Carl C. Koch

The synthesis and structure of nanocrystalline materials produced by mechanical attrition: a review

Abstract The evolution of the structure of nanophase materials prepared by mechanical attrition is reviewed. Fundamental studies of the microstructural development in ball-milled pure elements have defined the sequence of events leading to nanocrystalline grains (average grain diameter 2 to 20 nm). The grain sizes are found to decrease with milling time down to a constant value which appears to scale with the melting temperature of the given element. This implies a balance between defect creation and recovery during deformation. The prominent observation for the mechanical attrition of binary immiscible systems is the very large metastable solid solubilities that are attained. These solubilities are presumably related to solute segregation at the nanocrystalline grain boundaries. Recent evidence from Mossbauer spectroscopy and EXAFS experiments supports this assumption. Although the problems of 1) contamination from milling media and/or atmosphere, and 2) powder consolidation—without coarsening—need to be solved, mechanical attrition offers the possibility of production of nanocrystalline materials in tonnage quantities.

Synthesis of nanostructured materials by mechanical milling: problems and opportunities

Abstract Mechanical attrition as a method to produce nanocrystalline (nc) materials is reviewed. Its advantages include the fact that all classes of materials — including brittle compounds — are amenable to the method; it can be easily scaled up to tonnage quantities. The phenomenology and suggested mechanisms for formation of nc microstructures are discussed for ball milling of single component powders, mechanical alloying of multi-component powders, and mechanical crystallization of amorphous alloys. The phenomenology is well documented but microscopic mechanisms await better understanding of the nature of deformation processes in nc materials. The problems of contamination and powder consolidation are briefly considered.

Optimization of strength and ductility in nanocrystalline and ultrafine grained metals

Abstract The possible optimization of strength and ductility in nanocrystalline and ultrafine grained metals is assessed. Most nanocrystalline metals show little ductility while exhibiting enhanced strength. The possibility that these properties can be optimized is discussed for microstructures with appropriate grain size distributions, and the introduction of second phase particles.

Formation of amorphous alloys by the mechanical alloying of crystalline powders of pure metals and powders of intermetallics

Amorphous powders of Ni32Ti68 and of Ni45Nb55 were synthesized by mechanical alloying (MA) starting from either a mixture of pure metal powders (in the appropriate molar ratio) or from powders of the crystalline intermetallics NiTi2 and Ni45Nb55, respectively. For both alloys, the peak temperature increase (above the average processing temperature) in the powder particles trapped between colliding balls is estimated at 38 K. Thus, the amorphization is attributed to a process other than the formation of local melts followed by the rapid solidification of these melts into the amorphous phase. The amorphization by MA starting from a mixture of pure crystalline powders is attributed to a solid state interdiffusion reaction, the kinetics of which is controlled by the excess point and lattice defects generated by plastic deformation. The amorphization by MA starting from powders of crystalline intermetallics is attributed to the accumulation of point and lattice defects which raise the free energy of the faulted intermetallic above that of the amorphous alloy.

Grain growth in nanocrystalline iron prepared by mechanical attrition

Abstract The grain growth in nanocrystalline Fe produced by high energy ball milling is investigated. Grain growth data are analysed using two different models of grain growth, one of which takes pinning forces on the grain boundaries into account. The grain growth exponents n D 1 n − D 1 n o = kt for nanocrystalline and conventional polycrystalline Fe are compared. The use of the above mentioned equation yields an activation energy of 125 kJ/mol, while the second model gives 248 kJ/mol. These values are compared to those for grain boundary and lattice diffusion in Fe. Some evidence for two different sets of mechanisms governing the grain growth in nanocrystalline Fe are discussed.

Ultrahigh strength and high ductility of bulk nanocrystalline copper

We have synthesized artifact-free bulk nanocrystalline copper samples with a narrow grain size distribution (mean grain size of 23nm) that exhibited tensile yield strength about 11 times higher than that of conventional coarse-grained copper, while retaining a 14% uniform tensile elongation. In situ dynamic straining transmission electron microscope observations of the nanocrystalline copper are also reported, which showed individual dislocation motion and dislocation pile-ups. This suggests a dislocation-controlled deformation mechanism that allows for the high strain hardening observed. Trapped dislocations are observed in the individual nanograins.

Mechanical alloying of brittle materials

Mechanical alloying by high energy ball milling has been observed in systems with nominally brittle components. The phases formed by mechanical alloying of brittle components include solid solutions (Si + Ge → SiGe solid solution), intermetallic compounds (Mn + Bi → MnBi), and amorphous alloys (NiZr2 + Ni11Zr9 → amorphous Ni50Zr50). A key feature of possible mechanisms for mechanical alloying of brittle components is the temperature of the powders during milling. Experiments and a computer model of the kinetics of mechanical alloying were carried out in order to esti-mate the temperature effect. Temperature rises in typical powder alloys during milling in a SPEX mill were estimated to be ≤350 K using the kinetic parameters determined from the computer model. The tempering response of fresh martensite in an Fe-1.2 wt pct C alloy during milling was consistent with the maximum results of the computer model, yielding temperatures in the pow-ders of ≤575 Ki.e., ΔT ≤ 300 K). Thermal activation was required for mechanical alloying of Si and Ge powder. No alloying occurred when the milling vial was cooled by liquid nitrogen. The pos-sible mechanisms responsible for material transfer during mechanical alloying of brittle components are considered.

Nanocrystalline materials - Current research and future directions

Nanocrystalline materials, with a grain size of typically <100 nm, are a new class of materials with properties vastly different from and often superior to those of the conventional coarse-grained materials. These materials can be synthesized by a number of different techniques and the grain size, morphology, and composition can be controlled by controlling the process parameters. In comparison to the coarse-grained materials, nanocrystalline materials show higher strength and hardness, enhanced diffusivity, and superior soft and hard magnetic properties. Limited quantities of these materials are presently produced and marketed in the US, Canada, and elsewhere. Applications for these materials are being actively explored. The present article discusses the synthesis, structure, thermal stability, properties, and potential applications of nanocrystalline materials.

Amorphization and disordering of the Ni 3 Al ordered intermetallic by mechanical milling

The ordered fcc intermetallic compound Ni 3 Al was mechanically milled in a high energy ball mill. The severe plastic deformation produced by milling induced transformations with increasing milling time as follows: ordered fcc → 2; disordered fcc → 2; nanocrystalline fcc + amorphous. The milling time for complete disordering occurred at 5 h for stoichiometric Ni 3 Al milled at ambient temperature compared to 50 h for the first observation of an amorphous structure. The structural and microstructural evolution with milling time was followed by x-ray diffraction, TEM, hardness, and calorimetry. The major defect believed responsible for inducing the crystalline-to-amorphous transformation is the fine grain boundary structure with nanometer (∼2 nm diameter) dimensions. The calculated interfacial free energy of the grain boundaries is consistent with the estimated free energy difference between the fcc and amorphous phases in Ni 3 Al.

Mechanical milling/alloying of intermetallics

Abstract Recent research on the structure and properties of intermetallic compounds processed by ball milling—i.e. mechanical attrition—is reviewed. Mechanical attrition includes synthesis of intermetallics from the elemental powders of their constituents (mechanical alloying) and from milling the compound powder (mechanical milling) to provide changes in structure and/or microstructure. Mechanical attrition has been used to synthesize intermetallic compound compositions for a variety of reasons: for example (1) materials difficult to produce by conventional solidification because of phase equilibria constraints; (2) introduction of unique microstructures to enhance certain properties, and (3) formation of metastable structures at intermetallic compositions. The properties of intermetallics modified by the structures/microstructures induced by mechanical attrition include mechanical behavior, hard magnetic behavior, hydrogen storage, and catalysis. In addition the extensive plastic deformation observed by TEM in milled nominally brittle intermetallics may offer a means of studying deformation behavior in very brittle intermetallics at essentially ambient temperature.