From Ni–P Metastable Alloy Nanoparticles to Bulk Submicrometer Grain-Sized MMCs with Tunable Mechanical and Magnetic Properties

In this study, submicrometer grain-sized metal matrix composites (MMCs) based on nickel were elaborated via a bottom-up strategy combining the polyol process and a non-conventional heat treatment route. First, four sets of nano-sized Ni–P metastable alloy nanopowders with an average particle size centered at 50, 100, 130, and 220 nm were prepared by the polyol process modified by the addition of hypophosphite (strong reducing agent) and heterogeneous nucleation using silver nitrate and platinum salt (nucleating agents). The heat treatment step was realized by reactive spark plasma sintering (R-SPS) at identical heat treatment conditions (600 °C, 53 MPa, and 10 min as holding time). R-SPS transformed the Ni–P metastable alloys into bulk submicrometer grain-sized MMCs with Ni as the matrix and Ni3P as the reinforcement. Mechanical and magnetic properties of the four MMC samples were found to be closely related to the grain size of the Ni matrix, which varied from 247 to 638 nm. Yield stress, maximum stress, and coercive field increased when the grain size decreased, while plastic strain and magnetization saturation decreased. The reinforcement Ni3P phase enhanced the mechanical characteristics of the composite. Crossover behavior was observed at around 350 nm Ni grain size, where a ductile and soft magnetic composite was tuned into a hard mechanical and semi-hard magnetic one.

Synthesis of a bulk nanostructured metastable Al alloy with extreme supersaturation of Mg

Nanostructuring of bulk metals is now well documented with the development of severe plastic deformation (SPD) for improving the physical and mechanical properties of engineering materials. Processing by high-pressure torsion (HPT), which was developed initially as a grain refinement technique, was extended recently to the mechanical bonding of dissimilar metals during nanostrcturing which generally involves significant microstructural heterogeneity. Here we introduce, for the first time, a bulk metastable Al-Mg supersaturated solid solution by the diffusion bonding of separate Al and Mg metal solids at room temperature using HPT. Exceptional hardness was achieved homogeneously throughout the metastable alloy with a record maximum supersaturated Mg content of ~38.5 at.% in the Al matrix having a grain size of ~35–40 nm. Our results demonstrate the synthesis of a bulk nanocrystalline metastable alloy with good microstructural stability at room temperature where such bulk solids are not yet reported for mechanical alloying by powder metallurgy.

Metastable alloy nanoparticles, metal-oxide nanocrescents and nanoshells generated by laser ablation in liquid solution: influence of the chemical environment on structure and composition

Metastable nanoalloys, metal-oxide nanocrescents and metal@oxide nanoshells are generated by laser ablation in liquid solution (LASiS).