Kyung-Tae Park

Creep behavior of discontinuous SiCAl composites

Abstract A review of creep data of discontinuous SiCue5f8Al composites (whisker and particulate) shows that the creep behavior of these composites exhibits two main characteristics: (a) the stress dependence of the steady state (or minimum) creep rate, as described by the value of the stress exponent, is high and variable and (b) the temperature dependence of the steady state (or minimum) creep rate, which is measured by the creep activation energy, is much larger than that for self-diffusion in aluminum. These two characteristics are examined in the light of theoretical treatments describing the origin of high temperature strengthening in discontinuous metal matrix composites and dislocation models proposed for dispersion-strengthened alloys.

Thermal stability and mechanical properties of ultrafine grained low carbon steel

Ultrafine grained low carbon steel manufactured by equal channel angular pressing was annealed at 753 K, where negligible grain growth occurred, up to 72 h and the microstructural change and the mechanical properties were examined. This investigation was aimed at providing the guiding information for the effective use of ultrafine grained low carbon steel manufactured by severe plastic deformation processes. Under the present annealing conditions, the microstructural change was dominated by recovery. The tensile behavior of annealed ultrafine grained steel was characterized by much higher strength and the absence of strain hardening compared with that of large grained steel. In addition, the present ultrafine grained steel became mechanically stable by 24 h annealing treatment although recovery was in progress. The microstructure of the deformed sample of annealed ultrafine grained steel exhibited the elongated grains and dislocations distributed densely in the vicinity of grain boundaries. This finding indicated that dynamic recovery during deformation was associated with the absorption of dislocation by grain boundaries. The mechanical behavior of the present ultrafine grained low carbon steel was discussed in light of the recent development explaining that of nanocrystalline materials, i.e. the dislocation bow-out mechanism for high strength and the spreading kinetics of trapped lattice dislocation into grain boundary for the absence of strain hardening.

High temperature creep of silicon carbide particulate reinforced aluminum

Abstract The effect of stress on the creep properties of 30 vol.% silicon carbide particulate reinforced 6061 aluminum (SiC p -6061 Al), produced by powder metallurgy, has been studied in the temperature range of 618–678 K. The experimental data, which extend over seven orders of magnitude of strain rate, show that the creep curve exhibits a very short steady-state stage; that the stress exponent, n , is high ( n > 7.4) and increases with decreasing the applied stress; and that the apparent activation energy for creep, Q a , is much higher than the activation energy for self-diffusion in aluminum. The above creep characteristics of SiC p -6061 Al are similar to those reported for dispersion strengthened (DS) alloys, where the high stress exponent for creep and its variation with stress are explained in terms of a threshold stress for creep that is introduced by the dispersoid particles. Analysis of the creep data of SiC p -6061 Al using the various threshold stress models proposed for DS alloys indicates that the threshold stresses introduced by the SiC particulates are too small to account for the observed creep behavior of the composite. By considering an alternate approach for the source of the threshold stress in SiC p -6061 Al, an explanation for the asymptotic behavior of the creep data of the composite is offered. The approach is based on the idea that the oxide particles present in the Al matrix, as a result of manufacturing the composite by powder metallurgy, serve as effective barriers to dislocation motion and give rise to the existence of a threshold stress for creep.

Creep strengthening in a discontinuous SiC-Al composite

High-temperature strengthening mechanisms in discontinuous metal matrix composites were examined by performing a close comparison between the creep behavior of 30 vol pct SiC-6061 Al and that of its matrix alloy, 6061 Al. Both materials were prepared by powder metallurgy techniques. The experimental data show that the creep behavior of the composite is similar to that of the alloy in regard to the high apparent stress exponent and its variation with the applied stress and the strong temperature dependence of creep rate. By contrast, the data reveal that there are two main differences in creep behavior between the composite and the alloy: the creep rates of the composite are more than one order of magnitude slower than those of the alloy, and the activation energy for creep in the composite is higher than that in the alloy. Analysis of the experimental data indicates that these similarities and differences in creep behavior can be explained in terms of two independent strengthening processes that are related to (a) the existence of a temperature-dependent threshold stress for creep, τ0, in both materials and (b) the occurrence of temperature dependent load transfer from the creeping matrix (6061 Al) to the reinforcement (SiC). This finding is illustrated by two results. First, the high apparent activation energies for creep in the composite are corrected to a value near the true activation energy for creep in the unreinforced alloy (160 kJ/mole) by considering the temperature dependence of the shear modulus, the threshold stress, and the load transfer. Second, the normalized creep data of the composite fall very close to those of the alloy when the contribution of load transfer to composite strengthening is incorporated in a creep power law in which the applied stress is replaced by the effective stress, the stress exponent,n, equals 5, and the true activation energy for creep in the composite,Qc, is equal to that in the alloy.

Microstructural characteristics and thermal stability of ultrafine grained 6061 Al alloy fabricated by accumulative roll bonding process

Abstract An accumulative roll bonding process was employed to introduce a ultrafine grained structure into a commercial 6061 Al alloy. In performing the accumulative roll bonding process, the alloy was rolled with a 50% reduction ratio. Then, the rolled sheet was cut, stacked to be the initial thickness and the stacked piece was rolled again with the same reduction ratio. This procedure was repeated five times so that an effective strain of 4 was accumulated into the alloy. By 5-passes rolling, the grain size of ∼0.4 μm was obtained when the grain size was measured on the rolling plane and a remarkable enhancement in hardness was achieved, over twice than that before rolling. The microstructural examination revealed that, at relatively low strain, fine dislocation cells were formed by an operation of multi-directional slip. With increasing strain, dislocation cells were developed into ultrafine subgrains. At large strains, the microstructural change was dominated by a conversion of low-angled subboundaries to high-angled boundaries, rather than grain refinement. The 1 h static annealing treatment was carried out at temperatures of 373–773 K in order to examine the thermal stability of ultrafine grained 6061 Al alloy. The present ultrafine grained 6061 Al was found to be thermally stable up to 473 K. The microstructural change of 6061 Al alloy during accumulative roll bonding was compared with that observed in ultrafine grained Al alloys fabricated by the equal channel angular pressing technique which is another representative technique for fabricating ultrafine grained bulk materials. In addition, thermal stability of ultrafine grained 6061 Al alloy was discussed in terms of the grain growth kinetics.

High temperature deformation of 6061 Al

Abstract The creep behavior of powder metallurgy (PM) 6061 Al, which has been used as a metal matrix alloy in the development of discontinuous silicon carbide reinforced aluminum (SiCue5f8Al) composites, has been studied over six orders of magnitude of strain rate. The experimental data show that the steady-state stage of the creep curve is of short duration; that the stress dependence of creep rate is high and variable; and that the temperature dependence of creep rate is much higher than that for self-diffusion in aluminum. The above creep characteristics are different from those documented for aluminum based solid-solution alloys but are similar to those reported for discontinuous SiCue5f8Al composites and dispersion-strengthened (DS) alloys. Analysis of the experimental data shows that while the high stress dependence of creep rate in 6061 Al, like that in DS alloys, can be explained in terms of a threshold stress for creep, the strong temperature dependence of creep rate in the alloy is incompatible with the predictions of available threshold stress models and theoretical treatments proposed for DS alloys.

Effect of Fe on the superplastic deformation of Zn-22 pct Al

In this investigation, the creep behavior of three grades of the superplastic Zn-22 pct Al eutectoid was studied under identical conditions of grain size, temperature, and stress. These three grades were prepared from high-purity Al and Zn using the same procedure, but they have different Fe levels: grades A, B, and C contain 423, 120, and 40 ppm of Fe, respectively. The experimental results show that the creep behavior of the three grades exhibits a sigmoidal relationship between stress and strain rate, which is manifested by the presence of three regions: region I (low-stress region), region II (intermediate-stress region), and region III (high-stress region). In region II, the creep characteristics, including the stress exponent, n, and the activation energy for superplastic flow,Q, are insensitive to Fe level; n ≅ 2.5 andQ =Qgb, whereQgb is the activation energy for grain boundary diffusion. By contrast, the emergence of region I along with its stress exponent and activation energy is affected by Fe level; the higher the Fe level, the higher the stress exponent and the activation energy. The effect of Fe on region I behavior in Zn-22 pct Al is attributed to a threshold stress for creep, τ0, whose origin is related to Fe segregation at boundaries. An examination of the estimated values of threshold stresses in the three grades along with a comparison between these values and those reported earlier for two grades of Zn-22 pct Al containing 180 ppm and 100 ppm of impurities (120 and 50 ppm of Fe, respectively) reveals two findings. First, the threshold stress appears to approach a limiting value for Fe concentrations above 120 ppm with increasing Fe level. Second, for approximately the same Fe concentration, the presence of other impurities in Zn-22 pct Al leads to a higher value of τ0. These findings are discussed in terms of characteristics associated with grain boundary segregation (saturation and synergistic effects).

Formation of fine cementite precipitates in an ultra-fine grained low carbon steel

Abstract Low-carbon steel containing 0.06 wt.% vanadium was deformed by a severe plastic deformation technique and annealed in a temperature range from 420 to 600 °C. The treatment resulted in a formation of nano-sized cementite precipitates as well as refinement of ferrite grains to a submicrometer size. This phenomenon is discussed based on dislocation-cementite interactions.

Effect of Impurity Content on Cavitation in the Superplastic Zn-22 Pct Al Alloy

Micrograin superplasticity refers to the ability of some materials to exhibit neck-free strains of several hundred percent upon deformation. The two basic prerequisites for the observation of micrograin superplasticity are (a) a temperature greater than about one half of the melting point,Tm, and (b) a fine, stable, and equiaxed grain size that does not undergo significant growth during elevatedtemperature deformation. In addition to these two prerequisites, grain boundaries need to be mobile, high-angled, and able to resist tensile separation.

Boundary sliding behaviour in high-purity Pb-62 % Sn

Abstract Boundary sliding behaviour in high-purity Pb-62% Sn containing 5 ppm of impurities has been investigated in tension at T = 422 K by measuring the individual offsets across boundaries. In the investigation, the alloy was tested at various initial strain rates, in the range of 5·33 × 10−7 to 1·33 × 10−1 s−1. The experimental results show that the contribution of boundary sliding to total strain at low strain rates is significant (about 60%) and comparable to that at intermediate strain rates (the superplastic region in alloys of high impurity levels). Also, the results reveal that, in general, there is no considerable difference in the average value of sliding offsets at the three types of boundary: Pb-Pb, Sn-Sn and Pb-Sn. However, contribution of sliding at interphase boundaries, Pb-Sn, appears to be more significant than that at intercrystalline boundaries, Pb-Pb and Sn-Sn. The above results are discussed in the light of recent data regarding the effect of impurities on superplastic flow.