Self-Annealing Phenomena in the Cold-Rolled and Cryo-Rolled Copper Alloys: A Review

Severely deformed copper (Cu) alloys showed the self-annealing behaviour when exposed at room temperature (RT) for a prolonged time. This phenomenon results in the evolution of self-annealed grains, dislocations annihilations, and a decrease in the strength. The present review work addresses the evolution of self-annealed microstructures in room-temperature-rolled (RTR) and cryogenic-rolled (CR) Cu alloys, including various grades of Cu alloys such as ETP Cu, Cu-Fe-P, Cu-Sn-Mg alloys that were studied in terms of RT softening. Static recrystallization (SRX) at room temperature (RT) was observed in the highly deformed RTR and CR Cu alloy samples. Both discontinuous SRX (DSRX) and continuous SRX (CSRX) were responsible for the nucleation of small grains at RT. Particle-stimulated nucleation (PSN) also contributed to small-grain formations around Cu2O particles in ETP Cu. However, no PSN was reported for the Cu-Fe-P and Cu-Sn-Mg alloys. The formation of strain localizations (SLs) and shear bands (SBs) was observed in the deformed Cu alloys; these sites were preferred for grain nucleation during RT recrystallization.


Introduction
Cryo-rolling (CR) gained a lot of interest as an emerging severe plastic deformation method.Deformation at cryogenic temperature (CT) restricts recovery and grain growth during the deformation, resulting in higher dislocation densities compared to room temperature (RT) deformation [1].Additionally, significant work-hardening and ultra-fined grain microstructures have been observed during the CR of copper (Cu) [2].Konkova et al. [2] and Lapeire et al. [3] discussed the effect of CR on the microstructure and texture evolution in pure Cu.Heterogeneous deformation, formation of SBs, mechanical twinning, grain fragmentation were observed for the 93% CR Cu [2].The phenomenon of recrystallization at RT is rare in Cu alloys and has not been extensively discussed in works on thermomechanically processed Cu alloys [4,5].RT recrystallization, also known as self-annealing, was reported for CR Cu, indicating the nucleation of small grains, dislocation annihilation, and the evolution of a recovered microstructure in deformed Cu when samples are exposed to RT [2].Similarly, Lapeire et al. [3] observed the evolution of statically recrystallized (SRX) grains in CR Cu when the deformed samples were taken out from -17ºC for metallographic preparation.
Grain boundary bulging, as well as recovered microstructure, have been reported for the CR Cu [2].These bulges forms due to the pinning effect of dislocations at grain boundaries (GBs), which initiates the formation of new SRX grains [4].High defect densities, such as vacancy and other lattice defects, can arise during deformation at CT.A high density of lattice defects decrease the thermal stability of Cu and its alloys, causing grain boundary migration even at the RT [5].Self-annealing phenomena depend not only on material properties, such as pure metals/alloys or low/high melting point, but also on strain, strain rate, stacking fault energy, and deformation temperature [1].Self-annealing can also decrease the micro-hardness values, which were reported for CR FCC materials [4,6].
In the present review, we aim to discuss the evidences and implications of self-annealing phenomena on the microstructural evolution in room-temperature-rolled (RTR) and CR Cu alloys.Discussions on particle stimulated nucleation (PSN) were also made for CR Cu samples.High resolution electron back-scattered diffraction (EBSD) technique and transmission electron microscope (TEM) were used to capture SRX microstructural features.Grain orientation spread (GOS1.5)and confidence index (CI)>0.1 criteria were used to differentiate SRX/self-annealed grains from the deformed microstructure.

Experimental Details
Electrolytic tough pitch (ETP) Cu, as well as Cu-Fe-P alloys sheets with a thickness of 1.5 mm, were used for the RTR and CR processes.Both alloys were subjected to RTR and CR up to a thickness of 0.3 mm (80% reduction ratio (RR)) i.e., true strain of 1.6.For CR, specimens were dipped in the liquid nitrogen for more than 8 min before every pass.Detailed discussions on sample preparation for both alloys can be found elsewhere [4,7].Similarly, Cu-0.13Sn-0.04Mgalloys sheets were only subjected to RTR deformation until a 75% RR.Standard metallographic procedures were used to polish the deformed samples.An EBSD system attached to the FE-SEM JEOL JSM-7100F system was employed to determine the micro-texture.Samples for the EBSD measurements were prepared through final polishing with colloidal silica.EBSD measurements were done at 20 kV accelerating voltage and 15 mm working distance.High-resolution EBSD scans were captured for highly deformed samples using a step size of 0.1 µm.TSL-OIM software was utilized to process the EBSD data.It is essential to note that all the deformed samples were kept at RT just after rolling, and metallographic preparations were also performed at RT. Recrystallized microstructure was partitioned in the EBSD data using the grain orientation spread (GOS) criteria.GOS1.5 and CI>0.1 were used to observed the SRX microstructure.TEM was also used to observe the nature of self-annealed grains in RTR and CR Cu-Fe-P alloys, as discussed elsewhere [7].For TEM investigation, Cu-Fe-P samples were kept at -22C just after RTR and CR deformation.However, sample preparation and TEM measurements occurred at RT.

Self annealing in ETP Cu
Figure 1 displays the microstructural evolution in 80% CR ETP Cu.The average grain size (GS) was calculated to be 7.79±7.57µm after an 80% RR, while the initial as-received sample had a GS of 21.93±10.68µm [4].The reported GS values are the equivalent grain size measured from the EBSD data.The ND-IPF map in figure 1(a) reveals the presence of coarse and fine elongated grains, along with other significant features such as bulge GBs and PSN. Figure 1(b) shows the image quality (IQ) + GBs map, in which the line fraction of low angle GBs (LAGBs, 3-15) and high angle GBs (HAGBs, 15) were 34% and 40%, respectively.The increase in the fraction of HAGBs from 30% (for 40% RR sample) to 40% (for 80% RR sample) during CR indicates the formation of new grains either by grain fragmentation or via self-annealing phenomena [4].ETP Cu has a relatively higher concentration of oxygen than OHFC Cu, leading to the formation of Cu2O particles inside the pure Cu matrix [8].The presence of these hard Cu2O particles can be observed in the IQ map, marked with yellow arrows (figure 1(b)).Additionally, the formation of stain localization (SLs) was observed in the CR samples.These are inclined GBs along the RD and could be formed due to increased orientation gradients inside the parent grains.During self-annealing, these boundaries will respond early to the nucleation of new grains.Both features, i.e., SLs and hard-particles, responded to self-annealing and evolved the SRX grains at RT [4].Figure 1(c) exhibits self-annealed grains in 80% CR ETP Cu.GOS1.5 and CI>0.1 were used to partition self-annealed grains from the deformed grains.Many nucleated grains were observed either at the deformed GBs or around oxide particles.During deformation, dislocations accumulate at the GBs or other interfaces.This accumulation increases the dislocation density at the GBs, which in turn enhances the stored energy at boundaries.These GBs can be referred to as the GBs of deformed grains or deformed GBs.New grain formation at these deformed GBs occurs during the self-annealing phenomena.
Figure 1(d) illustrates GBs bulging (marked with a black arrow), which could signify the prelude for discontinuous SRX (DSRX) phenomena.Bulges in GBs were formed due to the pinning effect of dislocations to the GBs [9].Weygand et al. [10] discussed the role of bulged GBs on nucleation during SRX phenomena.The simulation results reported by the authors in [10] emphasized the importance of topological factors in triggering the bulging mechanism.Furthermore, anisotropy of GBs energy and mobility in the subgrain boundary population is also a decisive factor for the bulging mechanism.The high magnification image in figure 1(e) shows SLs (red colour boundaries), which have discontinuous LAGBs.Differences between the SLs and SBs can be observed based on the severity of deformation, in which SBs experience much higher deformation compared to SLs.Consequently, stored energy (SE) at the SBs regions will be higher than that of the SLs regions [4].Severe deformation could lead to the accumulation of a large number of dislocations that bow around Cu2O particles, forming an Orowan loop.A high density of dislocation enhances the SE around Cu2O particles, which could further cause the nucleation of small grains around these Cu2O particles during the RT recrystallization.The formation of new grains around the hard Cu2O particles is known as PSN [11].Figure 1(f) shows that SRX was observed not only at the deformed GBs but also around Cu2O particles.Humphreys [11] discussed the nucleation of grains at large second-phase particles in deformed Al single crystals in 1977, finding that PSN occurs above a critical particle size which increases with decreasing strain.SRX initiates within a zone of high dislocation density and large lattice misorientation around particles, and proceeds via a rapid polygonization process [11].A detailed description of self-annealing phenomena and microstructure evolution in RTR and CR ETP Cu can be found elsewhere [4].
The relationship between self-annealed grains and crystallographic texture has been reported for RTR and CR processed Cu. 80% CR ETP Cu exhibited the evolution of strong plane strain texture components [4].Brass, Copper, and S texture intensities were getting stronger with the increase in RR [4].Additionally, texture evolution for the self-annealed grains in 80% RTR and CR specimens showed the development of plane strain texture.Hence, both deformed and recrystallized grains evolved plane strain texture components [4].SRX phenomena can be divided into two types: (1) DSRX and (2) continuous SRX (CSRX).DSRX grains form at deformed GBs, in which new grains develop via HAGBs [12], whereas CSRX grains form due to progressive rotation of sub-grains [13].To understand the nucleation mechanism of these self-annealed grains, detailed analysis was performed using the high-resolution EBSD micrographs.Figure 2(a) provides details about the selfannealed grain in 80% CR Cu. Figure 2(a,b) demonstrates that self-annealed grains either evolved at deformed GBs or inside deformed grains, confirmed through GOS≤1.5ºcriteria.As seen in figure 2(b), the nucleated grains 1-3 formed inside deformed grains due to higher orientation gradients, while grains 4, 5, 7 formed at deformed GBs.The magnified image in figure 2(a) displays the positions of self-annealed grains (1-3) within the deformed matrix.Since CSRX grains formed due to the transformation of LAGBs to HAGBs, there could be some GBs that possess intermediate GBs (10-15º).As a result, CSRX grains might exhibit combined GBs [14].The (100)-pole figure map (figure 2(c)) of self-annealed grains and their neighboring grains indicates that the grains nucleated at deformed GBs (4,5,7) exhibited DSRX behavior, while those grains nucleated due to orientation gradient (1,2,3) displayed CSRX behavior.The magnified region of figure 2(a) reveals that grain 1, 2, and 3, formed due to CSRX phenomena, showed combined GBs of LABs (I; 2-10º), LABs (II; 10-15º), and HAGBs (15-65º).A large orientation gradient (figure 2(d)) was observed along line 1, causing the nucleation of new grains at RT inside deformed grains.Furthermore, other locations were analyzed to observe self-annealed grains.Figure 2(e) displayed a nucleated grain inside deformed grains, showing HAGBs (grain 1-3), which was also confirmed via GOS criteria.The (100)-pole figure confirmed that nucleated and its neighboring grains had nearby orientations, verifying the CSRX phenomena.Figure 2(g) displays the cumulative misorientation angle along line 2 as 16º, and the grains formed via CSRX showed combined grains boundaries (seen in the magnified region of IPF).Therefore, both CSRX and DSRX phenomena were observed, causing nucleation of grains at RT.

Self annealing in Cu-Fe-P alloys
Cu-alloys may also exhibit self-annealed grains, as they have lower recrystallization temperature [15].However, no previous reports discussing self-annealing in Cu-Fe-P alloys are available until 2022.Severe deformation, either during the RTR or CR, can increase dislocations density and enhance stored energy.Our group reported the self-annealing phenomena in Cu-Fe-P alloys for the first time [7].Samples were kept at -22C after the rolling deformation.However, samples preparation and TEM measurement was done at RT immediately after removing from the -22C.
Cu-Fe-P alloy samples were subjected to 80% RR by either RTR or CR deformation routes [16].TEM investigations revealed the presence of self-annealed grains in both samples.These alloys are known as precipitation-hardened alloys since they are strengthened by the presence of Fe3P/Fe2P/Fe precipitates [7,17].Similar precipitates can be seen in the TEM micrographs in figure 3.These Fe2P precipitates, marked with red arrows, are spherical in shapes.Since they do not deform during the RTR/CR process, this indicates the high strength of these precipitates.Most Fe2P precipitates were observed inside the deformed or self-annealed grains, not at the GBs.These precipitates are useful for pinning the GBs during annealing treatment, to maintain work-hardening effects.However, in this situation, Fe2P fractions were not significant enough to affect the self-annealing phenomena in Cu-Fe-P, due to their absence at GBs [16].Figure 3(a-d) shows TEM micrographs of the 80% RTR and CR Cu-Fe-P samples, with self-annealed grains marked with yellow arrows.Figure 3(a) exhibits the CSRX grains with LAGBs featured, while figure 3(b-d) presents DSRX grains surrounded by HAGBs and high dislocations density.GBs characteristics were analysed using convergent-beam electron diffraction techniques, with details provided elsewhere [7,16].Self-annealing phenomena could also affect the primary recrystallization kinetics of these alloys.Therefore, information about the mechanism of nucleation, i.e., CSRX and DSRX, will be very useful.It is vital to note that numerous reports exist on the RTR deformation of Cu alloys [18,19], but none discuss self-annealing phenomena.During plastic deformation, the formation of SLs occurred in Cu-Fe-P alloys.These regions of high dislocations density were inclined at 25-45 along the RD and formed due to plastic instability [7].As a result, SLs could be significant locations for self-annealing to occur [7,16].

Self annealing in Cu-Sn-Mg alloys
Cu-Sn based alloys are investigated for high strength and high conductivity properties.These alloys are generally used in applications such as conducting wires, railway contact wires, electrical connectors, and lead frames.The formation of shear bands (SBs)/SLs and mechanical twins has also been reported in the RTR deformation of Cu-based alloys [20,21].These SBs are inclined GBs, comprising highly deformed region, and can be considered prime locations for self-annealing, as seen in figure 4. Various characteristics of self-annealing in a 75% RTR Cu-0.13Sn-0.04Mgalloy sample are discussed in figure 4. Inclined SBs consisting of HAGBs and LAGBs can be seen in the IPF map.A high fraction of subgrains/LAGBs (red color boundaries, 42%) was observed after the 75% RTR (figure 4(b)).Deformed grains had an average GS of 2816 m.A similar region was taken for the observation of self-annealed grains, as shown in figure 4(c).GOS1.5 and CI>0.1 were used to partition self-annealed grains from the deformed microstructure.As evident from the microstructure, many small grains nucleated due to the self-annealing phenomena.Nucleation of small grains was observed at the SBs regions (figure 4(c)).TEM investigation of this alloy shows the homogenous distribution of Cu, Sn, Mg and O in the microstructure, indicating no precipitates formation.All these elements strengthened the materials via solid-solution hardening rather than precipitation hardening.A high amount of stored deformation energy generated during the heavy cold-rolling is the main cause of the self-annealing behavior in Cu-Sn-Mg alloys.The high-magnification region displayed small grains at the SBs region with different orientations compared to the parent matrix (figure 4(d)).SBs in the respective figures are marked with yellow arrows.Figure 4(f) demonstrates the formation of selfannealing grains at SBs regions.However, some grains at SBs were also found to have formed due to shearing.Cu-Sn-Mg alloy samples exhibited differences in microstructural features compared to what was observed in ETP Cu and Cu-Fe-P alloys [3,7,22].Microstructure distribution was more heterogenous in the case of RTR Cu-Sn-Mg.Heterogeneities indicates that some grains were deformed more severely compared to others and a few grains showed preferential formation of SBs, while others did not.IPF-maps seen in figure 1(a) and figure 4(a) clearly show the differences in the deformed microstructure, which was due to the prior thermomechanical processing of ETP Cu and Cu-Sn-Mg alloy, respectively.

Conclusions
Self-annealing phenomena were observed in highly deformed ETP Cu and Cu-Fe-P alloys.Softening occurred when the deformed samples were exposed to RT.The formation of self-annealed grains was observed at deformed GBs, triple junction boundaries, resulting from the DSRX mechanism.In contrast, the formation of grains inside the deformed/parent grains was due to the CSRX mechanism.PSN was also observed in ETP Cu, causing the nucleation of grains around the hard Cu2O particles.PSN led to the evolution of new grains via DSRX.Highly deformed Cu-Fe-P alloy samples shows the presence of self-annealed grains.Both RTR and CR deformation routes caused the evolution of SRX grains in Cu-Fe-P alloys, which was confirmed through TEM analysis.No PSN was observed in Cu-Fe-P due to the presence of nano-sized Fe2P precipitates.The formation of SBs was observed in the Cu-Sn-Mg alloy sample, which are regions of high SE, supporting self-annealing at those locations.

Figure 1 .
Figure 1.(a) ND-IPF map of 80% CR ETP Cu shows the bulge GBs, (b) formation of SLs and accumulations of dislocations/sub-boundaries around the Cu2O particles, (c) evolution of self-annealed grains at the deformed GBs and PSN around the Cu2O particles in 80% CR ETP Cu; Higher magnification regions shows (d) bulge GBs (marked with black arrows), (e) inclined SLs boundaries (marked with yellow arrows), and (f) nucleated grains around the Cu2O particles.Note: CR samples were kept at RT just after rolling and all the metallographic sample preparations were done at RT after 1 month of rolling.GOS1.5 was used to partition the self-annealed grains in (c) and (f).

Figure 2 .
Figure 2. (a) ND-IPF map 80% CR ETP Cu, (b) IPF map shows the SRX grains, (c) (100) pole figure shows CSRX (inside the grains) and DSRX (at the deformed GBs) phenomena, (d) misorientation angle distribution along line 1 in magnified region of (a), (e,f) ND-IPF map of SRX grains formed inside the deformed matrix and its (100) pole figure map, and (g) grain formation inside the deformed grains due to progressive rotation of sub-grains.Note: CR samples were kept at RT just after rolling and all the metallographic samples preparation was done at RT after 1 month of rolling.GOS1.5 was used to partition the self-annealed grains in (b) and (e).

Figure 3 .
Figure 3. TEM observation of selfannealing phenomena in 80% RTR and CR Cu-Fe-P alloy specimens.(a) BF-STEM shows the presence of nano-sized spherical Fe2P precipitates and CSRX grain with LAGBs, (b) formation of DSRX grains surrounded by high dislocations density region and presence of Fe2P inside the DSRX grains in 80% RTR Cu-Fe-P alloy specimens; (c) DSRX grains with HAGBs, (d) DSRX grains with HAGBs and surrounded by high dislocations density in 80% CR Cu-Fe-P alloy specimens.Note: Samples were kept at -22C after the rolling deformation.However, samples preparation and TEM measurement was done at RT immediately after removing from the -22C.

Figure 4 .
Figure 4. (a,b) ND-IPF and IQ maps shows the SBs and new orientations grain evolution at SBs in 75% RTR Cu-0.13Sn-0.04Mg,(c) evolution of self-annealed grains at the SBs in 75% RTR Cu-0.13Sn-0.04Mg;Higher magnification region shows (d) new grains evolution at SBs, (e) self-annealed grains formation at SBs. Note: Samples were kept at RT just after the rolling; And metallographic preparation was also performed at the RT.EBSD measurement done after 1 month of rolling.GOS1.5 was used to partitioned the self-annealed grains.