3D finite element and experimental study of the size requirements for measuring toughness on tempered martensitic steels

The fracture properties of the tempered martensitic steel Eurofer97, which is among the main candidates for fusion power plant structural applications, were studied with two sizes of pre-cracked compact specimens (0.35T C(T) and 0.87T C(T)). The fracture toughness behavior was characterized within the temperature range -80 to -40 ºC. The ductile-to-brittle transition reference temperature, as defined in the ASTM standard E1921, was around T0 ≈ -75 ºC. At -60 °C, it was found that two sets of toughness data obtained with 0.35T and 0.87T C(T) specimens are not consistent with the size adjustments recommended in the ASTM standard. It was then shown that the underlying reason of this inconsistency is an inappropriate specimen size limit of the ASTM standard for this type of steel. From published fracture toughness data on the tempered martensitic steel F82H steel, similar results were also highlighted. 3D finite elements simulations of the compact specimens were performed to compare the stresses and deformations at the onset of fracture. A local approach model based on the attainment of a critical stress and a critical volume was used to study the constraint loss phenomenon. Within the framework of this model, the strong toughness increase by reducing the specimen size could be satisfactorily explained

Measurement of residual stresses around the notch of tensile specimens of the high-Cr tempered martensitic steel F82H-mod

Residual elastic strains were measured by neutron diffraction using POLDI facility at PSI-Switzerland on pre-loaded notched flat tensile specimens made of the high-chromium tempered martensitic steel F82H-mod steel. To calculate the residual stresses using Hooke’s equation, three perpendicular components of the residual strain field were determined. The measured residual strains and stresses were compared with those deduced from finite element simulation calculations. A very good agreement was found for the strains in the loading plane of the specimen while a somewhat larger discrepancy was observed for the out-of-plane residual strain, which was tentatively attributed to an uncertainty in the initial lattice spacing in that directio

Loss of strength in Ni3Al at elevated temperatures

Stress decrease above the stress peak temperature (750 K) is studied in h123i single crystals of Ni3(Al, 3 at.% Hf ). Two thermally activated deformation mechanisms are evidenced on the basis of stress relaxation and strain rate change experiments. From 500 to 1070 K, the continuity of the activation volume/temperature curves reveals a single mechanism of activation enthalpy 3.8 eV/atom and volume 90 b3 at 810K with an athermal stress of 330 MPa. Over the very same temperature interval, impurity or solute diffusion towards dislocation cores is evidenced through serrated yielding, peculiar shapes of stress–strain curves while changing the rate of straining and stress relaxation experiments. This complicates the identification of the deformation mechanism, which is likely connected with cube glide. From 1070 to 1270 K, the high-temperature mechanism has an activation enthalpy and volume of 4.8 eV/atom and 20 b3, respectively, at 1250 K

Microstructure and tensile properties of ODS ferritic steels mechanically alloyed with Fe2Y

An oxide dispersion strengthened ODS ferritic steel has been produced by mechanical alloying of Fe–14Cr–2W–0.2Ti (wt.%) prealloyed powder with 0.55 (wt.%) Fe2Y intermetallic particles and consolidated by hot isostatic pressing. The microstructure after thermal treatments confirms the homogeneous precipitation of Y-Ti oxides with nanometric sizes. Tensile properties as a function of the testing temperature from room temperature to 973 K have been measured and the results are discussed with respect to similar ODS ferritic steels fabricated by a powder metallurgy route using Y2O3 powder

Multiscale modelling for fusion and fission materials: the M4F project

The M4F project brings together the fusion and fission materials communities working on the prediction of radiation damage production and evolution and its effects on the mechanical behaviour of irradiated ferritic/martensitic (F/M) steels. It is a multidisciplinary project in which several different experimental and computational materials science tools are integrated to understand and model the complex phenomena associated with the formation and evolution of irradiation induced defects and their effects on the macroscopic behaviour of the target materials. In particular the project focuses on two specific aspects: (1) To develop physical understanding and predictive models of the origin and consequences of localised deformation under irradiation in F/M steels; (2) To develop good practices and possibly advance towards the definition of protocols for the use of ion irradiation as a tool to evaluate radiation effects on materials. Nineteen modelling codes across different scales are being used and developed and an experimental validation programme based on the examination of materials irradiated with neutrons and ions is being carried out. The project enters now its 4th year and is close to delivering high-quality results. This paper overviews the work performed so far within the project, highlighting its impact for fission and fusion materials science.This work has received funding from the Euratom research and training programme 2014-2018 under grant agreement No. 755039 (M4F project)

On the strain-hardening of tempered martensitic alloys

The strain-hardening behaviour of a tempered martensitic iron-chromium-carbon model was examined in terms of dislocation mechanics. Tensile tests of this body-centered cubic alloy were carried out from 77 up to 293 K. A specific relationship between the flow stress a and the dislocation density p was considered below the transition temperature (T < 200 K) where the Peierls lattice friction becomes the single rate controlling mechanism. This relationship is different from the usual Taylor equation, where the flow stress scales with the square root of the dislocation density. This sigma-rho relationship was used in conjunction with an equation characterizing the dislocation evolution with strain to derive a strain- hardening law. The parameters that play a key role in the strain-hardening are the mean dislocation free path and a dislocation annihilation coefficient. The values of these parameters used to describe the strain-hardening law as well as their temperature dependence are found to be consistent with the microstructure. (c) 2005 Elsevier B.V. All rights reserved

On determination of the constitutive behavior of tempered martensitic steels from micro-indentations: Application to Eurofer97 steel

This paper proposes a simple but powerful approach to determine the constitutive equation of tempered martensitic steels from micro-hardness tests. Finite element simulations were used to investigate the plastic flow in the contact region between the tip and the specimen. The simulations were validated by experimental tests carried out on the tempered martensitic steel Eurofer97 using a simple constitutive Ludwik-type equation. A series of simulations using different constitutive behaviors representative of possible irradiation-induced changes were run. In all cases, a pile-up of material against the indenter tip was observed that is strongly dependent on the constitutive law. Considering the real contact height of the indenter with the material, it was shown that the hardness scales with an averaged value of the flow stress over 30% of plastic strain. In addition, the parameters of Ludwik equation were shown to be determinable from the two experimental quantities that are the hardness and the ratio between the contact height and the penetration depth of the tip. (C) 2013 Elsevier B.V. All rights reserved

Assessment of mechanical properties of the martensitic steel EUROFER97 by means of punch tests

The ball punch test technique was used to evaluate the conventional tensile and impact properties of the tempered martensitic steel EUROFER97 from room temperature down to liquid nitrogen temperature. The testing was carried out on unirradiated material only with small disks, 3 mm in diameter and 0.25 mm in thickness. For comparison, tensile tests were also performed over the same temperature range. Correlations between the load at the plastic bending initiation and the maximum load of the punch tests with the yield stress and the ultimate tensile stress of the tension tests could be established. The temperature dependence of the specific fracture energy of the punch test was used to define a ductile-brittle transition temperature (DBTT) and to correlate this with the DBTT measured from impact Charpy on KLST specimens. The results are compared with other available correlations done in the past on other ferritic steels. (C) 2002 Elsevier Science B.V. All rights reserved