A novel procedure for material quality assessment developed for castings like ductile irons and Al alloys, is based on the analysis of tensile strain hardening through dislocation-densityrelated constitutive equation, and consists of plotting the Voce equation parameters found through modeling the tensile flow curves with the Voce constitutive equation. In sound materials the Voce parameters have a regular trend, consistent with the physical meaning of the dislocation-densityrelated Voce constitutive equation. The Voce parameters identify a regular trend also in defective materials, even if defects and metallurgical discontinuities might be expected to add a random and unpredictable component to the plastic behavior. This unexpected regular behavior in defective materials has been called as Defects-Driven Plasticity (DDP), and its rationalization seems to be possible by coupling the concepts of Notch Strengthening (NS) of defects, and stable ductile fracture propagation of the Continuous Damage Mechanics (CDM). The rationalization of DDP and the experimental findings to support, are here reported in high Silicon strengthened ductile irons.

The microstructure and tensile behavior of two heavy section castings that had chemical compositions typical of GJS400 were investigated. Conventional metallography, fractography, and micro-Computer Tomography (?-CT) were employed, enabling the quantification of the volume fractions of eutectic cells with degenerated Chunky Graphite (CHG), which was identified as the major defect in the castings. The Voce equation approach was exploited to evaluate the tensile behaviors of the defective castings for integrity assessment. The results demonstrated that the Defects-Driven Plasticity (DDP) phenomenon, which refers to an unexpected regular plastic behavior related to defects and metallurgical discontinuities, was consistent with the observed tensile behavior. This resulted in a linearity of Voce parameters in the Matrix Assessment Diagram (MAD), which contradicts the physical meaning of the Voce equation. The findings suggest that the defects, such as CHG, contribute to the linear distribution of Voce parameters in the MAD. Furthermore, it is reported that the linearity in the MAD of Voce parameters for a defective casting is equivalent to the existence of a pivotal point in the differential data of the tensile strain hardening data. This pivotal point was exploited to propose a new material quality index assessing the integrity of castings.

At the end of 2022 the last creep test of the originally agreed test matrix reached its rupture. With the conclusion of such test, the task is concluded. The test data have been communicated to the relevant groups. Tests on interaction between fatigue and creep have been performed.

Risultati intermedi sull'indagine del comportamento a creep di una superlega base Fe-Ni per l'applicazione in una turbina in cui si usa CO2 supercritico come vettore termico. Nr. Prot. 0049206 del 20-02-2023

In 2022the creep tests on EUROFER 97/3 continued according to the details given in the test matrix M2412. In this year, some changes to the test matrix were applied. The present situation of the test campaign is described. Tests on neglegible creep in steel EUROFER97 have been performed.

Ductile irons were produced into different casting wall sections, that is, 25 mm, 5 mm and 3 mm. The alloys were then austenitized with the same conditions at 875 °C for 2 h and austempered for three different combinations of temperatures and times: 250 °C for 6 h, 310 °C for 3 h and 380 °C for 1 h. The aim of the investigation was to study the ausferrite stability of austempered ductile irons with three different nominal contents of nickel produced in thin sections through tensile testing. So, strain hardening analysis of tensile flow curves was carried out since it has been found to be a reliable support to ductility analysis in assessing the optimal austempering conditions. Because of different wall sections, round and flat tensile specimens with geometries complying with ASTM E8/E8M-11 were tested. Austempered ductile irons from 5 and 3 mm wall sections were tested through flat geometry specimens only, while 25 mm wall sections were tested through both round and flat geometries. Though the ausferrite was affected by Ni content and the graphite morphology was improved with reduced thin sections, the ausferrite stability and the tensile mechanical behavior were insensitive to Ni content and section thickness below 25 mm. Furthermore, it resulted that the tensile plastic behavior was sensitive to the specimen geometry in a consistent way, increasing the instability of ausferrite and indicating that a proper analysis and comparison of tensile properties of austempered ductile irons must take into account the tensile specimen geometry.

The structural stability of ausferrite in thin-walled Austempered Ductile Iron (ADI) castings with 5 mm wall thickness is compared to a reference casting with 25 mm wall thickness. The thin-walled and reference castings were first austenitized between 850 and 925 °C, and then austempered between 250 and 380 °C. X-ray diffraction (XRD) investigations with changing temperature were performed between - 260 up to + 450 °C to investigate the change of phase fraction, lattice parameters and strain in ausferrite. The role of the austenitization temperature on structural stability and homogeneity of the investigated ADI castings has been provided. In addition, the problem of the occurrence of "blocky" high-carbon austenite that was not completely involved during austempering, has been taken into account. Finally, it has been shown that the thin-walled castings provided higher structural homogeneity and stability if compared to the reference castings.

In 2021 three creep tests on EUROFER 97/2 were run, according to the details given in the text and in task MAT-T.01.01-T008-D001. The present situation of the test campaign is described.

Ductile Irons (DIs) with different nominal contents of nickel were produced into different casting wall sections, that is, 25 mm, 5 mm and 3 mm. The alloys were then austenitized at the same conditions, and austempered for three different combinations of temperatures and times: 250°C for 6 hours; 310 for 3 hours; 380°C for 1 hour. The aim of the investigation was to understand the combined effects of chemical composition, section thickness and austempering conditions on the tensile mechanical properties of ausferrite. So strain hardening analysis of tensile flow curves was carried out, since it has been found more reliable than ductility assessment in determining the optimal austempering conditions. Because of different wall sections, round and flat tensile specimens with geometries complying on ASTM E8/E8M-11 were tested. ADIs from 5 and 3 mm wall sections were tested through flat geometry specimens only, whilst 25 mm wall sections were tested through both geometries. From 25 mm wall section investigation it resulted that the tensile plastic behavior of ADIs was sensitive to the specimen geometry in a consistent way. The influence of chemical composition and wall section thickness on optimal austempering conditions was found through comparing tensile behaviors from the same geometry specimens, indicating that a proper analysis and comparison of tensile properties of ADIs has to take into account the geometry specimens.

Nickel base superalloys are candidates to be used in shafts for S-CO2 turbines for their excellent high temperature mechanical properties and corrosion resistance. In ICMATE-CNR in the framework of European Project H2020 SOLARSCO2OL, grant agreement n° 952953, several nickel-base superalloys are under investigation through creep tests, simulating the operational working conditions of a S-CO2 turbine.

Solid-state transformation during heat treatment is of great practical importance because it significantly affects the final structure, properties, and thermal stability of cast components. The present study highlights the issue of structure formation and its effect on the thermal stability of high-quality cast iron, namely, austempered ductile iron (ADI). In this study, experiments were carried out for castings with a 25-mm-walled thickness and under variable heat treatment conditions, i.e., austenitization and austempering within ranges of 850 degrees C to 925 degrees C and 250 degrees C to 380 degrees C, respectively. The X-ray diffraction (XRD) investigations were carried out within a range of - 260 degrees C to + 450 degrees C to study the structure parameters related to the XRD tests, which provided information related to the phase participation, lattice parameters, and stresses in the microstructure as well as with an expansion of the crystal lattice. The results also provide insight into the role of the structure and its homogeneity on the thermal stability of ADI cast iron. The present work also aims to develop strategies to suppress the formation of blocky-shaped austenite in the ADI structure to maintain a homogeneous microstructure and high thermal stability.

The analysis of the structural integrity of components in Al alloys produced by High Pressure Die Casting (HPDC) is paramount for identifying a correlation between process-microstructure-mechanical properties, a correlation that allows to classify the quality process parameters, and identify potential application areas. The tensile test represents the simplest method to pursue this aim, and the tensile strain hardening analysis seems to be capable of providing interesting correlations. In fact, an innovative method based on the analysis of strain hardening through the formalism of the constitutive equation of Voce has provided interesting results in other foundry materials, such as spheroidal cast irons. In this work, we report the results of the application of the innovative method to an Al alloy produced by HPDC in two components having two different geometries. One geometry is that of a round tensile specimen, adopted as a reference since it has an "ideal" microstructure according to the PD CEN/TR 16748:2014 standard, and a second one is a real U shape geometry. The innovative procedure has uniquely identified that the Al alloy with "ideal" microstructure has better microstructural integrity of the material from the U geometry casting, demonstrating that the innovative procedure can also be used successfully for Al alloys produced by HPDC.

The new nickel-base super alloy AF955 (R) (UNS N09955) was developed by Foroni S.p.A. in response to the industrial need for a material that preserves the excellent properties of the precipitation hardened nickel-based superalloys, while also providing adequate resistance to hydrogen embrittlement. A wide mechanical and microstructure characterization of the new superalloy was carried out to rationalize the effects of the production route and different heat treatments of alloy AF955 (R). Mechanical properties were evaluated by Rockwell hardness and tensile tests. The microstructure was investigated by conventional Optical Microscopy (OM) and Scanning Electron Microscopy (SEM) with micro-chemical measurements with Energy Dispersion Spectroscopy (EDS) for a general and phase precipitation assessment, and by Transmission Electron Microscopy (TEM) for a quantitative characterization of the nanometric precipitation strengthening phases, namely gamma prime (gamma') phase and gamma double prime (gamma '') phase. Quantitative relationships between mechanical properties and strengthening precipitation phases, were obtained by using the Weakly Coupled Dislocation (WCD) model and the Strongly Coupled Dislocation (SCD) model. The WCD model for deformable particles resulted successful to correlate the quantitative microstructure results obtained through TEM with the yield strength of the new superalloy after two different heat treatments.

Ferritic ductile iron (DI) GJS 400 and high-silicon strengthened ductile irons (HSSDIs) with 3.5 and 4.5 wt% of silicon contents were produced with different cooling rates, and the microstructures were analyzed to find the main microstructure parameters. The Y-blocks GJS 400 presented good nodularity, while the heavy section GJS 400 presented some graphite degeneracy with a lower nodularity. The 3.5 wt% HSSDI shown a good nodularity, while with increasing silicon content to 4.5 wt%, significant graphite degeneracy occurred with the appearance of chunky graphite. Samples were tensile tested and the tensile data were analyzed through the physical-based constitutive Voce equation, and the Voce parameters were plotted to produce a matrix assessment diagram (MAD). Y-blocks and heavy section GJS 400 data lied on two distinct lines in MAD, and they had, however, positive and negative intercepts which meant indeed two contradictory plastic behaviors. The intercept of the best fitting line of the GJS 400 Y-blocks data was positive and so consistent with the physical meaning of Voce equation, while the intercept of the best fitting line of the GJS 400 heavy section data was negative, which meant an unexpected opposite plastic behavior. Same behavior was reported for the investigated HSSDIs, resulting in positive intercept of the best fitting line in MAD for the 3.5 wt% HSSDI data and negative intercept for the 4.5 wt% HSSDI data. The uniform strain energy density (SEDU) that is the area below the tensile flow curve up to the uniform strain, i.e., the strain where necking begins, was also investigated. SEDU resulted to be almost constant for all the GJS 400 Y-blocks tensile flow curves and 3.5 wt% HSSDI, which was typical of a sound material, while in GJS 400 heavy section and 4.5 wt% HSSDI, SEDU changed significantly in a systematic way, indicating that metallurgical defects had affected the plastic behavior. So it was concluded that in the MAD the best fitting line of the tensile data with positive intercept was a possible indication of the material soundness, while the negative intercept was indication of defected material. The results suggested that the MAD analysis produced from tensile Voce parameters can be a useful and easy tool for industry not only to classify the production routes of DIs (Si content mainly and heat treatments), but also to identify possible microstructure poorness within a single DI grade.

A two diagram procedure was used to assess the microstructure effects on tensile strain hardening in different silicon strengthened ductile irons produced through different cooling rates. In the Matrix Assessment Diagram, the data from different ductile irons were identified through lines. While with Si 2.5 wt-% the intercept of the best fitting line was positive in agreement with the physical meaning of strain hardening, the intercept decreased with increasing silicon content and decreasing cooling rate, becoming unexpectedly negative with Si 4.5 wt-% and slow cooling rates. The negative intercepts were found to be related to graphite degeneracy. In the Integrity Assessment Diagram the effect of microstructure on ductility was investigated, reporting that ductility was reduced significantly by graphite degeneracy. So the two diagram procedure that was first proposed to classify ductile irons with different chemical compositions and production routes, can also indicate the presence of graphite degeneracy in ductile irons.

Austempered Ductile Irons (ADIs) with different contents of nickel and copper were austenitized at the same conditions, and austempered for 60, 90 and 180 min to investigate the austempering evolution. Besides the conventional ductility analysis to assess the best austempering times, strain hardening analysis of tensile flow curves was carried out, since it has been found reliable for this assessment if dislocation-density-related constitutive equations are used. Through plotting the Voce equation parameters 1/? vs. ? found from the strain hardening analysis, Matrix Assessment Diagrams (MADs) were drawn, and austempering conditions closest to the best austempering times were identified. Austempering times found with ductility-criterion and MAD approach did not match. Mg-Cu particles found in copper-containing ADIs affected the reliability of tensile ductility that could not be ascribed to ausferrite stability only, while the austempering times found with MAD approach seemed to be consistent with solid-state diffusion transformations.

Nell'ambito del contratto (Prot. 510 del 28/02/2018) sono richieste frattografie ed analisi di microscopia su campioni di lega SF286 fornita da EXERGY S.p.A., sottoposti a specifiche prove di creep,concordate con lo stesso committente. I campioni sono ricavati da una barra di lega SF286 e hanno le dimensioni riportate in Fig. 1. Su uno dei campioni è stata eseguita anche una prova di trazione.

The EDDI testing program at ENEA RU has been delayed during 2019 by a series of problems. However, it has been resumed now and it will continue with the programmed tests in 2020. Tests already agreed are concerning creep tests in negligible creep domain of EUROFER97/2 alloy. The testing plan for 2020 has been updated with the latest indications.

A series of samples made of ductile iron GJS 400 was cast with different cooling rates, and their microstructural features were investigated. Quantitative metallography analyses compliant with ASTM E2567-16a and ASTM E112-13 standards were performed in order to describe graphite nodules and ferritic grains. The occurrence of pearlite was associated to segregations described through Energy Dispersive X-ray Spectroscopy (EDS) analyses. Results were related to cooling rates, which were simulated through MAGMASOFT software. This microstructural characterization, which provides the basis for the description and modeling of the tensile properties of GJS 400 alloy, subject of a second part of this investigation, highlights that higher cooling rates refines microstructural features, such as graphite nodule count and average ferritic grain size.