Study On Microstructural And Crystallographic Characteristics Of Phase Transformation Induced By ECP In Annealed Cu-40%Zn Alloy

A thorough investigation has been conducted on Electric Current Pulse(ECP)treated Cu-40%Zn alloys in terms of the microstructural and crystallographic features of the parent and the product phases,the phase transformation orientation relationship(OR)and its correlation with crystal defects,the ECP induced crystal defects in the parent phase and the sub-structures in the ? precipitates as well as their formation mechanisms.The a to ? heating phase transformation can be induced by ECP treatment with the formation of fine ? precipitates that can be remained to the room temperature.With the increase the electric current density,the amount of precipitates is increased and the formation sites increase from a grain boundaries to grain interiors.The ? precipitates follow different ORs depending on the formation site.The grain boundary ? phase obeys the Kurdjumov-Sachs(K-S)OR;whereas the introgranular ? respects the Nishiyama-Wasserman(N-W)OR.In the former sites,the {111??/<110>? dislocation arrays are frequently observed,whereas in the latter,the {111l}?/<112>? stacking faults are often found.Transformation strain analyses revealed that under the K-S OR the maximum lattice deformation required is a shear on the {111}?/<110>? slip system,whereas under the N-W OR the maximum deformation is a shear on the {111} ?/<112>? system.Thus the existing {111}?/<110>? dislocation arrays along the a grain boundaries provide pre-strain required by the transformation via the K-S path,whereas the {111}?/<112>? stacking faults boarded by {11l}?/<112>? partial dislocations offer pre-strain facilitating the transformation via the N-W path.Different types of crystal defects are formed in the ? matrix by the ECP treatments depending on the current density.At low density,large amount of {111??/<112>? stacking faults and then nano twins are produced in the a matrix.At high density,dislocation nets are formed near the ? precipitates that are composed of edge typed {111}?/<110>a perfect dislocations and the Frank typed dislocations.The volume misfit between the ? and the ?phase analyzed with transformation deformation reveals that the transformation from a to ?requires an expansion along the[110]? direction and a contraction along[111]?direction.The former results in the appearance of the {111?<110>edge typed dislocation arrays in front of the {311}? broad faces and the latter induces the formation of the Frank typed dislocations in front of the ?121}? broad faces.Thus,dislocation nets formed along the edges of the broad faces of the ? precipitates when the two kinds of dislocations meet in these regions.Furthermore,the ? precipitates contain two kinds of nano-sized and diffuse atomic clusters with the ? structure obeying the Burgers OR and with the ? structure obeying the Blackburn OR with the ? matrix.They were each formed through a two-stepped atomic displacement.For the ? structure,the first step is the atomic shuffle of each second {110}?plane in the<110>? direction and the second is a structure change mainly by a shear on the{112}?<111>?.For the ? structure,the first is an atomic shuffle on each second and third{112}? plane in the ±[111]?directions and then normal strains in three mutually perpendicular directions(<111>?,<112>? and<110>?).The concomitant appearance of the two structures lies in the fact that the volume increase accompanying the ? formation can be canceled by the volume decrease accompanying the ? distortion,which minimizing the transformation strain energy.The results of this work provide fundamental crystallographic and microstructural information on the Cu-40%Zn alloys for interpreting the impact of the crystal defects on the solid phase transformation ORs,on the formation of various types of crystal defects induced by the ultra-rapid phase transformation and on the formation mechanisms of sub structures in the product phase.