Investigations On The Tensile Behavior Of Cu-Mn And Cu-Ni Alloys At Elevated Temperatures

The plastic deformation behavior of materials has been extensively investigated by researchers particularly focusing on the related deformation mechanisms.Stacking fault energy(SFE)and short-range ordered structures(e.g.,short-range ordering,SRO or short-range clustering,SRC)were recognized as the major criterion to impact the slip mode of dislocations in face-centered cubic(FCC)metallic crystals.Imposing a certain temperature under deformation will not only increase the SFE value of the alloy itself,but also change the content of short-range ordered structure in the alloy.Therefore,the influence of the joint changes in SFE and short-range ordered structure on the plastic deformation mechanism of metallic materials at elevated temperatures cannot be ignored.However,there are as yet few related studies.In order to systematically reveal the coupled effects of these two factors on the deformation behavior of metals at high temperatures,Cu-Mn and Cu-Ni alloys were selected as the target materials,and their tensile behavior was studied at elevated temperatures in the present work.The microstructures after tensile deformation at elevated temperatures were observed and characterized by scanning electron microscopy(SEM)and transmission electron microscopy(TEM).It is expected to provide a valuable reference for further enriching the understanding of high-temperature plastic deformation behavior and micro-mechanisms of FCC metallic materials.Experimental results of Cu-Mn alloys uniaxially tensioned at elevated temperatures indicate that,the range of temperature,at which a dynamic strain aging(DSA)effect occurs in Cu-Mn alloys with different Mn contents(5,10,20 at.%),is 200?350?,150?300?,100?300?,respectively.The yield strength of Cu-Mn alloys exhibits a slight decreasing tendency with increasing temperature.The ultimate tensile strength and uniform elongation(or ductility)of Cu-Mn alloys exhibit an overall decreasing tendency with increasing temperature.Due to a joint strengthening effect of DSA and SRO,the ultimate tensile strength and uniform elongation keep almost constant in the temperature range of 100?250?;however,owing to a lower content of SRO and the higher temperatures required for the occurrence of DSA,the ultimate tensile strength and uniform elongation slightly decreases in the Cu-5at.%Mn alloy.In the temperature range of 250?400?,with the increase of temperature,the ultimate tensile strength and uniform elongation decrease significantly.The dislocation structures of the Cu-5at.%Mn alloy exhibit varied characteristics at different deformation temperatures,e.g.,dislocation cells and cell blocks(RT?100?),dislocation cell blocks and extended dislocation walls(150?250?),and larger-sized dislocation cells and cell blocks(300?400?).The size of dislocation cells increases with increasing temperature,resulting in the decrease of ultimate tensile strength and ductility.The extended dislocation walls can weaken the strain localization and thus improve the work hardening capacity of the alloy;therefore,the alloy can maintain higher strength and ductility in the case of temperature increasing.The dislocation structures of the Cu-10at.%Mn alloy deformed at different temperatures are mainly composed of dislocation cells coexisted with planar slip bands(RT?100?),dislocation cells and planar-like slip structures(150?250?),and larger-sized dislocation cells(300?400?),respectively.The size of dislocation cells increases with the increase of temperature(SFE increased),leading to the decrease of tensile strength and ductility;however,the planar-like slip structures(high content of SRO)can inhibit the occurrence of dislocation cross-slip,thus improving dislocation storage ability.Therefore,the decreasing trend of tensile strength and ductility of the Cu-10at.%Mn alloy is somewhat alleviated.In contrast,the dislocation structures of the Cu-20at.%Mn alloy deformed at different temperatures are typical planar slip bands(RT?100?),planar slip bands coexisted with dislocation cells(150?300?),and smaller-sized dislocation cells and cell blocks(350?400?),respectively.Increasing temperature leads to the increase of SFE,which promotes the cross-slip of dislocations,promoting the change of dislocation structures from planar slip bands to dislocation cells;accordingly,the tensile strength and ductility of the alloy decrease with increasing temperature.Nevertheless,the formation of planar slip bands coexisted with dislocation cells(high content of SRO)can inhibit the transformation from planar slip bands to dislocation cells and cell blocks,and thus resist the decreases in tensile strength and ductility of this alloy.Experimental results of Cu-Ni alloys uniaxially tensioned at elevated temperatures demonstrate that,a DSA effect in the Cu-5at.%Ni,Cu-10at.%Ni and Cu-20at.%Ni alloys were found to take place at 200? and 250?.Interaction with moving dislocations by SRC and diffusing solute atoms is thought to be the major reason for the occurrence of DSA effect.Both of ultimate tensile strength and yield strength decrease with the increase of temperature,and the uniform elongation also simultaneously decreases.Although the dislocation structures of the Cu-20at.%Ni alloy deformed at 200? are composed of planar-like slip structures and a small amount of dislocation cells,since the SRC content is high,which promotes the planar slip of dislocations,the SFE of Cu-Ni alloys are very high,especially in the case of temperature rising,such that the dislocation substructures still exhibit the typical wavy-slip type morphologies at other temperatures.