Precipitation hardening in Cu 1.81 wt % Be 0.28 wt % Co

The structure of the precipitation hardening alloy Cu 1.81 wt % Be 0.28 wt % Co has been studied as a function of ageing temperature and time, by transmission electron microscopy. The continuous precipitation sequence found is: supersaturated solid solution G.P. zones .The G.P. zone is an ordered platelet precipitate, which is coherent on {100} matrix planes and is nucleated in very high densities (>10²⁴ m^–3). The coherency stress fields, due to the misfit of the G.P. zone and matrix, overlap to produce a net matrix contrast along {110} 10, and give the characteristic tweed structure, which can be described by the kinematical theory of diffraction. The semi-coherent intermediate precipitate is nucleated by the G.P. zones and the transformation is characterized from the changes in the arrowhead structure produced in the electron diffraction patterns. No transformation of to the equilibrium precipitate is found for the ageing times investigated.     

Internal friction due to G-P zones in pure Al-16 wt % Ag and Al-16 wt % Ag-0.2 wt % Fe-0.1 wt % Si alloys

A study has been made of an internal friction peak which occurs due to the precipitation of Guinier-Preston zones from two supersaturated Al-Ag alloys. The peak observed was interpreted as being due to relaxation around the zones of different elastic constants from that of the Al matrix. Differences in internal friction characteristics between specimens heated at 100 and 200 °C were attributed to the reversion process occurring around 200 °C in pure alloy. The height of the relaxation peak and the level of internal friction background were found to be highly affected by the presence of Fe and Si impurities in the doped alloy.     

Fatigue crack initiation in Ti-6wt % Al-4 wt % V castings

The lower ambient temperature fatigue life and higher fatigue data scatter of Ti-6 wt % Al-4 wt % V castings relative to wrought material, were investigated by analysing the nature of the crack initiation sites. Initiation mechanisms were characterized by precision sectioning the fracture surfaces at the crack nucleation locations. The combined effect of internal casting defects and a microstructure containing large colonies of similarly orientated -platelets was found to be responsible for the low smooth fatigue life. The large scatter in the fatigue data was attributed to the variety of casting defects in the material and their large size distribution. Most failures that were examined initiated from defects close to the specimen surface possibly due to some bending moment in the axial cyclic loading. The free dendrite arms in the large shrinkage pores were shown to contain -platelets in a colony arrangement. The shortest fatigue life associated with this type of defect was attributed to the notch effect of the dendrite arm interstices.     

Steady-state creep in a 25 wt % Cr-20 wt % Ni austenitic stainless steel

Steady-state creep behaviour of a 25 wt % Cr-20 wt % Ni stainless steel without precipitates was studied in the stress range 9.8 to 39.2 MPa at temperatures between 1133 and 1193 K. The results of stress-drop tests indicate that, in the steady-state creep region, diffusion-controlled recovery creep is dominant. Such recovery creep can be accounted for in terms of the composition of the internal stress, _i=_s+_c, except in the case of fine-grained specimens where d<80 m, whered is the mean grain diameter, _s is possible to reduce easily and is comparable to the driving stress for creep, and _c is the persistent stress field due to metastable substructure. In the fine-grained specimens, it is suggested that the steady-state creep is dominantly controlled by grain boundaries.     

The influence of 0.5 wt % Ag on defect structures in an Al/3.2 wt % Cu/1.5 wt % Mg alloy

A quantitative study has been made by electron microscopy of the defect structures in an Al/3.3 wt % Cu/1.6 wt % Mg alloy. The main defects observed in the quenched and lightly aged alloy were prismatic dislocation loops, while in an alloy of similar copper and magnesium contents, but containing also 0.5 wt % Ag, fewer defects were observed, these being mainly dislocation helices. In the quenched condition the average loop and helix diameters increased with increase in quenching bath temperature, and on ageing the loop and helix size increased with ageing time. These observations have been interpreted on the basis of silver increasing the solute/vacancy interaction, possibly with the formation of complex clusters of Cu, Mg and Ag atoms with vacancies.     

Micro-yielding in a precipitation hardening Cu 1.81 wt % Be 0.28 wt % Co alloy

The microstrain characteristics of a polycrystalline Cu 1.81 wt % Be 0.28 wt % Co precipitation hardening alloy have been determined for various precipitate conditions. The friction stress derived from measurements of closed hysteresis loops was found to remain constant ( 3 MN m^–2) for all the conditions investigated. In contrast, the microscopic yield stress (MYS) remained constant (18 to 24 MN m^–2) for most ageing conditions, but increased significantly (to 48 to 64 MN m^–2) for conditions associated with a high G.P. zone or precipitate density.     

Hydrogen trapping and repelling in an Al-6 wt % Zn-2 wt % Mg alloy

Hydrogen trapping in an Al-6 wt % Zn-2 wt % Mg alloy aged up to typical stages in the agehardening curve has been studied by measuring the tritium release rate after charging. The distribution of hydrogen in the aged alloy has been studied by tritium electron microautoradiography. It has been found that the Guinier-Preston zones in the alloy do not act as trapping sites but as a repeller for hydrogen, and that precipitate does not trap hydrogen, but the interface between the matrix and precipitate acts as a trapping site for hydrogen. Dislocation has been found to be capable of trapping hydrogen, while trapped hydrogen by the grain boundary has not been observed.     

Kinetics of alpha precipitation in Ti-6 wt % Cr and Ti-11 wt % Mo

The morphology and kinetics of the precipitation of the alpha phase produced by two different heat treatment routes, namely, (a) direct isothermal decomposition and (b)-quenching and subsequent ageing, were studied. In isothermally decomposed samples the (supersaturated) + transformation was seen to occur mainly through the discontinuous growth of the transformed zone consisting of groups of parallel side plates from the grain boundary regions towards the interior of the grain. Unlike for the case of a regular discontinuous precipitation, here the transformed regions are not separated from the untransformed by an incoherent interface and the growing-plates do obey a fixed orientation relationship with the grain from which they are evolved. The theory of cellular reaction has been applied to explain the growth rate of the duplex ( + ) region. The overall reaction kinetics were analysed on the basis of the Johnson-Mehl formulation and were found to be consistent with that of a discontinuous precipitation reaction, where grain boundary nucleation sites were saturated at an early stage of the transformation. The structure of the-quenched samples showed a uniform distribution of athermal omega particles which acted as precursors to the-precipitates. As a consequence, the reaction rate was greatly enhanced and-precipitation in the quenched and aged samples was seen to occur continuously in the entire body of the grain.     

Creep behaviour of a 25wt % Cr-20wt % Ni austenitic stainless steel doped with antimony

The effect of antimony on the steady state creep rates, , of a 25 wt% Cr-20wt% Ni austenitic stainless steel with 0.005 wt % C is studied. The effect on vacancy viscous creep (Coble creep) is shown to be different to that on dislocation creep (power law creep). The effect on Coble creep is particularly striking. The threshold stress is significantly increased by antimony additions.     

Metallurgical and electrochemical approach of Pb–(2wt.%)Sm and Pb(0.08wt.%)Ca(2wt.%)Sm lead alloys

The metallurgical study of Pb–2wt.%Sm and Pb–0.08wt.%Ca–2wt.%Sm has been performed from hardness measurements, DSC tests, TEM and SEM observations. It has been shown that the binary alloys do not hardened by microprecipitation of an intermetallic phase such as Pb₃Sm: after 2 years, the hardness of Pb–2wt.%Sm alloy is equal to that of pure lead, i.e. 6 HV. Moreover, for Pb–Ca alloy, it appears that the rare earth addition accelerates the three transformations of the ageing. In the simulated overcharge conditions of the acid battery, the Pb–2wt.%Sm grid alloy shows a lower corrosion resistance in 5 M sulphuric acid solution than pure lead. Indeed, the weight loss measured for the binary alloy is increased by 1.5 when it is compared to that of pure lead. This effect is probably due to the grain size decrease and, consequently, to the intergranular corrosion rising for the Pb–0.08%Ca–2.0%Sm with samarium additions in Pb–Ca alloy.     

The effect of hot-rolling on chill-cast Al,Al-2 wt % Ni and Al-4 wt % Ni alloys

The effect of hot-rolling on the mechanical properties and microstructure of directionallysolidified hypoeutectic Al-Al₃Ni alloys has been studied. Chill-cast hypoeutectic alloys were produced by casting into pre-heated mild-steel moulds placed on copper chills. The chill-cast Al-2 wt% Ni and Al-4 wt% Ni hypoeutectic alloys can be hot-rolled at 500 C to reductions of greater than 95%. Deformation is achieved by deforming the aluminium-rich dendrites in the rolling direction, followed by interpenetration of the Al₃Ni fibres into the dendrites resulting in a homogeneous microstructure. The variations of room-temperature tensile properties for the chill-cast hypoeutectic alloys were measured as a function of reduction of thickness during hot-rolling. The ultimate tensile strength and yield strength increase during rolling because of increasing Al₃Ni fibre alignment, homogeneous dispersion of the Al₃Ni fibres throughout the Al matrix, and work hardening in the Al matrix. The as-chill-cast alloys have strengths which agree with the composite law of mixtures for a combination of Al dendrites and Al-Al₃Ni eutectic. After hot-rolling, the alloy strengths can be predicted from discontinuous fibre reinforcement theory.     

Effects of Mo on microstructure of as-cast 28 wt.% Cr–2.6 wt.% C–(0–10) wt.% Mo irons

Microstructures of as-cast 28 wt.% Cr–2.6 wt.% C irons containing (0–10) wt.% Mo with the Cr/C ratio of about 10 were studied and related to hardness. The experimental irons were cast into dry sand molds. Microstructural investigation was performed by light microscopy, X-ray diffractometry, scanning electron microscopy, transmission electron microscopy and energy-dispersive X-ray spectrometry. It was found that the iron with about 10 wt.% Mo was eutectic/peritectic, whereas the others with less Mo content were hypoeutectic. The matrix in all irons was austenite, partly transformed to martensite during cooling. Mo addition promoted the formation of M₂₃C₆ and M₆C. At 1 wt.% Mo, multiple eutectic carbides including M₇C₃, M₂₃C₆ and M₆C were observed. M₂₃C₆ existed as a transition zone between eutectic M₇C₃ and M₆C, indicating a carbide transition as M₇C₃(M_2.3C) → M₂₃C₆(M_3.8C) → M₆C. At 6 wt.% Mo, multiple eutectic carbides including M₇C₃ and M₂₃C₆ were observed together with fine cellular/lamellar M₆C aggregates. In the iron with 10 wt.% Mo, only eutectic/peritectic M₂₃C₆ and M₆C were found without M₇C₃. Mo distribution to all carbides has been determined to be increased from ca. 0.4 to 0.7 in mass fraction as the Mo content in the irons was increased. On the other hand, Cr distribution to all carbides is quite constant as ca. 0.6 in mass fraction. Mo addition increased Vickers macro-hardness of the irons from 495 up to 674 HV₃₀. High Mo content as solid-solution in the matrix and the formation of M₆C or M₂₃C₆ aggregates were the main reasons for hardness increase, indicating potentially improved wear performance of the irons with Mo addition.     

Effect of grain size upon flow and fracture in a precipitation-strengthened Ti-8 wt % Al-0.25 wt % Si alloy

The interaction of grain size and precipitation strengthening has been studied in a Ti-8 wt % Al-0.25 wt %Si alloy. Grain sizes varying from 9 to 90m were produced by warm-working and annealing the alloy in the single--phase field. A uniform distribution of the coherent ₂ particles in the matrix was produced by ageing the alloy in the two-phase ( + ₂) field. The yield strength Hall-Petch slopes of the alloys with and without the ₂ precipitates were found to be nearly equal, indicating that the precipitation and grain-boundary strengthening are linearly additive. While specimens containing no precipitates exhibited a high ductility for all grain sizes, the ductility of the specimens with the ₂ particles decreased drastically with increasing grain size. TEM examination of the specimens containing the precipitates revealed a highly planar, localized slip and SEM examination of the fracture surfaces of these specimens revealed a transition in fracture behaviour from highly dimpled to mixed cleavage and intergranular with increasing grain size.     

Directional eutectoid decomposition in Cu-11.8 wt % Al

Directional eutectoid decomposition in a Cu-11.8 wt % Al alloy at a rate of 7×10^–5 cm/sec produced eutectoid colonies several inches in length, with lamellae parallel to the temperature gradient. Average lamellar spacing was 4000 Å. Significantly faster driving speeds produced martensitic structures. The maximum rate of eutectoid decomposition and the spacing observed were in agreement with earlier data from isothermal studies.     

Ion-nitriding of an Fe-19 wt % Cr alloy

The ion-nitriding behaviour of an Fe-18.75wt% Cr alloy was investigated at 803 K under constant plasma conditions. Both a thin surface layer of-Fe₄N and an internal-nitriding layer were observed. The nitride formed in the internal-nitriding layer was found to be CrN, rather than Cr₂N. The hardness of the nitriding layer rises to Hv=1200 due to small CrN precipitates. The growth rate of the internal nitriding layer, in the present alloy is controlled by a nitrogen diffusion process in the matrix metal,-iron. Because such ion-nitriding behaviour is analogous to that of internal-oxidation, the growth rate of nitriding was discussed according to the rate equation to that of internal-oxidation. The nitrogen diffusion in the present alloy is scarcely affected by the CrN precipitates.     

Hydrogen trapping in an Al-2.1 wt % Li alloy

Hydrogen trapping in an Al-2.1 wt % Li alloy aged up to typical stages in the age-hardening curve, has been studied by measuring the tritium release rate after charging. The distribution of hydrogen in the aged alloy has been studied by tritium electron microautoradiography. It has been found that the coherent precipitate and the incoherent precipitate act as a trapping site for hydrogen, while the semi-coherent precipitate does not trap hydrogen. A dislocation has been found to be capable of trapping hydrogen, while hydrogen trapping by the grain boundary has not been observed.