Anneal hardening in cold rolled PM Cu-Au alloy Ivana Marković a,n , Svetlana Nestorović a,1 , Boštjan Markoli b , Milena Premović c , Sašo Šturm d a University of Belgrade, Technical Faculty in Bor, Vojske Jugoslavije 12,19210 Bor, Serbia b University of Ljubljana, Faculty of Natural Sciences and Engineering, Aškerčeva 12, 1000 Ljubljana, Slovenia c University in Priština, Faculty of Technical Science, Kneza Miloša 7, 38220 Kosovska Mitrovica, Serbia d Jožef Štefan Institute, Department for Nanostructured Materials, Jamova cesta 39, 1000 Ljubljana, Slovenia article info Article history: Received 13 November 2015 Received in revised form 8 February 2016 Accepted 9 February 2016 Available online 10 February 2016 Keywords: Anneal hardening Solute segregation Short-range ordering Hardening mechanism abstract Pure copper and Cu-11.4wt%Au alloy were obtained by a powder metallurgy (PM) technique and sub- jected to thermomechanical treatment that was used to produce the conditions that led to an anneal hardening effect in the alloy. The results showed that low-temperature annealing of cold-rolled alloy caused a two-stage increase in hardness, microhardness and electrical conductivity. The mechanism of anneal hardening in this alloy was studied using differential thermal analysis (DTA), X-ray diffraction (XRD) and transmission electron microscopy (TEM). The analysis of the DTA curve showed the presence of one endothermic and three exothermic reactions during the linear heating, which confirmed the occurrence of both short-range ordering and solute segregation. A decrease in the lattice parameter of the cold-rolled αCu-Au solid solution at the annealing temperature, which corresponded to the second hardness peak, was explained by solute clustering. Formation of precipitates in the copper-based matrix during annealing was not observed by TEM. It was confirmed that the dominant hardening mechanism during annealing was solute segregation towards lattice defects. & 2016 Elsevier B.V. All rights reserved. 1. Introduction It is well known that a softening process occurs in cold de- formed metals and alloys during annealing as a result of recovery and recrystallization [1–5]. However, in the 1950s, a typical be- haviour of some cold deformed copper alloys was found during annealing, i.e. during annealing, strengthening was found to occur up to the recrystallization temperature. The described phenom- enon was termed “anneal hardening” by Nasiguti [6]. Since then, the anneal hardening effect has been found in several binary copper-based alloys with aluminium, gold, gallium, nickel, palla- dium, rhodium and zinc [7,8]. However, the origin of this phe- nomenon has not been fully explained. Some explanations have been offered by various research groups, among which two basic mechanisms are the most acceptable. The first mechanism in- volves the formation of ordered domains, which takes the de- crease in the dislocation movement into account. Sugino et al. [9] introduced the concept of a partial long-range order of the Cu 3 Al superlattice as an explanation of the anneal hardening phenom- enon in Cu-Al alloys. Using XRD and microhardness and specific heat measurements, Kuwano et al. [10] showed that annealing influenced the recovery of short-range ordering in cold-worked Cu-14.2 at%Al alloys and growth of coherent domains. However, the second mechanism related to the interactions of solute atoms with lattice defects represents the dominant strengthening me- chanism. Many authors have suggested that the combined effect of these mechanisms on properties change during the annealing of cold-deformed one-phase copper alloys. Popplewell and Crane [11] found that strengthening in Cu-Al alloys was associated with the development of ordered regions and Suzuki locking. Stacking faults had a very important role, representing the preferential nucleation sites for short-range ordering and chemical segregation of solute atoms. Bader et al. [8] cited the mutual interaction of the ordering effect and the solute segregation to lattice defects (va- cancies, dislocations and stacking faults). They stated that the anneal hardening effect in Cu-16 at%Al alloys was predominantly related to the segregation of solute atoms to dislocations, but the formation of ordered clusters, especially at the dissociated dis- locations, could not be excluded. Aruga et al. [12] studied low- temperature anneal hardening in very dilute Cu-Fe-P alloys with alloying concentrations of about 0.01%. They confirmed the de- crease in dislocation density by a half and the increase in volume fraction of Fe-P clusters for a quadruple in surface regions. According to the available literature, the anneal hardening ef- fect has not been studied in Cu-Au alloys, except for the paper by Contents lists available at ScienceDirect journal homepage: www.elsevier.com/locate/msea Materials Science & Engineering A http://dx.doi.org/10.1016/j.msea.2016.02.029 0921-5093/& 2016 Elsevier B.V. All rights reserved. n Corresponding author. E-mail address: imarkovic@tf.bor.ac.rs (I. Marković). 1 Deceased 3rd December 2015. Materials Science & Engineering A 658 (2016) 393–399