Thermal Fatigue of Polycrystalline Copper in CLIC Accelerating Structures: Surface Roughening and Hardening as a Function of Grain Orientation

The ac­cel­er­at­ing struc­tures of CLIC will be sub­mit­ted to 2 x 1010 ther­mal-me­chan­i­cal fa­tigue cy­cles, aris­ing from Radio Fre­quen­cy (RF) in­duced eddy cur­rents, caus­ing local su­per­fi­cial cyclic heat­ing. In order to as­sess the ef­fects of su­per­fi­cial fa­tigue, high tem­per­a­ture an­nealed OFE Cop­per sam­ples were ther­mal­ly fa­tigued with the help of pulsed laser ir­ra­di­a­tion. They un­der­went post­mortem Elec­tron Backscat­tered Diffrac­tion (EBSD) mea­sure­ments andμhard­ness ob­ser­va­tions. Pre­vi­ous work has con­firmed that sur­face rough­en­ing de­pends on the ori­en­ta­tion of near-sur­face grains*,**. It is clear­ly ob­served that, through ther­mal cy­cling, the in­crease of hard­ness of a crys­tal­lo­graph­ic di­rec­tion is re­lat­ed to the amount of sur­face rough­en­ing in­duced by fa­tigue. Near-sur­face grains, ori­ent­ed [1 0 0] with re­spect to the sur­face, ex­hibit­ing very low sur­face rough­en­ing, show lim­it­ed hard­en­ing where­as grains ori­ent­ed in [1 1 0], ex­hibit­ing se­vere sur­face rough­en­ing, show the most se­vere hard­en­ing. Con­sis­tent­ly, sur­face rough­en­ing and hard­en­ing mea­sured on [1 1 1] di­rec­tion lie be­tween the val­ues mea­sured for the other di­rec­tions men­tioned.