Microstructure modification of CGDS and HVOF sprayed CoNiCrAlY bond coat remelted by electron beam

In the present work two techniques are combined to optimize bond coat properties before thermal barrier coating (TBC) application, the cold gas dynamic spraying (CGDS) and electron beam remelting (EB). Results of the work focused on comparison of high velocity oxygen fuel (HVOF) and CGDS CoNiCrAlY bond coats are firstly presented. Than the effect of the electron beam remelting of the CoNiCrAlY coating manufactured by HVOF and CGDS deposition techniques is deeply investigated. The CoNiCrAlY bond coat to Inconel substrate interface displayed locations with very poor bonding, in larger extent for the states prepared by HVOF comparing to CGDS. The bond coats prepared by both ways being EB remelted are typically removal of the defects on the substrate to bond coat interface. The microstructure of the bond coat after this treatment is formed by Inconel fine grain layer being followed by the surface layer consisting of elongated dendritic microstructure. An increased porosity has been observed in interdendritical space in larger extent for CGDS samples

Temperature Dependence of Fracture Characteristics ofVariously Heat-Treated Grades of Ultra-High-Strength Steel:Experimental and Modelling

The temperature dependence of tensile characteristics and fracture toughness of the standardly heat-treated low-alloyed steel OCHN3MFA along with three additionally heat-treated grades was experimentally studied. In the temperature range of 196; 22 °C, all the additional heat treatments transferred the standard steel from a high- to ultra-high strength levels even with improved tensile ductility characteristics. This could be explained by a reduction of the inclusion content, refinement of the martensitic blocks, ductile retained austenite content, and homogenization of the shape ratio of martensitic laths as revealed by metallographic, X-Ray, and EBSD techniques. On the other hand, the values of the fracture toughness of all grades were found to be comparable in the whole temperature range as the cause of a high stress triaxiality in the pre-cracked Charpy V-notch samples. The values of the fracture toughness of the standard steel grade could be predicted well using the fracture model proposed by Pokluda et al. based on the tensile characteristics. Such a prediction failed in the case of additionally heat-treated grades due to the different temperature dependence of the fracture mechanisms occurring in the tensile and fracture-toughness tests. While the tensile samples fractured in a ductile-dimple mode at all temperatures, the fracture-toughness specimens exhibited a transition from the ductile to quasi-brittle fracture mode with decreasing temperature. This transition could be interpreted in terms of a transfer from the model proposed by Rice and Johnson to the model of Tvergaard and Hutchinson

Nitrogen Interstitial Alloying of CoCrFeMnNi High Entropy Alloy through Reactive Powder Milling

The present work is focused on the synthesis of CoCrFeMnNi high entropy alloy (HEA) interstitially alloyed with nitrogen via powder metallurgy routes. Using a simple method, nitrogen was introduced to the HEA from the protective N2 gas atmosphere during mechanical alloying (MA) processing. The lattice parameter and amount of nitrogen in HEA were observed to be linearly proportional to the milling duration. The limited solubility of nitrogen in the main face centered cubic (FCC) phase resulted in the in-situ formation of nitrides and, accordingly, significant increase in the hardness values. It has been shown that fabrication of such nitrogen-doped HEA bulk materials can be conveniently achieved by a simple combination of MA + spark plasma sintering processes, without the need for adding nitrogen from other source

Formation conditions of vortex-like intermixing interfaces in cold spray

Experimental investigation was conducted to explore the formation conditions and provide new insights into the formation mechanisms of the unexplained intermixing phenomenon observed at the substrate-coatings interface of cold sprayed materials. The results indicate that the formation of intermixing interface significantly depends on the extent of plastic deformation at the coating-substrate interface, with severe deformation creating favorable conditions for the intermixing interface. Two factors have been identified to be critical for inducing the severe interfacial plastic deformation: low deposition efficiency and material properties. During low deposition efficiency cold spraying, most of the accelerated particles rebound after impact while inducing accumulative plastic deformation and thus intermixing at the coating-substrate interface. Considering the material properties, the coating materials must have sufficiently high density to attain enough kinetic energy for creating substantial plastic deformation of the first coating layer and the substrate upon their impact. Also, the substrate materials cannot be too hard so that plastic deformation can be induced. Based on the experimental analyses, the hypothesis of the intermixing interface formation mechanism is proposed in this paper

The effect of high temperature annealing in different atmospheres on microstructure of ODS materials

Mechanically alloyed oxide dispersion strengthened materials MA 6000 and MA 956 have been annealed at 1100ºC for 24 and 240 hours in air, vacuum of 10 -3 Pa and hydrogen atmosphere.. Mass changes due to oxidation and evaporation were measured. Fracture surfaces, microstructure of the materials and changes in chemical composition were examined along with surface layers  evolution and pores formation. On fracture surfaces and polished sections, formation of voids and altered microstructure sub-surface zones in both materials has been studied. Different mechanisms causing void formation are discussed and proposed in discussion. Also the mechanisms and reaction sequences of surface oxidation reactions are examined

The effect of high temperature annealing in different atmospheres on microstructure of ODS materials

Mechanically alloyed oxide dispersion strengthened materials MA 6000 and MA 956 have been annealed at 1100ºC for 24 and 240 hours in air, vacuum of 10 -3 Pa and hydrogen atmosphere.. Mass changes due to oxidation and evaporation were measured. Fracture surfaces, microstructure of the materials and changes in chemical composition were examined along with surface layers  evolution and pores formation. On fracture surfaces and polished sections, formation of voids and altered microstructure sub-surface zones in both materials has been studied. Different mechanisms causing void formation are discussed and proposed in discussion. Also the mechanisms and reaction sequences of surface oxidation reactions are examined

The effect of high temperature annealing in different atmospheres on microstructure of ODS materials

Mechanically alloyed oxide dispersion strengthened materials MA 6000 and MA 956 have been annealed at 1100ºC for 24 and 240 hours in air, vacuum of 10 -3 Pa and hydrogen atmosphere.. Mass changes due to oxidation and evaporation were measured. Fracture surfaces, microstructure of the materials and changes in chemical composition were examined along with surface layers  evolution and pores formation. On fracture surfaces and polished sections, formation of voids and altered microstructure sub-surface zones in both materials has been studied. Different mechanisms causing void formation are discussed and proposed in discussion. Also the mechanisms and reaction sequences of surface oxidation reactions are examined

Potenciál technologie elektronového paprsku pro modifikaci rozhraní MCrAlY spojových nástřiků nanesených metodami kinetické depozice a HVOF

Electron beam (EB) technology treatment was carried out on CoNiCrAlY bond coats deposited on Inconel substrates via cold spray and HVOF techniques in dissimilar thicknesses. Such treatment was aimed with regard to the final materials microstructure, composition, surface roughness and the quality of the coating-substrate interface. Following a multiple-step optimization of the processing parameters (such as beam pattern configuration, accelerating voltage, longitudinal speed, multiple beam incidence), two final EB modifications were carried out on both coating types. It was found that the optimized EB treatment could lead to a significant alteration of the interface from a distinctive divide into smooth chemical and structural transition between the materials, significant decrease in surface roughness and porosity, as well as changes in mechanical properties (increase in Young's modulus and decrease in hardness of the coating).Pomocí technologie elektronového paprsku byly modifikovány spojové vrstvy CoNiCrAlY nanesené na Inconelových substrátech metodami kinetické depozice a HVOF v rozdílných tloušťkách. Tato modifikace byla uskutečněna s ohledem na konečnou strukturu, složení a povrchovou drsnost vrstev a rovněž na kvalitu rozhraní substrát-nástřik. Po několikastupňové optimalizaci řídicích parametrů (jako např. konfigurace svazku, urychlovací napětí, rychlost přejezdu, několikanásobné přetavení) byly provedeny dvě finální modifikace pro oba druhy nástřiků. Bylo zjištěno, že optimalizovaná modifikace paprskem může vést ke značné změně charakteru rozhraní z definovaného a jasně viditelného přechodu na plynulý přechod mezi materiály, ke značnému snížení povrchové drsnosti a vnitřní porozity a dále ke změnám mechanických vlastností (zvýšení Youngova modulu a snížení tvrdosti nástřiku)