Width Of Precipitate Free Zone Research Articles

Coarsening behavior of Ll 2 structured Al 3(Zr x V 1−x) precipitates in rapidly solidified AlZrV alloy

A supersaturated Al alloy with 1.1 wt.% of Zr and 1.7 wt.% of V, which gives rise to about 5 vol.% of metastable Ll/sub 2/ phase after aging, was prepared by melt spinning. Both spherical and cellular Ll/sub 2/ structured precipitates form during aging. A higher aging temperature suppresses the cellular and favors the spherical Ll/sub 2/ precipitates. Because of the growth of grain boundary Al/sub 10/V particles during aging, precipitate free zones (PFZs) form adjacent to the grain boundaries. The average PFZ width is proportional to the square root of the annealing time. A slow volumetric particle coarsening rate of 1.0 x 10/sup -28/ m/sup 3//h at 425/sup 0/C was measured for the Ll/sub 2/ structured Al/sub 3/(Zr/sub .25/V/sub .75/) precipitate. An interfacial energy of 0.03 J/m/sup 2/ is calculated for the Ll/sub 2/ particles. The resistance at 425/sup 0/C to transformation to the stable DO/sub 23/ form and to Ostwald ripening suggests that the phase is a promising dispersoid for high temperature Al alloys. Preliminary creep results support this suggestion.

Ａｌ‐１．８％Ｌｉ合金の時効析出と疲労挙動

Precipitation during aging, fatigue behavior of aged specimens and effects of grain size on fatigue strength in the peak aged condition of Al-1.8%Li alloy were investigated. Precipitation of the metastable δ'-phase occurs during quenching from the solution treating temperature. The average diameter of particles is about 360A in the peak-aged specimen. The width of precipitate free zones decrease during aging and the formation of PFZ in this aging process are explainable by a mechanism of the depletion of vacancies in the region along grain boundary. Fatigue strength increases with age-hardening. Fatigue cracks nucleate preferentially at the sharp or coarse slip bands in aged specimens and propergate mainly along them. PFZ is not main cause for the fatigue failure under the lowers tress. Fatigue strength of the peak-aged specimen increases with decreasing grain size.

The development of microstructures in AlLi alloys

The nucleation, growth and coarsening of precipitates of an AlLi alloy were investigated. The equilibrium phase AlLi nucleates on grain boundaries, while the coherent and metastable Al 3Li nucleates homogeneously within the matrix grains. Isothermal aging resulted in the growth of Al 3Li precipitates. The growth process was dependent on the cube root of the time. Together with the growth of the matrix precipitates, a preferential coarsening of the AlLi precipitates on the grain boundaries occurred. To accommodate the accelerated growth in the grain boundary regions a precipitate-free zone (PFZ) developed. The change in the PFZ width was a function of the square root of the time. The AlLi solvus was redetermined metallographically at three temperatures, the values corresponding closely to the AlLi solvus values given in the literature. From the coarsening experiments the product of the coefficient of diffusion and the interfacial free energy of the precipitates was found. The coefficient of diffusion was determined independently by measuring the PFZ growth rate.

Determination of the oxygen precipitate-free zone width in silicon wafers from surface photovoltage measurements

A procedure has been developed for applying the surface photovoltage technique to the measurement of the width of the oxygen precipitate-free zone present at the surface of a thermally processed, Czochralski-grown silicon wafer. This procedure was developed through the use of a numerical simulation program which models the experimental determination of an effective diffusion lenght, L 0, from surface photovoltage measurements on silicon wafers. The program predicts L 0 for a given precipitate-free zone diffusion length and thickness and bulk diffusion length. Results of the simulations show that for bulk diffusion lengths of 2 μ or less and a precipitate-free zone diffusion length greater than the thickness of the zone, W, the W ≡ 2.5 L 0. Experimental results are presented which support the numerical findings.

Determination of the oxygen precipitate-free zone width in silicon wafers from surface photovoltage measurements : Terry I. Chappell, Patrick W. Chye and Morton A. Tavel. Solid-St. Electron.26 (1), 33 (1983)

Determination of the oxygen precipitate-free zone width in silicon wafers from surface photovoltage measurements : Terry I. Chappell, Patrick W. Chye and Morton A. Tavel. Solid-St. Electron.26 (1), 33 (1983)

Elimination of precipitate free zones in an Fe−Nb creep-resistant alloy

Precipitation of the Fe2Nb intermetallic compound has previously been found to cause substantial hardening during aging of Fe rich Fe-Nb alloys. However, the formation of a wide precipitate free zone adjacent to the grain boundaries caused a degradation of creep resistance. In an effort to decrease the precipitate free zone width, thereby improving the creep resistance, an extensive study was made of the precipitation behavior of an Fe-1.7 at. pct Nb(Cb) alloy quenched from the δ-phase field. The quenched alloy was found to decompose via a two step reaction during aging at temperatures below 550°C. The first step in the decomposition reaction is thought to occur by clustering of Nb atoms in the ferrite matrix, similar to the clustering of Mo atoms which is known to occur during aging of Fe-Mo alloys. The second step in the reaction is not well understood. The precipitate free zones were formed by solute depletion in the vicinity of the grain boundary and the subsequent difficulty of nucleation of the Fe2Nb precipitates in the regions of lowered solute concentration. Using two step aging treatments, an initial low temperature step to develop the Nb atom clusters followed by a higher temperature step to cause Fe2Nb precipitation, the precipitate free zones were eliminated from the aged alloys. The origin of this effect is thought to be the heterogeneous nucleation of Fe2Nb precipitates on the clusters developed during the initial aging step.

Creep-fracture initiation in 2¼%Cr-1%Mo steel

AbstractA detailed study has been made of grain-boundary damage in a 2¼%Cr-1%Mo steel after creep at 565°C. By subjecting the material to specific heat-treatments, a quantitative description of the resultant structures together with an assessment of creep damage has enabled creep cavitation to be correlated with certain microstructural features. Large numbers of cavities were distributed heterogeneously on prior austenite grain boundaries and an association with coarse carbide particles was evident. The cavity population was relatively insensitive to changes in the prior austenite grain size but was found to be inversely related to the width of precipitate-free zones (PFZ's) at the prior austenite grain boundaries. This result was linked with surface observations of zone shear and suggested that stress concentrations necessary for cavity nucleation were induced by local accommodation strains adjacent to the grain boundaries. On these lines, a theoretical nucleation model has been proposed and discussed in...

Influence of Precipitate Free Zones on the Stress Corrosion Susceptibility of a Ternary Al-5.9 Wt% Zn-3.2 Wt% Mg Alloy

Abstract A range of precipitate free zone (PFZ) widths containing different solute composition profiles have been developed in an Al-5.9 Wt% Zn-3.2 Wt% Mg alloy aged to produce similar overall microstructures. The initiation of stress corrosion cracking is shown not to be related to the PFZ width, but rather to the width of solute depletion within it. This behavior is explained in terms of a simple electrochemical model based on the polarization characteristics of the PFZ and grain boundary precipitates.

Influence of the precipitate-free zone width on the tensile properties of an Al-6 Wt pct Zn-1.2 Wt pct Mg alloy

The mechanical properties of an Al-6 wt pct Zn-1.2 wt pct Mg alloy with various width of precipitate-free zones have been investigated. The width of the precipitate-free zone (PFZ) has been changed by the quench interruption technique without any appreciable change in the size and distribution of precipitates. An important relationship has been observed between the width of the PFZ and the quench-interruption period;i.e., the width of the PFZ increases in proportion to the square root of the holding time at 200°C. From the analysis of stress-strain curves as well as the observation of dislocation arrangements in slightly deformed specimens, the plastic deformation has been found to occur preferentially in the PFZ. The initial stage of deformation is much affected by the change in the width of the PFZ, but in the later stage, the work-hardening rate seems to be almost independent of the PFZ width. Tensile tests show that the ultimate tensile strength and the 0.2 pct proof stress decrease very little with increasing width of the PFZ, while the uniform elongation is practically constant regardless of the reduction in the nonuniform elongation. The work-hardening rate at the initial stage of deformation is found to decrease in proportion to the reciprocal of the PFZ width. This relationship can be explained from the dislocation model for work hardening.

A study of grain boundary precipitate-free zone formation in an Al−Zn−Mg alloy

The effect of quench rate and boundary type on the width of precipitate-free zones in an Al-6.86 wt pct Zn-2.35 wt pct Mg alloy that was aged at two different temperatures was studied by transmission electron microscopy. The width of the precipitate-free zones increased with decreasing quench rate, but by a considerably smaller extent than that predicted from calculations using previous models based upon quenched vacancy profiles. The simple tilt-type boundaries were associated, in general, with narrower precipitate-free zones than those occurring at complex-type boundaries. The width of precipitate-free zones was also observed to decrease with increasing precipitate coverage in the boundary. A mechanism of precipitate-free zone formation based on the modification of the as-quenched vacancy profile by the vacancies generated due to grain boundary precipitation was proposed to explain the observed results.

The relation between microstructure and toughness in 7075 aluminum alloy

Electron microscopy, fractography, and notched tear tests have been used to investigate the effects of heat treatment upon the fracture behavior of aged 7075 aluminum alloy sheet. Toughness, as measured by crack propagation energy, decreases as the yield stress increases; the toughness of an overaged structure is inferior to that of an underaged structure at the same yield stress. The decrease of toughness with increased aging time is accompanied by a change in fracture mode from predominantly transgranular to intergranular. Transgranular fracture proceeds by dimple rupture and is facilitated by chromium-rich particles which are dispersed throughout the microstructure. Intergranular fracture proceeds by the fracture of grain boundary precipitate particles. The variation of fracture mode with aging time is attributed to a steady decrease of the intergranular fracture stress relative to the transgranular fracture stress, due to increasing grain boundary particle size. A possible explanation of this effect is discussed using the stress concentration due to colinear crack arrays as an analogy. The effects of quenching variations and two-step aging are discussed. It is shown that, in aged 7075, microstructural variables such as the width of precipitate-free zones and the nature of the matrix precipitate do not have a controlling effect on toughness.

The effect of quench rate and silver addition on the precipitate free zone width in an Al-4.3 pct Zn-1.3 pct Mg alloy

The effect of quench rate and silver addition on the precipitate free zone width in an Al-4.3 pct Zn-1.3 pct Mg alloy

The effect of multiple-step aging on the strength properties and precipitate-free zone widths in Al−Zn−Mg alloys

The effect of multiple-step aging treatments on the strength properties of Al-5 pct Zn−(2 and 4 pct)Mg alloys has been investigated. Some correlation has been made with precipitate-free zone widths and precipitation within the grains. Results show that the highest strengths are reached by multiple-step aging when the temperatures of all the aging steps are kept relatively low,i.e., less than about 120°C. The maximum strength is attained when the precipitate-free zone width is minimized and no evidence of the conversion of G.P. zones to M’ within the grains is observed in the electron microscope. A wide precipitate-free zone is indicative of a relatively low-strength, highly ductile Al−Zn−Mg alloy. Al−Zn−Mg alloys in the composition range studied when aged to maximum strength are believed to be principallychemically hardened. The high strengths exhibited by these alloys are attributed to the difficulty of dislocations to break the strong Zn−Mg bonds within the G.P. zones.

On the influence of step aging on the stress corrosion cracking of Al-Zn-Mg alloy

These studies were made to examine the effects of two-stepped aging on the stress corrosion cracking of Al-Zn-Mg alloys.Al-6Zn-3Mg alloy was aged in two steps under several conditions. The stress corrosion cracking test were performed on the above two-stepped aged alloys to pursure factors affecting the stress corrosion cracking.The results obtained were summarized as follows:(1) The stress corrosion cracking tests were performed on the alloy which had the maximum strength on the final aging curve. The stress applied was 75% of the proof stress of the specimens. The resistance to the stress corrosion cracking was lower when the preaging temperature was higher and its time was longer.(2) In the alloy which was sensitive to the stress corrosion cracking on the above tests, the size of the precipitated particles was fine and the change in surface roughness due to the tensile deformation was great.The sensitivity to the stress corrosion cracking did not depend only on the width of precipitate free zone on the grain boundary, but also on other factors.(3) It has been thought that the two-stepped aging reduces the sensitivity to the stress corrosion cracking of Al-Zn-Mg allys. It may be due to the following two reasons.In general, the stepped-aged alloy has higher strength than the alloy ordinally aged. Then, the former specimen will have higher resistance to deformation and also to stress corrosion cracking than the latter when the both specimens were subjected to the stress corrosion cracking tests under the load of the same weight.The other reason would be due to the difference in aging stage. When the final aging of the both specimens was performed under the same conditions, the former specimen will be more advanced in aging than the latter. The sensitivity to stress corrosion test is generally low in over-aged alloys. Then, the former specimen, which is more advanced in aging, will have higher resistance to stress corrosion cracking than the latter.(4) The electrode potential and the weight loss by normal corrosions were not closely related with the stress corrosion cracking. As well known, pitting corrosions often occurred on the specimens sensitive to the stress corrosion cracking.

Mechanical properties of age-hardened Al-Zn-Mg alloys

Al-Zn-Mg alloys such as compositions of Al-6%Zn-1%Mg (η-alloy) and Al-6%Zn-3%Mg (T-alloy) were subjected to various ageing treatments and their age-hardening mechanism and factors for controlling grain boundary fractures were investigated by tension tests and transmission electron microscopy.The effect of test temperature on the yield stress was very little in the range between room temperature and 77°K for as-quenched and overaged alloys. However, the effect was appreciable for these alloys containing G. P. zones only at temperatures of about 200°K or lower, or containing intermediate precipitates at below room temperature.The elongation to rupture was decreased with the increase of strength, independent of the width of precipitate free zone (PFZ) in η alloy: while it was dependent on the width of PFZ in T-alloy, since it was observed to be less in smaller width for the same strength.As the results of these tests, the age-hardening mechanism and the important factors for controlling grain boundary fractures in these ternary alloys such as the state of slip band and the width of PFZ were discussed.