Ni-Cr Alloy Research Articles

Unraveling the role of tungsten as a minor alloying element in the oxidation NiCr alloys

Ni-based superalloys offer a unique combination of mechanical properties, corrosion resistance and high temperature performance. Near ambient pressure X-ray photoelectron spectroscopy was used to study in operando the initial steps of oxidation for Ni-5Cr, Ni-15Cr, Ni-30Cr and Ni-15Cr-6W at 500 °C, p(O2)=10−6 mbar. The comparison of oxide evolution for these alloys quantifies the outsized impact of W in promoting chromia formation. For the binary alloys an increase in chromia due to Cr-surface enrichment is followed by NiO nucleation and growth thus seeding a dual-layer structure. The addition of W (Ni-15Cr-6W) shifts the reaction pathways towards chromia thus enhancing oxide quality. Density functional theory calculations confirm that W atoms adjacent to Cr create highly favorable oxygen adsorption sites. The addition of W supercharges the reactivity of Cr with oxygen essentially funneling oxygen atoms into Cr sites. The experimental results are discussed in the context of surface composition, chemistry, reactant fluxes, and microstructure.

Chemical Analysis and Electrochemical Monitoring of Extremely Low-Concentration Corrosive Impurity MgOHCl in Molten MgCl2–KCl–NaCl

MgCl2–KCl–NaCl is a promising thermal energy storage (TES) material and heat transfer fluid (HTF) with high operating temperatures of >700°C for next-generation concentrating solar power (CSP) plants. One major challenge for future implementation of the molten chloride TES/HTF technology arises from the presence of some corrosive impurities, especially MgOHCl, a hydrolysis product of hydrated MgCl2. Even extremely low-concentration MgOHCl (tens of ppm O in weight) can cause unneglectable corrosion of commercial Fe–Cr–Ni alloys, which limits their service time as the structural materials in the molten chloride TES/HTF system. Thus, the chemical analysis and monitoring techniques of MgOHCl at the tens of ppm O level are vital for corrosion control. In this work, a chemical analysis technique based on direct titration and a high-precision automatic titrator was developed for an exact measurement of MgOHCl at the tens of ppm O level. It shows a standard deviation below 5 ppm O and an average error below 7 ppm O when the concentration of MgOHCl is 36 ppm O. Moreover, compared to other methods available in some literature reports, it can exclude the influence of co-existing MgO on the MgOHCl concentration measurement. This chemical analysis technique was used to calibrate the previously developed electrochemical method based on cyclic voltammetry (CV) to achieve reliable in situ monitoring of MgOHCl in the MgCl2–KCl–NaCl molten salt at a concentration as low as the tens of ppm O level. The in situ monitoring technique shows a monitoring limitation of <39 ppm O. The two techniques for MgOHCl measurement developed in this work could be used to develop an in situ corrosion control system to ensure the long service time of the molten chloride TES/HTF system in next-generation CSP plants.

Fracture resistance to fixed partial denture abutments with direct and indirect posts

The objective of this study was to evaluate the resistance to fracture of three-element fixed partial dentof glasstment teeth restored with glass fiber posts (GFP) and cast post-and-core (CPC) retainers. Herein, 60 bovine mandibular incisors were divided into two groups (n = 30) according to the retainer type in: 1) resin composite core reinforced with GFP; and II) CPC (Cu-Al). Each pair of roots consisted of one sample, simulating a three-element fixed partial denture. The structures of the fixed three-element partial dentures were made in Ni-Cr alloy. A conventional dual resin cement (RelyX ARC - 3M Espe, USA) was used for all cementation procedures. Samples were subjected to a compressive load onto an orifice at occlusal contact region of the pontic until collapse or fracture. Normal data distribution was verified by D'Agostino & Pearson normality test. Unpaired t test was used for data analysis (α = 5%). The results showed that the the three-element fixed partial denture abutment teeth restored with composite resin cores retained by GFP presented higher fracture resistance compared to the ones restored with CPC (p=0.0009). In conclusion, the resistance to fracture of three-element fixed partial denture restored with resin composite core reinforced with glass fiber posts was higher to the one restored with post-and-core retainers.

Aging process and strengthening mechanism of Cu–Cr–Ni alloy with superior stress relaxation resistance

With the development trend of connectors to high-current and high-temperature service, copper alloy connectors are required to have superior stress relaxation resistance. In this work, the optimal aging parameter was explored via stress relaxation resistance, strength, microhardness, and conductivity. And the strengthening mechanism of stress relaxation was investigated through the TEM and EBSD microscopic characterization. The results show that the Cu–Cr–Ni alloy reached the peak aging state under 450 °C for 1 h and obtained excellent properties. Especially, the percentage of residual stress in the relaxation test reached 93.1%. The microhardness associated with aging strengthening showed a positive correlation with yield strength and stress relaxation resistance. Then the microhardness after aging can be used as a simple evaluation index of stress relaxation resistance. The Cu–Cr–Ni alloy with superior stress relaxation resistance is mainly related to dislocation strengthening, second phase strengthening, and the coexistence of spinodal decomposition and ordering.

Visualizing the Nanoscale Oxygen and Cation Transport Mechanisms during the Early Stages of Oxidation of Fe–Cr–Ni Alloy Using In Situ Atom Probe Tomography

AbstractUnderstanding the early stages of interactions between oxygen and material surfaces—especially at very high spatial resolutions—is highly beneficial for fields ranging from materials degradation, corrosion, geological sciences, forensics, and catalysis. The ability of in situ atom probe tomography (APT) is demonstrated to track the diffusion of oxygen and metal ions at nanoscale spatial resolution during the early stages of oxidation of a model Fe–Cr–Ni alloy. Using 18O isotope tracers in these in situ APT experiments and complementary ex situ multimodal microscopy, spectroscopy, and computational simulations allows to precisely analyze the kinetics of oxidation and determine that outward cation diffusion to oxide/air interface is the primary mechanism for intragranular oxide growth in this alloy at 300 °C. This unique in situ isotopic tracer APT approach and the insights gained can be highly beneficial for studying early stages of gas–surface reactions in a broad array of materials.

Study of the Pattern Preparation and Performance of the Resistance Grid of Thin-Film Strain Sensors.

The thin-film strain sensor is a cutting-force sensor that can be integrated with cutting tools. The quality of the alloy film strain layer resistance grid plays an important role in the performance of the sensor. In this paper, the two film patterning processes of photolithography magnetron sputtering and photolithography ion beam etching are compared, and the effects of the geometric size of the thin-film resistance grid on the resistance value and resistance strain coefficient of the thin film are compared and analyzed. Through orthogonal experiments of incident angle, argon flow rate, and substrate negative bias in the ion beam etching process parameters, the effects of the process parameters on photoresist stripping quality, etching rate, surface roughness, and resistivity are discussed. The effects of process parameters on etching rate, surface roughness, and resistivity are analyzed by the range method. The effect of substrate temperature on the preparation of Ni Cr alloy films is observed by scanning electron microscope. The surface morphology of the films before and after ion beam etching is observed by atomic force microscope. The influence of the lithography process on the surface quality of the film is discussed, and the etching process parameters are optimized.

Oxidation Behavior and Interfacial Bonding Properties of NiCrAl Alloys

AbstractThe oxidation behavior and oxidation kinetics of NiCrAl alloy were studied by XRD, SEM and the thermogravimetric analysis method. The adhesion strength of the films was characterized by micron scratch tester. The results show that the oxide film has not formed at 600 °C and 700 °C, and an oxide layer of Al2O3 has formed at 800 °C. Moreover, an oxide layer composed of Cr2O3 and Al2O3 has formed at 900 °C. After oxidation at 800 °C for 100 h, partial θ‐Al2O3 has transformed to steady‐state α‐Al2O3 and a dense oxide film of Al2O3 with the thickness of about 620 nm has formed on the surface of the NiCrAl alloy. The oxidation kinetics curve of the alloy obeys the parabolic law and there are two oxidation rate constants in the oxidation process. The oxidation rate constant of 1.58×10−4 mg2⋅cm−4⋅h−1 in the early oxidation period (0∼12 h) and the oxidation rate constant of 1.04×10−4 mg2⋅cm−4⋅h−1 in the late oxidation period (12∼100 h) indicate that the oxidation resistance ability of the alloy increases with the extension of oxidation time. With the increase of the oxidation time, the interface bonding strength between the oxide film and the substrate can be improved and reaches 5.21 N at 800 °C for 100 h.

An In Vitro Study on the Shear Bond Strength of Feldspathic Porcelain to Nickel Chromium Alloy and Cobalt Chromium Alloy after Various Surface Treatments.

Background To evaluate and compare the shear bond strength of feldspathic porcelain to four distinctively surface-treated Ni-Cr and Co-Cr alloys and to assess the impact of oxidation-heat treatment on porcelain to base metal alloy bond strength. Methods 40 specimens each of nickel-chromium alloy and cobalt-chromium alloy were cast. A total of four groups of specimens were created. Group I was surface-treated by sandblasting with 50 μm alumina particles, Group II was surface-treated by sandblasting with 110 μm alumina particles, Group III and Group IV were surface-treated with 250 μm alumina particles. In Group IV, after sandblasting initially with 250 μm alumina particles, the alloys were subjected to oxidation and resandblasting with 250 μm alumina particles. Each of the specimen was coated with opaque and body porcelain and fired to a total thickness of 2 mm porcelain. A universal measuring machine was used to assess shear bond strength at a crosshead speed of 0.5 mm/min. Results Two-way ANOVA followed by Tukey's post hoc test was used to assess the significant difference within the groups. Unpaired t-test was used for the intergroup comparison of the obtained data. The study showed that the size of the air abrasion particles used for sandblasting significantly influenced the porcelain to metal surface bond strength, with p value <0.001. The bond strength values of the two alloys tested showed no major variations. Result also showed that oxidation influences the metal-ceramic bond strength. Conclusions The bond strength of the metal-ceramic interface is influenced by the alloy's surface treatment. The oxidation process impacts the bond strength of the metal-ceramic system.

Ni-Cr Powders Modified with Rhenium as a Novel Coating Material-Physical Properties, Microstructure, and Behavior in Plasma Plume.

The aim of this work was to develop a new coating material based on Ni20Cr alloy modified with up to 50%wt. rhenium. The modification was carried out by the mechanical mixing of the base powder and ammonium perrhenate with the subsequent thermoreduction in an H2 atmosphere. The obtained powder consists of a nickel–chromium core surrounded by a rhenium shell. The characterization of the powders—including their microstructure, phase and chemical composition, density, flowability, particle size distribution, and specific surface area—was performed. The influence of plasma current intensity and hydrogen gas flow on in-flight particle temperature and velocity were investigated. The results indicate that there is interdiffusion between the base Ni20Cr and the rhenium shell, resulting in intermediary solid solution(s). The modified powders have a higher specific surface area and a lower flowability, but this does not prevent them from being used as feedstock in plasma spraying. In-flight measurements reveal that increasing the content of rhenium allows for the higher temperature of particles, though it also reduces their speed.

Towards tailorable interface microstructure through Solid-state interface reaction between synthetic diamond grits and sputtered Ni-Cr binary alloy

In this work, the solid-state (S/S) interface reaction between single-crystal synthetic diamond grits and binary Ni-Cr alloys has been comprehensively studied on (100) and (111) crystal planes using Raman spectroscopy, SEM, TEM, and XPS techniques, in combination with first-principles calculation. The early-stage interfacial chromium carbides formation was found to be dependent on crystal orientation of synthetic diamond, which can be explained by geometric optimization that explores the underlying catalytic mechanisms of Ni-Cr alloys on the relaxation and bonds construction of C atoms on (100) and (111) crystal planes of diamond. More importantly, a unique interfacial microstructure, namely a Ni-rich layer instead of interfacial carbides was formed directly on diamond surface of S/S reaction samples at 700 and 900 °C. In addition, the phase transformation between Ni-rich phase and chromium carbides formed in the S/S interface reaction between Ni-Cr alloy and synthetic diamond were comparatively examined at 700 and 900 °C and investigated through a careful TEM lattice mismatches analysis. Ultimately, a mechanism to tailor the S/S interface microstructure by simply designing alloy compositions and reaction temperatures was proposed with experimental validation.

Surface texture characterization for thin-wall NASA HR-1 Fe–Ni–Cr alloy using laser powder directed energy deposition (LP-DED)

Additive Manufacturing (AM) offers new design and manufacturing opportunities of thin-wall microchannel heat exchangers for aerospace and industrial applications. Laser Powder Directed Energy Deposition (LP-DED) is an AM process providing large scale manufacturing of thin-wall microchannel heat exchangers. Successful industrialization of the LP-DED process requires critical quantification and understanding of the metallurgical, geometric, and process limitations. Specifically, understanding the as-built surface texture, inclusive of roughness and waviness, is significant due to its effects on the friction factor and pressure drop within a heat exchanger. This experimental study completed a design of experiments (DOE) to determine the critical build parameters that impact surface texture for enclosed thin-wall samples. This study summarizes the characterization work of the LP-DED process for 1 mm enclosed walls with an Fe–Ni–Cr (NASA HR-1) alloy. The LP-DED parameters including laser power, powder feedrate, travel speed, layer height, and rotary atomized powder feedstock were modified in the experiment. An evaluation of the DOE samples and resulting surface texture is provided along with conclusions from these experiments. Results indicate that 3D areal and 2D profile (directional) surface texture is estimated by 2x the powder diameter that becomes captured or partially melted on the trailing edge of the melt pool. The fine powder showed a higher sensitivity to parameter changes but resulted in a higher density material and 23% reduction in roughness. Surface texture was also shown to vary between closed channel shapes (internal) due to ricochets, recirculation, and higher volume of powder available to bond compared to external (outer) surfaces. The understanding of the LP-DED process as-built surface texture is essential to fluid flow applications such as heat exchanges and can modify performance for enhanced heat transfer or can be a detriment to pressure drop.

水素チャージ下引張試験におけるNi-Cr合金の局所塑性発達および破壊挙動に及ぼすCrの影響

水素チャージ下引張試験におけるNi-Cr合金の局所塑性発達および破壊挙動に及ぼすCrの影響

The Laser Deposited Nickel-Aluminum Bronze Coatings on SUS630 Stainless Steel and Its Corrosion Resistance in 3.5 wt.% NaCl Solution

In this work, three composite structures of nickel-aluminum bronze (NAB) bonded with SUS630 stainless steel with different intermediate layers were fabricated by laser deposition. The microstructure and corrosion behavior of NAB in 3.5 wt.% NaCl solution were studied. The NAB coating directly deposited on steel substrate contains a large number of Fe-rich dendrites due to the dilution by laser energy and the Cu-Fe liquid phase separation characteristics. The microstructures of NAB were independent and isolated well from the steel substrate when the nickel intermediate layer was applied. Immersion corrosion and electrochemical tests indicated that the composite structure with the nickel intermediate layer presented better corrosion resistance than direct deposited the NAB coating, especially with the NiCr alloy intermediate layer, which led to a shallower corrosion depth and formed a denser layer of protective corrosion products.

Surface Characteristics and Adhesion of Veneering Composite Resin to PAEK-Based Substructure Restorative Materials.

To evaluate and compare the shear bond strength (SBS) of composite veneering material to polyetherketoneketone (PEKK), polyetheretherketone (PEEK), zirconia (YZ), and nickel-chromium alloy (NiCr) substructure restorative materials. Forty samples (12 × 2 mm) were prepared from four materials: PEKK, PEEK, zirconia, and NiCr alloy (n = 10). The Vickers hardness was evaluated before preparing the surface for bonding by shot-blasting using 110 μm Al2 O3 particles. The surface roughness (Ra) of each sample was determined using a noncontact optical profilometer. The veneering resin was bonded onto each sample following primer application. The prepared samples were then subjected to an SBS test using a universal testing machine at 0.5 mm/min crosshead speed. Failure modes and surface topography following debonding were assessed. The data were statistically analyzed using ANOVA and Tukey'spost-hoccomparison test (p < 0.05). RESULTS: The highest and lowest mean surface roughness was observed in PEEK (3.45 ±0.13 μm) and NiCr (1.87 ±0.07 μm) materials, respectively. A significant difference in roughness values was observed between the materials except for NiCr and YZ (p = 0.547). Concerning SBS, PEEK and NiCr exhibited the highest (16.23 ±0.96 MPa) and lowest (10.1 ±0.63 MPa) values. The mean difference in SBS indicated a statistically significant difference between the material groups (p < 0.01). PEKK materials demonstrated significantly lower SBS than PEEK and significantly higher SBS values than conventional zirconia and alloy materials. A positive and significant correlation between mean roughness and SBS was observed, but the causality could be either intrinsic to the material or the roughness.

Investigations on TaHf alloys for thin film resistor applications

TaHf thin films were deposited by performing the cosputtering of Ta and Hf targets. The results indicated that TaHf alloy is suitable for use as a resistive layer for thin film chip resistors and a replacement for NiCr and TaNx alloys. Maintaining the Hf:Ta ratio at approximately 1:1 produced the highest resistivity of 5.7 × 10−4 Ωcm when compared with TaNx and NiCr alloy thin films. This ratio also yielded a smaller temperature coefficient of resistance of −330 ppm/°C compared with TaNx thin films. Furthermore, the temperature coefficient of resistance of most TaHf thin films could be adjusted closer to zero through annealing. The increased Hf content in TaHf alloy was attributed to its poorer thermal stability and anticorrosion property relative to pure Ta films. However, the anticorrosion property of TaHf thin films was still stronger than that of NiCr thin films.

Comparison of stress distribution between zirconia/alloy endocrown and CAD/CAM multi-piece zirconia post-crown: three-dimensional finite element analysis.

This study aimed to evaluate a digital multi-piece zirconia post-crown to restore a mandibular second molar with extensive coronal loss and limited restoration space, and to compare the stress distribution between endocrowns made of zirconia or alloy and CAD/CAM multi-piece zirconia post-crowns. Four three-dimensional finite element analysis models of a mandibular second molar with extensive coronal loss and limited restoration space were created as follows: (A) intact molar; (B) zirconia endocrown restored molar; (C) multi-piece post-crown restored-molar with tapered nail; (D) multi-piece post-crown restored molar with T-shaped nail. Models C and D were divided into two subgroups according to the material type: C1/D1, zirconia; C2/D2, NiCr alloy. The maximum modified von Mises failure criterion (mvM) stresses were calculated, and the stress distribution was recorded to analyze the effects of the restoration and material types on the biomechanical properties of dentin and prosthesis. The maximum mvM stress of dentin in model B (33.80MPa) was lower compared with models C (C1, 37.81MPa; C2, 36.36MPa) and D (D1, 36.34MPa; D2, 34.97MPa) under vertical load, but the opposite was observed under oblique load. The highest mvM stress was concentrated in the nail region located in the root canal, and the T-shaped nail values were greater than the tapered nail, whereas model D with T-shaped nail showed a lower mvM stress level in dentin compared with Model C with tapered nail. The digital multi-piece zirconia post-crown is a potential approach to restore mandibular second molars with extensive coronal loss and limited restoration space. The digital multi-piece zirconia post-crown has potential to restore mandibular second molars with extensive coronal loss and limited restoration space using an innovative approach.

Corrosive Studies of a Prosthetic Ni-Cr Alloy Coated with Ti(C,N) Type Layers.

Background: Investigating the general corrosion resistance of Ti(C,N) type coatings on a prosthetic nickel alloy in the aspect of their use as protective coatings on prosthetic and orthodontic elements. Methods: Five groups of Ni-Cr alloy samples covered with Ti(C,N) type coatings differing in their carbon and nitrogen contents were used for the tests. The reference group included alloy samples without coatings. The samples were held for 105 days (2520 h) in salt spray chambers and examined by means of the NSS (neutral salt spray) and SWAAT (sea water acetic acid test) tests. After the periods of 14, 28, 81 and 105 days, the samples were removed and weighed, and their weight losses were determined. Results: In the case of each type of Ti(C,N) coating, the mass loss was lower than the mass loss of a sample without a coating, which makes it possible to state that coatings improve the corrosion resistance. No significant differences in the resistance were observed between the particular coatings. The corrosion rate of the examined coatings is close to parabolic. Conclusions: Ti(C,N) type coatings improve the resistance of a prosthetic Ni-Cr alloy and can be used as protective coatings for prosthetic and orthodontic elements.

Influence of Al and Fe additions on metal dusting of NiCr alloys

AbstractCorrosion by metal dusting causes problems in various industries that process carbonaceous gases. Recently, the chromium‐ and aluminum‐rich Ni‐based Alloy 699 XA was introduced as a material with high resistance against metal dusting attacks. In this study, the metal dusting degradation of Alloy 699 XA and model alloys was investigated. In the model alloys, the aluminum, chromium, and iron contents were varied to analyze the role of each element systematically. Alloy 602 CA was included as a reference material. It was found that the alloys with the highest chromium and aluminum contents (30 and 2–3 wt%, respectively) showed the highest resistance against metal dusting. Also, the limited addition of iron enhanced the aluminum activity and thereby promoted the formation and maintenance of an aluminum‐rich oxide scale. This effect at low iron levels is contrary to the negative impact of iron at higher levels, which typically leads to an increased metal dusting susceptibility. Exposure tests were performed with two gas mixtures having similar compositions, but different carbon activities. It was found that both gases had a similar aggressiveness regarding metal dusting attacks.

Microstructure and properties at bonds of diamond grains and Ni[sbnd]Cr filler alloy by fiber laser brazing

Advanced laser technology has been used in laser brazing to bond diamond grains in metal matrix to achieve some distinctive advantages such as high protrusion heights and large gaps of diamond grains. However, laser brazing has not been widely adopted as expected to produce various diamond tools since that bonding mechanisms, microstructures and characteristics of bonded interfaces have not fully investigated. In this paper, the experimental study was performed for using fiber laser brazing to bond diamond grains on steel substrate with NiCr filler alloy. Microstructures, graphitization, and shear force at the bonded interface were especially analyzed. The experiments led to the following outcomes: (1) the interface of diamond grains and NiCr alloy was a wavy surface with an average thickness of 7 μm. (2) Two types of carbide, i.e., Cr3C2 and Cr7C3, were generated on the brazed surface; Cr3C2 was lath-shaped and generated at first, and Cr7C3 was prismatic and built upon Cr3C2. (3) The interface of diamond grains and NiCr alloy was rough and accompanied by graphitization. (4) The shearing test showed that an average force of 138 N was applied to debone single diamond grain from filler metal.

Effect of concurrent grain growth on radiation-induced segregation in nanocrystalline Fe–Cr–Ni alloys

Irradiation of crystalline materials modifies their microchemistry and microstructure. This includes solute segregation toward defect sinks such as grain boundaries (GBs), a phenomenon commonly known as radiation-induced segregation (RIS). Unlike in coarse-grained alloys where GBs are nearly static, RIS is usually accompanied and affected by either thermal or irradiation-induced grain growth in nanocrystalline materials. This work presents a modeling study of concurrent grain growth and RIS in austenitic Fe–Cr–Ni adopting realistic 2D grain structures. RIS can be significantly affected by concurrent grain growth due to (i) increasing grain size, (ii) motion of GBs as defect sinks, and (iii) their combined effect. Consequently, RIS is enhanced by grain growth due to increased grain size and sink motion. More notably, RIS in nanocrystalline materials were found to induce grain-level compositional redistribution in addition to RIS at defect sinks, resulting in grain-size-dependent compositions in individual grains. Without concurrent grain growth, elements depleted at the sinks due to RIS, such as Cr in austenitic steels, will have lower concentrations in smaller grains than in larger grains. The opposite trend becomes true when concurrent grain growth takes place. The compositional difference in individual grains can be significant enough to affect local phase stability. These findings are not discernible with the classical 1D bicrystal model, and they highlight the different effects of RIS in nanocrystalline alloys compared to their coarse-grained counterparts.