Bulk Nanocrystalline Materials Research Articles

Structure and Mechanical Behavior of Bulk Nanocrystalline Materials

The reduction of grain size to the nanometer range (˜2-100 nm) has led to many interesting materials properties, including those involving mechanical behavior. In the case of metals, the Hall-Petch equation, which relates the yield stress to the inverse square root of the grain size, predicts great increases in strength with grain refinement. On the other hand, theory indicates that the high volume fraction of interfacial regions leads to increased deformation by grain-boundary sliding in metals with grain size in the low end of the nanocrystalline range. Nanocrystalline ceramics also have desirable properties. Chief among these are lower sintering temperatures and enhanced strain to failure. These two properties acting in combination allow for some unique applications, such as low-temperature diffusion bonding (the direct joining of ceramics to each other using moderate temperatures and pressures). Mechanical properties sometimes are affected by the fact that ceramics in a fine-grained form are stable in a different (usually higher pressure) phase than that which is considered “normal” for the ceramic. To the extent that the mechanical properties of a ceramic are dependent on its crystal-lographic structure, these differences will become evident at the smaller size scales.It is uncertain how deformation takes place in very fine-grained nanocrystalline materials. It has been recognized for some time that the Hall-Petch relationship, which usually is explained on the basis of dislocation pileups at grain boundaries, must break down at grain sizes such that a grain cannot support a pileup. Even some of the basic assumptions of dislocation theory may no longer be appropriate in this size regime. Recently considerable progress has been made in simulating the behavior of extremely fine-grained metals under stress using molecular-dynamics techniques. Molecular-dynamics (MD) simulations of deformation in nanophase Ni and Cu were carried out in the temperature range of 300–500 K, at constant applied uniaxial tensile stresses between 0.05 GPa and 1.5 GPa, on samples with average grain sizes ranging from 3.4 nm to 12 nm.

Processing and mechanical properties of nanocrystalline 5083 al alloy

A commercial aluminum alloy, 5083, was processed using a cryomilling synthesis approach to produce nanocrystalline powders. The powders were subsequently degassed, HIPped, and extruded to produce a thermally stable bulk nanocrystalline material. The grain size at each processing step was measured utilizing both X-ray diffraction and transmission electron microscopy. The mechanical properties of the n-5083 extruded material were determined utilizing ASTM E8-93, Standard Test Methods for Tension Testing of Metallic Materials. The nanocrystalline 5083 was found to have a 30% increase in yield strength and ultimate strength over the strongest commercially available form of 5083 with no corresponding decrease in elongation. Grain boundary sliding is proposed as a possible mechanism for the observed enhanced ductility.

Fabrication of bulk amorphous alloys by powder consolidation: A.Inoue et al. (Tohoku University, Sendai, Japan.) Int. J. Powder Metall., Vol 33, No 2, 1997, 50–61

The first successful synthesis of glassy alloy compacts with full strength by warm extrusion of atomized glassy powder in the supercooled liquid state is reported. The effects of powder processing and the extrusion conditions on the mechanical properties of the compacts were investigated for amorphous Zr 65 Al l0 Ni l0 Cu l5 with a wide supercooled liquid region of 105K The tensile strength and Young's modulus of the amorphous powder compacts were 1520MPa and 80GPa, respectively which are similar to valves obtained in the as-cast bulk alloy and melt-spun ribbon. The high tensile strength is attributed to full densification, perfect bonding between powder particles and retention of ductility by extrusion at extrusion ratios above 4 in the supercooled liquid state. Powder processing, improved particle bonding, resulting in high strength at smaller extrusion ratios. The results of this study have major implications regarding the future development of bulk amorphous and nanocrystalline materials with complex shapes and novel properties.

Effect of high pressure on the preparation of Pd–Si–Cu bulk nanocrystalline material

A Pd–Si–Cu bulk nanocrystalline material was prepared by quenching the melted Pd78Si16Cu6 alloy at a cooling rate of 200 K/s under 2–6 GPa. It was found that the nanocrystalline material consists of Pd(Cu) disordered solid solution and a metastable phase-II, Pd4Si. The grain size was found to decrease with increasing pressure. The influence of high pressure on the grain size of bulk nanocrystalline material is discussed, and a possible formation mechanism is proposed.

Preparation of CuSi bulk nanometre alloy under high pressure

A CuSi bulk nanocrystalline material was prepared by quenching of the melted Cu 70Si 30 alloy at a cooling rate of 200 K s −1 under 6 GPa. It was found that the nanocrystalline consists of γ-Cu 5Si and an unknown phase. The study on the crystal structure and thermal stability of the unknown phase was carried out. The formation mechanism of bulk nanocrystalline material under high pressure was discussed.