Electrodeposition on magnesium alloy diecasting

Magnesium alloy diecasting AZ91CC, AZ61CC', AZ91HC and AZ71HC were electroplated using different pretreatment sequences which incorporated conventional zincate immersion processes. Satisfactory peel adhesion in excess of 7. 7 KNm -1 was achieved on AZ61CC using a sequence which was designated Canning. The comparatively low adhesion achieved on the AZ91HC was due to its poor surface quality as cast. Growth of deposits was monitored using a strip-and-analysis technique and the morphology of the various deposits were studied using scanning electron microscopy. Different pretreatment sequences resulted in different surface responses for the alloys but all alloys behaved in a similar manner in a particular sequence with regard to potential time-curves and the rate of zinc deposition. The role of fluoride in both the second stage solution and zinc immersion stages of the Canning pretreatment sequence was studied using techniques listed above and Auger electron spectroscopy. Complete coverage of the magnesium alloy surface with immersion zinc was achieved when fluoride was absent from the zincating solution. However, a zero adhesion value was indicated in both thermal cycling and peel tests. The presence of fluoride in the immersion zinc solution suppressed the rate of zinc deposition and affected the time taken to reach equilibrium during potential-time determinations. A mechanism is suggested to explain the significance of fluoride additions to the processing solutions. pH and composition of the zincating solution had a significant effect on the time taken to produce the step observed in the potential/time curves and hence equilibrium potential. Immersion zinc deposition occurred rapidly at first but then changed to a lower uniform rate at a point corresponding approximately to the step in the potential/time curve. Although the minimun levels of adhesion, using the Canning sequence, varied from 7.72 KNm-1 for alloy AZ61CC to 1.54 KNm-1 for alloy AZ91HC, all the alloys revealed ductile failure characteristics in the surface layer of the substrate after peel testing. Plated magnesium alloys exhibited good corrosion resistance when appropriately pretreated and overplated with adequate nickel chromium coatings. The immersion zinc layer was not preferentially attacked when pits penetrated to the coating/substrate interface. Hemispherical pits formed and attack on the substrate was severe. Of the pretreatment sequences investigated, the Canning one was the most premising with respect to peel adhesion and corrosion behaviour.

Autocatalytic deposition of composite coatings

The deposition and properties of electroless nickel composite coatings containing graphite, PTFE and chromium were investigated. Solutions were developed for the codeposition of graphite and chromium with electroless nickel. Solutions for the deposition of graphite contained heavy metal ions for stability, with non-ionic and anionic surfactants to provide wetting and dispersion of the particles. Stability for the codeposition of chromium particles was achieved by oxidation of the chromium. Thin oxide layers of 200 nm thick prevented initiation of the electroless reaction onto the chromium. A mechanism for the formation of electroless composite coatings was considered based on the physical adsorption of particles and as a function of the adsorption of charged surfactants and metal cations from solution. The influence of variables such as particle concentration in solution, particle size, temperature, pH, and agitation on the volume percentage of particles codeposited was studied. The volume percentage of graphite codeposited was found to increase with concentration in solution and playing rate. An increase in particle size and agitation reduced the volume percentage codeposited. The hardness of nickel-graphite deposits was found to decrease with graphite content in the as-deposited and heat treated condition. The frictional and wear properties of electroless nickel-graphite were studied and compared to those of electroless nickel-PTFE. The self-lubricating nature of both coatings was found to be dependent on the ratio of coated area to uncoated area, the size and content of lubricating material in the deposit, and the load between contacting surfaces. The mechanism of self-lubrication was considered, concluding that graphite only produced an initial lubricating surface due to the orientation of flakes, unlike PTFE, which produced true self-lubrication throughout the coating life. Heat treatment of electroless nickel chromium deposits at 850oC for 8 and 16 hours produced nickel-iron-chromium alloy deposits with a phosphorus rich surface of high hardness. Coefficients of friction and wear rates were intially moderate for the phosphorus rich layer but increased for the nickel-iron-chromium region of the coating.

Fundamental mechanisms affecting service performance of electroplated plastics

Ten grades of ABS and four grades of polypropylene have been plated with various copper + nickel + chromium coatings and subjected to a variety of tests. In corrosion studies the pre-electroplating sequence and plastics type have been shown to influence performance. One ABS pre-electroplating sequence was consistently associated with better corrosion performance; two factors were responsible for this, namely the more severe nature of the etch and the relatively more noble electroless nickel. Statistical analysis has indicated that order of severity of the corrosion tests was static-mobile-CASS, the latter being the least severe. In mechanical tests two properties of ABS and polypropJylene, ductility and impact strength, have been shown to be adversely affected when electrodeposited layers were applied. The cause of this is due to a complex of factors, the most important of which is the notch sensitivity of the plastics. Peel adhesion has been studied on flat panels and also on ones which had a ridge and a valley moulded into one face. High adhesion peaks occurred on the flat face at regions associated with the ridge and valley. The local moulding conditions induced by the features were responsible for this phenonemon. In the main programme the thermal cycling test was shown to be more likely than the peel adhesion test to give an indication of the service performance of electroplated plastics.

Study of the deposition, properties and applications of electroless deposits containing particles

The deposition efficiencies of a number of electroless nickel and cobalt plating solutions were studied and in the case of nickel compared with a commercial plating solution Nifoss 80. At the optimum plating conditions (92ºC and pH 4.5) Nifoss 80 produced nickel layers most efficiently, the alkaline cobalt solution operated most efficiently at 90ºC and pH 9. The methods of producing compostte layers containing 2-3 µm carbide particles and chromium powder is described. Nickel and cobalt layers containing approximately 27% carbide particles, or 40% (Ni) and 30% (Co) chromium particles by volume were obtained. This value is independent of the particle concentration in the plating solution within the range (20~200 g/l). Hardness of the nickel. as deposited was 515 Hv, this was increased to a maximum of 1155 Hv by heat treatment at 200ºC for 5 hours in vacuum. Incorporation. of .chromium carbide particles resulted in a maximum hardness of 1225 Hv after heating at 500ºC for 5 hours in vacuum and chromium particles resulted in a maximum hardness of 16S0 Hv after heat treatment at 400ºC for 2 hours in vacuum. Similarly the hardness of cobalt as deposited was 600 Hv, this was increased to a maximum of 1300 Hv after heat treatment at 400ºC for 1 hour. Incorporation of chromium carbide particles resulted jn a maximum hardness of 1405 Hv after heating at 400ºC for 5 hours in vacuum and chromium particles resulted in a maximum hardness of 1440 Hv after. heat treating for 2 hours at 400ºC in vacuum. The structure of the deposits was studied by optical and scanning electron microscopy. The wear rate and coefficient of friction was determined by a pin and disc method. Wear rate and coefficient of friction decreased with increase in hardness. The wear resistance of the materials was also determined using a simulated forging test. Dies made of standard die steel were coated and the wear rates of the layers as deposited and after heat treatment were compared with those of uncoated tools. The wear resistance generally increased with hardness, it was 50-75% more than the uncoated die steel. Acetic acid salt spray test and outdoor exposure for six months was used to study the corrosion behaviour of the deposits and potentiodynamic curves plotted to find their corrosion potential. Nickel deposit exhibited less staining than carbide composite deposits and nickel-chromium deposits had the most noble corrosion potential.

Cracking in the welding of cupro-nickel alloys

The problem of variation in weld crack susceptibility caused by small variations in alloy and impurity elements for the 70-30 cupro-nickel alloy has been investigated. Both wrought and cast versions of the alloy have been studied, the main techniques employed being the Varestraint test and weld thermal simulation. In the wrought alloys, cracking has been found to occur mainly in the weld metal, whilst in the cast alloys cracking is extensive in both weld metal and heat affected zone. The previously reported effects of certain impurities (P,S,Si) in increasing cracking have been confirmed, and it has also been shown that Ti and Zr may both have a crack promoting effect at levels commonly found in cupro-nickels, whilst C can interact with several of the other elements investigated to produce a beneficial effect. The testing carried out using the weld thermal simulator has shown that a relationship does exist between hot ductility and weld cracking. In particular, the absence of the peak in ductility in the range 1100°C-900°C on cooling from a temperature near to the solidus is indicative of a highly crack susceptible alloy. Principal practical implications of the investigation concern the relationship of weld metal cracking to alloy composition, especially the level of certain impurities. It would appear that the upper limits permitted by the alloy specifications are unrealistically high. The introduction of lower impurity limits would alleviate the current problems of variability in resistance to cracking during welding.

Electrodeposition on aluminium and its alloys

Aluminium alloys S1C, NS4, HE9, LM25 and the 'difficult' zinc containing U.S. specification alloy used for automobile bumpers (X-7046), have been successfully electroplated using pretreatments which utilized either conventional immersion, elevated temperature or electrolytic modified alloy zincate (M.A.Z.) deposits. Satisfactory adhesion in excess of 7•5 KN m -I was only achieved on X-7046 using an electrolytic M.A.Z. pretreatment. The limitations of simple zincate solutions were demonstrated. Growth of deposits ~as monitored using a weight loss technique and the morphology of the various deposits studied using scanning electron microscopy. The characteristics of a specific alloy and processing sequence selected had a significant influence on the growth and morphology of the N.A.Z. deposi t. These all affected subsequent adhesion of electrodeposited nickel. The advantages of double-dip sequences were confirmed. Superior adhesion was associated with a uniform, thin, fine grained M.A.Z. deposit which exhibited rapid and complete surface coverage of the aluminium alloy. The presence of this preferred type deposit did not guarantee adhesion because a certain degree of etching was essential. For a satisfactory combination of alloy and M.A.Z. pretreatment, there was a specific optimum film weight per unit area which resulted in maximum adhesion. An ideal film weight of 0•06 :!: 0•01 mg cm-2was determined for S1C. Different film weights were required for the other alloys due to variations in surface topography caused by pretreatment. S1C was the easiest alloy on which to achieve high bond strength. Peel adhesion was not directly related to tensile strength of the alloy. The highest adhesion value was obtained on S1C which had the lowest strength of the alloys studied. The characteristics of the failure surfaces after peeling depended on alloy type, adhesion level and pretreatment employed. Plated aluminium alloys exhibited excellent corrosion resistance when appropriately pretreated. The M.A.Z. layer was not preferentially attacked. There was a threshold value of adhesion below which corrosion performance ~a8 poor. Alloy type, pretreatment and coating system influenced corrosion performance. Microporous chromium gave better corrosion protection than decorative chromium.

Fracture toughness of a series of high chromium abrasion resistant alloys

The fracture properties of a series of alloys containing 15% chromium and 0.8 to 3.4% carbon are investigated using strain fracture toughness testing techniques. The object of the work is to apply a quantitative method of measuring toughness to abrasion resistant materials, which have previously been assessed on an empirical basis; and to examine the relationship between microstructure and K10 in an attempt to improve the toughness of inherently brittle materials. A review of the relevant literature includes discussion of the background to the alloy series under investigation, a survey of the development of fracture mechanics and the emergence of K10 as a toughness parameter. Metallurgical variables such as composition, heat treatment, grain size, and hot working are ???? to relate microstructure to toughness, and fractographic evidence is used to substantiate the findings. The results are applied to a model correlating ductile fracture with plastic strain instability, and the nucleation of voids. Strain induced martensite formation in austenitic structures is analysed in terms of the plastic energy dissipation mechanisms operating at the crack tip. Emphasis is placed on the lower carbon alloys in the series, and a composition put forward to optimise wear resistance and toughness. The properties of established competitive materials are compared to the proposed alloy on a toughness and cost basis.

Precipitation studies on alloys from the system copper chromium zirconium

The precipitation reactions occurring in a series of copper-based alloys selected from the system copper-chromium-zirconium have been studied by resistometric and metallographic techniques. A survey of the factors influencing the development of copper-based alloys for high strength, high conductivity applications is followed by a more general review of contemporary materials, and illustrates that the most promising alloys are those containing chromium and zirconium. The few systematic attempts to study alloys from this system have been collated, discussed, and used as a basis for the selection of four alloy compositions viz:- Cu - 0.4% Cr Cu - 0.24. Zr Cu - 0. 3% Cr - 0.1% Zr Cu - 0.2% Cr - 0.2% Zr A description of the experimental techniques used to study the precipitation behaviour of these materials is preceeded by a discussion of the currently accepted theories relating to precipitate nucleation and growth. The experimental results are presented and discussed for each of the alloys independently, and are then treated jointly to obtain an overall assessment of the way in which the precipitation kinetics, metallography and mechanical properties vary with alloy composition and heat treatment. The metastable solid solution of copper-chromium is found to decompose by the rejection of chromium particles which maintain a coherent interface and a Kurdjumov-Sachs type crystallographic orientation relationship with the copper matrix. The addition of 0.1% zirconium to the alloy retards the rate of transformation by a factor of ten and modifies the dispersion characteristics of the precipitate without markedly altering the morphology. Further additions of zirconium lead to the growth of stacking faults during ageing, which provide favourable nucleation sites for the chromium precipitate. The partial dislocations bounding such stacking faults are also found to provide mobile heterogeneous nucleation sources for the precipitation reactions occurring in copper-zirconium.

Preferred orientation in 18/10, 18/12 and 18/14 chromium/nickel steels and its relation to tensile and press forming properties

DUE TO COPYRIGHT RESTRICTIONS ONLY AVAILABLE FOR CONSULTATION AT ASTON UNIVERSITY LIBRARY AND INFORMATION SERVICES WITH PRIOR ARRANGEMENT

Precipitation of chromium containing phases in aluminide coated nickel-base superalloy single crystals

The precipitation of chromium-containing phases, in both the B2 type β-phase coating matrix (nominally NiAl) and the substrate of high-activity-pack-aluminized single crystals of a nickel-base superalloy, is considered in this paper. An ‘edge-on’ transmission electron microscopy (TEM) technique is employed to examine the precipitation of M23X6, σ, α-Cr and other phases after coating and diffusion treatment and subsequent post-coating treatment at 850 and 950 °C. Initial precipitation is dominated by the formation of M23X6 in both the coating and substrate, however, in the case of single-crystal substrates the formation of this carbon-rich phase is not sustained. M23X6 precipitation is superceded by the formation of coherent precipitates of the α-Cr phase which effectively retains the basis but removes the superlattice of the β-matrix. Extensive precipitation of α-Cr has the effect of changing the balance of chromium to molybdenum in solution in the β-phase and further precipitation is dominated by Σ-phase intermetallics and other Cr-Mo-containing phases.

Brush plating of bearing alloys on aluminium alloy shells

The turbocharging of diesel engines has led to increase in temperature, load and corrosive attack of plain bearings. To meet these requirements, overlay plated aluminium alloys are now preferred. Currently, lead-tin alloys are deposited using a zincate layer and nickel strike, as intermediate stages in the process. The nickel has undesirable seizure characteristics and the zincate can given rise to corrosion problems. Consequently, brush plating allows the possible elimination of these stages and a decrease in process together with greater automation. The effect of mode application, on the formation of zincate films, using film growth weight measurements, potential-time studies, peel adhesion testing and Scanning Electron Microscopy was studied, for both SIC and AS15 aluminium alloys. The direct plating of aluminium was also successfully achieved. The results obtained indicate that generally, although lower adhesion resulted when a brush technique was used, satisfactory adhesion for fatigue testing was achieved. Both lead-tin and tin-cobalt overlays were examined and a study of the parameters governing brush plating were carried out using various electrolytes. An experimentally developed small scale rig, was used to produce overlay plated bearings that were fatigue tested until failure. The bearings were then examined and an analysis of the failure mechanisms undertaken. The results indicated that both alloy systems are of the regular codeposition type. Tin-cobalt overlays were superior to conventional lead-tin overlays and remained in good condition, although the lining (substrate) failed. Brush plated lead-tin was unsatisfactory. Sufficient understanding has now been gained, to enable a larger scale automated plant to be produced. This will allow a further study of the technique to be carried out, on equipment that more closely resembles that of a full scale production process.

The effects of metallurgical variables on the mechanical properties of high-chromium cast irons

The.use of high-chromium cast irons for abrasive wear resistance is restricted due to their poor fracture toughness properties. An.attempt was made to improve the fracture characteristics by altering the distribution, size and.shape of the eutectic carbide phase without sacrificing their excellent wear resistance. This was achieved by additions of molybdenum or tungsten followed by high temperature heat treatments. The absence of these alloying elements or replacement of them with vanadium or manganese did not show any significant effect and the continuous eutectic carbide morphology remained the same after application of high temperature heat treatments. The fracture characteristics of the alloys with these metallurgical variables were evaluated for both sharp-cracks and blunt notches. The results were used in conjunction with metallographic and fractographic observations to establish possible failure mechanisms. The fracture mechanism of the austenitic alloys was found to be controlled not only by the volume percent but was also greatly influenced by the size and distribution of the eutectic carbides. On the other hand, the fracture mechanism of martensitic alloys was independent of the eutectic carbide morphology. The uniformity of the secondary carbide precipitation during hardening heat treatments was shown to be a reason for consistant fracture toughness results being obtained with this series of alloys although their eutectic carbide morphologies were different. The collected data were applied to a model which incorporated the microstructural parameters and correlated them with the experimentally obtained valid stress intensity factors. The stress intensity coefficients of different short-bar fracture toughness test specimens were evaluated from analytical and experimental compliance studies. The.validity and applicability of this non-standard testing technique for determination of the fracture toughness of high-chromium cast irons were investigated. The results obtained correlated well with the valid results obtained from standard fracture toughness tests.

Effect of solute segregation on the strength of nanocrystalline alloys:Inverse Hall-Petch relation

We have used a high-energy ball mill to prepare single-phased nanocrystalline Fe, Fe90Ni10, Fe85Al4Si11, Ni99Fe1 and Ni90Fe10 powders. We then increased their grain sizes by annealing. We found that a low-temperature anneal (T < 0.4 Tm) softens the elemental nanocrystalline Fe but hardens both the body-centered cubic iron- and face-centered cubic nickel-based solid solutions, leading in these alloys to an inverse Hall–Petch relationship. We explain this abnormal Hall–Petch effect in terms of solute segregation to the grain boundaries of the nanocrystalline alloys. Our analysis can also explain the inverse Hall–Petch relationship found in previous studies during the thermal anneal of ball-milled nanocrystalline Fe (containing ∼1.5 at.% impurities) and electrodeposited nanocrystalline Ni (containing ∼1.0 at.% impurities).

Modification of the B2-type matrix of aluminide diffusion coatings on nickel-base superalloys-bulk aluminide analogues

Pack aluminide coating is a useful method for conferring oxidation resistance on nickel-base superalloys. Nominally, these coatings have a matrix composed of a Ni-Al based B2-type phase (commonly denoted as Β). However, following high-temperature exposure in oxidative envi-ronments, aluminum is depleted from the coating. Aluminum depletion in turn, leads to de-stabilization of the Β phase, resulting in the formation of a characteristic lathlike Β-derivative microstructure. This article presents a transmission electron microscopy study of the formation of the lathlike Β-derivative microstructure using bulk nickel aluminides as model alloys. In the bulk nickel aluminides, the lathlike microstructure has been found to correspond to two distinct components: L10-type martensite and a new Β derivative. The new Β derivative is characterized and the conditions associated with the presence of this feature are identified and compared with those leading to the formation of the L10 martensitic phase. © 1995 The Minerals, Metals & Material Society.

Microstructural degradation of aluminide coatings on single crystal nickel based superalloys during high temperature exposure

An investigation, employing edge-on transmission electron microscopy, of the microstructure of aluminide diffusion coatings on a single crystal y' strengthened nickel base super alloy is reported. An examination has been made of the effect of postcoating exposure at 1100°C on the stability of the coating matrix, a B2 type phase, nominally NiAl. Precipitation in the coating is considered with respect to both decomposition of the B2 matrix to other Ni-Al (plus titanium) phases and the formation of chromium bearing precipitates. A comparison is drawn with behaviour at lower temperatures (850-950°C). © 1995 The Institute of Materials.