Sc-Zr Alloy under Plane Strain Condition

This study investigates the hot deformation behavior of a new Al-Mg-Sc-Zr alloy under plane strain conditions. Flow curves corrected for deformation heating were calculated for strain rates between 0.01 and 10s-1 in a temperature range of 200 to 400°C. To evaluate the deformation behavior, strain rate sensitivity as well as flow localization parameter maps were calculated for strains of 0.2, 0.4, and 0.6. In addition, microstructural investigations and hardness measurements were performed for selected samples. It was shown that the flow stress decreased with deacreasing strain rate and increasing temperature. The best formability was observed for high strain rates and low temperatures as well as for low strain rates and high temperatures. In these cases no flow instabilities were observed.

Investigation of Pre‐Existing Pores in Creep Loaded 9Cr Steel

Creep failure of materials under service conditions strongly rely on the formation and growth of cavities, encouraging the characterization and modelling of the cavitation process. In the present work pre-existing pores from manufacturing process are investigated in 9Cr steel creep loaded for up to 9000 hours. Scanning electron microscopy (SEM) is used for 2D analysis while Computer tomography (CT) is employed for 3D exploration. Nearest neighbours distances in 3D are calculated from 2D measurements and are decreasing with creep exposure time. The pore growth is studied applying a physical growth model, and experimental results are compared with numerical simulation. From this research it is deduced that damage occurs by agglomeration and growth of pre-existing cavities. The developed model can predict the growth of pores as a function of temperature and load at service.

Microstructure Evolution of Allotropic Materials during Thermomechanical Processing

The microstructure developed during hot deformation is the result of deformation mechanisms such as dynamic recovery and dynamic recrystallization. Hot deformation can also result in damage and flow localisation, especially in multiphase metal based materials. Several models have been proposed to correlate the parameters of the deformation process (temperature, strain and strain rate) with the flow behaviour such as the processing maps. They were developed based on the dynamic materials model (DMM) and later a modified DMM introduced some changes in the calculation of the processing maps. The correlation of the relevant microstructural changes with thermodynamic parameters are tested and discussed. The data was obtained by using the Gleeble simulator with in situ quenching facilities. Microstructural studies related to the hot deformation of metals were carried out based on alpha-beta and near beta titanium alloys and on low carbon steels. The results are correlated with the efficiency of power dissipation, and the constitutive equations. In diffusion controlled processes such as dynamic recovery, dynamic recrystallization, phase transformation and pore coarsening are related to high power efficiency, and to low n exponent. The efficiency of power dissipation is more sensitive to the deformation parameters than the constitutive equations for materials with phase transformation.

Agile Multiscale Modelling of the Thermo-Mechanical Processing of an Aluminium Alloy

The multiscale modelling of the behaviour of metal alloys during processing is often limited by the computing power required to run them. The Agile Multiscale Methodology was conceived to enhance the designing and controlling of complex multiscale models through an automatic run-time adaptation of its constitutive sub-models. This methodology is used to simulate the behaviour of an 6082 aluminium alloy during its thermomechanical treatment. The macroscopic deformation, the work-hardening and the state of precipitation are computed in different modules, allowing the coupling of several software solutions (DEFORMTM2D and

Friction Stir Welding of Aluminum Metal Matrix Composite Containers for Electric Components

For aerospace applications, light-weight boxes to protect and carry electronic equipment need to be sealed. The main requirements on the components are low thermal expansion and gas tightness. The common material for such an application is a metal matrix composite (MMC). The MMC suggested here consists of A356 aluminum alloy matrix with 15 vol.% SiC particle reinforcement. A safe limit for the electronic component inside the boxes during sealing is determined to be 180°C. Due to the boundary conditions gas tightness and low heat input, Friction Stir Welding (FSW) might be an alternative to the employed joining techniques. For the FSW process the T-Joint is the most appropriate joint geometry in respect to the box design. The geometry of the lid has to ensure the backing system for the stir zone inside the box. A successful welding of the box was done after a joint geometry optimization. The examination of the welded box concerns material characterization with microscopic methods, measuring thermal expansion in base material and stir zone and temperature measurement while FSW.

Determination of the Mechanism of Restoration in Subtransus Hot Deformation of Ti-6Al-4V

Titanium alloys are attractive for structural applications in the aerospace industry due to their high specific strength in comparison with other engineering materials. These properties are strongly related to the microstructure obtained during thermo-mechanical processes. The influence of the processing parameters on the microstructure is investigated to determine criteria for the control of the forming processes. Pre-forged specimens of alpha-beta Ti-6Al-4V alloy with elongated primary alpha grains are deformed below the beta transus temperature between 0.1 and 10/s of strain rate. Compression is carried out parallel and perpendicular to the preferential orientations of the primary alpha grains. The local strain within the compressed samples is determined by finite element methods and correlated to the microstructure observed there. The alpha content is affected by the temperature of deformation and the morphology of the alpha grains is influenced by the strain and strain rate. Specimens with previous primary alpha grains parallel to the compression axis show a rotation of the alpha grains which were oriented almost perpendicular to the load axis. EBSD measurements are used to determine the restoration mechanism involved during hot deformation. Continuous dynamic recrystallization in the alpha grains is revealed by increasing the cumulative crystallographic misorientation towards the grain boundary and the formation of new grains. This misorientation increases with increasing values of the Zener Hollomon parameter (Z). For lower values of Z restoration occurs mainly in the beta phase.

Enhancing mechanical properties of wires by a novel continuous severe plastic deformation method

Abstract Commercially pure Al wires are severely plastic deformed by a novel method called equal channel angular torsion drawing (ECATD) up to four passes. Initial wires are drawn through an equal channel angular die and simultaneously torsion deformed by turning the ECATD die. The wires are deformed up to an equivalent strain of 1 to 4 (based on FE result) at room temperature. The microstructural evolution of the wires is investigated using optical microscopy of both longitudinal and transverse cross-sections. A grain refinement from 100 μm to a mean grain size of 1–10 μm is achieved mainly at the areas near the surface of the wires. A decreasing trend of grain refinement is observed from the edge area to the wire center due to the non-uniform strain distribution, resulting in an inhomogeneous hardness. A significant increase in hardness is obtained from ∼22 HV to ∼43 HV at the wire center and to ∼60 HV at the wire edge, this confirms simulated equivalent plastic strain. The most important advantage of this process is the ability to impose continuous large plastic deformation on wires. It can be used as an industrial method for continuous strain hardening and grain refinement of wires.

Flow Localization Modelling in Ti Alloys and Ti Matrix Composites

Titanium-based alloys are commonly applied to aerospace, medicine and energy due mainly to their high specific mechanical properties and high corrosion resistance. Reinforcement with particles further improves their specific strength and stiffness. In previous studies, the hot formability of both unreinforced Ti6Al6V2Sn alloy, and reinforced with 12%vol of TiC particles was analyzed by hot compression tests carried out by means of Gleeble device and metallography. It was observed that the hot workability of these materials is limited at given forming conditions by non-desirable shear bands, voids, as well as micro- and macro cracks, especially in the composite. In this work, damage during hot deformation is predicted by damage models coupled to FEM. Therefore, the flow localization parameter α described by thermal and microstructural softening and the strain rate sensitivity are computed and implemented in DEFORMTM 2D to describe the localization of the plastic flow. The results show intense flow localization as a combination of low dynamic restoration (given by small m values) and temperature gradient. The damage analysis combined with the Cockcroft and Latham continuum cumulative stress model.