Behaviour of hardening and serration yield of a Fe-Ni-Cr alloy under isothermal cycling （ISC） and out-phase TMF was studied on the basis of varied hysteresis loops. Cycling hardening and serrated yielding for ISC de...Behaviour of hardening and serration yield of a Fe-Ni-Cr alloy under isothermal cycling （ISC） and out-phase TMF was studied on the basis of varied hysteresis loops. Cycling hardening and serrated yielding for ISC depend on the temperature and the total strain range, stronger hardening with serrated yielding at higher strain range under ISC at 600 ℃, but no hardening and serrated yielding occurred under ISC at 800 ℃. Stronger hardening with stress serration occurred at the thermal path going to the lowest temperature, no stress serration occurred at the highest temperature under the out-phase. The hardening also depends on the total strain range, higher total strain range with lower cycling temperature resulted in a stronger hardening and remarkable serration yielding behavior. Weaker hardening without serrated yielding occurred at near 800℃ may due to an obvious cycling stress drop under out-phase TMF. Change in the shape of the hysteresis loops also expresses the degree of the damage of the tested alloy under out-phase and ISC.展开更多
The mechanism of the critical strain of serrated yielding is studied via tension tests at various strain rates. Betore the critical strain, it is deduced that dislocations are not pinned at high strain rates, and disl...The mechanism of the critical strain of serrated yielding is studied via tension tests at various strain rates. Betore the critical strain, it is deduced that dislocations are not pinned at high strain rates, and dislocations at low strain rates are pinned but cannot escape. The critical strain depends on the first pinning process at high strain rates and on the first unpinning process at low strain rates. The calculated results based on the two criteria are in good consistency with the experiment.展开更多
The apparent activation energy of serrated yielding was measured by two different methods for a commercial brass H68.The results showed that the apparent activation energy of serrat- ed yielding measured by a method i...The apparent activation energy of serrated yielding was measured by two different methods for a commercial brass H68.The results showed that the apparent activation energy of serrat- ed yielding measured by a method involving(m+β)values increases with the grain size.In contrast,those measured by another method without involving(m+β)values are all the same for specimens with different grain sizes.Combining with the observation of the microstructures,the above phenomena have been explained.The method to measure the true activation energy of serrated yielding has also been proposed.展开更多
The temperature dependence of critical strain for serrated yielding in high strength A1-Zn-Mg-Cu alloy may be divided into two temperature regions.Their temperature coefficients of critical strain will be negative and...The temperature dependence of critical strain for serrated yielding in high strength A1-Zn-Mg-Cu alloy may be divided into two temperature regions.Their temperature coefficients of critical strain will be negative and positive,respectively.展开更多
Due to the gradually prominent impact of fracture characteristics and serrated yielding in the application of nickel-based superalloys,the hot tensile properties of Inconel 718 superalloy were studied,including fractu...Due to the gradually prominent impact of fracture characteristics and serrated yielding in the application of nickel-based superalloys,the hot tensile properties of Inconel 718 superalloy were studied,including fracture behavior,mechanical properties,and plastic behavior.The experiments adopted three heat treatment regimes and two tensile directions.Results show that various heat treatments make grain sizes different.The larger-sized grains make the vertical surface uneven,which also decrease the number of grain boundaries and carbides,restricting the occurrence of dimples and ultimately reducing the material plasticity.The reduced grain boundaries can decrease dislocations,increase the demand for thermal activation energy,and transform the serration mode of serrated yielding.In addition,various heat treatments also make precipitates different.Carbides can promote the formation of dimples.The needle-shaped δ phase precipitates at grain boundaries and twin boundaries,and slightly inclines towards the rolling direction.Therefore,its pinning effect is outstanding along the transverse direction,which can affect the dimple aggregation and the dislocation movement,ultimately exhibiting anisotropy in fracture characteristics,mechanical properties,and serrated yielding.展开更多
Deformation behavior and microstructure of AlMg6Mn alloy subjected to shear spinning were studied by means of mechanical characterization, optical and SEM＋EDS microscopy. Specimens were shear spun on an industrial sp...Deformation behavior and microstructure of AlMg6Mn alloy subjected to shear spinning were studied by means of mechanical characterization, optical and SEM＋EDS microscopy. Specimens were shear spun on an industrial spinning machine using different mandrels, providing reductions of wall thickness of 30%, 50% and 68%. The grain structure developed during shear spinning refines gradually. The grains elongate in axial direction with increase of reduction, and also stretches along circumferencial direction. Optimal combination of strength and elongation is observed. This is attributed to grain refinement and dislocation reactions with particles and atoms of Mg and Mn in solid solution.展开更多
Nanocrystalline surface layers and gradient nanostructure in 5182 aluminum alloy have been produced through surface mechanical attrition treatment(SMAT). The results indicate that the gradient nanostructure can not on...Nanocrystalline surface layers and gradient nanostructure in 5182 aluminum alloy have been produced through surface mechanical attrition treatment(SMAT). The results indicate that the gradient nanostructure can not only improve the mechanical properties of 5182 Al alloy, but also has a certain effect on the Portevin-Le Chatelier(PLC) effect. The yield and ultimate tensile strength of 5182 Al alloy with SMAT are significantly improved combining with the decrease of fracture elongation compared with the as-received one. The PLC effect of 5182 Al alloy could be effectively postponed by the formation of gradient nanostructure after SMAT. It leads to the increase of critical strain of the PLC effect, more concentrated distribution of serrated strain, and increase of average stress amplitude in special strain range. The influence of grain size and gradient nanostructure on the PLC effect of 5182 Al alloy was also discussed in detail. Grain refinement could sharply increase the density of dislocations and hinder the movement of dislocations, which results in the decrease of moving speed of dislocations and the more concentrated distribution of solute atoms. The solute atoms would aggregate to form nano precipitates and further impede movement of dislocation. The stronger interaction between the dislocations and the nano precipitates is the main mechanism of postponed PLC effect.展开更多
文摘Behaviour of hardening and serration yield of a Fe-Ni-Cr alloy under isothermal cycling （ISC） and out-phase TMF was studied on the basis of varied hysteresis loops. Cycling hardening and serrated yielding for ISC depend on the temperature and the total strain range, stronger hardening with serrated yielding at higher strain range under ISC at 600 ℃, but no hardening and serrated yielding occurred under ISC at 800 ℃. Stronger hardening with stress serration occurred at the thermal path going to the lowest temperature, no stress serration occurred at the highest temperature under the out-phase. The hardening also depends on the total strain range, higher total strain range with lower cycling temperature resulted in a stronger hardening and remarkable serration yielding behavior. Weaker hardening without serrated yielding occurred at near 800℃ may due to an obvious cycling stress drop under out-phase TMF. Change in the shape of the hysteresis loops also expresses the degree of the damage of the tested alloy under out-phase and ISC.
基金Supported by the National Natural Science Foundation of China under Grant Nos 11332010,51271174 and 51571013the Fundamental Research Funds for the Central Universities
文摘The mechanism of the critical strain of serrated yielding is studied via tension tests at various strain rates. Betore the critical strain, it is deduced that dislocations are not pinned at high strain rates, and dislocations at low strain rates are pinned but cannot escape. The critical strain depends on the first pinning process at high strain rates and on the first unpinning process at low strain rates. The calculated results based on the two criteria are in good consistency with the experiment.
文摘The apparent activation energy of serrated yielding was measured by two different methods for a commercial brass H68.The results showed that the apparent activation energy of serrat- ed yielding measured by a method involving(m+β)values increases with the grain size.In contrast,those measured by another method without involving(m+β)values are all the same for specimens with different grain sizes.Combining with the observation of the microstructures,the above phenomena have been explained.The method to measure the true activation energy of serrated yielding has also been proposed.
文摘The temperature dependence of critical strain for serrated yielding in high strength A1-Zn-Mg-Cu alloy may be divided into two temperature regions.Their temperature coefficients of critical strain will be negative and positive,respectively.
基金National Natural Science Foundation of China(52175353)Shanxi Province Key Research and Development Program(202102150401002)Shanxi Province Major Science and Technology Special Project(202101110401009)。
文摘Due to the gradually prominent impact of fracture characteristics and serrated yielding in the application of nickel-based superalloys,the hot tensile properties of Inconel 718 superalloy were studied,including fracture behavior,mechanical properties,and plastic behavior.The experiments adopted three heat treatment regimes and two tensile directions.Results show that various heat treatments make grain sizes different.The larger-sized grains make the vertical surface uneven,which also decrease the number of grain boundaries and carbides,restricting the occurrence of dimples and ultimately reducing the material plasticity.The reduced grain boundaries can decrease dislocations,increase the demand for thermal activation energy,and transform the serration mode of serrated yielding.In addition,various heat treatments also make precipitates different.Carbides can promote the formation of dimples.The needle-shaped δ phase precipitates at grain boundaries and twin boundaries,and slightly inclines towards the rolling direction.Therefore,its pinning effect is outstanding along the transverse direction,which can affect the dimple aggregation and the dislocation movement,ultimately exhibiting anisotropy in fracture characteristics,mechanical properties,and serrated yielding.
文摘Deformation behavior and microstructure of AlMg6Mn alloy subjected to shear spinning were studied by means of mechanical characterization, optical and SEM＋EDS microscopy. Specimens were shear spun on an industrial spinning machine using different mandrels, providing reductions of wall thickness of 30%, 50% and 68%. The grain structure developed during shear spinning refines gradually. The grains elongate in axial direction with increase of reduction, and also stretches along circumferencial direction. Optimal combination of strength and elongation is observed. This is attributed to grain refinement and dislocation reactions with particles and atoms of Mg and Mn in solid solution.
基金financially supported by the National Key Research and Development Plan(No.2017YFB1103700)the Natural Science Foundation of China(Nos.51671101 and51464034)+1 种基金the Natural Science foundation of Jiangxi Province(Nos.20172BCB22002,20171BCD40003,20161ACB21003,20162BCB23013)the Science and Technology Key Research Plan in Jiangxi Educational Department(No.GJJ150010)
文摘Nanocrystalline surface layers and gradient nanostructure in 5182 aluminum alloy have been produced through surface mechanical attrition treatment(SMAT). The results indicate that the gradient nanostructure can not only improve the mechanical properties of 5182 Al alloy, but also has a certain effect on the Portevin-Le Chatelier(PLC) effect. The yield and ultimate tensile strength of 5182 Al alloy with SMAT are significantly improved combining with the decrease of fracture elongation compared with the as-received one. The PLC effect of 5182 Al alloy could be effectively postponed by the formation of gradient nanostructure after SMAT. It leads to the increase of critical strain of the PLC effect, more concentrated distribution of serrated strain, and increase of average stress amplitude in special strain range. The influence of grain size and gradient nanostructure on the PLC effect of 5182 Al alloy was also discussed in detail. Grain refinement could sharply increase the density of dislocations and hinder the movement of dislocations, which results in the decrease of moving speed of dislocations and the more concentrated distribution of solute atoms. The solute atoms would aggregate to form nano precipitates and further impede movement of dislocation. The stronger interaction between the dislocations and the nano precipitates is the main mechanism of postponed PLC effect.
