The impact of chemical short-range order on the thermophysical properties of medium- and high-entropy alloys

The unusual behavior observed in the coefficient of thermal expansion and specific heat capacity of CrFeNi, CoCrNi, and CoCrFeNi medium/high-entropy alloys is commonly referred to as the K-state effect. It is shown to be independent of the Curie temperature, as demonstrated by temperature-dependent magnetic moment measurements. CoCrFeNi alloy is chosen for detailed characterization; potential reasons for the K-state effect such as texture, recrystallization, and second-phase precipitation are ruled out. An examination of the electronic structure indicates the formation of a pseudo-gap in the Density of States, which suggests a specific chemical interaction between Ni and Cr atoms upon alloying. Hybrid Monte Carlo/Molecular Dynamic (MC/MD) simulations indicate the presence of non-negligible chemical short-range order (CSRO). Local lattice distortions are shown to be negligible, although deviations around Cr and Ni elements from those expected in a fully disordered structure are experimentally observed by X-ray absorption spectroscopy. The determined bonding distances are in good agreement with MC/MD calculations. A mechanism is proposed to explain the anomalies and calorimetric experiments and their results are used to validate the mechanism

Endovascular Abdominal Aortic Aneurysm Repair With Ovation Alto Stent Graft: Protocol for the ALTAIR (ALTo endogrAft Italian Registry) Study

Background: Since 2010, the Ovation Abdominal Stent Graft System has offered an innovative sealing option for abdominal aortic aneurysm (AAA) by including a sealing ring filled with polymer 13 mm from the renal arteries. In August 2020, the redesigned Ovation Alto, with a sealing ring 6 mm closer to the top of the fabric, received CE Mark approval. Objective: This registry study aims to evaluate intraoperative, perioperative, and postoperative results in patients treated by the Alto stent graft (Endologix Inc.) for elective AAA repair in a multicentric consecutive experience. Methods: All consecutive eligible patients submitted to endovascular aneurysm repair (EVAR) by Alto Endovascular AAA implantation will be included in this analysis. Patients will be submitted to EVAR procedures based on their own preferences, anatomical features, and operators experience. An estimated number of 300 patients submitted to EVAR with Alto stent graft should be enrolled. It is estimated that the inclusion period will be 24 months. The follow-up period is set to be 5 years. Full data sets and cross-sectional images of contrast-enhanced computed tomography scan performed before EVAR, at the first postoperative month, at 24 or 36 months, and at 5-year follow-up interval will be reported in the central database for a centralized core laboratory review of morphological changes. The primary endpoint of the study is to evaluate the technical and clinical success of EVAR with the Alto stent graft in short- (90-day), mid- (1-year), and long-term (5-year) follow-up periods. The following secondary endpoints will be also addressed: operative time; intraoperative radiation exposure; contrast medium usage; AAA sac shrinkage at 12-month and 5-year follow-up; any potential role of patients' baseline characteristics, valuated on preoperative computed tomography angiographic study, and of device configuration (number of component) in the primary endpoint. Results: The study is currently in the recruitment phase and the final patient is expected to be treated by the end of 2023 and then followed up for 5 years. A total of 300 patients will be recruited. Analyses will focus on primary and secondary endpoints. Updated results will be shared at 1- and 3-5-year follow-ups. Conclusions: The results from this registry study could validate the safety and effectiveness of the new design of the Ovation Alto Stent Graft. The technical modifications to the endograft could allow for accommodation of a more comprehensive range of anatomies on-label

In situ studies of non-equilibrium crystallization of Alx_xCoCrFeNi (x = 0.3, 1) high-entropy alloys

To understand the effect of aluminum addition on the non-equilibrium solidification of the equiatomic CoCrFeNi alloy, the Al$_{0.3}$CoCrFeNi and AlCoCrFeNi high-entropy alloys (HEAs) have been investigated in situ using electromagnetic levitation and high-energy synchrotron X-ray diffraction (XRD). Because the five- component alloys could not be deeply undercooled, the studies were confined to the low undercooling regime. The in situ XRD revealed that a fcc single-phase solid solution is the primary crystalline phase nucleating in Al$_{0.3}$CoCrFeNi melt undercooled up to 80 K. In contrast, the B2 ordered phase appears as the primary phase in the AlCoCrFeNi melt, at least at a low undercooling of about 30 K. These results correlate with the equilibrium phase diagrams available. Crystal growth velocities of the Al$_{0.3}$CoCrFeNi alloy are shown to be smaller than for CrFeNi and CoCrNi equiatomic alloys; nonetheless, they are in the same order of magnitude. Spontaneous grain refinement was observed in the Al$_{0.3}$CoCrFeNi alloy solidified at a low undercooling of $ΔT$ = 70 K

Gas atomization of AA2017 aluminum alloy: Effect of process parameters in the physical properties of powders for additive manufacturing

Close-coupled gas atomization (CCGA) uses pressurized gas jets to atomize molten metals, producing powders suitable for additive manufacturing (AM). Optimization of processing parameters is crucial to achieve powders with good fluidity and high apparent density for laser powder bed fusion (L-PBF) and increase production yield. This study evaluated the influence of melt feed nozzle diameter, superheating temperature, and atomization pressure on AA2017 aluminum alloy powders' physical properties. Parameters variation poorly affected the median particle diameter (d50), while significantly altering the particle size distribution curves width (IDR = d90-d10). Suitable powders for AM/L-PBF were produced using a specific set of parameters (d0= 1.5 mm; ΔT= 150 °C; PG= 20 Bar), presenting appropriate fluidity and apparent density due to an optimal granulometric distribution and morphological characteristics. Mathematical analysis showed a good correlation between experimental and calculated mean particle size, suggesting an equation to predict percentile d90 based on previous correlations

In situ study of non-equilibrium solidification of CoCrFeNi high-entropy alloy and CrFeNi and CoCrNi ternary suballoys

The solidification behavior of the CoCrFeNi high-entropy alloy and the ternary CrFeNi and CoCrNi medium-entropy suballoys has been studied in situ using high-speed video and synchrotron X-ray diffraction on electromagnetically levitated samples. In all alloys, the formation of a primary metastable bcc phase was observed if the melt was sufficiently undercooled. The delay time for the onset of the nucleation of the stable fcc phase, occurring within bcc crystals, is inversely proportional to the melt undercooling. The experimental findings are corroborated by thermodynamic calculations of stable and metastable phase diagrams for the (CoCrNi)-Fe section. Crystal-growth velocities for the CrFeNi, CoCrNi, and CoCrFeNi medium- and high-entropy alloys extracted from the high-speed video sequences in the present study are comparable to the literature data for Fe-rich Fe-Ni and Fe-Cr-Ni alloys, evidencing the same crystallization kinetics. The effect of melt undercooling on the microstructure of solidified samples is analyzed and discussed

The impact of chemical short-range order on the thermophysical properties of medium- and high-entropy alloys

The unusual behavior observed in the coefficient of thermal expansion and specific heat capacity of CrFeNi, CoCrNi, and CoCrFeNi medium/high-entropy alloys is commonly referred to as the K-state effect. It is shown to be independent of the Curie temperature, as demonstrated by temperature-dependent magnetic moment measurements. CoCrFeNi alloy is chosen for detailed characterization; potential reasons for the K-state effect such as texture, recrystallization, and second-phase precipitation are ruled out. An examination of the electronic structure indicates the formation of a pseudo-gap in the Density of States, which suggests a specific chemical interaction between Ni and Cr atoms upon alloying. Hybrid Monte Carlo/Molecular Dynamic (MC/MD) simulations indicate the presence of non-negligible chemical short-range order (CSRO). Local lattice distortions are shown to be negligible, although deviations around Cr and Ni elements from those expected in a fully disordered structure are experimentally observed by X-ray absorption spectroscopy. The determined bonding distances are in good agreement with MC/MD calculations. A mechanism is proposed to explain the anomalies and calorimetric experiments and their results are used to validate the mechanism