Improving corrosion resistance of additively manufactured WE43 magnesium alloy by high temperature oxidation for biodegradable applications

Laser powder bed fusion(L-PBF)has been employed to additively manufacture WE43 magnesium(Mg)alloy biodegradable implants,but WE43 L-PBF samples exhibit excessively rapid corrosion.In this work,dense WE43 L-PBF samples were built with the relativity density reaching 99.9%.High temperature oxidation was performed on the L-PBF samples in circulating air via various heating temperatures and holding durations.The oxidation and diffusion at the elevated temperature generated a gradient structure composed of an oxide layer at the surface,a transition layer in the middle and the matrix.The oxide layer consisted of rare earth(RE)oxides,and became dense and thick with increasing the holding duration.The matrix was composed ofα-Mg,RE oxides and Mg_(24)RE_(5) precipitates.The precipitates almost disappeared in the transition layer.Enhanced passivation effect was observed in the samples treated by a suitable high temperature oxidation.The original L-PBF samples lost 40%weight after 3-day immersion in Hank’s solution,and broke into fragments after 7-day immersion.The casted and solution treated samples lost roughly half of the weight after 28-day immersion.The high temperature oxidation samples,which were heated at 525℃ for 8 h,kept the structural integrity,and lost only 6.88%weight after 28-day immersion.The substantially improved corrosion resistance was contributed to the gradient structure at the surface.On one hand,the outmost dense layer of RE oxides isolated the corrosive medium;on the other hand,the transition layer considerably inhibited the corrosion owing to the lack of precipitates.Overall,high temperature oxidation provides an efficient,economic and safe approach to inhibit the corrosion of WE43 L-PBF samples,and has promising prospects for future clinical applications.

Effect of Rare Earth Doping on the Tensile Properties of High Temperature Oxidation Coating

The failure process was characterized by complex diffusion of elements in the bonding layer,TGO growth and growth stress inside the coating.We studied the aluminum migration phenomenon of NiCoCrAlY and NiCoCrAlYHf coatings under high temperature oxidation,TGO growth characteristics,the microstructure and composition of the bonding layer,and integrates them into the description of the surface strain under coating tension.The experimental results show that the TGO growth rate of NiCoCrAIYHf coating is lower than that of NiCoCrAIY coating,and the formed TGO is thinner.After high temperature oxidation,the cracking time of NiCoCrAIY coating is advanced,while the cracking time of rare earth doped coating is delayed.The addition of rare earth elements can effectively inhibit the generation of spinel phase,improve the fracture toughness of TGO,refine the grains in the bonding layer,and increase the grain boundary strengthening by 29.1 MPa which is consistent with the experimental value.Therefore,the yield strength of the doped coating is improved and the crack time of the coating is delayed.

Effect of Yttrium on High Temperature Oxidation Resistance ofa Directionally Solidified Superalloy

The effect of rare earth element yttrium on the high temperature oxidation resistance of a directionally solidified Ni-base superalloy was studied with scanning electron microscopy(SEM), energy dispersive spectrum(EDS)and X-ray diffraction(XRD)techniques. The results show that the oxidation resistance of the alloy is substantially improved by adding proper amount of yttrium.

Interfacial Behavior of Sulfur and Yttrium in Yttrium Modified Ni _3Al Based Alloy IC6 during High Temperature Oxidation Process

The interfacial behavior of sulfur and yttrium in the yttrium modified Ni 3Al based alloy IC6 during oxidation at 1100 ℃ was analyzed by X ray line scan of electron probe microstructural analysis(EPMA). The results show that the migration and segregation of sulfur to the interface between oxide scale and the substrate at high temperature is retarded owing to the presence of yttrium. This is attributed to the desulfurization by yttrium in the melt and the trapping of sulfur by yttrium rich phases during oxidation, which leads to improving the coherence between oxide scale and substrate. Another reason of increasing the high temperature oxidation resistance of alloy IC6 by the addition of yttrium is that yttrium migrates to the grain boundaries of oxides during oxidation and hence improve their strength. This results in the transformation of the oxide scale spallation cracks from intergranular cracks for alloy without yttrium to transgranular ones for yttrium modified alloy.

High temperature oxidation behavior of high speed steel for hot rolls material

The oxidation characteristics of high speed steel (HSS) were studied at 500 to 800°C. The non-isothermal oxidation and isothermal oxidation (500, 575, 650, 725, 800°C) of HSS were investigated by thermo-gravimetric analysis (TGA). The microstruc- ture, morphology and oxide scale thickness of the isothermal oxidation samples were analyzed by optical microscope (OM), electron probe micro analyzer (EPMA), X-ray diffraction spectrum (XRD) and scanning electron microscope (SEM). The results indicate that the oxidation rate of HSS is very slow at 500 to 650°C, increasing gradually at 650 to 750°C, and drastically at 750 to 800°C, be- cause the phase transformation happens at about 750°C.

Influence of yttrium addition on high temperature oxidation resistance of single crystal superalloy

Two experimental single crystal superalloys Y-free alloy and Y-containing alloy were cast in the directionally solidified furnace, while other alloying element contents were basically kept unchanged. The isothermal oxidation behavior of two single crystal superalloy was studied at 1100 ℃ in ambient atmosphere. Morphology of oxides was examined by scanning electron microscopy （SEM） and their compostion was analyzed by X-ray diffraction （XRD） and energy dispersive spectroscopy （EDS）. The effect of yt- trium on the high temperature oxidation resistance of the single crystal superalloy was investigated. The results showed that the oxi- dation resistance of Ni-base single crystal superalloy was substantially improved by adding a little amount of rare earth yttrium. Yt- trium could promote the oxide of A1 formation and decreased the proportion of NiO. Yttrium increased the coherence between the oxide scale and the alloy substrate to decrease the spallation of oxide scale. Yttrium reduced the oxide grain size and the thickness of oxide layer.

Effect of Oxidation-Induced Material Parameter Variation on the High Temperature Oxidation Behavior of Nickel

In high temperature oxidation environment, the oxidation reaction will induce variations in material parameters, such as Young's modulus, thermal expansion coefficient (CTE), coefficient of oxygen diffusion (COD), etc. The oxidation -induced material parameter variations should be considered in high temperature mechanical analysis. In this paper, high temperature oxidation behavior of an oxide film/metal substrate system was investigated through a modified phase field approach. The oxidative stress and oxidation weight gain induced by high temperature oxidation were studied. Effects of Young's modulus, COD and CTE on oxidative stress in the oxide film were studied particularly. The simulation results showed that a better agreement with the experimental results could be obtained when considering the oxidation -induced material parameter variations in the high temperature mechanical analysis of oxide film/metal substrate system. The simulation results demonstrated that oxidative stress and oxidation weight gain were more sensitive to the variation of Young's modulus than to the variations of COD and CTE.

HIGH-TEMPERATURE OXIDATION OF FGH96 P/M SUPERALLOY

High temperature oxidation behaviors of FGH96 P/M superaUoy have been studied in air at temperatures ranging from 600 to 1000℃. By means of isothermal oxidation testing, X-ray diffraction, SEM （scanning electron microscopy）, and EDS （energy dispersive X-ray spectroscopy） analyses, the oxidation kinetics as well as the composition and morphology of scales were investigated. Thermodynamic calculations were used to explain the oxidation mechanism. The results showed that as the oxidation temperature increased, the oxidation rate, the scale thickness, and scale spallation increased. FGH96 P/M superalloy exhibits good oxidation resistance at temperature below 800℃. The oxidation kinetics follows an approximately parabolic rate law, and the oxide layer was mainly composed of Cr2O3 TiO2 and a little amount of NiCr2O4. The oxidation is controlled by the transmission of chromium. titanium, and oxygen through the oxide scale.

Protective Effect of the Sputtered TiAlCrAg Coating on the High-Temperature Oxidation and Hot Corrosion Resistance of Ti-Al-Nb Alloy

The effect of a sputtered Ti-48AI-8Cr-2Ag (at. pct) coating on the oxidation resistance of the cast Ti-46.5AI-5Nb (at. pct) alloy was investigated in air at 1000-1100℃. Hot corrosion in molten 75 wt pct Na2SO4+25 wt pct K2SO4 was investigated at 900℃. The scale on the cast TiAINb tends to spall in air, while the scale on coating is very adherent. The sputtered Ti-48AI-8Cr-2Ag coating remarkably improved high temperature oxidation resistance of the cast Ti-46.5AI-5Nb alloy because of the formation of an adherent Al2O3 scale. Due to the inward diffusion of Cr, Kirkendall voids were found at the coating/substrate interface. TiAICrAg coating provided excellent hot corrosion resistance for TiAINb alloy in molten 75 wt pct Na2SO4+25 wt pct K2S04 at 900℃ due to the formation of a continuous Al_2O_3 scale.

Development of high-strength magnesium alloys with excellent ignition-proof performance based on the oxidation and ignition mechanisms: A review

High reactivity and ease of ignition are the major obstacles for the application of Mg alloys in aerospace.Thus,the ignition mechanisms of Mg alloys should be investigated systematically,which can guide the ignition-proof alloy design.This article concludes the factors influencing the ignition resistance of Mg alloys from oxide film and substrate microstructure,and also the mechanisms of alloying elements improving the ignition resistance.The low strength is another reason restricting the development of Mg alloys.Therefore,at the last section,Mg alloys with the combination of high strength and good ignition-proof performance are summarized,including Mg-Al-Ca based alloys,SEN(Mg-Al-Zn-Ca-Y)alloys as well as Mg-Y and Mg-Gd based alloys.Besides,the shortages and the future focus of theses alloys are also reviewed.The aim of this article is to promote the understanding of oxidation and ignition mechanisms of Mg alloys and to provide reference for the development of Mg alloys with high strength and excellent ignition-proof performance at the same time.

Effect of Si on high temperature oxidation of 30Cr13 stainless steel

The effect of Si on the high temperature oxidation behavior of the 30Cr13 martensitic stainless steels was investigated. The underlying mechanism was further discussed by oxidation kinetics curves, scanning electron microscopy （SEM） and X-ray diffraction （XRD） analysis. It was ob- served that the addition of Si can significantly reduce the mass gain per unit area and the oxidation rate of the experimental steels. With the increase of Si content, the film surface composed of the wheat-like oxides turned into the small granular oxides after oxidation for 120 h. The SiO_2 film formed on the metal substrate inhibited the outward diffusion of the metal cation and the inward diffusion of the oxygen anion; thus, the high temperature oxidation resistance was enhanced.

High Temperature Oxidation Resistance of 9Ni Steel

Oxidation rates and scale/steel interface configuration of 9Ni steels were investigated at 1000--1 250 ℃ in air. The results revealed that Cu addition caused high temperature oxidation resistance to deteriorate. High tempera ture oxidation rates increased and scale/steel interface configuration became complicated due to Cu addition. Scale/ steel interface appeared to be network above certain temperature. Temperature required to form network scale/steel interface dropped more than 100 ℃ for 1.5% Cu-containing steel. （Fe,Ni,Cu）x Oy in inner oxidation layer dissocia ted to Fe-Ni-Cu phase and released active oxygen which can react with base steel easily. So the inner oxidation layer became the second source of oxidizing agent besides atmosphere. Internal stress at austenite grain boundary caused local oxide to fragment. So the scale/steel interface appeared to be network. Liquid Si-rich phase formed at sufficient ly high temperature. Penetration of the liquid Si-rich phase along austenite grain enhanced austenite grain oxidizing.

Oxidation Behavior of Pd-Modified Aluminide Coating at High Temperature

(Ni,Pd)AI coating, prepared by low pressure pack cementation on the Ni-base superalloy M38 where Pd-20 wt pct Ni alloy was predeposited, consists of a single β-(Ni,Pd)AI phase. The initial isothermal oxidation behavior of (Ni,Pd)AI coating was investigated by TGA, XRD, SEM/EDS at 800-1100℃. Results show that oxidation kinetics accord preferably with parabolic law at 800, 900 and 1100℃, but not at 1000℃. θ-AI203 was observed at 800-1100℃. It is found that Pd plays an important role in accelerating the diffusion of Ti from the substrate to the coating surface in the aluminide coating.

High-temperature oxidation behavior of a cast Ti-47.5Al-2.5V-1.0Cr-0.2Zr alloy

The oxidation behavior of the Ti-47.5Al-2.5V-1.0Cr-0.2Zr alloy at 900℃ was investigated at different oxidation times(5,20,60 and 100 h).The results show that the total weight gain of the alloy after 100 h at 900℃ oxidation is 9.1 g·m^(-2),and the oxidation rate decreases with oxidation time.The oxides on the alloy surface are mainly TiO_(2) and Al_(2)O_(3).At the beginning of oxidation(5 h),the oxide film is relatively complete,thin,and the interface between the oxide layer and the matrix is virtually flat.At the end of oxidation(100 h),the thickness of the oxide film is expanded,cracking and spalling occur,and the spalling form is intra-film spalling.At the same time,oxygen is mainly distributed in the oxide film and the oxygen content in the alloy substrate is reduced,confirming that the TiAl alloy has a certain oxidation stability at 900℃.From the outer surface of the oxide layer to the matrix,the TiO_(2) content increases and the Al_(2)O_(3) content decreases.Oxidation proceeds to completion in this system via the dissolution and diffusion of O atom.

High Temperature Protection Ability of Oxide Scale Formed on Fe_3Al Enhanced by Surface-applied Y_2O_3 Thin Film

The Y_2O_3 thin film was applied on Fe-3Al intermetallic compound by electrodeposition and thermal decomposition. The cyclic oxidation of the Fe-3Al specimens with and without surfaceapplied Y_2O_3 thin film was carried out at 900 ℃ in air. The results show that the selective oxidation of Al in Fe-3Al was promoted, and both of the plasticity and the adhesion of the oxide scale formed on Fe3Al were improved and the high temperature oxidation resistance of Fe3Al was enhanced markedly.

Protective Properties of High Temperature Oxide Films on Ni-based Superalloys in 3.5% NaCl Solution

The electrochemical behaviors of high temperature oxide film formed on the sputtered microcrystalline coating of M38 alloy （mc-M38） were investigated by potentiodynamic and electrochemical impedance spectroscopy （EIS） techniques in 3.5% NaCl solution. Mott-Schottky analysis was used to study the semi-conductive properties of the surface oxide. The results of the capacitance measurements showed that the oxide films on both the coating and the cast alloy were p-type semiconducting characteristics. Both the carrier density （Na）and the flat band potential （Efb） were obviously frequency-dependent, and the optimal frequency range was from 1000 to 1500 Hz. The oxidized coating exhibited higher protectivity than the oxidized cast alloy due to the lower carrier density compared with that of the oxidized cast alloy. The EIS data of the long-term immersing tests suggested that the oxide film served as an inner-barrier layer against chloride ions. The penetration of the aggressive ions into the surface oxide resulted in the decreased polarization resistance as a function of the immersion time.

Effect of Ce^+ Ion Implantation upon Oxidation Resistance of Superalloy K38G

The oxidation behavior (isothermal and cyclic oxidation) of cast superalloy K38G and the effect of Ce^+ ion implantation with dose of 1×10^(17) ions/cm^2 upon its oxidation resistance at 900 and 1000 ℃ in air were investigated. Meanwhile, the influence of Ce^+ implantation on oxidation behavior of K38G with pre-oxide scale at 1000 ℃ in air was compared. The pre-oxidation was performed at 1000 ℃ in static air for 0.25 and 1.5 h, respectively. It is shown that the homogeneous external mixture oxide of rutile TiO_2+Cr_2O_3 and non-continuous internal oxide of Al_2O_3 are formed during the oxidation procedure in all the cases. The isothermal oxidation resistance and the cracking or spallation resistance of superalloy K38G implanted with Ce^+ by both of the two different implantation ways are not improved notably. This may be attributed to the mixed oxide composition characteristics and the blocking effect differences of Ce^+ segregation along the oxide grain boundaries on the transport process for different diffusing ions.

Mullite Coating on Recrytallized Silicon Carbide and Its Cycling Oxidation Behavior

Mullite coating on recrystallized silicon carbide was successfully prepared by the sol-gel route. The cycling oxidation of coated recrystallized silicon carbide was performed at 1500℃. For comparison, the oxidation of uncoated recrystallized silicon carbide was also carried out at the same condition. The results in- dicated that a layer of compact, adhesive and crack free mullite coating was found on the recrystallized silicon carbide. After oxidation, the new coatings exhibit adherence and crack resistance under thermal cycling between room temperature and 1500℃, therefore the oxidation resistance capability of silicon carbide was enhanced. With the increase of the dipping frequencies, namely, the increase of the thickness of mullite coating, the oxidation resistance of silicon carbide would be futher improved. The formation mechanism of mullite coating was analyzed and discussed and the oxidation dynamics model of coatedmullite silicon carbide has been also proposed.

Air oxidation of Cu-50Ni and Cu-70Ni alloys at 800℃

The air oxidation of Cu Ni alloys with 50% and 70% nickel (mole fraction) at 800?℃ was studied. The kinetic curves for the oxidation of the two alloys are complex and deviate from the parabolic rate law. Typical double layered scales are produced, which consist of a CuO outer layer and an inner layer containing a mixture of Cu 2O and NiO with many pores. Cu 50Ni presents a small degree of internal oxidation of nickel, which is observed in many binary double phase systems, but is quite rare in single phase systems.

DEFLECTION METHOD FROM TWO SIDES OXIDATION TO STUDY EFFECT OF Y ON INTERNAL STRESS OF OXIDE SCALES

Au investigation was carried ont of the effect of Y addition upon the internal stress of Al_2O_3 scale formed during oxidation of sputtering coating on Co-30Cr-6Al alloy and the growth stress of oxidized film on Fe-23Cr-5Al alloy heating up to 900℃ in air,using a specially designed deflection method from thin strip specimen with coat- ings,ion-implanted Y on one side and oxidized onto both sides.Results indicate that Y may decrease the internal stress of oxide scale on 2×10^(17) Y^+/cm^2 implanted Co- 30Cr-6Al coating,and increase one on 2×10^(16) Y^+/cm^2 implanted Fe-23Cr-5Al alloy. This seems due to whether or not Y promotes the plastic deformation of oxide scale.In addition,at initial oxidation stage,the change of growth stress of oxide scale formed on Y-implanted Co-30Cr-6Al coating may be related to the influence of Y on oxidation process of the coating.