Ni-containing Alloys Research Articles

Oxygen Evolution Behavior of Ni-Containing Alloy Electrodes in NaOH–KOH Hydrate Melt

We investigated the oxygen evolution behavior of nine types of commercially available Ni-containing alloys in a NaOH–KOH hydrate melt. The oxygen overpotential of all Ni-containing alloys was lower than that of the Ni electrode (453 mV) at 500 mA cm–2 and 150°C. The Kovar (Fe–Ni–Co alloy) electrode exhibited the lowest oxygen overpotentials of 280 mV at 150°C and 227 mV at 200°C. The Tafel slope of the Kovar electrode at 150°C was 63 mV dec–1, which was slightly lower than that of the Ni electrode (67 mV dec–1). The upper limit of the Tafel region for the Kovar electrode was extended to a higher current density of 300 mA cm–2 compared to that for the Ni electrode (100 mA cm–2) at 150°C. It further extended to 1000 mA cm–2 at 200°C, indicating that highly efficient water electrolysis can be expected.

Microstructure and mechanical properties of cast, eutectic Al-Ce-Ni-Mn-Sc-Zr alloys with multiple strengthening mechanisms

The effects of Ni additions on microstructure and microhardness of near-eutectic Al-9Ce-xNi-0.75Mn-0.18Sc-0.12Zr (x = 2.5, 3.2, and 3.9 wt.%) alloys are investigated for various cooling rates. The as-cast microstructure consists of fine, micron-scale Al11Ce3 and Ni-rich phases formed during eutectic solidification. Two Ni-rich phases are observed: (i) Al27Ce3Ni6 at higher Ni contents and slower solidification rates, and (ii) Al9(Ni,Mn,Fe)2 at lower Ni contents and faster solidification rates. While these Ni-containing alloys have higher microhardness than a Ni-free control alloy, varying the Ni concentration in the 2.5–3.9 wt.% range does not significantly affect the microhardness, indicative of competing strengthening and weakening effects from adding Ni. The alloy with intermediate Ni content (3.2 wt.%) is selected for further microstructural and mechanical characterization. It contains four coarsening-resistant strengthening constituents: (i) micron-scale Al11Ce3 and (ii) Al27Ce3Ni6 or Al9(Ni,Mn,Fe)2 platelets, all formed during eutectic solidification, (iii) L12-Al3(Sc,Zr) nanoprecipitates formed on aging, and (iv) Mn atoms in solid solution in the α-Al matrix. The combined strengthening mechanisms impart high strength at ambient and elevated temperatures, as measured by microhardness (as a function of aging time), by compression and tensile experiments, and by compressive creep measurements. Alloys previously reported in the literature - with various combinations of one, two or three of these four strengthening phases - show lower hardness and creep resistance, indicating that cumulative strengthening can be achieved when combining mechanisms; the present alloy containing all four phases shows a remarkably high creep threshold stress of 62 MPa at 300 °C.

Microstructural Examination and Thermodynamic Analysis of Sn-1.5Ag-0.5Cu-x mass% Ni Lead-Free Solder Alloys

The diminutive additions of nickel (Ni) element have been fused to Sn-1.5Ag-0.5 mass% Cu (SAC155) lead-free solder alloy. The study was examined experimentally and computationally (using JMatPro software program) the microstructural features, thermal behavior, density, thermal diffusivity, and conductivity as well as tensile stress strain of the Sn-1.5Ag-0.5Cu-x mass %Ni (x = 0.00, 0.05, 0.10, 0.20, and 0.50) solder alloys (SAC155-xNi). The fusing additions of Ni have a little impact on the melting point of SAC155 alloy which increasing from 502 to 504.2K\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$504.2 K$$\\end{document}. The microstructure of SAC155 solder alloy included coarse grains of β-Sn besides, large eutectic regions, and embedded IMCs of Ag3Sn and Cu6Sn5. The computed values of Gibbs free energy (G) during solidification of the β-Sn phase, Ag3Sn, and Sn3Sn4 IMCs show the stability at -130.7×103J.kg-1\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$- 130.7 \ imes 10^{3} {\ ext{J}}{\ ext{.kg}}^{ - 1}$$\\end{document}, -157.7×103J.kg-1\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$- 157.7 \ imes 10^{3} {\ ext{J}}{\ ext{.kg}}^{ - 1}$$\\end{document}, and -377.13×103J.kg-1\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$- 377.13 \ imes 10^{3} {\ ext{J}}{\ ext{.kg}}^{ - 1}$$\\end{document}, respectively. The SAC155-xNi alloys involved finer β-Sn grains, large fibrous eutectic regions, (Cu,Ni)6Sn5 and (Cu,Ni)3Sn4 IMCs. The G of Cu6Sn5 decreased from -233.1×103J.kg-1\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$- 233.1 \ imes 10^{3} {\ ext{J}}{\ ext{.kg}}^{ - 1}$$\\end{document} to -317.9×103J.kg-1\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$- 317.9 \ imes 10^{3} {\ ext{ J}}{\ ext{.kg}}^{ - 1}$$\\end{document} when Ni content increased up to 0.25 mass %, then stable and steady at -318.4×103J.kg-1\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$- 318.4 \ imes 10^{3} {\ ext{J}}{\ ext{.kg}}^{ - 1}$$\\end{document} with more addition of Ni element. The formation of the (Cu, Ni)6Sn5 and (Ni, Cu)3Sn4 IMCs is motivated by the lowest Gibbs free energy, especially when Ni mass% is added to a sufficient level. The measured and computed values of specific heat at constant pressure (Cp) of SAC155-xNi alloys show a good matching especially at lower temperatures and a little mismatch at high temperatures which decreases with increasing Ni content. The activation energy of atomic arrangement (Q) increased from 11.36 to 13.41kJ.mole-1\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$13.41 {\ ext{kJ}}{\ ext{.mole}}^{ - 1}$$\\end{document} with increasing Ni content in SAC155-xNi alloys. Thermal diffusivity (α) and conductivity (κ) of SAC155-xNi gradually decreased with increasing temperature in the range from 303 to 423 K and/or Ni content in alloys. The measured values of (κ) are slightly lower than the computed value at similar temperatures of the SAC155-xNi alloys. The decrease in (κ) may be assigned to scattering the electrons or reducing the phonon contributions due to the presence of various solute atoms and different IMCs. The results of stress-strain graphs reveal the enhancement of YS and UTS for all Ni-containing alloys. The improvement of the yield stress (YS) and ultimate tensile strength (UTS) of Ni-containing alloys is attributed to the uniform distribution of the IMCs, the reduction of β-Sn grain size, and the smoothed enlargement of the eutectic region.

Internal oxidation of austenitic Fe-Ni-Cr alloys at high temperatures: Deduction of oxygen permeability and the influence of carbon-bearing gases

Internal oxidation was employed to determine oxygen permeability in the Fe-Ni system between 1000 ºC and 1150 ºC at the oxygen partial pressures of Fe/FeO equilibrium. Fe-Ni-Cr alloys were exposed to environments with and without carbon to assess the influence of dissolved carbon on oxygen permeability. A detailed examination of the classical diffusion model for internal oxidation allowed increased accuracy of the permeability measurement. Oxygen permeability decreased nonlinearly with Ni content in the alloy, driven by a decrease in oxygen solubility. No carbon effect was detected in Ni-containing alloys.

Influence of Ni Contents on Microstructure and Mechanical Performance of AlSi10Mg Alloy by Selective Laser Melting.

To improve the tensile strength and wear resistance of AlSi10Mg alloys, a novel in situ synthesis method of selective laser melting (SLM) was used to fabricate the Ni-reinforced AlSi10Mg samples. The eutectic Si networks formed around the α-Al crystals by diffusion and transportation via Marangoni convection in the SLM process. Moreover, the XRD and TEM results verified that the Al3Ni nanoparticles were created by the in situ reaction of the Ni and aluminum matrix in the Ni/AlSi10Mg samples. Therefore, the microstructure of the Ni-containing alloys was constituted by the α-Al + Si network + Al3Ni phases. The dislocations accumulated at the continuous Si network boundaries and cannot transmit across the dislocation walls inside the Si network. SEM results revealed that the continuity and size of eutectic Si networks can be tailored by adjusting the Ni contents. Furthermore, the Al matrix also benefited from the Al3Ni nanoparticles against the dislocation movement due to their excellent interfacial bonding. The 3Ni-AlSi10Mg sample exhibited high mechanical properties due to the continuous Si networks and Al3Ni nanoparticles. The tensile strength, elongation, Vickers hardness, friction coefficient, and wear volumes of the 3Ni-AlSi10Mg samples were 401.15 ± 7.97 MPa, 6.23 ± 0.252%, 144.06 ± 0.81 HV, 0.608, 0.11 mm3, respectively, which outperformed the pure AlSi10Mg samples (372.05 ± 1.64 MPa, 5.84 ± 0.269%, 123.22 ± 1.18 HV, 0.66, and 0.135 mm3).

Insights into the microstructures and mechanical properties of magnesium–calcium–transition elements: A combined experimental and simulation study

Small amounts of transition elements: Ti, Mn, Fe, Co, and Ni, have been added to a Mg–0.1Ca (wt%) alloy to elucidate their additions on the microstructure and room temperature (RT) mechanical properties. Only Ni was effective in weakening the texture intensity and thus led to the improvement in RT formability and ductility, whereas the other elements were basically no effect. The microstructural characterization by high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) in conjunction with energy-dispersive X-ray spectrometry (EDS) and numerical simulation using a grain boundary (GB) phase model indicates that the Ca and Ni co-segregated to the GB of the Ni-containing alloy, while the GB of the other alloys was enriched with Ca only. Further investigations by a correlative electron backscatter diffraction and STEM-EDS reveal that the transverse direction (TD) oriented GBs had significantly lower concentration of Ca and Ni compared to the basal-oriented GBs, which resulted in the development of the TD-split texture in the Ni-containing alloy. A compositionally optimized Mg–0.1Ca–0.1Ni (wt%) alloy showed a large index Erichsen value of 8.5 mm in the sheet form, and exhibited a high tensile yield strength of 238 MPa with a moderate fracture elongation of 14% in the form of an extruded rod. Due to the addition of small amounts of alloying elements, the newly developed Mg–0.1Ca–0.1Ni alloy had an excellent thermal conductivity of 154 W/(m·K) which is even higher than that of a commercial 5052 Al alloy.

Unusual alloying effects of Co and Ni on structure and magnetic properties of Fe-Si-B-Cu nanocrystalline alloys with pre-existing α-Fe nanocrystals

Alloying Co and Ni in traditional Fe-based nanocrystalline alloys always brings about crystals refinement and magnetic softness improvement, while we found opposite phenomena in Fe81.3Si4B13Cu1.7 nanocrystalline alloy with pre-existing α-Fe nanocrystals in present work and clarified the related mechanism by investigating the structure evolution of melt-spun (Fe, Co, Ni)81.3Si4B13Cu1.7 alloys. The Co and Ni alloying lowers the number density (Nd) of pre-existing α-Fe nanocrystals in the as-spun alloys by suppressing the formation of Cu-enriched clusters. The effect of Ni alloying is more distinct than that of Co and adding 10 at.% Ni completely eliminates the nanocrystals. The decreased Nd leads to the weakened competitive growth between the nanocrystals, combined with the absence of diffusion-suppression, resulting in the slightly crystals coarsening and corresponding magnetic softness deterioration in the annealed Co-containing alloys and much more distinct ones in the annealed Ni-containing alloys.

Experimental studies of the quasi-binary system Na2SO4–NiSO4 by differential thermal analysis, calorimetry and X-ray diffraction

This work is dedicated to experimental studies of the quasi-binary sub-system Na2SO4–NiSO4 with Differential Thermal Analysis (DTA) with simultaneous Thermal Gravimetry (TG), Differential Scanning Calorimetry (DSC), and X-ray Diffraction (XRD) to obtain the transition temperatures, the phase compositions, and thermodynamic data of Na2Ni(SO4)2, (Fp: 982 ± 3 K, ΔHfus = 59 ± 3 kJ mol−1), compound in this sulfate system. A compound Na6Ni(SO4)4) with the decomposition temperature 682 ± 3 K was confirmed. The lack in the data of the NiSO4-rich region was clarified. The entire composition range could be measured and the structure change at 1243 ± 3 K (“dkostruct”) that enables to describes corrosion processes on Ni-containing alloys in a sulphatic-rich environmental, as well as the melting point of NiSO4 at 1483 ± 3 K could be confirmed in this work. A new and complete phase diagram was determined.

The effect of electrospark deposition using zirconium electrodes on structure and properties of nickel-containing alloy obtained selective laser melting

Protective coatings were applied by electrospark deposition (ESD) using zirconium electrodes to improve the performance of the Ni-containing alloy obtained using the selective laser melting (SLM) technology. The kinetics of mass transfer was studied in 5 different frequency-energy processing modes. An analog-to-digital converter was used to determine the average number of pulse discharges, single-pulse energy, and the total energy of pulse discharges for 1 min of processing (ΣЕ) for all the modes used. In low-energy processing modes (ΣЕ = 1459÷2915 J), a weak mass transfer was observed, and the cathode weight gain was recorded only in the first minutes. As the processing time increased, a decrease in the substrate weight was observed. The roughness of coatings (Ra) varied in the range of 3.9–7.2 μm. In high-energy modes (ΣЕ = 5197÷17212 J), due to intense electrode heating, a steady cathode weight gain was observed, but the formed coatings featured by increased roughness: Ra = 7.4÷8.6 μm. The Ra parameter for the original SLM samples was 10.7 μm. The formed coatings featured by a thickness of 15–30 μm, high continuity (up to 100 %), hardness of 9.0–12.5 GPa, elastic modulus of 122–145 GPa, and friction coefficient of 0.36–0.49. The ESD processing promoted an increase in wear resistance of the SLM alloy by 7.5–20 times, and oxidation resistance by 10–20 % (t = 1150 °C, τ = 30 h). It was found that the coating obtained in the low-energy ESD mode with energy ΣЕ = 2915 J featured the best performance (hardness, modulus of elasticity, roughness, wear resistance and oxidation resistance).

Investigation on the robust boronizing strategy for the surface strengthening of CoCrNi medium-entropy alloy

Boronizing has been considered as a promising strategy to enhance the surface hardness and corrosion resistance of the new emerged alloys, like CoCrNi medium-entropy alloy (MEA). However, recent studies showed that a brittle silicide layer was generated on the surface for Ni-containing alloys boronizied with the SiC-containing powder (SiC+KBF4+B4C). In this study, Si-free powder packing (900 °C 4 h) and electrochemical boronizing (900 °C 30 min) methods were explored to obtain the boronized layer for CoCrNi MEA. The morphology, element distributions and phases of the boronized layer were characterized by Scanning Electron Microscopy (SEM) and Energy Dispersive Spectrometer (EDS) and X-ray diffraction (XRD). The results show that the powder-pack boronizing method with 90 wt% B4C + 10 wt% NaBF4 (B4C) and electrochemical (EC) methods have the thickest boronized layer (~70 μm) with hardness of about 22 GPa. The boronized layer is dominated by columnar CrB and Co2B with obvious preferred orientation and strong texture. In addition, CoB, Ni2B and Ni3B also exists in the boronized layer, and these two optimal boronizing processes avoid the influence of Si. This study shows that the B4C and EC boronizing methods can be considered as the viable alternative strategy for the preparation of strengthened layers.

Understanding the effect of Ni content on microstructures and tensile properties of AlSi10Mg alloy samples under a variety of solidification rates

It is critical to gain a better knowledge of the impact of varying Ni contents on the microstructural changes of the AlSi10Mg alloy solidified at various cooling severities. Understanding the combined features of Si and Al3Ni phases constituting this modified alloy may also have a significant impact on understading its properties. In order to advance in these topics, AlSi10Mg alloys modified with, 1.0, 2.0 and 3.0 wt% Ni are either directionally solidified (DS) at lower cooling rates or laser surface remelted (LSR) at higher rates. α-Al dendritic branches are found in the microstructure of all tested alloys either for DS or LSR specimens, surrounded by the eutectic constituent. Moreover, the dendritic array is defined by the secondary dendritic spacing, λ2, which is found to be 65% lower for the Ni-containing alloys than in the AlSi10Mg alloy. The microstructure of the quaternary alloys is constituted by the α-Al+Si+Al3Ni phases, with the Al3Ni phase having a prevalent fishbone shape and being found in higher proportions as a result of slow cooling and increasing Ni content. Ultimate tensile strength of the AlSi10Mg alloy is 190 MPa for a specified range of λ2 (22–30 µm), which is higher than that of Ni-containing alloys for the same range. However, Ni-containing alloys are more sensitive to λ2, and the DS AlSi10Mg-1Ni alloy, as well as a combination of samples solidified at no less than 3 °C/s, fine dendritic spacing (<18 µm) and fine fishbone+plate-like Al3Ni mixture, shows the best tensile results: 4.6% and 208 MPa. This appears to be the optimal condition for cooperative reinforcement throughout these hardening phases. The laser treated AlSi10Mg-2Ni alloy samples have a superior hardness of 145 HV, much higher than the original substrate, demonstrating the effectiveness of the laser treatment in improving the surface mechanical properties of a Ni-modified AlSi10Mg alloy.

Laser remelting of AlSi10Mg(-Ni) alloy surfaces: influence of Ni content and cooling rate on the microstructure

AlSi10Mg alloys are widely employed in a variety of industries, including aerospace, automotive, and microelectronics. This is because of its low density, acceptable mechanical properties, acceptable corrosion resistance, and inexpensive application cost. Advantageous fluidity, a short solidification period, and minimal volumetric contraction are beneficial characteristics under processing such alloys. Despite being used as commercial alloys, the mechanical properties of the AlSi10Mg alloys still need to be improved. In line with this, the current focus of Al-based alloy development is mostly on modifying commercially available alloys. Under such context, Ni was used as an alloying element in this study to generate the Al3Ni intermetallics, distinguished by its improved mechanical strength. Furthermore, the thermal stability of the Al3Ni may be a benefit, particularly for high-temperature applications. The present study aims to investigate the solidification under low and high cooling rates of four alloys: AlSi10Mg, AlSi10Mg-1Ni, AlSi10Mg-2Ni, and AlSi10Mg-3Ni (wt.%). Samples were obtained by directional solidification (DS) and laser surface remelting (LSR) processes. The cooling rates were calculated for the DS samples and with extrapolation for LSR samples as well as with the use of a model from the literature. After testing several laser conditions, the results also include an examination of microstructural and hardness changes in the treated and untreated zones. The produced gradient of microstructures is fully characterized as well as used to evaluate cooling rates inside the laser molten pools. For energy densities of 400 J/mm2 and 100 J/mm2, the mean dendritic spacings, λ, of the three Ni-containing alloys at the laser molten pool yielded estimated cooling rates of approximately 1.5 × 104 °C/s and 4.7 × 104 °C/s, respectively. A model explaining the reversion of λ across the molten pool will be outlined.

Mechanical Performance and Precipitation Behavior in Al-Si-Cu-Mg Cast Alloys: Effect of Prolonged Thermal Exposure.

Al-Si-Cu-Mg cast (354) alloys are used in the automotive sector owing to their remarkable properties which are achievable after applying appropriate thermal treatments. Zirconium, Nickel, and Manganese were added to this category of Al-alloys to preserve good mechanical properties while being exposed to elevated temperatures for long times. The ultimate and yield strength values obtained at room temperature for the stabilized (thermally-exposed) T5-treated condition were comparable to those of the stabilized T6-treated condition, whereas the same properties for T5-treated alloys were higher than those of T6-treated ones for elevated-temperature tensile testing. Interestingly, the results showed that the addition of 0.75 wt.% Mn was competitive with the addition of 2 and 4 wt.% Ni with respect to the elevated-temperature and ambient temperature strength values, respectively. In addition, the Mn-containing alloy M3S exhibited improved ductility values at ambient temperature and at 250 °C, compared to the Ni-containing alloys. Examination of the fracture surface of tested samples revealed the advantageous role of sludge particles in enhancing the performance of Mn-containing alloys through their resistance to the propagation of cracks that developed in many intermetallic phases. This finding is considered to be economically significant in view of the lower price of manganese compared to that of nickel.

Effects of Ni and Co on the Corrosion Resistance of Al-Si-Cu-Zn-Fe Alloys in NaCl Solution

The corrosion behavior of Fe-containing directionally solidified (DS) and centrifugally cast (CC) Al-Si-Cu-Zn alloys with either Co or Ni additions has been investigated. Electrochemical and immersion corrosion methods were used to investigate the corrosion behavior in 0.6 M NaCl after short (1-h) and long (30-day) exposure periods. The employed solidification methods allowed the production of samples with a wide range of secondary dendrite arm spacing (SDAS) while preserving Si and Fe-containing phases. The 0.5 wt.% Ni and Co additions led to the growth of the AlFeSi(Ni) and AlFeSi(Co) phases, but no binary AlNi nor AlCo intermetallic particles have been generated. Potentiodynamic polarization studies at early exposure revealed an increase in the corrosion potential as the Ni was added for either fast or slow solidified samples. The electrochemical impedance spectroscopy at early exposure demonstrated that the Ni-modified alloy, on the other hand, was associated with smaller charge transfer resistances, indicating a reduction in the corrosion resistance after a short elapsed time into the electrolyte. However, the 30-day immersion tests revealed much lower corrosion rate of the Ni-modified alloy than the other alloys, while the corrosion rates of the Co-modified and non-modified alloys were similar. In the Ni-containing alloy, a decreased corrosion rate under a long-term corrosion process was attributed to the formation of a thick and dense alumina layer, effectively protecting the surface under such conditions. This work contributes to better knowledge of the corrosion behavior of Ni- and Co-corrected Al industrial scrap compositions.

Facilitation of detwinning through controlling crystal structure in Ti–Zr–Ni–Pd high temperature shape memory alloys

Ti–(Zr, Hf)–Ni alloys, in which Zr or Hf is added to Ti–Ni shape memory alloys, are expected to be excellent high temperature shape memory alloys because they have martensitic transformation temperatures above 100°C and high strength at elevated temperatures by utilizing H-phase precipitation. However, during reorientation of martensite variants, deformation stress increases with applied strain without showing a plateau region, making it difficult to obtain a large shape recovery strain. In the martensite phase of Ti–(Zr, Hf)–Ni alloys, thin and dense (001)B19′ compound twins are introduced during the martensitic transformation, and the detwinning of martensite variants becomes difficult to occur, resulting in the increase in deformation stress during the variant reorientation. Therefore, we propose a new alloy-design concept to control the crystal structure by adding a fourth element in order to facilitate detwinning. In this paper, we first considered the cause of (001)B19′ compound twinning in Ti–(Zr, Hf)–Ni alloys, and then selected Pd as the fourth element to change the crystal structure for preventing the formation of (001)B19′ compound twins. Experimental investigations of the effects of alloy composition on the crystal structure, transformation temperatures, and precipitate formation realized this alloy-design concept through presenting a candidate composition range of Ti–(15–20)Zr–49.7Pd and surrounding Ni-containing alloys for new high temperature shape memory alloys which readily undergo detwinning.

Development of long-period ordered structures during crystallisation of amorphous Mg80Cu10Y10 and Mg83Ni9Y8

Abstract The different transformations occurring during the crystallisation of amorphous Mg80Cu10Y10 and Mg83Ni9Y8 alloys have been elucidated. The formation of several unknown metastable phases was observed. The present work confirms that such phases exhibit long-period structures originated by the periodic alignment of (Y, TM)-layers (TM: transition metal). The long-period stacking-ordered phase, identified in the crystallised Ni-containing alloy, is stable at higher temperatures than that in the Cu-containing alloy.

Synergy of Ni micro-alloying and thermomechanical processing in Al–Mg–Si–Cu–Zn–Fe–Mn alloys with enhanced bendability

Synergy of Ni micro-alloying and thermomechanical processing on the phase distribution, formability and bendability of Al–Mg–Si–Cu–Zn–Fe–Mn alloys was systematically studied in this paper. With the addition of micro-alloying Ni, the Ni-containing Fe-rich phase can be formed, which not only serves as nucleation sites of Mg–Si precipitates (such as, Q phase) during the casting process, but also improves the uniform distribution level of Fe-rich phases after homogenization. The formability and bendability of Ni-containing alloy can be both improved to a certain level due to the positive effect of Ni micro-alloying. In comparison, if increasing the cold rolling deformation between hot rolling and annealing, the distribution of multi-scale Fe-rich phases can be significantly improved based on the synergy of Ni micro-alloying and thermomechanical processing. And finally, this improvement further results in the great improvements in the microstructure, texture, formability (average r = 0.688, △r = −0.09) and bendability of the alloy together. Based on the microstructure evolution, the synergy mechanism of Ni micro-alloying and thermomechanical processing is put forward in this paper.

Dealloying Behavior in Ni-Cr-Fe Alloys As a Precursor to SCC in Boiling Caustic Environments

Experiments have shown that the susceptibility of Ni-Cr-Fe Alloys to dealloying, and subsequently stress corrosion cracking (SCC), highly depends on alloy composition and electrochemical potential. Dealloying is defined as the selective dissolution of the more active elements in an alloy (e.g., Cr and Fe in a Ni-based alloy). A dealloying process results in a remnant nano-porous layer on the surface that could be a precursor to SCC by a mechanism known as film-induced cleavage (FIC) where cracks propagate through this layer with sufficient velocity to promote energy transmission into the underlying metal resulting in cleavage fracture. Presence of a threshold amount of a more noble element could theoretically suppress this phenomenon by surface diffusion. This was investigated further by previous researchers, and the percolation theory of materials was hypothesized that states that the more noble element in an alloy creates a percolating mass that would limit the ability to retain a connected pathway of the reactive elements from the surface given it reaches a certain threshold of the mass of the material. This percentage was estimated to be between 50-60% for most alloying systems and termed the parting limit. In this study, percolation theory and dealloying is further explored by exposing several Ni-based and Fe-based alloys with varying percentages of Ni to boiling caustic solutions (50% NaOH) and evaluating their dealloying behavior.As mentioned, dealloying highly depends on electrochemical potential and is believed to occur above a threshold potential known as the dealloying critical potential. The dealloying critical potential is defined as the condition at which a transition occurs from a “passivated” alloy surface to the sustained formation of a bi-continuous porous structure. It can be identified by running an anodic polarization scan and initially matching anodic peaks to a pH-potential diagram (Pourbaix diagram) to evaluate the potential dissolution behavior. Bench-top electrochemical measurements were conducted to find the dealloying critical potential and make a correlation between this potential and alloys’ composition. The figure attached shows the anodic polarization scan results for all alloys included in the study in 50% NaOH at 140°C. It was observed that for the samples with high Ni content (i.e., Alloy 600 and Alloy 690) there was only a single active peak, which indicates transitioning to passivation and inhibition of dealloying. All other materials were susceptible to dealloying. Electron microscopy techniques were used to analyze all the samples used in the figure after exposure to 50% NaOH at 140°C at open circuit potential (OCP) and during a potentiostatic hold at the dealloying critical potential. Scanning electron microscopy (SEM) imaging followed by energy dispersive X-ray spectroscopy (EDS) revealed varying degrees of dealloying, with low Ni-containing alloys showing greater severity.The role of dealloying as a precursor to SCC was also evaluated by utilizing U-bend samples for Alloy 690, Alloy 800, and SS 304. The findings were in-line with previous dealloying result; the alloys with Ni content below the parting limit, SS304 and the Alloy 800, both underwent SCC after 48 hours of exposure to 50% NaOH solution at 135 °C, while the Alloy 690 sample (~60Ni) did not show any signs of cracking. A peculiar phenomenon was also noted for alloys containing Mo where it is seen that their dissolution current is approximately two folds higher than their counterparts with approximately the same Ni content. For example, when comparing SS 304 and SS 316 or Alloy C-276 and Alloy G-35. A possible explanation for that is the electrochemical behavior of Mo in caustic conditions where it is shown in its pourbaix diagram that dissolution occurs over a wider range of pH and potential in comparison to the other alloying elements. This earlier dissolution can possibly create more terrace sites on the surface that could accelerate the dissolution of iron and chromium. However, further studies are needed before a conclusion can be reached. Further work has also been conducted on Alloy 800 under the same conditions to evaluate the possibility of controlling the geometry of the dealloyed layer. The results so far indicate that the time of exposure and the electrochemical potential are critical in controlling the geometry of the dealloyed layer. Parallel studies are being conducted on SS 304 to evaluate the effect of Pb on extending the dealloying regime. The results so far have shown that the addition of Pb would shift the OCP more positively towards a region of passivity. It was also noted that dealloying kinetics is hindered in comparison to pure caustic conditions. However, SCC may still be possible with dissolved Pb present. Figure 1

Strengthening precipitates and mechanical performance of Al–Si–Cu–Mg cast alloys containing transition elements

This study investigated the active strengthening precipitates existing in the structure of 354-type Al–Si–Cu–Mg cast alloys following the addition of Zr as a primary alloying element in addition to Ni and Mn in different amounts and combinations. The investigation comprised three main parts: the first part simulated the formation and dissolution of strengthening precipitates and their precursors using differential scanning calorimetric (DSC) heating analysis of as-quenched samples, the second part presented transmission electron microscopy (TEM) observations of the fine precipitates existing in the structure of the alloys studied, and the third part examined the mechanical performance (tensile properties and hardness values) of the alloys studied. Interpretation of the DSC heating data in accordance to the available literature showed that the alloys are strengthened primarily by θ-Al2Cu and S–Al2CuMg precipitates and their precursors. Furthermore, some of the copper content was consumed in forming Al–Ni–Cu particles instead of the θ-Al2Cu strengthening phase and its precursors and hence the Ni-free alloys contain higher fractions of the θ-Al2Cu phase and its precursors than the Ni-containing alloys. TEM investigations confirmed the results obtained from the DSC heating analysis regarding the combined strengthening effect of θ-Al2Cu and S–Al2CuMg precipitates and their precursors as well as the fraction of θ-Al2Cu phase in Ni-free and Ni-containing alloys. TEM observations revealed, also, that the alloys studied have a secondary strengthening effect which is exerted by precipitates in the form of Alx (Zr,Ti)Si that formed following the addition of Zr. The Mechanical performance showed to be highly dependent on the chemical composition which controls alloys structure and response to the applied heat treatment and hence the formation of the fine precipitates which are mainly responsible to the strengthening effect.

Stress-induced crystallographic and microstructural evolution for hierarchically twinned martensite in single- and dual-phase Ni-Mn-Ga alloys

Polycrystalline Ni54+xMn25Ga21-x high temperature shape memory alloys were developed to examine the Ni-content dependent crystallographic features, phase transformation and deformation behavior. The alloys were characterized by a hierarchically twinned martensite structure with the co-existence of ductile γ phase in Ni57 and Ni58 alloys. Two types of twinning relationships existed, i.e., (112) compound twin in the internal nano-lamellae and 11¯2 type-I twin in the adjacent micro-variants. The martensitic start temperature, compressive strength and ductility increased with increasing Ni content. However, the corresponding shape recovery capability was significantly deteriorated in the higher Ni-containing alloys. For the single-phase alloy, the detwinning/reorientation of internal nano-lamellae and activation of deformation twins contributed to the macroscopically recoverable strain in case of low pre-strains. In contrast, large pre-strains led to piles-up of dislocation, bending and kinking of twinning interfaces, formation of deformation bands and crossing of twin structures. These irreversible processes produced massive unrecoverable strain. Almost no detwinning and twining processes were observed in the dual-phase alloys due to the severe lattice distortion. Instead, most deformation occurred via dislocation motion in γ phase.