Cr-Ni-Mo-Co surface alloying layer formed by plasma surface alloying in pure iron

Using double glow plasma alloying technique, a multi-elements alloyed layer containing elements of Cr, Ni, Mo and Co was formed on the surface of pure iron. After undergoing suitable aging treatment followed solid solution treatment, the formed alloying layer keeps a good combination of corrosion resistance and wear resistance. The relationship between the process parameters of heat treatments and the properties of the formed Cr-Ni-Mo-Co alloying layer, such as the chemical composition, hardness, corrosion resistance and wear resistance, was investigated in this study. It was revealed that the formed alloying layer exhibits a better behavior than that of 304 stainless steel and pure iron by employing a suitable heat treatment system. The temperature employed in solid solution treatment is 1453 K (1180 °C) followed by water quenching and the aging temperature is 813 K (540 °C) followed by water cooling.     

The effect of nanostructured surface layer on the fatigue behaviors of a carbon steel

A nanostructured surface layer was formed on a carbon steel by means of surface mechanical attrition treatment (SMAT). The microstructure of the surface layer of the SMATed sample was characterized by using scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Microhardness and residual stress distribution along the depth from the SMATed surface layer were measured at the same time. Fatigue behaviors of the carbon steel subjected to the SMAT process were investigated. A nanostructured layer with average grains size of ∼12.7 nm was formed, of which microhardness is more than twice as high as that in matrix and residual compressive stress can reach about −400 MPa with maximum depth of ∼600 μm. The fatigue strength of as-received sample is 267 MPa and that of SMATed sample is 302 MPa based on fatigue life 5 × 10⁶ cycles. The SMAT process has improved the fatigue strength by as much as 13.1% for the carbon steel. It is shown that the SMAT is an effective method to render the material with the features, such as a nanostructured and work-hardened surface layer as well as compressive residual stresses, which can pronouncedly improve the fatigue strength of the carbon steel.     

Fabrication of Zr and Zr-N surface alloying layers and hardness improvement of Ti-6Al-4V alloy by plasma surface alloying technique

Titanium alloys are very attractive materials because they have high specific strength, excellent corrosion and erosion resistance in many active environments. However, their low hardness values and poor tribological properties require improvement of their surface properties. The present study is concerned with the fabrication of Zr and Zr-N alloying layers in the surfaces of Ti-6Al-4V substrates by plasma surface alloying technique. The microstructure, chemical composition and hardness of the surface alloying layers were analyzed to understand the mechanisms of surface alloying and hardness improvement. The Zr and Zr-N surface alloying layers formed were homogeneous and compact, in which the surface alloying elements all displayed gradient distributions. The Zr and Zr-N surface alloying layers all enhanced the surface hardness of Ti-6Al-4V alloy. Zr-N surface alloying resulted in greater improvement in hardness and the maximum microhardness of (1.37 ± 0.04) × 10³ HK was obtained at the subsurface, which was much higher than that of the untreated Ti-6Al-4V alloy. The Zr-N surface alloying layer consisted of an outer nitride layer and an inner diffusion zone of Zr and N, and its very high hardness owed to the formation of the nitride layer. The mechanism of hardness improvement of Zr surface alloyed Ti-6Al-4V alloy was solid solution strengthening.     

Laser surface alloying of Ni-plated steel with CO2 laser

Laser surface alloying of low carbon steel electroplated with thin (10 μm) Ni using an 850 W CW CO₂ laser is reported for the first time. Fe-Ni binary alloys of different concentrations are formed by varying laser traverse speed from 0.5 to 5 m/min. The phase transformation from α to α + γ is discussed as a function of Ni contents. Development of microstructure in the modified zone is analysed in terms of solidification rate and Ni concentration. A three-fold increase in the microhardness of the binary alloy is observed. Formation of homogenous, adherent and crack free surface alloys is reported.     

The effect of surface nanocrystallization on plasma nitriding behaviour of AISI 4140 steel

A plastic deformation surface layer with nanocrystalline grains was produced on AISI 4140 steel by means of surface mechanical attrition treatment (SMAT). Plasma nitriding of SMAT and un-SMAT AISI 4140 steel was carried out by a low-frequency pulse excited plasma unit. A series of nitriding experiments has been conducted at temperatures ranging from 380 to 500 °C for 8 h in an NH₃ gas. The samples were characterized using X-ray diffraction, scanning electron microscopy, optical microscopy and Vickers microhardness tester. The results showed that a much thicker compound layer with higher hardness was obtained for the SMAT samples when compared with un-SMAT samples after nitriding at the low temperature. In particular, plasma nitriding SMAT AISI 4140 steel at 380 °C for 8 h can produced a compound layer of 2.5 μm thickness with very high hardness on the surface, which is similar to un-SMAT samples were plasma nitrided at approximately 430 °C within the same time.     

Investigating the magnetic properties of soft magnetic composites based on mechanically alloyed nanocrystalline Fe-5 wt% Ni powders

In this work, the soft magnetic composites (SMCs) of the nanocrystalline Fe-5 wt% Ni powders coated with phenolic resin were studied. The nanocrystalline powders with an average diameter of 10 nm were obtained by mechanical alloying up to 96 h milling in a high-energy planetary ball mill. The microstructure and magnetic properties of the milled powders were characterized by X-ray diffraction, energy dispersive X-ray spectroscopy and a vibrating sample magnetometer. The results of X-ray diffraction showed that the bcc Fe(Ni) solid solution is formed after 24 h milling. Magnetic measurements indicated that the 96 h milled powders with a steady-state grain size of 10 nm have the highest saturation magnetization and the lowest coercivity. The SMCs based on nanocrystalline powders showed higher electrical resistivity and magnetic permeability up to 1 MHz, as compared with the pure iron-based composites. Besides, the nanocrystalline-based SMCs exhibited higher relaxation frequency and a significantly lower loss factor up to 1 MHz.     

Study on bacteria adherence properties of Mo surface-modified layer in Ti6Al4V alloy prepared by plasma surface alloying technique

Mo surface-modified layer in Ti₆Al₄V alloy was prepared using plasma surface alloying technique. Microstructure of the modified layer was analyzed using X-ray photoelectron spectroscopy (XPS), rough-meter and GDA750 glow discharge optical emission spectrometer. Phase composition of the Mo surface-modified layer was characterized by D/max 2500 X-ray diffraction. Results show that the Mo surface-modified layers consist of pure Mo surface layer with 〈1 1 0〉 and 〈2 1 1〉 orientations and diffusion layer. Mo 3d, O 1s, C 1s and Ti 2p, O 1s, C 1s XPS spectra are recorded at topsurface in the Mo-modified layer and titanium substrate respectively. Because of the different roughness and microstructure, the Mo surface-modified layer can to some extent inhibit bacteria adherence.     

Investigation on corrosion and wear behaviors of nanoparticles reinforced Ni-based composite alloying layer

In order to investigate the role of amorphous SiO₂ particles in corrosion and wear resistance of Ni-based metal matrix composite alloying layer, the amorphous nano-SiO₂ particles reinforced Ni-based composite alloying layer has been prepared by double glow plasma alloying on AISI 316L stainless steel surface, where Ni/amorphous nano-SiO₂ was firstly predeposited by brush plating. The composition and microstructure of the nano-SiO₂ particles reinforced Ni-based composite alloying layer were analyzed by using SEM, TEM and XRD. The results indicated that the composite alloying layer consisted of γ-phase and amorphous nano-SiO₂ particles, and under alloying temperature (1000 °C) condition, the nano-SiO₂ particles were uniformly distributed in the alloying layer and still kept the amorphous structure. The corrosion resistance of composite alloying layer was investigated by an electrochemical method in 3.5%NaCl solution. Compared with single alloying layer, the amorphous nano-SiO₂ particles slightly decreased the corrosion resistance of the Ni-Cr-Mo-Cu alloying layer. X-ray photoelectron spectroscopy (XPS) revealed that the passive films formed on the composite alloying consisted of Cr₂O₃, MoO₃, SiO₂ and metallic Ni and Mo. The dry wear test results showed that the composite alloying layer had excellent friction-reduced property, and the wear weight loss of composite alloying layer was less than 60% of that of Ni-Cr-Mo-Cu alloying layer.     

Effect of substrate temperature on the surface and interface oxidation of NiTi thin films

NiTi shape memory alloy thin films are deposited on pure Cu substrate at substrate ambient temperatures of 300 °C and 450 °C. The surface and interface oxidation of NiTi thin films are characterized by X-ray photoelectron spectroscopy (XPS). After a subsequent annealing treatment the crystallization behavior of the films deposited on substrate at different temperatures is studied by X-ray diffraction (XRD). The effects of substrate temperature on the surface and interface oxidation of NiTi thin films are investigated. In the film surface this is an oxide layer composed of TiO₂. The Ni atom has not been detected on surface. In the film/substrate interface there is an oxide layer with a mixture Ti₂O₃ and NiO in the films deposited at substrate temperatures 300 °C and 450 °C. In the films deposited at ambient temperature, the interface layer contains Ti suboxides (TiO) and metallic Ni.     

Growth of Ni-Al alloys on Ni(1 1 1), from Al deposits of various thicknesses: (II) Formation of NiAl over a Ni3Al interfacial layer

This paper describes the second part of a study devoted to the growth of thin Ni-Al alloys after deposition of Al on Ni(1 1 1). In the previous paper [S. Le Pévédic, D. Schmaus, C. Cohen, Surf. Sci. 600 (2006) 565] we have described the results obtained for ultra-thin Al deposits, leading, after annealing at 750 K, to an epitaxial layer of Ni₃Al(1 1 1). In the present paper we show that this regime is only observed for Al deposits smaller than 8 × 10¹⁵ Al/cm² and we describe the results obtained for Al deposits exceeding this critical thickness, up to 200 × 10¹⁵ Al/cm². Al deposition was performed at low temperature (around 130 K) and the alloying process was followed in situ during subsequent annealing, by Auger electron spectroscopy, low energy electron diffraction and ion beam analysis-channeling measurements, in an ultra-high vacuum chamber connected to a Van de Graaff accelerator. We evidence the formation, after annealing at 750 K, of a crystallographically and chemically well-ordered NiAl(1 1 0) layer (whose thickness depends on the deposited Al amount), over a Ni₃Al “interfacial” layer (whose thickness—about 18 (1 1 1) planes—is independent of the deposited Al amount). The NiAl overlayer is composed of three variants, at 120° from each other in the surface plane, in relation with the respective symmetries of NiAl(1 1 0) and Ni₃Al(1 1 1). The NiAl layer is relaxed (the lattice parameters of cc-B2 NiAl and fcc-L1₂ Ni₃Al differ markedly), and we have determined its epitaxial relationship. In the case of the thickest alloyed layer formed the results concerning the structure of the NiAl layer have been confirmed and refined by ex situ X-ray diffraction and information on its grain size has been obtained by ex situ Atomic Force Microscopy. The kinetics of the alloying process is complex. It corresponds to an heterogeneous growth leading, above the thin Ni₃Al interfacial layer, to a mixture of Al and NiAl over the whole Al film, up to the surface. The atomic diffusion is very limited in the NiAl phase that forms, and thus the progressive enrichment in Ni of the Al film, i.e. of the mean Ni concentration, becomes slower and slower. As a consequence, alloying is observed to take place in a very broad temperature range between 300 K and 700 K. For annealing temperatures above 800 K, the alloyed layer is decomposed, Al atoms diffusing in the bulk of the substrate.     

Morphology and composition of nanoscale surface oxides on Fe–20Cr–18Ni{1 1 1} austenitic stainless steel

Surface oxidation ranging from initial stages to the onset of passive oxide layer formation have been investigated on Fe–20Cr–18Ni{1 1 1} single crystal surface by X-ray photoelectron spectroscopy (XPS). Surface segregation of the alloying elements and the morphology of the surface oxide nanostructure were characterized quantitatively by inelastic electron background analysis. Our results demonstrate that by increasing the oxidation temperature the relative concentrations of Fe²⁺ and Fe³⁺ cations increase due to their enhanced mobility. Higher temperature also improves the mobility of chromium, thus enhancing its segregation to the oxygen-rich surface and thereby reinforcing the passive layer on the alloy. This is in agreement with the results showing the sudden decrease in oxide film thickness at the oxidation temperatures exceeding 600 K. Additionally, a pronounced segregation of metallic nickel is found in the interface between the surface oxide layer and the bulk alloy.     

Laser surface alloying of an electroless Ni–P coating with Al-356 substrate

Laser surface alloying of an electroless plating Ni–P coatings on an Al-356 aluminium alloy was carried out using a 1-kW pulsed Nd:YAG laser. The microstructure, chemical composition and phase identification of the alloyed layer were determined using scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDX) and X-ray diffractometry (XRD), respectively. It was shown that laser surface treatment produced a relatively smooth, crack-free and hard surface layer. The hardness of the surface significantly increased due to the formation of the uniformly distributed fine Ni–Al intermetallic phases. The corrosion behaviour of the surface alloyed specimens in 3.5% NaCl solution at 23 °C was also determined by electrochemical techniques. The laser-alloyed surface showed an improved corrosion and pitting potential compared to the substrate as well as the plated Ni–P coating.     

Growth of Ni-Al alloys on Ni(1 1 1): (I) Formation of epitaxial Ni3Al from ultra-thin Al deposits

In this paper we describe the alloying process of ultra-thin Al layers (below 8 × 10¹⁵ Al/cm²) deposited on Ni(1 1 1). For this purpose Auger electron spectroscopy, low energy electron diffraction, and ion beam analysis-channelling measurements have been performed in situ in an ultra-high vacuum chamber. Al deposits formed at low temperature (about 130 K) are strained defective crystalline layers retaining the substrate orientation. Alloying takes place, with very progressive Ni enrichment, in a very broad temperature range between 250 K and 570 K. This feature shows that diffusion of the alloy species is more and more difficult when the Ni concentration increases. At 570 K a crystallographically and chemically ordered Ni₃Al phase is formed, and its order continuously improves upon annealing, up to 750 K. We have shown by ion beam methods that this alloy is three-dimensional, extending up to 16 (1 1 1) planes for the thickest deposits. The Ni₃Al phase can also be obtained directly by Al deposition at 750 K, but its crystalline quality is lower and the layer is probably formed of grains elongated along 〈1 1 −2〉 directions. The Al content of the thin Ni₃Al layers formed mostly dissolves in the bulk above 800 K. However a small amount of Al remains segregated at the Ni crystal surface.     

In vitro corrosion behavior of Ti-O film deposited on fluoride-treated Mg-Zn-Y-Nd alloy

In this paper, a new composite coating was fabricated on magnesium alloy by a two-step approach, to improve the corrosion resistance and biocompatibility of Mg-Zn-Y-Nd alloy. First, fluoride conversion layer was synthesized on magnesium alloy surface by immersion treatment in hydrofluoric acid and then, Ti-O film was deposited on the preceding fluoride layer by magnetron sputtering. FE-SEM images revealed a smooth and uniform surface consisting of aggregated nano-particles with average size of 100 nm, and a total coating thickness of ∼1.5 μm, including an outer Ti-O film of ∼250 nm. The surface EDS and XRD data indicated that the composite coating was mainly composed of crystalline magnesium fluoride (MgF₂), and non-crystalline Ti-O. Potentiodynamic polarization tests revealed that the composite coated sample have a corrosion potential (E_corr) of −1.60 V and a corrosion current density (I_corr) of 0.17 μA/cm², which improved by 100 mV and reduced by two orders of magnitude, compared with the sample only coated by Ti-O. EIS results showed a polarization resistance of 3.98 kΩ cm² for the Ti-O coated sample and 0.42 kΩ cm² for the composite coated sample, giving an improvement of about 100 times. After 72 h immersion in SBF, widespread damage and deep corrosion holes were observed on the Ti-O coated sample surface, while the integrity of composite coating remained well after 7 d. In brief, the data suggested that single Ti-O film on degradable magnesium alloys was apt to become failure prematurely in corrosion environment. Ti-O film deposited on fluoride-treated magnesium alloys might potentially meet the requirements for future clinical magnesium alloy stent application.     

Formation and structure of a (√19 × √19)R23.4°-Ge/Pt(1 1 1) surface alloy

An ordered (√19 × √19)R23.4°-Ge/Pt(1 1 1) surface alloy can be formed by vapor depositing one-monolayer Ge on a Pt(1 1 1) substrate at room temperature and subsequently annealing at 900-1200 K. The long-range order of this structure was observed by low energy electron diffraction (LEED) and confirmed by scanning tunneling microscopy (STM). The local structure and alloying of vapor-deposited Ge on Pt(1 1 1) at 300 K was investigated by using X-ray Photoelectron Diffraction (XPD) and low energy alkali ion scattering spectroscopy (ALISS). XPS indicates that Ge adatoms are incorporated to form an alloy surface layer at ∼900 K. Results from XPD and ALISS establish that Ge atoms are substitutionally incorporated into the Pt surface layer and reside exclusively in the topmost layer, with excess Ge diffusing deep into the bulk of the crystal. The incorporated Ge atoms at the surface are located very close to substitutional Pt atomic positions, without any corrugation or “buckling”. Temperature Programmed Desorption (TPD) shows that both CO and NO adsorb more weakly on the Ge/Pt(1 1 1) surface alloy compared to that on the clean Pt(1 1 1) surface.     

Thermal stability of nanocrystalline layers fabricated by surface nanocrystallization

A nanocrystalline layer with ultrafine grains (about 30-40 nm) on the surface of 7050 aluminum alloy was fabricated by a new technique called High Pressure Shot Peening (HPSP) which is the combination of common Shot Peening equipment with a pressurizing vessel. Relationship between hot flow and temperature was observed by Differential Scanning Calorimetry (DSC) and the activation energy, calculated by Kisssinger equation, of the as-treated sample increased 26.6 kJ/mol when it is compared with the as-reserved sample. The Bragg peaks of the as-prepared samples, respectively treated with various annealing treatments were characterized by XRD and the microhardness distribution along the depth from the treated surface were measured at the same time, which indicated that the broadening of Bragg peaks decreased with the increasing of anneal temperature; the grain size, calculated by Scherrer-Wilson equation, increased obviously during 180-220 °C, accordingly, the microhardness obviously decreased. According to the results of DSC, XRD and microhardness, it is reasonable to deduce that the temperature range of thermal stability for aluminum alloy nanocrystalline layer is lower than 200 °C.     

Synthesis, characterization, and microwave absorption properties of Fe-40 wt%Ni alloy prepared by mechanical alloying and annealing

Fe-40 wt%Ni alloys with granular shape and flake shape were prepared by a mechanical alloying (MA) and annealing method. The phase composition and morphology of the FeNi alloys, electromagnetic parameters, and microwave absorbing properties of the silicone rubber composite absorbers filled with the as-prepared FeNi alloy particles were characterized using X-ray diffraction (XRD), scanning electron microscope (SEM) and vector network analyzer. The XRD results indicate that the crystalline structures of the Fe-40 wt%Ni alloys prepared by both one-step and two-step MA processes are face-centered cubic (fcc) Ni (Fe) solid solutions, and the structures can be retained after annealing at 600 °C for 2 h. SEM images show that the FeNi alloy powders for one-step process have a granular shape; however the particles turned into flake form when they were sequentially milled with absolute ethyl alcohol. With the increase in thickness of composite absorber, the reflection loss (RL) decreases, and the peak for minimum reflection loss shifts towards the lower frequency range. Compared to the absorbers filled with the granular FeNi alloy, the absorbers filled with flaky FeNi alloys possess higher complex permittivities and permeabilities and have a lower RL and peak frequency under the same thickness. Microwave absorbing materials with a low reflection loss peak in the range of 1-4 GHz are obtained, and their microwave absorbing properties can be adjustable by changing their thicknesses.     

Effect of surface nanocrystallization induced by fast multiple rotation rolling on hardness and corrosion behavior of 316L stainless steel

A nanostructured layer was fabricated by using fast multiple rotation rolling (FMRR) on the surface of 316L stainless steel. The microstructure in the surface was characterized by transmission electron microscopy and X-ray diffraction. The effects of FMRR on the microhardness, surface roughness and corrosion behavior of the stainless steel were investigated by microhardness measurements, surface roughness measurements, potentiodynamic polarization curves and pitting corrosion tests. The surface morphologies of pitting corrosion specimens were characterized by scanning electron microscopy. The results show that FMRR can cause surface nanocrystallization with the grain size ranges from 6 to 24 nm in the top surface layer of the sample. The microhardness of FMRR specimen in the top surface layer remarkably increases from 190 to 530 HV. However, the surface roughness slightly rises after FMRR treatment. The potentiodynamic polarization curves and pitting corrosion tests indicated that the FMRR treated 316L stainless steel with a surface nanocrystallized layer reduced the corrosion resistance in a 3.5% NaCl solution and enhanced the pitting corrosion rate in a FeCl₃ solution. Possible reasons leading to the decrease in corrosion resistance were discussed.     

The role of process parameters in plasma surface chromising of Ti2AlNb-based alloys

In order to enhance the wear resistance of Ti₂AlNb-based alloy (O-phase), surface chromising was performed by double glow plasma process in this study. The effect of process parameters, such as temperature, time and pressure, on the microstructure, thickness, and micro-hardness of the alloyed layers was investigated. Scanning electron microscope (SEM), X-ray photoelectron spectroscopy (XPS) and X-ray diffraction (XRD) were employed to analyze the composition distribution and microstructure of the alloy. The results showed that the optimum process parameters were as follows: 970 °C for temperature, 4 h for chromising time, and 30 Pa for pressure. Following the optimization the thickness, microstructure and micro-hardness of the modified layer achieved the designed requirements. The results of tribological tests showed that the friction coefficient of the chromised layer was lower than that of the matrix at the room temperature or 500 °C, and the specific wear rate of samples with plasma chromising at either room temperature or high temperature was decreased markedly.