NZ30K Alloy Research Articles

CORROSION BEHAVIOUR OF MAGNESIUM ALLOYS NZ30K AND NZ30K ALLOYED WITH SILVER IN THE MODEL SOLUTION OF THE OSTEOSYNTHESIS PROCESS

The corrosion behaviour of magnesium alloys NZ30K and NZ30K alloyed with 0.1 wt.% silver in Ringer's Locke solution has been studied, since their components are not toxic to the human body and do not cause clinical complications in the treatment of bone fractures, and silver has antibacterial properties inherent in antibiotics. It has been found that the Ecor potential of the silver-alloyed NZ30K sample was -1.57V during the first 100 seconds of testing, but then it intensively shifted to the positive side to -1.54V within 512 seconds at a rate of 0.051 mV/s, which decreased to 0.014 mV/s after the next 1000 seconds, and a stationary value of the potential Ecor on the sample has been recorded. The sample to uniform general corrosion has been subjected, and the improvement of its potential Ecor during its corrosion study was due to the most intense selective dissolution of magnesium, which has the most negative value of the standard potential among the alloy components, and the enrichment of its surface with Zn, Nd, Zr, Ag, which have a positive value of the standard potential. This trend contributed to a decrease in the rate of general corrosion and made it impossible to develop local corrosion. The NZ30K alloy alloyed with 0.1 wt.% silver is recommended for further potentiodynamic and volumetric corrosion studies to justify its selection as a structural material for the production of biodegradable implants in osteosynthesis.

BIOSOLUBLE MAGNESIUM-BASED ALLOYS FOR OSTEOSYNTHESIS

The impact of micro-alloying of AZ91 and NZ30K magnesium alloys with silver and scandium to produce biosoluble implants for osteosynthesis is studied in the paper. The application of these alloys is limited due to the significant reduction of the mechanical properties of magnesium during the regeneration of fractures. It was found that the higher the concentration of the above-mentioned elements, the bigger the amount of intermetalloids that appear and the smaller they become. There were different mechanical properties observed at the sample cross-section, but the difference was successfully eliminated by thermal treatment. It reduced the chemical heterogeneity of the workpiece at the cross-section and, respectively, the value of micro-hardness. It is shown that microalloying these alloys with 0.05…0.1 % of silver and scandium ensures the grinding of the structural elements of the metal and a significant increase of the set of their properties. The research of the developed alloys showed that NZ30K alloy, which additionally contains 0,05 ... 0,1% of silver, after a three-month treatment with gelofusine saves its mechanical properties like those of the bone tissue. Pre-clinical research revealed that they are non-toxic, have an antibacterial effect and preserve the whole set of properties till the complete fracture consolidation.

STUDY OF THE INFLUENCE OF SILVER ALLOYING ON THE MICROSTRUCTURE AND PROPERTIES OF MAGNESIUM ALLOY NZ30K FOR IMPLANTS IN OSTEOSYNTHESIS

Purpose. Study of the influence of silver alloying of magnesium alloy NZ30K on its mechanical properties for use in the manufacture of biodegradable implants. Research methods. The tensile strength and relative elongation of the samples were determined on a P5 tensile machine at room temperature. The microstructure of the samples was studied using a “Carl Zeiss” optical electron microscope using the “Observer.D1m” software. Samples were used after etching with a reagent containing 1 % nitric acid, 20 % acetic acid, 19 % distilled water, and 60 % ethylene glycol. Fractographic analysis of sample fractures was performed on a JSM-6360LA scanning electron microscope. The phase analysis of the structural components of magnesium alloys was studied using an electron microscope – a microanalyzer with an energy dispersive attachment РEMMA 202М and РЕМ 16I. Research on biocorrosion was carried out by keeping the samples in a solution of gelofusin – an artificial blood substitute for 2, 4, 6 weeks, using a TC-20 MICROmed thermostat. Results. The study showed that silver was a part of the complex alloyed intermetallic phases, which were additional crystallization centers. In this case, the average grain size decreases by almost three times compared to the original alloy. It was found that the optimal set of properties is achieved by the introduction of 0.1 % Ag, while the tensile strength increases by 7.9 % and the relative elongation almost doubles. It was found that the magnesium alloy with silver (0.1 %) slows down biocorrosion processes and helps to maintain a high level of tensile strength (σB = 205 MPa) after 6 weeks of exposure to a solution of helofusine. Thus, silver is a promising material for improving the structure and increasing the mechanical properties of biodegradable magnesium alloy implants. Scientific novelty. The silver content for alloying the NZ30K alloy was determined to be 0.1 % Ag, which forms the optimal ratio of strength and ductility, grain structure refinement, and slowing down biocorrosion processes. Practical value. NZ30K alloy with the addition of 0.1 % silver is promising for use in the manufacture of implants. The studied alloy provides the required level of properties until complete fracture consolidation.

Characterization of complex surface oxide layers formed during the solidification of distinct Mg-RE alloys

Surface oxide layers play a significant role in forming secondary oxidation inclusions during the casting process. In this study, three typical Mg-RE alloys (Mg-3Nd (NZ30K), Mg-3Nd-3Gd (EV33) and Mg-3Nd-4Y (WE43A)) are selected. Their surface oxide layers formed during the solidification are characterized in detail, and the corresponding oxidation mechanisms are discussed. The results reveal that RE elements obviously influence the characteristics of surface oxide layers, which depends on their ability to purify the formed MgO in the melt via the reaction (2RE + 3MgO = 3Mg + RE2O3). On the one hand, as Nd and Gd do not easily displace MgO already formed in the melt, the loose oxide layers in NZ30K and EV33 alloys are mainly composed of MgO matrix with embedded RE-rich oxide particles. On the other hand, due to the strong ability of Y to purify MgO in the melt, the oxide layer of WE43A alloy becomes a denser and thinner Y2O3 oxide layer. Note that the differences in surface oxide layers well explain the different secondary inclusions that occur in three typical Mg-RE alloys during the casting process.

Effects of processing parameters on fabrication defects, microstructure and mechanical properties of additive manufactured Mg–Nd–Zn–Zr alloy by selective laser melting process

Mg–3Nd–0.2Zn–0.4Zr (NZ30K, wt.%) alloy is a new kind of high-performance metallic biomaterial. The combination of the NZ30K Magnesium (Mg) alloy and selective laser melting (SLM) process seems to be an ideal solution to produce porous Mg degradable implants. However, the microstructure evolution and mechanical properties of the SLMed NZ30K Mg alloy were not yet studied systematically. Therefore, the fabrication defects, microstructure, and mechanical properties of the SLMed NZ30K alloy under different processing parameters were investigated. The results show that there are two types of fabrication defects in the SLMed NZ30K alloy, gas pores and unfused defects. With the increase of the laser energy density, the porosity sharply decreases to the minimum first and then slightly increases. The minimum porosity is 0.49 ± 0.18%. While the microstructure varies from the large grains with lamellar structure inside under low laser energy density, to the large grains with lamellar structure inside & the equiaxed grains & the columnar grains under middle laser energy density, and further to the fine equiaxed grains & the columnar grains under high laser energy density. The lamellar structure in the large grain is a newly observed microstructure for the NZ30K Mg alloy. Higher laser energy density leads to finer grains, which enhance all the yield strength (YS), ultimate tensile strength (UTS) and elongation, and the best comprehensive mechanical properties obtained are YS of 266 ± 2.1 MPa, UTS of 296 ± 5.2 MPa, with an elongation of 4.9 ± 0.68%. The SLMed NZ30K Mg alloy with a bimodal-grained structure consisting of fine equiaxed grains and coarser columnar grains has better elongation and a yield drop phenomenon.

A comparison of low-cycle fatigue behavior between the solutionized and aged Mg-3Nd-0.2Zn-0.5Zr alloys

The present paper compares the low cycle fatigue characteristics between the solution-treated (T4) and peak-aged (T6) Mg-3Nd-0.2Zn-0.5Zr magnesium alloys produced by semi-continuous casting. The cyclic stress amplitudes of the T6-treated counterpart are higher than those of the T4-treated alloy, which is due to precipitate strengthening. At the same stress amplitude, the T4-treated alloy with higher hysteresis energies undergoes more fatigue damage, resulting in shorter fatigue lives. For the T4-treated alloy, the cyclic stress amplitude is found to increase with increasing the cycle number, which is attributed to its soft matrix and the increased volume fraction and number density of twins under continuous loading. Cyclic hardening followed by the decrease of the stress is observed in the T6-treated alloy. It is shown that the cyclic deformation behavior and failure mechanism of the T4-treated alloy are dependent on the dislocations-slip plus twinning, while those of the T6-treated counterpart are dependent on the dislocations-slip. Based on discussion, the Coffin-Manson law and Basquin equation or the energy-based concepts can be well applied to the prediction of the LCF lives of the NZ30K alloys.

Tensile crack initiation behavior of cast Mg–3Nd–0.2Zn–0.5Zr magnesium alloy

This paper studied the tensile crack initiation behaviors in cast NZ30K alloys under as-cast, T4- and T6-treated conditions. Results indicate that tensile failure of cast magnesium alloys mainly nucleates from the tensile-sensitive microstructural constituent such as eutectic particles, grain boundaries and twin grain boundaries. For the as-cast alloy, the micro-cracks form mainly by cracking of the eutectic phases (~83%). In addition, twin grain boundaries are also found to be preferential sites for crack nucleation (~17%). The tensile failure in the T4- and T6-treated alloys is mainly caused by twin nucleation (T4~60% and T6~44%) and grain boundaries cracking (T4~28% and T6~52%). Interactions among dislocation-slip, twinning and grain boundaries as well as eutectic phases determine the crack initiation behavior of cast magnesium alloys.

Preparation of Mg–Nd–Zn–(Zr) alloys semisolid slurry by electromagnetic stirring

The effects of electromagnetic stirring (EMS) including stirring time, applied voltage and rotational frequency on the microstructure of semisolid slurries of the Mg–Nd–Zn–(Zr) alloys were investigated. The results indicate that all of the parameters have large effects on the microstructure of the slurries. After being treated by EMS, the morphology of primary α-Mg phases in the Mg–3Nd–0.2Zn (NZ30) alloy slurries evolves from dendrite to three types: rosette, dendrite, and spheroid, while a semisolid microstructure with small and spheroidal particles is obtained in the Mg–3Nd–0.2Zn–0.4Zr (NZ30K) alloy slurries. The data for solid fraction with stirring time can be fitted to linear equations. The increase of applied voltage and rotational frequency makes the primary α-Mg phases initially refined and then coarsened. The optimal processing parameters are stirring time 120–180s, applied voltage 300–350V, rotational frequency ~20Hz for NZ30 alloy, and stirring time 30–180s, applied voltage ~350V, rotational frequency ~20Hz for NZ30K alloy. The average particle size of NZ30 alloy can be refined from ~879 to ~457μm. Furthermore, the effects of applied voltage and rotational frequency on undercooling and melt temperature were also discussed on the basis of thermodynamic theory.

Damage morphology study of high cycle fatigued as-cast Mg–3.0Nd–0.2Zn–Zr (wt.%) alloy

Laser scanning confocal microscopy (LSCM) and Electron back-scattered diffraction (EBSD) were applied to the study of surface morphology variation of as-cast Mg–3.0Nd–0.2Zn–Zr (NZ30K) (wt.%) alloy under tension-compression fatigue test at room temperature. Two kinds of typical damage morphologies were observed in fatigued NZ30K alloy: One was parallel lines on basal planes led by the cumulation of basal slips, called persistent slip markings (PSMs), and the other was lens shaped, thicker and in less density, led by the formation of twinning. The surface fatigue damage morphology evolution was analyzed in a statistical way. The influences of stress amplitude and grain orientation on fatigue deformation mechanisms were discussed and the non-uniform deformation among grains and the PSMs, within twinning were described quantitatively.

Low cycle fatigue of an extruded Mg–3Nd–0.2Zn–0.5Zr magnesium alloy

The purpose of this study was to evaluate strain-controlled cyclic deformation behavior of an extruded Mg–3Nd–0.2Zn–0.5Zr (NZ30K) magnesium alloy. The microstructure of this alloy consisted of a bimodal microstructure with equiaxed recrystallized grains and unrecrystallized coarse grains along with a large number of smaller second-phase particles present inside the grains and larger particles along the grain boundaries alongside a characteristic precipitate free zone (PFZ). The average grain size was about approximately 5–7μm. It was observed that unlike the higher RE-containing Mg–10Gd–3Y–0.5Zr (GW103K) magnesium alloy, the NZ30K alloy exhibited asymmetrical hysteresis loops in tension and compression in the fully reversed strain-control tests at a strain ratio of Rε=−1. This was mainly due to the presence of relatively stronger crystallographic texture, PFZ, and the resultant twinning–detwinning activities during cyclic deformation. While this alloy exhibited cyclic softening at lower strain amplitudes and cyclic hardening at higher strain amplitudes, it had an equivalent fatigue life to that of other extruded Mg alloys. Fatigue crack was observed to initiate from the specimen surface with some isolated facets of the cleavage-like planes near the initiation site. Crack propagation was basically characterized by serrated fatigue striations.

Characterization of highly corrosion-resistant nanocrystalline Ni coating electrodeposited on Mg–Nd–Zn–Zr alloy from a eutectic-based ionic liquid

A dense nanocrystalline Ni coating was successfully electrodeposited on Mg–3.0Nd–0.2Zn–0.4Zr (wt.%, NZ30K) alloy from a choline chloride–urea (1:2 molar ratio) eutectic-based ionic liquid (1:2 ChCl–urea IL) on the basis of an appropriate pretreatment. A Cu underlayer was pre-plated from a pyrophosphate based solution to avoid the severe corrosion and replacement reaction of NZ30K alloy in the IL electrolyte. The evolution of surface morphologies, compositions and phase structures of the pretreatment films and Ni coating were studied in detail. The uniform, smooth and dense nanocrystalline Ni coating possessing good adhesion to substrate has been proved by thermal shock test. Potentiodynamic polarization test in 3.5 wt.% NaCl solution showed that the Ni coating significantly improved the corrosion resistance of NZ30K alloy by nearly two orders of magnitude. Open circuit potential (OCP) and electrochemical impedance spectroscopy (EIS) during long-term immersion test further demonstrated that the Cu/Ni coating could provide an effective protection for NZ30K alloy from corrosion up to ∼336 h. The high corrosion resistance of the coating can be attributed to the dense nanocrystalline Ni structure and the synergistic effect of electrochemical protection between Ni coating and Cu underlayer.

Effect of pouring and mold temperatures on hot tearing susceptibility of AZ91D and Mg–3Nd–0.2Zn–Zr Mg alloys

Pouring and mold temperatures are two important parameters during casting magnesium components. The present study examined their influence on hot tearing susceptibility (HTS) of commercial AZ91D and newly developed Mg–3Nd–0.2Zn–Zr (mass fraction, %; NZ30K) magnesium alloys in gravity permanent mold casting condition. The results indicate that mold temperature shows much more significant influence on the HTS of both alloys than pouring temperature whose influence only can be distinguished at low mold temperature (341 K for AZ91D alloy and 423 K for NZ30K alloy). Hot tearing susceptibility prediction model concerning feeding parameters, grain size and solidification range, is more suitable to estimate the HTS of different magnesium alloys than the model only concerning feeding parameters. In order to achieve better hot tearing resistance, the ranges of pouring and mold temperatures are suggested to be 961–991 K and ≥641 K for AZ91D alloy, 1003–1033 K and ≥623 K for NZ30K alloy, respectively.

Precipitate morphology and effects on strength of laser welded Mg–Nd–Zn–Zr after aging treatment

Magnesium rare earth alloys have been received extensive attention due to their attractive mechanical properties. The precipitation sequence in laser welded Mg–3Nd–0·2Zn–0·4Zr (NZ30K) alloy during aging treatment has been investigated using transmission electron microscopy (TEM). The results show that the tensile strength of laser welded NZ30K can be improved significantly after aging treatment at 200°C for 8 h. The precipitation in laser welded NZ30K alloy follows the sequence of super saturated solid solution→β″(DO19)→β′(fcc)→β. The results of incremental of critical shear stress component are Δτs/Δτp/Δτr/Δτf = 1·86∶3·57∶3·14∶1. The platelet of precipitates with high volume fraction is the most important factor that enhances aging treatment strengthening.

Fluidity of AZ91D and Mg–3Nd–0·2Zn–Zr (wt-%) magnesium alloys: response to pouring and mould temperature

The fluidities of commercial AZ91D and newly developed Mg–3Nd–0·2Zn–Zr (wt-%) (NZ30K) magnesium alloys were investigated at the same superheat temperature (Tpouring−Tliquidus) and temperature difference between solidus line and mould temperature (TDSM) (Tsolidus−Tmould) conditions. The results indicate that in the range of superheat 60–150°C and TDSM 400–100°C, the increases in both pouring temperature and mould temperature can effectively improve the fluidities of both AZ91D and NZ30K alloys. The flow lengths of AZ91D and NZ30K alloys could be estimated approximately by and (ΔT: superheat; T: pouring temperature; T0: mould temperature). In order to achieve better fluidity in permanent mould casting, the ranges of pouring and mould temperatures are suggested to be 718–748 and ≧268°C for AZ91D alloy, and 760–790 and ≧350°C for NZ30K alloy respectively.

High Cycle Fatigue of Cast Mg-3Nd-0.2Zn Magnesium Alloys

This paper investigates the high cycle fatigue properties of a recently developed high-strength cast magnesium alloy [Mg-3Nd-0.2Zn (all compositions in wt pct except when otherwise stated)] with varied Zr contents for grain refinement (NZ30K) and the influence of heat treatment conditions. The NZ30K alloy containing 0.45Zr and heat treated to the peak-aged T6 condition [14 hours at 473 K (200 °C)] shows the highest fatigue strength, about 100 MPa, which is about 25 pct higher than that of commercial AZ91D-T6 alloy. In the absence of casting flaws, the high cycle fatigue properties of the NZ30K alloy strongly depend on its grain size and heat treatment conditions. The dependency of fatigue strength on grain sizes follows the Hall–Petch relationship. The NZ30K alloy also shows a significant response to heat treatments. The fatigue strength increases in a near linear fashion with increasing yield strength of the material through heat treatment.

Hot Deformation Behavior and Microstructural Evolution of Mg-Nd-Zn-Zr Magnesium Alloy

The hot deformation behaviors and microstructural evolution of Mg-3.0Nd-0.2Zn-0.4Zr (wt. %, NZ30K) alloy were investigated by means of the isothermal hot compression tests at temperatures of 350-500 °C with strain rates of 0.001, 0.01, 0.1 and 1s-1. The results showed that the flow stress increased to a peak and then decreased which showed a dynamic flow softening. The flow stress behavior was described by the hyperbolic sine constitutive equation with an average activation energy of 193.8 kJ/mol. The average size of dynamically recrystallized grains of hot deformed NZ30K alloy was reduced by increasing the strain rate and/or decreasing the deformation temperature. A large amount of fine particles precipitated in the grains interior and at the grain boundaries when heated to the compression temperatures and soaked for 5min below 450 °C. However, the volume fraction of particles decreased significantly when soaked for 5 min at 500 °C, and the coarse particles precipitated mainly at the grain boundaries. Hot deformation at the temperature of 500 °C around and at the strain rate range of 0.1s-1 was desirable for NZ30K alloy.

Effects of Aging Treatment on Laser-Welded Mg-Rare Earth Alloy NZ30K

Magnesium-rare earth alloys have received extensive attention due to their attractive mechanical properties resulting from high density of precipitation. The precipitation sequence in laser-welded Mg-3Nd-0.2Zn-0.4Zr (NZ30K) alloy during aging treatment at 200°C and 225°C has been investigated using transmission electron microscopy (TEM). The results indicate that the tensile strength of laser-welded NZ30K can be improved significantly after aging treatment at 200°C for 8 h. It is found that the precipitation in laser-welded NZ30K alloy follows the sequence of supersaturated solid solution → β′′(DO19) → β′(fcc).

Effects of grain size and heat treatment on the tensile properties of Mg–3Nd–0.2Zn (wt%) magnesium alloys

This paper reports the tensile properties and deformation behavior of a newly cast magnesium alloy Mg–3Nd–0.2Zn–xZr (NZ30Kx, x=0, 0.1, 0.3, 0.5, 1.0, 2.0wt%) with different grain sizes and heat treatment conditions. Zirconium (Zr), as an effective grain-refiner in NZ30K alloys, is found to significantly improve the yield strength (YS), ultimate tensile strength (UTS) and elongation of these alloys. The NZ30K alloys show a significant response to heat treatment, achieving 60 and 47% increases in YS and UTS, respectively, in the peak-aged condition (14h at 200°C), compared with those of the as-cast alloy. Both grain refinement and age hardening significantly improve work-hardening at strains below 1%, leading to higher yield strength. At large strains, however, only grain refinement continues to show a slight strain hardening. The strain-hardening exponents of both NZ30Kx–T4 and NZ30Kx–T6 alloys decrease with reducing grain size at large strains. The Hall–Petch relationships of true flow stresses of NZ30Kx–T6 alloys at low strains (0.001–0.04) show a tendency of two-slope feature as plastic strains. All tensile failures occur prior to global instability, indicating the existence of localized damage.

Prediction of flow stress of Mg–Nd–Zn–Zr alloy during hot compression

Isothermal hot compression tests were carried out on Mg–3.0Nd–0.2Zn–0.4Zr (mass fraction, %, NZ30K) alloy using a Gleeble-3500 thermo-simulation machine at temperatures ranging from 350 to 500 °C and strain rates from 0.001 to 1 s−1. A correction of flow stress for deformation heating at a high strain rate was carried out. Based on the corrected data for deformation heating, a hyperbolic sine constitutive equation was established. The constants in the constitutive equation of the hyperbolic sine form were determined as a function of strain. The flow stresses predicted by the developed equation agree well with the experimental results, which confirms that the developed constitutive equations can be used to predict the flow stress of NZ30K alloy during hot deformation.

Laser Hybrid Welding Processing of Mg-Rare Earth Alloy

The welding processing of Mg-rare earth alloy NZ30K was studied using laser-TIG hybrid welding. For comparison the NZ30K alloy was also welded by the gas tungsten arc (TIG) and laser beam respectively. The microstructure of the welded joints had been analyzed. The hybrid welding method could refine the grains in the fusion and improve the tensile strength of the welded joints obviously. The arc plasma and the laser-induced plasma during welding were recorded by a high speed camera and the area of the plasma was calculated through image processing technology. Among the three welding processes the plasma area of the hybrid welding is the largest, but not a simple addition of the TIG welding and laser welding. The results show that Mg-rare earth alloy NZ30K can be well joined using the laser-TIG hybrid welding method.