Abstract
Nano-crystalline metals have attracted considerable attention over the past two decades due to their increased mechanical properties as compared to their microcrystalline counterparts. However, the behaviour of nano-crystalline metals is influenced by imperfections introduced during synthesis or heat treatment. These imperfections include pores, which are mostly located in the area of grain boundaries. To study the behaviour of multiphase nano-crystalline materials, a novel fully parametric algorithm was developed. The data required for implementing the developed numerical model were the volume fraction of the alloying elements and their basic properties as well as the density and the size of randomly distributed pores. To validate the developed algorithm, the alloy composition 75 wt% tungsten and 25 wt% copper was examined experimentally under compression tests. For the investigation, two batches of specimens were used; a batch having a coarse-grained microstructure with an average grain diameter of 150 nm and a nanocrystalline batch having a grain diameter of 100 nm, respectively. The porosity of both batches was derived to range between 9% and 10% based on X-ray diffraction analyses. The results of quasi-static compression testing revealed that the nanocrystalline W-Cu material exhibited brittle behaviour which was characterised by an elastic deformation that led to fracture without remarkable plasticity. A compressive strength of about 1100 MPa was derived which was more than double compared to conventional W-Cu samples. Finite element simulations of the behaviour of porous nano-crystalline materials were performed and compared with the respective experimental compression tests. The numerical model and experimental observations were in good agreement.
Highlights
Owing to the desirable properties of materials with nanocrystalline microstructure, notable attempts have been invested in recent years to their production and characterisation, e.g., [1,2,3]
Various manufacturing methods have been applied to obtain alloys with ultrafine-grain (UFG, 100 nm < d < 500 nm) or nanocrystalline grain including electrodeposition [2,4], powder metallurgy [3,5], inert gas condensation followed by consolidation of powders [2] and severe plastic deformation (SPD) [6,7]
SPD is a “one-step” manufacturing technique that begins with a bulk workpiece with coarse-grained microstructure and refines its grain size down into the ultrafine or nanocrystalline regime with increased plastic straining
Summary
Owing to the desirable properties of materials with nanocrystalline (nc) microstructure (grain size d < 100 nm), notable attempts have been invested in recent years to their production and characterisation, e.g., [1,2,3]. SPD is a “one-step” manufacturing technique that begins with a bulk workpiece with coarse-grained microstructure and refines its grain size down into the ultrafine or nanocrystalline regime with increased plastic straining. The SPD method is a cost-effective technology with the absence of contaminations for the production of bulk ultrafine grain metals and alloys. With respect to limitations in the manufacturing methods of nano-crystalline materials, their mass production remains up to now difficult and has not been industrialised. For the Metals 2020, 10, 821; doi:10.3390/met10060821 www.mdpi.com/journal/metals
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