Annealing behavior of Cu-7at.%Pd alloy deformed by cold rolling

Highlights

• Atypical hardening was appeared in cold deformed Cu-7at.%Pd alloy after low-temperature annealing.

• Significant microstructural changes occurred during the thermo-mechanical treatment.

• Strong segregation of palladium atoms along the grain boundaries during the annealing has been confirmed.

Abstract

The anneal hardening behavior of Cu-7at.%Pd alloy after deformation by cold rolling was investigated by means of hardness, microhardness, electrical conductivity, light optical microscopy (LOM), scanning electron microscopy (SEM), and energy dispersive spectroscopy (EDS). In this regard, the cast Cu-7at.%Pd alloy after quenching was deformed with 20% and 40% reductions and isochronally annealed between 100 °C and 700 °C. After annealing up to 400 °C, the deformed samples exhibited atypical anneal hardening behavior in combination with an increase in electrical conductivity. In addition, the recrystallization was substantially retarded by the applied thermo-mechanical treatment which included solution annealing, quenching, cold rolling, and annealing. The microstructural evolution during a low-temperature annealing proved a strong pinning of grain boundaries by solute palladium atoms, indicating that grain boundaries have an important role during the anneal hardening.

Introduction

The copper-palladium system belongs to binary systems which have an affinity for ordering [1]. According to the phase diagram, copper and palladium form a continuous series of fcc solid solution at higher temperatures. Under critical temperatures, depending on composition, some superlattices are formed [2]. Long range ordering of Cu₃Pd (α′) superlattice with a fcc structure of the L1₂ (Cu₃Au) type typically occurs in the concentration range of ∼12 wt.% Pd to 32 wt.% Pd [3,4]. One dimensional anti-phase domain (APD) or long period superlattice (1D LPS) was confirmed in alloys with ∼26 wt.%Pd - 39 wt.%Pd. Two-dimensional APD (or 2D LPS) occurs in alloys with ∼28 wt.%Pd – 43 wt.%Pd [4,5]. Both LPSs have ordered face-centered tetragonal structure of the ordered Cu₃Au type. In alloys with ∼49 wt.%Pd to 60 wt.% Pd, ordered lattice CuPd (β) with bcc structure of the B₂ (CsCl) type occurs [2,[6], [7], [8]].

The Cu-Pd alloys are used extensively in many applications such as oxidation catalysts [9], hydrogen separation membranes [10,11], functional materials [6], etc. A wide range of applications for these alloys is due to their unique properties such as good corrosion resistance and high electrical conductivity. Wider usage of these materials is limited by their lower mechanical properties. An improvement in mechanical properties can be achieved by described long-range ordering, which usually is not efficient enough [6]. However, Vitek and Warlimont [12] have shown a significant hardening in dilute Cu-Pd alloys, where long-range ordering does not occur generally. This hardening occurs only after cold deformation and annealing up to the recrystallization temperature, and it is called anneal hardening [12]. Anneal hardening effect was mostly studied in copper based alloys with aluminum [[13], [14], [15]], and it was manifested by an increase in the mechanical properties of annealed samples compared to deformed ones. It means that samples in annealed state have higher mechanical properties than in deformed state. Vitek and Warlimont [12] have confirmed the anneal hardening effect in some other binary copper based alloys with palladium, zinc, gold, gallium, nickel, and rhodium. Volkov et al. [16] obtained an increase in microhardness values of deformed Cu-8at.%Pd alloy after annealing at 200 °C and 250 °C for 15 min, but this anomaly during short time annealing was not considered in the paper but only prolonged annealing. However, other studies about the intensity and mechanism of anneal hardening effect in the Cu-Pd alloys could not be found in the literature. Therefore, in this investigation, the annealing was performed on the cold-rolled Cu-7at.%Pd samples to study the effect of thermo-mechanical treatment on the anneal hardening effect. This paper correlates the mechanical properties and electrical conductivity with microstructure during the thermo-mechanical treatment which included solution annealing, quenching, cold rolling, and annealing, and it evaluates the contribution of anneal hardening effect to the overall hardening of the cast Cu-7at.%Pd alloy.