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.     

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.     

The effect of direct electric current on precipitation in a bulk Al-4 wt % Cu alloy

The effect of the stress of direct electric current on precipitation in a quenched Al-4.15 wt % Cu alloy is investigated by measurement of the change in electrical resistance of a bulk specimen as a function of current density up to about 3000 A cm^–2 at a given annealing time. A similar effect of d.c. stress is observed at 50, 75 and 100 C. For a lower current density, a constant or slightly increasing resistance is observed, while for a current density higher than a critical value, the resistance decreases with an increase in density. On the assumption that the temperature increase of a specimen due to the Joule heating is proportional to the square of the current density, it is concluded that there exists some effect of d.c. stress per se to prevent the precipitation reaction, aside from the effect of the temperature increase due to the Joule heating, and it is suggested that this retarding effect is related to the sweeping out of quenched-in excess vacancies into grain boundaries by electromigration.     

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.     

The variation of tensile characteristics of Al-2 wt.%Ag and Al-2 wt.%Ag-1 wt.%Sn alloys during transformation

Effect of tin addition and heat treatment on the work hardening characteristics of Al-2 wt.%Ag alloy during phase transformation has been studied in the deformation temperature range from 503 K to 583 K. The fracture strain ?_f, and dislocation slip distance L increased with increasing deformation temperature. On the other hand the coefficient of work hardening χ = δσ²/δ?, the fracture time t_f, yield stress, σ_y fracture stress, σ_f decreased with increasing deformation temperature and exhibited an abrupt increase at about 553 K. The Sn-free samples were generally harder than the ternary alloy. The activation energy of the fracture mechanism in both alloys was around 26.8 ± 3 kJ/mol and 34.6 ± 3 kJ/mol before and after transformation temperature (553 K), respectively. The quenched samples are harder than those of slowly cooled samples. From X-ray analyses it is clear that when lattice strain ? and dislocation density ρ increases, the crystallite size η decreases.     

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.     

Transformation behaviour of laser processed Fe-5% Cr,Fe-5% Ni and Fe-6% Cr-2% Ni alloys

The effects of rapid cooling, of the order of 10⁴? C sec^?1, achieved through the laser surface alloying process, on the transformation behaviour of Fe-5% Cr, Fe-5% Ni and Fe-6% Cr-2% Ni alloys have been investigated. The solidification structures and microstructures were characterized by optical and transmission electron microscopy. The significant findings are that Fe-5% Cr alloy undergoes a massive transformation to ferrite while Fe-5% Ni and Fe-6% Cr-2% Ni alloys undergo a transformation to a structure consisting of both ferrite and martensite. The substructures of ferrite and martensite were observed to contain essentially dislocations. These alloys also exhibited two carbide precipitations namely epsilon carbide and cementite. Solidification studies revealed that cellular solidification structures were present in 5% Ni and 6% Cr-2% Ni alloys with a cell spacing of 3 Μm but no solidification structures were observed in 5% Cr alloy. The influence of nature of solute and cooling rate on solidification behaviour, transformation mode and morphology of structures have been discussed.     

A re-examination of the Kraft and Flinn diffraction data for Cr2B, (Cr,Fe)2B, and the boride phase in Fe + 18.5 wt % Ni + 20 wt % Cr + B alloys

Kraft and Flinn's [2] powder X-ray diffraction data obtained from excess-phase particles in Fe + 18.5 wt% Ni + 20 wt% Cr + B alloys have been analyzed. The major boride phase was determined to be (Cr,Fe)2B (orthorhombic, Fddd, isomorphous with Cr2B). Its lattice parameters, a = 1.4585 ± 0.0010 nm, b = 0.7331 ± 0.0005 nm, c = 0.4223 ± 0.0003 nm, are indicative of the composition (Cr1.04±0.08Fe0.96 ± 0.08)B. It was not possible to identify the phase(s) giving rise to a much weaker set of diffraction lines in the patterns. Of the two alloys used by Kraft and Flinn as standards, the one intended to be Cr2B was found to be a mixture of Cr2B and Cr5B3. The second, intended to be Cr1.0Fe1.0B was determined to be Cr0.67± 0.07Fe1.33± 0.07B.     

The growth of germanium precipitates in an Al-4.0 wt % Ge alloy

The growth of germanium precipitates in an Al-4.0 wt % Ge alloy aged at 145, 160 and 190° C has been studied by small-angle X-ray diffraction. The intensity curves show that the precipitates are not spherical. By assuming that the precipitates are long cylindrical rods the experimental and theoretical intensity distributions are in a good agreement. The radius of the rods was determined as a function of ageing time.     

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.     

The structure and properties of a cold-rolled and annealed Al-0.8 wt % Zr alloy

The effect of annealing at 400? C on the microstructure of a cold-worked Al-0.8 wt % Zr alloy is reported. It is shown that the initially high dislocation density in the cold-rolled material is progressively reduced, although the grains and subgrains were exceptionally resistant to coarsening. Precipitation of the metastable cubic Al₃Zr phase occurred, both discontinuously in the form of fan shaped precipitates and also on the grain boundaries and within the grains as small, nearly spherical particles. The mechanical properties of the alloy at 400? C are consistent with a major dislocation contribution to the overall deformation process, in contrast with most other fine grained materials which are superplastic.     

喷射沉积含Ni铝合金的力学性能研究

利用电子显微镜、电子万能拉伸机等设备分析了喷射沉积含Ni铝合金的显微组织和力学性能,结果表明：Ni元素大部分分布在晶粒边界,与沿晶析出相固溶在一起。过多的Ni元素沿晶析出影响了合金的力学性能,降低合金的强度,使合金易产生沿晶断裂,降低了合金的韧性。通过调整元素Ni含量,合金的抗拉强度可以达到790MPa,延伸率达到了7%以上,达到了国内的领先水平。     

Annealing of supersaturated and deformed Al-0.042 wt % Fe solid solutions

The recrystallisation and precipitation behaviour of a quenched Al-Fe alloy (0.042 wt % Fe) during isothermal annealing was observed, after different amounts of plastic deformation, by microscopy and measurement of electrical conductivity. Three temperature ranges with different types of reaction can be distinguished with decreasing temperature: (i) The recrystallisation of solid solution; (ii) A two-step reaction: the recrystallisation is complete before precipitation starts; (iii) One-step reactions: characterised by a mutual influence of recrystallisation and precipitation.The combined reactions can take place by two different mechanisms. These are continuous recrystallisation, by growth of sub-boundaries, with simultaneous precipitation and coarsening of particles, and discontinuous recrystallisation, by motion of high-angle boundaries dissolving dislocations and redistributing particles. Conclusions of practical annealing treatments are drawn from the knowledge of the micromechanisms.     

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.     

Precipitation hardening and recrystallization in Cu-4% to 7% Ni-3% Al alloys

The effects of microstructure on mechanical properties in three cold-worked Cu-4% to 7% Ni-3% Al alloys have been investigated by changing ageing time at 500 °C. Hardness and strength in the Cu-7% Ni-3% Al and Cu-5.5% Ni-3% Al alloys increase with ageing time and have maximum values at an ageing time of 10³–10⁴s at 500 °C, then decrease. During ageing of Cu-7% Ni-3% Al at 500 °C, the coherent Ni₃Al phase was first precipitated out and later incoherent NiAl phase was formed. Ni₃Al formed during the initial stage of ageing is likely to be a transient phase. The increases in hardness and strength are due to the precipitation of coherent Ni₃Al phase. Coherent Ni₃Al particles are effective in increasing the strength and retarding the recrystallization process. On the other hand, the hardness and strength in the Cu-4% Ni-3% Al alloy gradually declined with ageing time. Only incoherent NiAl phase was formed during ageing at 500 °C. Decreases in hardness and strength in the Cu-4% Ni-3% Al alloy are attributed to softening during recovery and recrystallization, because incoherent NiAl particles have an insufficient effect to increase the strength.     

变形程度对强变形Al-4%Cu合金退火组织性能的影响

为了研究变形程度对强变形Al-4%Cu合金退火行为的影响,通过透射电镜观察和拉伸试验,研究了人工时效Al-4%Cu合金经过不同变形量的多向压缩变形(MAC),退火(120℃/60 min)后的显微组织和力学性能.研究表明:试样中的第二相在MAC过程中破碎溶回基体后,会在后续退火过程中再次析出,且析出相回溶的程度对退火组织性能的影响很大.含θ″相试样和含θ’相试样经MAC变形后,析出相完全回溶于基体,在退火过程中有新的第二相析出,试样强度升高;同时试样的的塑性也得到了提高,这与再析出粒子对超细晶粒长大的阻碍作用有关.析出相未完全回溶的含θ相试样,退火后强度低于退火前,析出相回溶和再析出交替进行.析出相基本回溶态强韧化效果最佳.