Serrated flow behavior in a Pd-based bulk metallic glass under nanoindentation
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The serration behavior of solution treated Al-Mg-Si alloys containing Cu up to 0.6% has been investigated in relation to precipitates. The strong serration is observed on each alloy at an early stage of room temperature aging and on fully annealed excess Mg alloys containing Cu. However, no serration can be observed on artificially aged specimens. It is thought that the serration is caused by dynamic segregation of solute atoms on moving dislocations and appearance of the serration seems to reduce with decrease in the amount of solute Mg and Cu atoms during aging. The results of TEM observation and EDX analysis indicate that the composition of precipitates of fully annealed specimens depends on Mg, Si and Cu contents. Q-phase precipitates are observed on excess Si alloys containing Cu. But in excess Mg alloys which show strong serration, Cu content of precipitates is less than Q-phase. Serration behavior of annealed specimens may be related to the solute Cu content.
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Many types of ferrite stainless alloys have deformation that is accompanied by serration. However, in many such cases, the actual cause of the serration still remains uncertain. Observations on Fe-30%Cr alloys revealed that 0.2%-proof stress increases at two different ranges of the deformation temperature. When the 0.2%-proof stress increased at the lower temperature range, two different types of serration could be observed. The strain hardening coefficient was calculated from the stress-strain curves. The strain hardening coefficient increased in the low temperature range but did not so in the high temperature range. The critical conditions for serration were determined by measuring the changes in the temperature and strain rate. When the critical conditions are determined, then the migration activation energy of moving solute atoms can be calculated. The solute atom that plays a role in causing the serration was determined from the activation energy of the solute atoms. Calculated activation energies were found to be 79.1kJ/mol and 216kJ/mol, respectively. Based on these results, we consider the cause of the serration observed in the low temperature range to be associated with the interaction of carbon and moving dislocations, while the serration evident in the higher temperature range is regarded as being due to the interaction between chromium and interstitial solute atom pairs with moving dislocations. Similar analyses were made for the pre-strain required to cause serration. The required pre-strain was observed for serration both in the low temperature range and that in the high temperature range. We also discuss the pre-strain required for causing serration.
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The effect of heat treatment conditions and small addition of Fe on occurrence of serration in Al-Zn alloys was investigated. Specimens were aged for various times up to 5Ms at 293K or 273K after quenching from various temperatures (T Q ), 398K to 853K, and tensile-tested at room temperature. Serration occurred more easily according as T Q became lower and the aging time became shorter: in the case that T Q =473K serration was observed even after aging for 2.6Ms, while in the case that T Q =773K serration did not occur irrespective of aging conditions. Serration was also recognized when the specimens were furnace-cooled from 773K to room temperature. Thus, for the binary alloy serration was observed only when the aging period was short enough, but addition of Fe to the binary alloy prolonged the aging period where serration could be recognized. Aging rate measured by hardness was remarkably retarded with the increase of Fe addition. These results together with those obtained by the electrical resistometry suggest that the serration in Al-Zn alloys occurs in the early stage of aging where small GP zones or solute clusters are formed.
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The serration flow and its morphology of Al-Li single crystal were investigated under different temperatures(T) and strain rates(e). The critical strain of serration(ec) first increased and then decreased with increasing T or decreasing e. Temperature and strain rate have some influences on the frequency and amplitude of serration which changed irregularly with deformation. The relations between the type of the serration and the mechanisms for the occurrence of serration, and the changes of frequency and amplitude were obscure in the Al-Li single crystal.
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Concerning computer simulation of serration at very low temperatures, calculation procedure and its several applications have been mentioned. Simulation methods by other researchers have several shortcomings, for instance, deformation beeing unreasonably considered to occur homogeneously. Using the present author's method, characteristics of serration and the accompanied phenomena can be simulated very well, and the effects of many factors on serration can be clarified.
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Abstract The deformation curve characteristics of rapeseeds and sunflower seeds compressed using the equipment ZDM 50-2313/56/18 and varying vessel diameters (40, 60, 80, and 100 mm) were investigated. Maximum compressive force of 100 kN was applied on bulk oilseeds of rape and sunflower of measured height 20-80 mm and deformed at a speed of 60 mm∙min-1. The compression test using the vessel diameters of 40 and 60 mm showed a serration effect while the vessel diameters of 80 and 100 mm indicated an increasing function effect on the force-deformation characteristic curves. Clearly, the increasing function effect described the region with oil flow and that of serration effect described the region without any oil flow. However, it was observed that the serration effect could be due to the higher compressive stress inside the smaller vessel diameters (40 and 60 mm) compared to those with bigger vessel diameters (80 and 100 mm). Parameters such as deformation, deformation energy, and energy density were determined from the force-deformation curves dependency showing both increasing function and serration effect. The findings of the study provide useful information for the determination of specific compressive force and energy requirements for extracting maximum oil from oilseed crops such as rape and sunflower.
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The influence of phase composition on the manifestations of the Portevin–Le Chatelier effect was studied in Al-Mg alloys using the digital image correlation technique. The smooth deformation curves were observed for Al-3Mg and Al-4.57Mg-0.35Mn-0.2Sc-0.09Zr alloys, whereas type A serration can be distinguished on the deformation curve of Al-5.4Mg-0.52Mn-0.1Zr alloy. At the same time, both the smooth plastic deformation and the stress serration are related to the propagation of a deformation band. The band velocity and the band passed distance for investigated alloys were estimated.
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