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Process optimization is one pathway to maximizing strength of a given alloy. Thixomolding® is a semi-solid casting process that combines pores reduction with a typical bimodal grain size distribution that can lead to enhanced strength. AZ91D and AZX911 were processed via Thixomolding® using two different processing conditions to change fraction solid of primary particles at the point of injection into the mould. The tensile properties, creep resistance and corrosion behaviour of the alloys were investigated. The creep resistance was measured in the range of 135–150 °C for stresses of 50–85 MPa. The corrosion behaviour was measured via hydrogen evolution for the two alloys and was smaller than that for die-cast AZ91. The AZX911 alloy showed improved creep resistance compared to the AZ91D. The differences in the property profile of the chosen alloys are correlated with their chemical compositions as well as with different microstructures obtained through the different processing conditions.

Cold spraying was applied to deposit Ti2AlC on different substrate materials. The study of single impacts by scanning electron microscopy indicates that bonding of the first layer is mainly attributed to the deformation and shear instabilities occurring at substrate sites. Nevertheless, as compared to the feedstock particles, the splats appear flattened by the impact. This deformation seems to be attributed not only to local, internal shear but also to internal fracture. By applying up to five passes under optimized spray parameters, Ti2AlC-coatings with thicknesses of about 110-155 μm were achieved. XRD analysis of the coating proved that the crystallographic structure of the feedstock was retained during cold spraying. The coating microstructures show rather low porosity of about

The corrosion resistance of die cast and freely solidified or electromagnetically stirred thixocast AZ91D alloy has been studied using electrochemical noise technique and electrochemical impedance spectroscopy in a 0.05 M NaCl solution. Specimens polished at different depths were immersed in the solution in order to assess the influence of the microstructure on corrosion kinetics and morphology. At depths between 10 and 50 μm (skin), all specimens showed general non-uniform corrosion with the lowest corrosion resistance. Between 100 and 200 μm (interior skin), the observed corrosion was accompanied by superficial undefined pits due to metastable pitting. The corrosion form in the interior skins gave the best corrosion resistance. Stable pitting corrosion was observed beyond 400 μm deep on the bulk specimens. The skin of all thixocast specimens prepared from both types of billets showed a more corroded surface than that of die cast. On the other hand, the interior skin as well as the bulk of thixoc...

The effect of a silicate-based plasma anodization treatment on the corrosion and stress corrosion cracking behaviour of a cast AM50 magnesium alloy was studied. Electrochemical tests revealed the beneficial effect of the plasma electrolytic oxidation (PEO) in improving the corrosion resistance of the alloy. Although the coating had provided an improved resistance to stress corrosion cracking in this test environment at a nominal strain rate of 10 −6 s −1 , it could not completely eliminate the SCC susceptibility of the alloy. Cracking of the coating under conditions of straining was found to be the reason for SCC of PEO-coated alloy.

An attempt was made to understand the effect of silicate based plasma electrolytic oxidation (PEO) coating on the stress corrosion cracking (SCC) behaviour of an AZ61 wrought magnesium alloy. The SCC behaviour of untreated and PEO coated specimens was assessed using slow strain rate tensile tests at two different nominal strain rates, viz. 1 × 10−6 s−1 and 1 × 10−7 s−1, in ASTM D1384 test solution at ambient conditions. The PEO coating was found to improve the general corrosion resistance to a significant extent; however, the improvement in the resistance to stress corrosion cracking was only marginal.

The corrosion properties of the skin and the bulk of semi-solid processed and high pressure die cast AZ91 alloy were studied in 5% NaCl solution by a) analyzing the corrosion morphology, b) measuring electrochemical polarization curves, c) performing electrochemical impedance spectroscopic measurements, and d) immersion tests at constant pH values. Using X-ray diffraction (XRD), light microscope (LM), scanning electron microscope (SEM) and energy dispersive X-ray (EDX) to evaluate the microstructure of skin and bulk specimens, the corrosion results were related to their respective microstructures of the particular alloys. However, microstructure and corrosion performance of the alloy were found to be similar for both production processes and only slight but distinct differences were found. Generally, the skin corrosion properties are inferior to those of the bulk with the exception of the high pressure die cast AZ91 alloy, where the finer microstructure of the skin layer exhibits a better perform...

It is shown that oxide-ceramic coatings on magnesium alloys are, in general, characterized by high corrosion resistance depending on the porosity of the coating. The surface topology of oxide-ceramic coatings on IMV2 and VMD10 alloys is studied to reveal discontinuities by using special procedures of immersion and electrochemical deposition. Craters detected as a specific feature of the topology of the coatings and not always through and, in this case, they do not affect the corrosion resistance of the alloys. Isolated pores are formed, most often around the craters, in the process of oxidation under the action of high thermal stresses. On the basis of the comparative analysis of various materials and corrosive media, it is possible to make a conclusion that the corrosion resistance of magnesium alloys with oxide-ceramic coatings in acid media is mainly determined by the chemical composition of the alloy. At the same time, in neutral media, it is determined by the porosity of the coatings.

Ferritic alloys with different contents of carbon, aluminum and chromium are nitrided by plasma immersion ion implantation at 300 °C for 3 h. The thermal stability of the nitrided surface layers is evaluated by annealing the specimens in vacuum and in air both at 400 °C. The composition and the structure of the surface layers is studied by means of GDOES-spectroscopy and X-ray diffraction. In comparison to the nitrided reference specimens the nitrogen concentration decreases after heat treatment at 400 °C in air as well as under vacuum condition. The possibly main reasons are outward (degassing) and inward diffusion of nitrogen at elevated temperature. The e-nitride layer of a Fe–Cr alloy with high chromium content transforms to a stable Fe–Cr–N solid solution.

The X6CrNiTi1810 austenitic stainless steel samples nitrided by plasma immersion ion implantation at 300 °C/3 h are studied by slow positron implantation and conversion electrons Mossbauer spectroscopies completed with X-ray diffraction (XRD) measurements. The surface layers consist of γ N -Fe expanded austenite and e-Fe 2 N nitride. The changes in hyperfine parameters and relative representation of both phases during the interrupted long-time annealing at 150 °C are discussed from the point of view of nitrogen migration and changes in defect structure.

Expanded austenite can be formed either by nitrogen or carbon plasma immersion ion implantation (PI3 ™) from a nitrogen or methane plasma at elevated temperatures. The structure and properties of nitrogen and carbon expanded austenite layers produced on austenitic stainless steel X5CrNi189 are compared. A new structural model of expanded austenite based on a defect rich face centred cubic (fcc) lattice is proposed. Although the structure of the two expanded austenite layers is similar, there is a remarkable difference in the uptake of nitrogen or carbon, despite the use of similar treatment conditions. The modified surfaces have different hardness, corrosion and wear properties.

Plasma immersion ion implantation has been used for simultaneous implantation of carbon and nitrogen into austenitic stainless steel X5CrNi189 at 400°C. Duplex layer structures are formed with a nitrogen-rich layer close to the surface and a carbon-rich layer at greater depth. The influence of different gas compositions on the structure and phase composition of the layers is examined and compared with results from pure nitrogen and methane. The results provide further information about ‘expanded austenite’ (S-phase) which is formed during the low temperature nitriding of austenitic stainless steels.

Plasma immersion ion implantation is an effective surface treatment for stainless steels. The influence of treatment parameters (temperature, plasma density and pressure) on the sliding wear resistance are studied here. At moderate temperatures, nitrogen remains in solid solution without forming nitrides. This increases the surface hardness and the wear resistance without affecting the passivation of the steel. This may allow the use of such steels in applications where their poor wear resistance would normally prohibit their use.

The structure and phase composition of the modified surface of two different stainless steels (austenitic steel X6CrNiTi1810 and austenitic–ferritic steel X2CrNiMoN2253) after 3 h plasma immersion ion implantation of nitrogen at 400°C were compared using XRD, SEM, TEM and Mossbauer spectroscopy. Both steels show a surface layer with very similar composition. The major phase found is expanded austenite. Very fine precipitates in the near surface region are most likely decomposition products (ferrite/martensite and CrN) of the metastable expanded austenite. The stability of the expanded austenite in the austenitic–ferritic steel is lower and the amount of decomposition products is higher under the same treatment conditions.

Plasma immersion ion implantation technology was used to produce expanded austenite and to study the influence of treatment conditions and starting material on its formation and structure. Standard materials characterisation techniques, XRD, TEM, conversion electron Mossbauer spectroscopy, and Rutherford back scatter spectrometry analysis were used to determine the structure of the layers, thereby revealing the complex structure of expanded austenite. The properties of the expanded austenite layers (hardness, wear, and corrosion) were determined and correlated with the structure. Most work was concentrated on the austenitic stainless steel X6CrNiTi1810 after treatments at 300 and 400°C. Based on these results, the possibilities of influencing properties by varying the process conditions are discussed.

Nitriding of steels by Plasma Immersion Ion Implantation (PI 3 ) allows access to a large process parameter space. Although the parameters associated with the high-energy ion bombardment (implantation energy, high-voltage pulse length and frequency, ion current density and time-averaged dose rate) are important, the treatment temperature and plasma parameters such as ion density, excited neutral density and plasma potential also play a vital role. Previous investigations have been hampered by the use of the high-energy ion bombardment to heat the workpiece. In this paper, we present the results of a study in which the treatment temperature and the ion bombardment were decoupled by radiatively heating. The effect of varying high-voltage pulse length, repetition rate, total implanted dose, plasma density and potential on the nitrogen uptake during PI 3 treatment depends strongly on whether nitrides are formed in the surface (e.g. Ck45 mild steel) or nitrogen is incorporated in solid solution (e.g. X6CrNiTi 1810 austenitic stainless steel). In the first case, nitride formation can be suppressed by increasing the high-voltage pulse frequency or can be enhanced by treating at a high pressure or plasma potential. In the second case, the thickness of the modified layer can be increased by increasing the ion current density or time-averaged dose rate. In both cases, nitrogen uptake by direct thermochemical absorption from the plasma is significant.

It has been shown previously in the literature that plasma immersion ion implantation (PHI) can increase the wear resistance of austenitic stainless steel without losing its corrosion resistance. In this work, the effect of PHI treatment on the microstructure and the properties of an austenitic (X6CrNiTi1810, AISI 321) and a duplex austenitic-ferritic (X2CrNiMoN2253, AISI 318) stainless steel has been studied and the results compared. Three different treatment temperatures and treatment times were used. The microstructures were studied by optical metallography and glancing angle X-ray diffraction (XRD). The formation of expanded austenite was observed in both steels up to treatment temperatures of 400 °C. The ferrite in the duplex austenitic-ferritic steel was also transformed to expanded austenite. At 500 °C, a surface layer consisting of CrN was formed on the duplex austenitic-ferritic steel whereas the modified layer on the austenitic steel was still expanded austenite with a small amount of CrN precipitation. Elemental depth profiling by sputtered neutral mass spectrometry (SNMS) revealed a similar treatment depth for both materials up to 400 °C, which was a function of treatment temperature and time. A pin on disc tribometer was used to determine the tribological behaviour. A change in the wear behaviour was observed and the wear depth decreased relative to untreated material. This was due to an increase in the surface hardness and a decrease in the coefficient of friction. The decrease in wear depth correlated with the thickness of the modified layer. The best results were found with the duplex austenitic-ferritic steel at a treatment temperature of 500 °C and can be attributed to the formation of a CrN layer. Corrosion tests have shown that good corrosion resistance was preserved up to 400 °C for both materials with only a small decrease being observed. This is due to nitrogen remaining in solid solution without CrN-precipitation. At a treatment temperature of 500 °C, the corrosion resistance decreased dramatically, especially for the duplex austenitic-ferritic steel where a layer of CrN was formed. These results show the capability of PIII treatment to increase the wear resistance of these stainless steels without losing their good corrosion performance. This may allow the use of such steels in applications where the poor wear resistance of the untreated material would normally prohibit their use. In comparison to the austenitic steel, the duplex austenitic-ferritic steel performed better after PIII treatment. For an optimum surface treatment, it is necessary to consider the substrate material as well as the treatment parameters.

Most low alloy steels are tempered at relatively low temperatures, limiting the opportunities for improving their wear resistance and surface hardness by traditional heat treatment processes. In this work, we use plasma immersion ion implantation (PI 3 ) to apply a hybrid ion implantation/thermochemical diffusion treatment to ball bearing steel 100Cr6 (1% C, 1.5% Cr). By a judicious choice of implantation voltage, temperature and time, it is possible to combine the PI 3 treatment with the tempering stage of heat treatment, without compromising the hardness of the alloy. Up to 200 °C, the bulk hardness (750 HV tempered) of the material is maintained, and nitrogen implantation increases the surface hardness due to the precipitation of fine Fe 2 N. At 300 °C, where enhanced nitrogen and carbon diffusion can occur, the bulk hardness decreases, although in many applications this can be compensated by the increase in surface hardness to around 900 HV0.1 due to Fe 3 N and Fe 4 N formation. Wear and corrosion tests reveal significant improvements in comparison with untreated steel.

Macro test samples of magnesium alloy AM60B were melted and quenched at maximum instantaneous cooling rates ranging from -5°C/s to -500°C/s and the resultant cooling curves were analyzed. Characteristic reactions on these curves corresponding to formation of individual phases were identified with the aid of literature data as well as metallographic and micro-chemical analysis. The results indicate that these phases, their size and location in the micro structure, their chemistry and their relative proportions all change in response to the increase in the cooling rate. These rapid cooling rates are typical of real industrial solidification processes such as die casting. These findings can be used to improve future computer models of casting solidification processes for magnesium and for other alloys.

Ultrahigh molecular weight polyethylene (UHMWPE) has been the material of choice in orthopaedic joint prostheses for over three decades owing to its proven low friction and excellent wear resistance when coupled with smooth Co-Cr-Mo alloy or austenitic stainless steel surfaces in vivo [1]. However, it is gradually recognized that the principal constraint on the longevity of total joint replacements is the body’s reaction to debris, which is mostly UHMWPE [2]. Therefore, how to improve the wear resistance of UHMWPE and thus reduce the volume of wear debris, which promotes implant loosening, is a timely task from both a scientific and technological point of view. Nitrogen ion implantation of UHMWPE has been proved to improve its tribological properties in terms of low friction and extremely low wear rate [3], however, it is difficult in practice to uniformly treat such three-dimensional objects as hip joint sockets without using sophisticated manipulating devices. Recently, an innovative hybrid technology, plasma immersion ion implantation (PI3), has been developed, which can, to a large extent, address the problems with ion implantation of components with complex shapes [4]. However, there appears to have been little, if any, work reported on the surface modification of UHMWPE using the innovative, hybrid PI3 technology with a view to improve its mechanical and tribological properties [5]. In the present investigation, an attempt has been made to explore the feasibility of surface modification of UHMWPE using the novel PI3 technology. The material used in the study was medical grade UHMWPE (GUR-412) in the form of 12.5 diameter, extruded and annealed rod which was obtained from Poly Hi Solidur Ltd (GB). The density of the homogeneous material and the average molecular weight are 0.932 g cm−3 and 3.6× 106 g mol−1, respectively. Specimens were treated in a Mark 1 PI3 unit at the Technical University of Clauthal with the pulse source being (PI3 treatment) or not being (plasma treatment) applied. Specimens were immersed in a low pressure (0.17 Pa) radio frequency plasma discharge, which was generated by a 300 W r.f. power at 13.56 MHz. Table I lists the treatment conditions for plasma treatment (PT) and PI3 treatments. By selecting appropriate pulse length and pulse frequency, the treatment temperature was controlled at below 90 ◦C. Although it has been claimed that ion implantation can decrease the surface roughness of polymeric materials [6], such an improvement was not observed in the present study. Indeed, surface relief appeared on PI3-H specimens largely because of the ion etching effect resulting from nitrogen ion bombardment. Slight surface discoloration was observed for all surface treated specimens, increasing in the order of PT, PI3-L and PI3-H. In view of the fact that the surface modified layers of UHMWPE material usually are very thin and the substrate very soft, the mechanical properties of the untreated and treated UHMWPE specimens were measured using a NanoTest 500. A standard procedure developed by Oliver and Pharr was adopted to process the load-depth data [7]. Fig. 1 depicts characteristic load versus depth hysteresis curves, showing the loadingunloading response of the PI3 treated as well as untreated material. It can be clearly seen that, to reach the same penetration depth (250 nm), a higher load was need for the treated surface relative to the untreated surface. It also demonstrated that PI3 treated material exhibited a higher elastic recovery and less creep during the dwelling time (10 s) than the untreated material.

The effect of plasma electrolytic oxidation (PEO) treatment on the stress corrosion cracking (SCC) of a cast magnesium alloy was assessed. Even though the PEO coating has offered improved corrosion resistance, as evidenced in electrochemical tests, its effect in improving the SCC resistance in ASTM D1384 test solution was only marginal. The SCC behaviour of PEO-coated specimens was attributed to the development of microcracks in the coating, leading to cracking of the substrate under slow strain rate test conditions.