Low Ni Alloys Research Articles

Influence of Nickel on High‐Temperature Oxidation and Characteristics of Oxide Layers in Two High‐Strength Steels

The oxidation behaviors of two high‐strength medium carbon steels with different Ni concentrations are investigated by means of differential thermal analysis equipped with thermogravimetry (TG). Four different oxidation temperatures and five oxidation times are tested. A parabolic behavior is observed for the oxide growth in both alloys; however, the relative increases in the oxidation rate from one temperature to another are not similar for both alloys, and the oxidation kinetics of the Ni‐rich steel are significantly lower than those of the low Ni alloy. On the basis of the TG results, the activation energy for the oxidation of the two alloys is determined. The influence of Ni content on the microstructure and characteristics of the different oxide layers is studied using a combination of laser confocal microscopy, electron microscopy, and X‐ray diffraction (XRD) analysis. The results revealed a clear influence of Ni on the nature and relative presence of different oxide layers. The results are interpreted in terms of the influence of Ni on the diffusion of oxygen through the oxide layer.

The role of nickel in radiation damage of ferritic alloys

According to modern theory, damage evolution under neutron irradiation depends on the fraction of self-interstitial atoms (SIAs) produced in the form of one-dimensional glissile clusters. These clusters, having a low interaction cross-section with other defects, are absorbed mainly by grain boundaries and dislocations, creating the so-called production bias. It is known empirically that the addition of certain alloying elements influences many radiation effects, including swelling; however, the mechanisms are unknown in many cases. In this paper we report the results of an extensive multi-technique atomistic level modeling study of SIA clusters mobility in body-centered cubic Fe–Ni alloys. We have found that Ni interacts strongly with the periphery of clusters, affecting their mobility. The total effect is defined by the number of Ni atoms interacting with the cluster at the same time and can be significant, even in low-Ni alloys. Thus a 1nm (37SIAs) cluster is practically immobile at T<500K in the Fe–0.8at.% Ni alloy. Increasing cluster size and Ni content enhances cluster immobilization. This effect should have quite broad consequences in void swelling, matrix damage accumulation and radiation induced hardening and the results obtained help to better understand and predict the effects of radiation in Fe–Ni ferritic alloys.

Stacking fault energies of Mn, Co and Nb alloyed austenitic stainless steels

The alloying effects of Mn, Co and Nb on the stacking fault energy (SFE) of austenitic stainless steels, Fe–Cr–Ni with various Ni contents, are investigated via quantum–mechanical first-principles calculations. In the composition range ( c Cr = 20%, 8 ⩽ c Ni ⩽ 20%, 0 ⩽ c Mn, c Co, c Nb ⩽ 8%, balance Fe) studied here, it is found that Mn always decreases the SFE at 0 K but increases it at room temperature in high-Ni ( c Ni ≳ 16%) alloys. The SFE always decreases with increasing Co content. Niobium increases the SFE significantly in low-Ni alloys; however, this effect is strongly diminished in high-Ni alloys. The SFE-enhancing effect of Ni usually observed in Fe–Cr–Ni alloys is inverted to an SFE-decreasing effect by Nb for c Nb ≳ 3%. The revealed nonlinear composition dependencies are explained in terms of the peculiar magnetic contributions to the total SFE.

The Effect of Alloy Solidification Path on Sulfide Formation in Fe–Cr–Ni Alloys

Solidification and sulfide precipitation in Fe–Cr–Ni alloys were investigated. Five samples containing 0.3 wt% S and 1.5 wt% Mn were investigated and each sample chemistry was adjusted to solidify as primary austenite, primary ferrite or eutectic by tailoring the Cr and Ni contents. The solidification was observed in-situ in a Confocal Scanning Laser Microscope (CSLM) equipped with a gold-image furnace, and the morphology and distribution of sulfides were characterized through SEM and Optical Microscopy. For high Ni and low Cr alloys which solidified in the primary austenite field, sulfides formed in the enriched melt ahead of the dendrite fronts as a eutectic metal-sulfide structure. On the other hand, for high Cr and low Ni alloys which solidified in the primary ferrite field, liquid sulfide phase precipitated in the inter-dendritic enriched melt first, and it appeared to deform and to concentrate in the dendrite boundaries during cooling. In the near eutectic intermediate Cr and Ni containing alloy, sulfides formed in the solid state. The sulfides formed in all the alloys were Mn and/or Cr rich.

Analysis of multistep transformations in single-crystal NiTi

The effects of composition and heat treatment on the thermally induced phase-transformation behavior of single-crystal NiTi with compositions of 50.1, 50.4, 50.8, and 51.5 at. pct Ni are presented in this article. Differential scanning calorimetry (DSC) experiments reveal that a heat-treated 50.1 at. pct Ni alloy exhibits an unprecedented multiple-step transformation (MST) on both heating and cooling, with up to four peaks. This behavior is absent in the higher-Ni-content alloys. In polycrystalline NiTi alloys, MSTs have been attributed to microstructure heterogeneities such as grain boundaries and dislocations, which influence precipitation. In-situ scanning electron microscopy (SEM) results show that the MST in the 50.1 at. pct Ni alloy is associated with single-crystal defects such as dendrites and low-angle boundaries. A heterogeneous precipitate distribution is observed in transmission electron microscopy (TEM) images of the same low-Ni alloy, also associated with the defects, creating conditions that have been shown in other studies to promote the MST in polycrystals. These MSTs are not observed for high-Ni single-crystal alloys containing the same defects. In this article, we describe the origin of the extraordinary forward and reverse MSTs in the low-Ni alloy and the absence of the MST in high-Ni alloys. Transformation sequences are proposed based on the contrasting precipitate microstructures.

Microstructure and corrosion behaviour in biological environments of the new forged low-Ni Co–Cr–Mo alloys

Corrosion behaviour and microstructure of developed low-Ni Co–29Cr–(6, 8)Mo (mass%) alloys and a conventional Co–29Cr–6Mo–1Ni alloy (ASTM F75-92) were investigated in saline solution (saline), Hanks’ solution (Hanks), and cell culture medium (E-MEM+FBS). The forging ratios of the Co–29Cr–6Mo alloy were 50% and 88% and that of the Co–29Cr–8Mo alloy was 88%. Ni content in the air-formed surface oxide film of the low-Ni alloys was under the detection limit of XPS. The passive current densities of the low-Ni alloys were of the same order of magnitude as that of the ASTM alloy in all the solutions. The passive current densities of all the alloys did not significantly change with the inorganic ions and the biomolecules. The anodic current densities in the secondary passive region of the low-Ni alloys were lower than that of the ASTM alloy in the E-MEM+FBS. Consequently, the low-Ni alloys are expected to show as high corrosion resistance as the ASTM alloy. On the other hand, the passive current density of the Co–29Cr–6Mo alloy with a forging ratio of 50% was slightly lower than that with a forging ratio of 88% in the saline. The refining of grains by further forging causes the increase in the passive current density of the low-Ni alloy.

Auger Electron Spectroscopy Study of Grain Boundary Segregation in Alloy K-500: Part I. Behavior in As-Processed State

Increased interest has been paid to grain boundary segregation in alloy K-500 due to severe intergranular cracking recently observed in forged bars. However, little systematic study of this segregation has been performed so far. A detailed auger electron spectroscopy (AES) study of grain boundary segregation in alloy K-500 has been carried out as a function of alloy chemistry. To determine C segregation, the C and O contamination rates in a vacuum chamber were measured and the necessary condition for C grain boundary segregation determination was established. It has been found that severe C, Al, and Cu segregation to grain boundaries occurred and depended on alloy chemistry. High bulk Ni and low bulk Al promoted C and Al grain boundary segregation, and low bulk Ni and high bulk Al significantly enhanced Cu segregation to grain boundaries. The depth profiles of intergranularly segregated elements also showed different features for high and low Ni content alloys. In high Ni alloys, C and Al levels dropped continuously as a function of distance from the grain boundaries but the Cu level dropped only slightly. In low Ni alloys, the Al and C levels rose from relatively low grain boundary levels to a peak at a certain distance from the grain boundary where the high grain boundary Cu level dramatically dropped. Transmission electron microscope (TEM) observation revealed a grain boundaryγ′-depleted zone followed by a region with coarser and denserγ′ particles in low Ni and high Al alloys but quite uniformly distributedγ′ particles with no depleted zone in high Ni and low Al alloys. These can be explained by the observed segregation behavior. The occurrence of Cu segregation is explained according to available theories about surface segregation in binary Ni-Cu alloys, and the segregation of C and Al to grain boundaries is suggested to be probably due to their interaction with Ni and Cu.

Preferential sputtering in dilute CuNi alloys

Preferential sputtering of Cu-Ni alloys has been investigated using the Ion Scattering Spectroscopy (ISS). We have observed the preferential sputtering of Ni in low Ni alloys and of Cu in low Cu alloys when sputtering them with 0.8 and 3.0 keV Ne + beams. Current models which consider mass and surface binding energy predict the behavior for Cu but not for Ni.

鋼の燒入殘留應力に及ぼず變態及び熱收縮の影響

Using cylindrical Ni-Fe älloy, specimens, we measured the residual stress of quenching by G. Sachs' method, and studied the mechanism of the growth-of quenching stress by changing the method of the heat treatment and the alloy elements.By this study, we found the fact that, in comparatively low nickel alloy (16% Ni), when quenching tempsrature rises higher than thansformation point, it takes the thermal stress type (Outsidecompression, inside-tension), and when quenching temperature approaches immediately above the transformation point, it takes the transformation stress type (outside-tension, inside-compression). In high Ni alloy (26% Ni), however, no matter how the quenching temperature may be, the stress distribution takes the type of transformation stress, and its value is little, too. As we (authors) thought the cause of the latter depends on "the transformation plasticity" as reported by G. Wasserman and E. Scheil, we made a simple experiment and could assure it to be true. In other words, in the case of low Ni alloy (16% Ni), transformation takes place it the viscous state of high temperature and the stress changes into non-elastic strain when it passes the range of transformation, so it is not affected by "the transformation plasticity". On the contrary, we may conclude that in the case of high Ni alloy, the transformation takes place in the elastic state in low temperature, so it reduces the stress by "the transformation plasticity".