N. Boucharat

New Routes for Synthesizing Massive Nanocrystalline Materials

New opportunities for fabricating massive nanocrystalline materials in bulk quantities are required for facilitating the transition of nanocrystalline solids from laboratory samples to technologically relevant materials. Advanced options might be based on combining different nonequilibrium processing routes sequentially, such that an initially metastable state is continuously energized and successively driven farer away from thermodynamic equilibrium. The current paper presents recent results on the evolution of nanoscaled microstructures resulting from combinations of different plastic deformation treatments or of vitrification and severe plastic deformation.

Development of nanocrystals in amorphous Al-alloys

Abstract Rapidly quenched aluminum-based metallic glasses have been found to undergo a partial transformation upon heating that yields a fine-scaled dispersion of Al-nanocrystals within an amorphous matrix. The origin of the nanocrystallisation process is discussed based upon calorimetric and structural investigations of Al92Sm8 samples prepared using different processing pathways. Additionally, the early stages of the nanocrystal development was monitored by isothermal calorimetry techniques on long time scales. The results strongly indicate that the nanocrystals originate from quenched-in nuclei. Isothermal annealing of Al88Y7Fe5 glasses at 245 °C revealed that α-Al nanocrystals develop at temperatures well below the primary crystallisation onset temperature after short time periods. On annealing, an additional phase evolves, which gives an exothermic signal in DSC at about 330 °C. XRD measurements have been performed to study the phase formation sequence at high temperature. The formation of additional phases is related to a modification of the matrix composition near the nanocrystal/amorphous interface due to a pronounced concentration gradient as a result of the unequal component diffusivities.

Deformation-Induced Nanocrystallization in Al-Rich Metallic Glasses

Deformation-induced nanocrystallization has been investigated in a marginally Al88Y7Fe5 glass forming alloy. Conventional calorimetry and microstructural analyses of materials that have been subjected to high pressure torsion straining (HPT) at room temperature indicate the development of an extremely high number density of small Al nanocrystals. The nanocrystals appear to be distributed homogeneously throughout the sample without any evidence of strong coarsening. Moreover, the comparison between nanocrystallization caused by the application of either HPT, cold-rolling or in-situ TEM tensile straining yielded the identification of the probable mechanisms underlying the formation of nanocrystals. These results form the basis for the development of advanced processing strategies for producing new nanostructures with high nanocrystal number densities which allow increased stability and improved performance.

Powder Metallurgical Processes for NiTi Shape Memory Alloys

Two promising powder metallurgy (PM) processes were used for the fabrication of NiTi shape memory alloys (SMA): Mechanically Activated Reactive FOrging Synthesis (MARFOS) and Mechanically Activated Reactive Extrusion Synthesis (MARES). In these two processes, equimolar powder mixtures of elemental Ni and Ti are first mechanically activated and then forged/extruded at relatively low temperature. Afterwards, heat treatments are used to promote homogenization and to adjust the composition of the NiTi matrix. When MARFOS and MARES processes are compared some differences have been observed but only in relation to the extent of phase transformation and to the degree of densification. The crystallite size was less than 100 nm for all the phases which indicates nanostructured materials and multi-step martensitic transformations could be observed in heat treated materials.