A. Casagrande

Enhanced reactive NiAl coatings by microwave‐assisted SHS

Purpose – The paper aims to focus on microwave (2.45 GHz) assisted SHS (MA‐SHS) preparation of NiAl intermetallic coatings on titanium substrates conducted in single mode applicator in order to promote the formation of a complex Ni‐Al‐Ti interface. This enhances the NiAl coating adhesion to the Ti substrate and presents high hardness, high toughness and the capability of stopping the fracture propagation.Design/methodology/approach – Numerical modelling, coupling electromagnetic and heat transfer, allowed to demonstrate that the interface cooling rate can be controlled immediately after SHS using microwave heating, benefiting from the possibility of conveying energy to the newly formed intermetallic compounds, despite an adverse temperature gradient which would negatively affect conventional heating techniques, based exclusively on heat transfer. Experimental validation of the modelling results confirmed that by altering the synthesis conditions (load geometry, microwave power, auxiliary microwave absorbe...

Microwave Assisted Combustion Synthesis of Non-equilibrium Intermetallic Compounds

Abstract A simplified model of the microwave-assisted combustion synthesis of Ni and Al metal powders to form the NiAl intermetallic on titanium and steel substrates is presented. The simulation couples an electro-thermal model with a chemical model, accounting for local heat generation due to the highly exothermic nature of the reactions between the powders. Numerical results, validated by experimental values, show that the capability of microwaves to convey energy, and not heat, can be used to alter the temperature profiles during and after the combustion synthesis, leading to unique intermetallic microstructures. This phenomenon is ascribed to the extended existence of high temperature liquid intermetallic phases, which react with the metallic substrates at the interface. Moreover, microwave heating selectivity allows to maintain the bulk of the substrate metallic materials to a much lower temperature, compared to combustion synthesis in conventionally heated furnaces, thus reducing possible unwanted transformations like phase change or oxidation.

A study on the role of Oxygen in the inelastic behaviour of YBa2Cu3O7 − x

Abstract The inelastic behaviour of YBa 2 Cu 3 O 7 − x has been investigated by the vibrating reed technique in the 300–900 K range. In particular the mobility of oxygen and its contribution to the inelastic spectra, measured both in isochronal and isothermal conditions, was studied by testing samples with different initial oxygen contents. Two inelastic peaks of relaxational type were observed at approximately 560 and 680 K associated to stress induced reorientation of structural defects. The relaxation strenght in both cases strongly depends on the oxygen content. The dynamic modulus shows anomalous behaviour at temperatures higher than 740 K where the orthorombic to tetragonal transformation sets up. Some hypothesis on the inelastic behaviour of YBa 2 Cu 3 O 7−x in terms of oxygen content, thermally activated diffusional jumps and structural transformations are discussed.

Ni–Al–Ti coatings obtained by microwave assisted combustion synthesis

Abstract Microwaves at 2·45 GHz have been used to ignite the self-propagating high temperature synthesis of Ni and Al powder mixtures (50∶50 at-%) to produce a duplex intermetallic coating on Ti substrates. Owing to the high β-NiAl synthesis reaction enthalpy and the energy provided by the electromagnetic field, the newly formed aluminide is in the liquid phase. Thus, it can react with the underlying Ti substrate, forming a ternary eutectic at the interface, belonging to the Ni–Al–Ti system. The peculiar interlocking microstructure of such ternary layer is responsible for an increase in toughness and hardness compared to the parent NiAl coating. Moreover, the ternary layer’s oxidation resistance was tested, showing the possibility of replacing the functionality of hard and brittle NiAl, but limitedly up to 750°C. This can be ascribed to the relatively low Al content in the ternary layer, which hinders the formation of a continuous and protective Al2O3 scale. For this reason, the increase in Al content, or the addition of further elements such as Cr or Si, in the starting powder mixture, helps improve the oxidation resistance of the complex intermetallic coating at the interface, up to the tested temperature of 900°C.

Upward modulus trend in NiAl and NiFeAl single crystals

Abstract Dynamic Young modulus, torsion modulus and elastic energy dissipation were measured in Ni 56.2 Fe 0.3 Al 43.1 Ta 0.3 Mo 0.1 (NiAl crystal) and Ni 56.6 Fe 12.3 Al 30.2 Hf 0.6 (Al 2 O 3 ) 0.3 (NiFeAl crystal). The elastic constant C ′=( C 11 − C 12 )/2 has in both alloys a positive temperature derivative: (d C ′/d T )>0 from 150 K up to 1300 K. The elastic energy dissipation exhibits a Debye-like peak whose activation energy is 2.36±0.05 eV in NiAl and 1.98±0.09 eV in NiFeAl. The peak is described as a Zener’s pair reorientation.

SPS-assisted Synthesis of SICp reinforced high entropy alloys: reactivity of SIC and effects of pre-mechanical alloying and post-annealing treatment

ABSTRACT In this work a traditional high entropy alloy (FeCoNiCrAl) was reinforced by uniformly distributed reactive silicon carbide (SiC) particles by a powder metallurgy synthetic route, using as precursors simply mixed powders or mechanically prealloyed ones. The reactive sintering produced a single isomorphic BCC structure. The sample microstructure resulted equiassic, more homogenous in samples based on prealloyed powders. The instability of SiC in the presence of metal precursors resulted in the formation of more stable carbides and silicides, as well as in carbon diffusion in the high entropy alloy matrix and partially unreacted SiC particles. The formation of these newly formed fine precipitates, as well as the presence of residual SiC were useful to increase the hardness of the alloy.

Nutzung des instrumentierten Kerbschlagbiegeversuches zur Bestimmung von Parametern der Bruchzähigkeit

Kurzfassung Es gibt viele Ansätze zur Korrelation der Kerbschlagarbeit mit Bruchparametern. Die meisten davon liefern empirische oder halbtheoretische Gleichungen, die diesen Zusammenhang belegen. In der vorliegenden Arbeit wird die dynamische Streckgrenze genutzt, um die Gültigkeit der Beziehung zwischen der an spitzgekerbten Proben ermittelten Kerbschlagarbeit KV [J] und der Bruchzähigkeit KIC [MPa √m] herzustellen. Dazu werden drei Bustähle (1.0570 – 1.0511 – 1.8900) und die Ti-Legierung Ti-AI6-V4 Vergleichend betrachtet. Bei den Stählen stimmen die experimentell ermittelten Werte der Bruchzähigkeit mit denen anhand der abgeleiteten Gleichung berechneten überein. Für die Legierung Ti-Al6-V4 stimmen die Werte nicht überein, besonders nicht bei Raumtemperatur, wenn der Beitrag der α- Phase größer ist.