Strengthening mechanisms in a high-strength bulk nanostructured Cu–Zn–Al alloy processed via cryomilling and spark plasma sintering

Abstract

A bulk nanostructured alloy with the nominal composition Cu–30Zn–0.8Al wt.% (commercial designation brass 260) was fabricated by cryomilling of brass powders and subsequent spark plasma sintering (SPS) of the cryomilled powders, yielding a compressive yield strength of 950 MPa, which is significantly higher than the yield strength of commercial brass 260 alloys (∼200–400 MPa). Transmission electron microscopy investigations revealed that cryomilling results in an average grain diameter of 26 nm and a high density of deformation twins. Nearly fully dense bulk samples were obtained after SPS of cryomilled powders, with average grain diameter 110 nm. After SPS, 10 vol.% of twins is retained with average twin thickness 30 nm. Three-dimensional atom-probe tomography studies demonstrate that the distribution of Al is highly inhomogeneous in the sintered bulk samples, and Al-containing precipitates including Al(Cu,Zn)–O–N, Al–O–N and Al–N are distributed in the matrix. The precipitates have an average diameter of 1.7 nm and a volume fraction of 0.39%. Quantitative calculations were performed for different strengthening contributions in the sintered bulk samples, including grain boundary, twin boundary, precipitate, dislocation and solid-solution strengthening. Results from the analyses demonstrate that precipitate and grain boundary strengthening are the dominant strengthening mechanisms, and the calculated overall yield strength is in reasonable agreement with the experimentally determined compressive yield strength.

Introduction

The family of Cu–Zn alloys, known as brasses, represents one of the most widely used classes of materials in the world. Because of their unique combination of properties, specifically strength, ductility, machinability, electrical conductivity, corrosion resistance and cost effectiveness, brasses are heavily used in a wide range of applications, including automotive, building construction, electrical, ammunition, precision engineering industry and in high-strength, high-integrity equipment used in mines [1]. For a Zn concentration of approximately <35 wt.%, brass is a single-phase face-centered cubic (fcc) substitutional alloy. The C260 brass (CuZn30) with a nominal composition of 70Cu–30Zn wt.% is the most popular brass [1]. The industrial 260 alloy contains ∼0.8 wt.% Al, which is also in solid solution, to increase the strength and corrosion resistance, with a yield strength of ∼200–400 MPa, depending on processing conditions [1]. This level of strength, however, limits the range of potential applications, and hence it is highly desirable to improve the strength of brass.

Dislocations, grain boundaries (GB), solute atoms and second-phase particles are barriers that impede dislocation motion and, therefore, they constitute the main types of strengthening mechanisms in metallic materials. GB strengthening is an effective strengthening mechanism, and bulk nanocrystalline (nc) materials (grain diameter ⩽100 nm) with a significant volume fraction of GB have emerged as a new class of materials, which has drastically improved strength over their conventional coarse-grained (CG) counterparts [2]. Additionally, coherent twin boundaries (TB) inside grain interiors have a strengthening effect similar to that of GB, and ultrafine-grained (UFG) copper containing a high density of growth twins, with nanoscale twin thickness, possesses ultrahigh strength [3], [4], [5]. Since a lower stacking fault energy (SFE) leads to easier formation of both growth and deformation twins, adding alloying elements, such as Zn and Al to Cu, reduces the SFE and has emerged as a viable strategy to tailor mechanical behavior [6], [7], [8], [9], [10], [11], [12], [13]. Brass 260 alloy is also therefore scientifically interesting. UFG materials (100 nm < grain diameter < 1 μm) or nc materials, processed by severe plastic deformation (SPD) techniques, usually have a high dislocation density, which also contributes to significantly improved strength [14], [15]. Second-phase particle strengthening, usually in the form of precipitation strengthening, has been used as the major strengthening mechanism in CG or UFG Al-alloys and steels, with a precipitate diameter of a few nanometers [16], [17], [18], [19], [20]. Different strengthening mechanisms may contribute simultaneously to the overall strength of real materials, but few studies have quantified their respective contributions [21], [22], [23]. Understanding the respective quantitative contributions of different strengthening mechanisms provides useful guidelines that may be used to engineer high-strength materials with multiple strengthening mechanisms.

The cost-effective fabrication of bulk UFG or nc metallic materials, with the physical dimensions required for many engineering applications, remains a formidable barrier for the implementation of this novel class of materials. It is generally challenging to attain grain sizes <100 nm via equal-channel angular pressing (ECAP) or high pressure torsion (HPT), the two most prominent examples of SPD techniques [14], [15]. Electrodeposition is generally limited by material chemistry, with Ni–Fe, Ni and Cu usually produced as foils or thin films [3], [4], [24]. The family of SPD techniques includes powder milling methods, which can achieve grain sizes on the order of ∼20 nm; nanostructured milled powders can be consolidated into bulk form with large sample dimensions [25]. The high temperature and extended exposure time required for conventional sintering/consolidation processes, however, usually lead to significant grain growth, causing degradation in the bulk strength [26]. Spark plasma sintering (SPS) is a consolidation technique involving the simultaneous application of current and pressure. When SPS is applied to nc powders, densification can be achieved at significantly lower temperatures and shorter times than conventional sintering, thereby limiting grain growth and preserving the microstructure [27]. Because metallic powders possess a high affinity for oxygen, and powders are exposed to nitrogen during milling (for cryomilling in liquid nitrogen) or during handling in a laboratory environment, oxygen and nitrogen in certain forms (e.g., oxides and/or nitrides), which are regarded as impurities, will be present in the powders and in the sintered bulk samples. It is established that oxides, nitrides and other second-phase particles influence mechanical behavior, but quantitative information on their influence is lacking [23], [28], [29].

The present study was undertaken with three primary objectives: first, to fabricate bulk nc brass 260 alloy with a high strength; second, to characterize all microstructural features, including second-phase particles; and third, to quantify the contributions of different strengthening mechanisms, calculate the overall strength and validate experimentally the calculated yield strength. This was accomplished by fabricating a brass 260 alloy using cryomilling and SPS. Microstructures of powders and bulk samples were characterized by transmission electron microscopy (TEM), X-ray diffraction (XRD) and atom probe tomography (APT). The mechanical behavior of sintered bulk samples was investigated using compressive tests, and the strengthening contributions were calculated using strengthening theories.