C. Argyrakis

Bio: C. Argyrakis is an academic researcher from Rolls-Royce Holdings. The author has contributed to research in topics: Recrystallization (metallurgy) & Grain size. The author has an hindex of 2, co-authored 2 publications receiving 67 citations.

The Effect of Strain Distribution on Microstructural Developments during Forging in a Newly Developed Nickel Base Superalloy

Abstract: In the current study, the effect of strain distribution in a simple forging geometry on the propensity for recrystallization, and its impact on mechanical properties has been investigated in a newly developed experimental nickel-based superalloy. The new alloy was produced via a Powder Metallurgy (PM) route and was subsequently Hot Isostatic Processed (HIP), isothermally forged, and heat treated to produce a coarse grain microstructure with average grain size of 23–32 μm. The alloy was examined by means of Electron Back-Scatter Diffraction (EBSD) to characterise the microstructural features such as grain orientation and morphology, grain boundary characteristics and the identification of potential Prior Particle Boundaries (PPBs) throughout each stage of the processing route. Results at the central region of the cross-section plane parallel to the loading direction showed significant microstructural differences across the forging depth. This microstructural variation was found to be highly dependent on the value of local strain imparted during forging such that areas of low effective strain showed partial recrystallisation and a necklace grain structure was observed following heat treatment. Meanwhile, a fully recrystallised microstructure with no PPBs was observed in the areas of high strain values, in the central region of the forging.

Development of a Novel Methodology to Study Fatigue Properties using the Small Punch Test

Abstract: Small scale mechanical test methods are now widely recognised as an established and quantifiable means of obtaining useful mechanical property information from limited material quantities. Much research has been gathered employing such approaches, but to date these methods have largely been restricted to characterising the creep, tensile and fracture characteristics of numerous materials and alloys through the small punch (SP) test. Clearly, a key element that is missing from this list of fundamental mechanical properties is understanding the cyclic response of the material, a significant form of damage that accounts for a large proportion of in-service failures in critical structural components. Therefore, in order to profit from the numerous benefits that SP testing has to offer, including a small sample size and hence reduced cost, a small scale fatigue testing methodology is now required to provide a holistic mechanical property evaluation. Such an innovative approach would provide real potential benefit to the engineering mechanical characterisation community. This paper will discuss the development and implementation of this highly bespoke SP fatigue testing methodology that can accommodate alternative loading ratios and frequencies to mimic conventional fatigue data. A number of novel experiments have been performed on the titanium alloy Ti-6Al-4V with accompanying analysis and fractography detailed. Numerical correlations to uniaxial fatigue data is also presented through the use of Finite Element Analysis.

Development of the Metal Rheology Model of High-Temperature Deformation for Modeling by Finite Element Method

Abstract: It is shown that when modeling the processes of forging and stamping, it is necessary to take into account not only the hardening of the material, but also softening, which occurs during hot processing. Otherwise, the power parameters of the deformation processes are precisely determined, which leads to the choice of more powerful equipment. Softening accounting (processes of stress relaxation) will allow to accurately determine the stress and strain state (SSS) of the workpiece, as well as the power parameters of the processes of deformation. This will expand the technological capabilities of these processes. Existing commercial software systems for modeling hot plastic deformations based on the finite element method (FEM) do not allow this. This is due to the absence in these software products of the communication model of the component deformation rates and stresses, which would take into account stress relaxation. As a result, on the basis of the Maxwell visco-elastic model, a relationship is established between deformation rates and stresses. The developed model allows to take into account the metal softening during a pause after hot deformation. The resulting mathematical model is tested by experiment on different steels at different temperatures of deformation. The process of steels softening is determined using plastometers. It is established experimentally that the model developed by 89 ... 93 % describes the rheology of the metal during hot deformation. The relationship between the components of the deformation rates and stresses is established, which allows to obtain a direct numerical solution of plastic deformation problems without FED iterative procedures, taking into account the real properties of the metal during deformation. As a result, the number of iterations and calculations has significantly decreased.