Corrosion behavior of Ni-based amorphous alloys and their crystalline counterparts

In this paper, we report the corrosion behavior of Ni₅₉Zr₂₀Ti₁₆Si₂Sn₃ and Ni₅₃Nb₂₀Ti₁₀Zr₈Co₆Cu₃ (at.%) amorphous alloys with glass forming ability (GFA) of ∼3 mm, and their crystalline counterparts in severe corrosive environment of 1 M HCl aqueous solution. From the results of polarization curves and X-ray photoelectron spectroscopy (XPS), it was found that the corrosion behavior in Ni-based amorphous alloys is very sensitive to the compositions and the structural homogeneity is favorable for their corrosion-resistance in 1 M HCl aqueous solution.     

Investigations of mechanically alloyed Ni–Zr–Ti–Si amorphous alloys with significant supercooled regions

This study examined the amorphization behavior of Ni₅₇Zr₂₀Ti_23−xSi_x (x=0, 1, 3) alloy powders synthesized by mechanical alloying technique. According to the results, after 5 h of milling, the mechanically alloyed powders were amorphous at compositions of Ni₅₇Zr₂₀Ti_23−xSi_x (x=0, 1, 3). The amorphization behavior of Ni₅₇Zr₂₀Ti₂₀Si₃ was examined in details. The conventional X-ray diffraction and synchrotron EXAFS results confirm that the fully amorphous powders formed after 5 h of milling. The thermal stability of the Ni₅₇Zr₂₀Ti_23−xSi_x amorphous powders was investigated by differential scanning calorimeter (DSC). As the results demonstrated, the amorphous powders were found to exhibit a large supercooled liquid region before crystallization. The supercooled liquid regions, defined by the difference between T_g and T_x, (i.e. ΔT=T_g−T_x), are 95 K, 66 K, and 88 K, for Ni₅₇Zr₂₀Ti₂₃, Ni₅₇Zr₂₀Ti₂₂Si₁, and Ni₅₇Zr₂₀Ti₂₀Si₃, respectively.     

Electrochemical behavior of multicomponent amorphous and nanocrystalline Zr-based alloys in different environments

Electrochemical behavior of multicomponent melt spun Zr-based amorphous as well as nanocrystalline alloys have been studied in three different corrosive media (neutral NaCl, basic NaOH and acidic H₂SO₄ solutions). Due to the presence of strong passivating elements, (Zr and Ti) melt spun ternary alloys (Zr₅₅Ti₂₅Ni₂₀) have shown complete passivation in NaCl solution even as they contain a small amount of crystallinity. Amorphous multicomponent alloys containing Cu (Zr₅₈Cu₂₈Al₁₀Ti₄ and Zr₆₅Cu_7.5Al_7.5Ni₁₀Pd₁₀) show active nature in NaCl solution and this is strongly related to selective dissolution of base metal (Zr) and enrichment of Cu in the pit region. In NaOH and H₂SO₄ solutions, all the alloys have shown complete passivation irrespective of the alloy composition.     

Microstructure and properties of cold consolidated amorphous ribbons from (NiCu)ZrTiAlSi alloys

Amorphous ribbons of compositions (Ni₅₆Cu₂)Zr₁₈Ti₁₃Al₆Si₅ and (Ni₃₆Cu₂₃)Zr₁₈Ti₁₄Al₅Si₄ were consolidated by high pressure torsion (HPT) at room temperature. In the HPT experiments a 6 GPa pressure and two turns were applied. Samples in the form of discs, 6–7 times thicker than the ribbons and about 10 mm in diameter were achieved. The minimal deformation for the homogenous consolidation was estimated to be in the range of 400%. XRD showed that the microstructure was dependent on the composition. The sample with high Cu content remained amorphous while the sample with low Cu content revealed some crystallization. DSC experiments allowed a comparison of the glass transition temperature T_g and crystallization process of the amorphous ribbon and HPT sample which were different. The glass transition temperature T_g of the amorphous HPT sample of (Ni₃₆Cu₂₃)Zr₁₈Ti₁₄Al₅Si₄ composition decreased. For both alloys the nanohardness and the elastic modules showed decrease for cold consolidated samples in comparison to the ribbons.     

Effect of Nb addition on microstructure evolution and nanomechanical properties of a glass-forming Ti–Zr–Si alloy

The glass-forming Ti₇₅Zr₁₀Si₁₅ alloy is regarded as a potential material for implant applications due to its composition of non-toxic, biocompatible elements and some interesting mechanical properties. The effects of partial substitution of 15 at.% Ti by Nb on the microstructure and the mechanical behaviour have been investigated by X-ray diffraction, scanning electron microscopy with energy-dispersive X-ray analysis, transmission electron microscopy and nanoindentation techniques. Copper mold casting and melt-spinning methods have been applied to study the influence of the cooling rate on the properties of both alloys, Ti₇₅Zr₁₀Si₁₅ and Ti₆₀Zr₁₀Nb₁₅Si₁₅. As a result of different cooling rates, significant microstructural variations from multiphase crystalline states in cast rods to nanocomposite structures in ribbons were observed. The limited glass-forming ability (GFA) of the Ti₇₅Zr₁₀Si₁₅ alloy results for melt-spun ribbons mainly in nanocomposite structures with β-type nanocrystals being embedded in a glassy matrix. Addition of Nb increases the glass-forming ability. Raising the overheating temperature of the melt prior to melt-spinning from 1923 K to 2053 K yields for both alloys a higher amorphous phase fraction. The mechanical properties were investigated using compression tests (bulk samples) and the nano-indentation technique. A decrease of hardness (H), ultimate stress and reduced Young's modulus (E_r) is observed for Ti₆₀Zr₁₀Nb₁₅Si₁₅ rods as compared to Ti₇₅Zr₁₀Si₁₅ ones. This is attributed to an increase of the fraction of the β-type phase. The melt-spun ribbons show an interesting combination of very high hardness values (H) and moderate reduced elastic modulus values (E_r). This results in comparatively very high H/E_r ratios of >0.075 which suggests these new materials for applications demanding high wear resistance.     

Bulk amorphous Ni59Zr20Ti16Sn5 alloy fabricated by powder compaction

The possibility of fabrication of bulk amorphous Ni₅₉Zr₂₀Ti₁₆Sn₅ alloy by hot isostatic pressing of powders was investigated. The amorphous powders were obtained by ball milling of amorphous melt spun ribbon and by mechanical alloying of a mixture of powders of pure crystalline elements. Fully amorphous bulk samples were successfully obtained by hot isostatic pressing of both types of powders. However, at least 10% porosity of the sample fabricated from the ball milled ribbon was observed. Further optimisation of the compaction process needs to be performed.     

Designing a toxic-element-free Ti-based amorphous alloy with remarkable supercooled liquid region for biomedical application

A series of toxic-element-free Ti–Zr–Ta–Si amorphous alloy ribbons have been successfully prepared by melt-spinning. The differential scanning carlorimetry (DSC), X-ray diffraction analysis, bending test and microhardness test are conducted for studying the thermal and mechanical properties. The results show that the Ti₄₂Zr₄₀Ta₃Si₁₅ metallic glass ribbon present excellent ductile behavior by the bending testing, without any fracture cracking after bending over 180 degree. In addition, this amorphous alloy possesses a very high glass transition and crystallization temperature of 799 and 898 K, respectively, as well as a very wide supercooled liquid region of 99 K. This amorphous alloy exhibits promising thermal stability during isothermal annealing at the middle temperature of its supercooled region, with more than 3000 s incubation time for isothermal annealing at 823 K (550 °C). This amorphous alloy also shows much lower value of corrosion current density (2.27 × 10⁻⁹ A/m²) than the 304 stainless steel in the 0.3 mass% sodium cloride solutions. This Ti₄₂Zr₄₀Ta₃Si₁₅ alloy is believed to be a promising based alloy for fabricating the bulk metallic glass foam by the spacer technique in the application of biomedical implants.     

Effects of electrochemical hydrogenation of Zr-based alloys with high glass-forming ability

Zr–(Ti)–Cu–Al–Ni metallic glasses exhibit a high thermal stability corresponding to a wide undercooled liquid region. Depending on their composition, the formation of metastable intermediate phases, e.g. a quasicrystalline phase is possible. The combination of early and late transition metals makes these alloys very interesting regarding their interaction with hydrogen. Amorphous Zr₅₅Cu₃₀Al₁₀Ni₅, Zr₆₅Cu_17.5Al_7.5Ni₁₀ and Zr₅₉Ti₃Cu₂₀Al₁₀Ni₈ ribbons were prepared by melt spinning and their microstructure and thermal behaviour was checked by X-ray diffraction, transmission electron microscopy and differential scanning calorimetry. The cathodic reactivity of alloy samples at different microstructural states and after pre-etching in 1 vol.-% HF was investigated in 0.1 M NaOH by applying potentiodynamic polarisation techniques. Galvanostatically hydrogenated samples were characterised by XRD, DSC, TEM and thermal desorption analysis (TDA). For amorphous Zr₅₉Ti₃Cu₂₀Al₁₀Ni₈ samples an increase in electrochemical surface capacity by two orders of magnitude is observed after pre-etching. Compared to the quasicrystalline and crystalline alloy, the hydrogen reduction takes place at significantly lower overpotentials. Zr-based alloys cathodically absorb hydrogen up to H/M=1.65 while keeping the amorphous structure. Already small amounts of hydrogen cause a significant decrease of the thermal stability and changes in the crystallisation sequence. The hydrogen desorption is a two-stage process: (T<623 K) hydrogen desorption from high interstitial-site energy levels and (T>623 K) zirconium hydride formation and subsequent transformation under hydrogen effusion. Hydrogen suppresses the oxygen-triggered formation of metastable phases upon heating and supports primary copper segregation. At very high H/M ratios, severe zirconium hydride formation causes the crystallisation of new compounds.     

Corrosion behavior of Zr-based metallic glass coating on type 304L stainless steel by pulsed laser deposition method

The surface morphology and corrosion behavior of Zr-based amorphous metallic glass (MG) of Zr₅₉Ti₃Cu₂₀Al₁₀Ni₈ alloy and MG coated type 304L stainless steel in different nitric acid media of 1 M, 6 M and 11.5 M HNO₃ is reported. Zirconium based MG of Zr₅₉Ti₃Cu₂₀Al₁₀Ni₈ alloy was successfully deposited on type 304L stainless steel using pulsed laser deposition technique. The SEM morphology revealed a scattered particles of “Donut” shaped features distributed in the amorphous matrix. The atomic force microscope measurement indicated the formation of dense metallic deposited layer of agglomerate of granular clusters with negligible pores or micro-crack in metallic glass coated sample. The results of the potentiodynamic polarization shows that the amorphous MG coated type 304L stainless steel exhibited marginally lower corrosion resistance than MG alloy which is attributed to the presence of corrosion-induced defects in the coated layer. This work reports suitability of using pulsed laser deposition for the preparation of thin film amorphous metallic coating to achieve improved corrosion resistance in nitric acid medium.     

The corrosion behaviour of Zr-based bulk metallic glasses in 0.5 M NaCl solution

The corrosion behaviour of eutectic Zr₅₀Cu₄₀Al₁₀ and hypoeutectic Zr₇₀Cu₆Al₈Ni₁₆ bulk metallic glasses (BMGs) was studied by electrochemical measurements, scanning electron microscope (SEM), energy dispersive X-ray spectroscopy (EDX) and X-ray photoelectron spectroscopy (XPS). Zr₅₀Cu₄₀Al₁₀ BMG was highly susceptible to pitting corrosion in naturally aerated 0.5 M NaCl solution at 30 °C. In contrast, Zr₇₀Cu₆Al₈Ni₁₆ BMG passivated spontaneously under the same condition. EDX results for Zr₅₀Cu₄₀Al₁₀ indicated that enrichment of Cu, Cl and O occurred in the pitted region, while for Zr₇₀Cu₆Al₈Ni₁₆ BMG, no significant difference was found in the surface composition from the specimens before and after immersion in the solution. XPS analysis including angle-resolved measurements for Zr₇₀Cu₆Al₈Ni₁₆ BMG revealed that zirconium cation (Zr⁴⁺) was highly concentrated in both air-formed and passive films. Furthermore, the concentration of Zr⁴⁺ ions after immersion for 24 h or more showed tendency to increase with decreasing take-off angle, indicating that the exterior part of the passive film consisted exclusively of zirconium oxyhydroxide. The high corrosion resistance of Zr₇₀Cu₆Al₈Ni₁₆ BMG was attributed to the formation of homogeneous and stable passive film enriched with zirconium.     

Electrochemical behaviour of amorphous and nanoquasicrystalline Zr-Pd and Zr-Pt alloys in different environments

The corrosion behaviour of melt spun amorphous and nanoquasicrystalline Zr₇₀Pd₃₀ and Zr₈₀Pt₂₀ alloy ribbons has been investigated using potentiodynamic polarization study in NaCl, H₂SO₄ and NaOH solutions at different concentrations. The amorphous and nanoquasicrystalline alloys show better corrosion resistance than Zr in all the solutions studied. Both the alloys are susceptible to chloride attack and pitting has been observed. Complete passivation has been observed in H₂SO₄, while gradual break down of passivating layer occurs in NaOH. In general, nanoquasicrystalline state in both the alloys shows better corrosion resistance than amorphous state in all the solutions studied.     

Formation,thermal stability and corrosion behavior of glassy Ti45Zr5Cu45Ni5 alloy

Ti-rich Ti₄₅Zr₅Cu₄₅Ni₅ bulk metallic glass with critical diameter reaching 3 mm and supercooled liquid region of 42 K was prepared by copper mold casting. The glass transition temperature and onset temperature of crystallization are 673 K and 715 K, respectively. The glassy Ti–Zr–Cu–Ni alloy is passive in 3 mass% NaCl, 1 N HCl and 1 N H₂SO₄ solutions, although pitting corrosion occurred by anodic polarization at higher potential in the Cl⁻-containing solutions. Corrosion rates of the glassy Ti₄₅Zr₅Cu₄₅Ni₅ alloy are of the order of 10⁻³ mm/year in the NaCl and H₂SO₄ solutions and about 1 × 10⁻² mm/year in the HCl acid.     

Effect of surface finishing of a Zr-based bulk metallic glass on its corrosion behaviour

Zr-based metallic glasses passivate spontaneously, but exhibit also a certain pitting susceptibility. On the example of the Zr₅₉Ti₃Cu₂₀Al₁₀Ni₈ alloy studied in 0.01 M Na₂SO₄ + x M NaCl (x = 0-0.1) electrolytes it is demonstrated that the surface finishing state and the pre-exposure conditions can significantly influence the free corrosion and anodic polarisation behaviour. Mechanical fine-polishing procedures can lead to extremely smooth topographies but also to Cu enrichment at the surface. This yields a pronounced Cu dissolution at low anodic polarisation prior to stable passivity and increases the pitting initiation susceptibility as compared to mechanically ground surface states.     

Mechanical alloyed Ti–Cu–Ni–Si–B amorphous alloys with significant supercooled liquid region

This study examined the glass formation range of Ti_94–x–yCu_xNi_ySi₄B₂ alloy powders synthesized by mechanical alloying technique. According to the results, after 5–7 h of milling, the mechanically alloyed powders were amorphous at compositions with (x+y) equal to 20–40%. For the compositions with (x+y) larger than 45% or smaller than 10%, the structure of ball-milled powders is a partial amorphous single phase or coexistent partial amorphous and crystalline phases, respectively. The thermal stability of the amorphous powders was also investigated by differential thermal analysis. As the results demonstrated, several amorphous powders were found to exhibit a wide supercooled liquid region before crystallization. The temperature interval of the supercooled liquid region defined by the difference between T_g and T_x, i.e. ΔT(=T_x–T_g), are 52 K for Ti₇₄Ni₂₀Si₄B₂, 74 K for Ti₆₄Ni₃₀Si₄B₂, 58 K for Ti₆₄Cu₂₀Ni₁₀Si₄B₂, and 61 K for Ti₇₄Cu₁₀Ni₁₀Si₄B₂.     

Improved mechanical behavior of Cu–Ti-based bulk metallic glass by in situ formation of nanoscale precipitates

The mechanical properties of Cu₄₇Ti₃₄Zr₁₁Ni₈, Cu₄₇Ti₃₃Zr₁₁Ni₈Fe₁, and Cu₄₇Ti₃₃Zr₁₁Ni₈Si₁ bulk metallic glasses prepared by copper mold casting are investigated. Room temperature compression tests reveal fracture strengths above 2000 MPa, Young’s moduli around 100 GPa and elastic strains slightly exceeding 2.0%. Only Cu₄₇Ti₃₃Zr₁₁Ni₈Si₁ exhibits distinct plastic strain due to a unique composite microstructure with in situ formed nanoscale precipitates in the glassy matrix.     

Hot corrosion behaviour of Ni3Al in sulphate-chloride mixtures in the atmosphere

Hot corrosion of Ni₃Al intermetallic compound in the presence of sulphate-chloride mixtures was studied. A comminuted Ni₃Al mixed with NaCl-Na₂SO₄, NaCl-Li₂SO₄, LiCl-Na₂SO₄, LiCl-Li₂SO₄ additions was oxidized in the air up to 1000 °C with linearly increasing temperature and isothermally within the temperature range of 500-700 °C. The corrosion process was observed by thermogravimetric method using Mettler thermoanalyzer.The experiments indicated that LiCl (∼10 wt.%)-Li₂SO₄ mixture was the most corrosive agent. It was also found that by addition of MgO the corrosion of Ni₃Al was reduced. Phase composition of the corrosion products was established by X-ray diffraction analysis; there were detected Al₂O₃, Al₂S₃, NaAlO₂ (or LiAlO₂) as intermediate products, nickel sulphides, NiO and NiAl₂O₄. NiAl₂O₄ spinel was formed only at the highest temperatures, while at lower temperatures alumina was present instead of spinel.Hot corrosion behaviour of Ni₃Al in sulphate-chloride mixtures in air atmosphere.     

Microstructure and electrochemical behavior of Ti-coatedZr55Al10Ni5Cu30 bulk metallic glass

In this study, pure Ti was coated on Zr₅₅Al₁₀Ni₅Cu₃₀ bulk metallic glass (BMG) using a physical vapour deposition (PVD) technique with magnetron sputtering. Microstructures of Ti coating, BMG substrate and interface were investigated by conventional and high-resolution transmission electron microscopy (TEM and HREM). The electrochemical behavior of Ti-coated Zr₅₅Al₁₀Ni₅Cu₃₀ BMG was studied by potentiodynamic polarization curves and electrochemical impedance spectroscopy (EIS) in Hanks' solution. Scanning electron microscopy (SEM) was used to characterize the surface morphology of the coating after electrochemical testing. HRTEM observation reveals that the sputtering Ti coating consists of α-Ti nano-scale particles with the size about 10 nm. The polarization curves revealed that the open-circuit potential shifted to a more positive potential and the passive current density was lower after Ti coating was applied in comparison with that of the monolithic Zr₅₅Al₁₀Ni₅Cu₃₀ BMG. Electrochemical impedance spectroscopy (EIS) measurements showed that the Bode plots of Ti-coated Zr₅₅Al₁₀Ni₅Cu₃₀ BMG presented one time constant for 1 h and 12 h immersion and two time constants after 24 h immersion. The good bonding condition between Ti coating and Zr₅₅Al₁₀Ni₅Cu₃₀ BMG substrate may be responsible for the high corrosion resistance of Ti-coated Zr₅₅Al₁₀Ni₅Cu₃₀ BMG.     

High-temperature SO2-gas corrosion of TiN–Ti5Si3 composites prepared by polymer pyrolysis

By pyrolyzing a mixture of Si-containing pre-ceramic polymers and TiH₂ powders in a N₂ atmosphere, a TiN–Ti₅Si₃ composite was synthesized. The composite was then corroded between 700 °C and 1000 °C for 20 h in an Ar–0.2% SO₂ atmosphere. TiN was mainly oxidized to rutile TiO₂. Ti₅Si₃ was oxidized to TiO₂ supersaturated with Si ions, and sulfidized to Ti₂S supersaturated with Si ions. At initial stage of corrosion, oxidation dominated sulfidation. As corrosion proceeded, sulfidation progressively occurred underneath the oxide scale based on the decreased oxygen potential and increased sulfur potential near the scale/matrix interface.     

Interface structure and properties of a brass-reinforced Ni59Zr20Ti16Si2Sn3 bulk metallic glass composite

Interfaces between a Ni₅₉Zr₂₀Ti₁₆Si₂Sn₃ bulk metallic glass (BMG) and crystalline brass reinforcements were characterized using transmission electron microscopy and nanoindentation. An interfacial layer with a thickness of ∼50–100 nm was observed in the composite prepared by warm extrusion of gas atomized powders. Microstructural characterization and chemical analysis suggest that the formation of interfacial layer was caused by interdiffusion between the BMG and brass during the warm extrusion. Nanoindentation in the vicinity of BMG–brass interfaces does not cause interface decohesion or crack formation, suggesting a strong interface bonding. Apparently, the resultant interfacial layer not only enhances interfacial bonding but also provides a buffer zone to prevent the catastrophic shear band propagation in the BMG matrix.     

Peculiarities of ball-milling induced crystalline–amorphous transformation in Cu–Zr–Al–Ni–Ti alloys

An amorphization process in (Cu₄₉Zr_45−xAl_6+x)_100−y−zNi_yTi_z (x = 1, y, z = 0; 5; 10) induced by ball-milling is reported in the present work. The aim was investigation of the effect of Ni and Ti addition to Cu₄₉Zr₄₅Al₆ and Cu₄₉Zr₄₄Al₇ based alloys as well as type of initial phases on the amorphization processes. Also the milling time sufficient for obtaining fully amorphous state was determined. The entire milling process lasted 25 h. Drastic structural changes were observed in each alloy after first 5 h of milling. In most cases, after 15 h of milling the powders had fully amorphous structure according to XRD except for those ones, where TEM revealed a few nanosized crystalline particles in the amorphous matrix. In (Cu₄₉Zr₄₅Al₆)₈₀Ni₁₀Ti₁₀ alloy the amorphization process took place after 12 h of milling and the amorphous state was stable up to 25 h of milling. In the case of (Cu₄₉Zr₄₄Al₇)₈₀Ni₁₀Ti₁₀ alloy the powders have fully amorphous structure between 12 h and 15 h of milling.