Structural Evolution during Reactive Mechanical Milling of TiC/Ti‐Al Nanocomposites

Abstract

High-energy Mechanical Alloying (MA) has been used successfully to produce numerous equilibrium or nonequilibrium alloy phases starting from blended elemental or pre-alloyed powders. The non-equilibrium phases synthesized include supersaturated solid solutions, metastable crystalline and quasicrystalline phases, nanostructures, and amorphous alloys. [1][2][3] MA, as a non-equilibrium, low temperature, and solid-state powder treatment process, involves repeated cold-welding, fracturing, and re-welding of powder particles in a high-energy ball mill.Titanium aluminides (TiAl) are of great importance for intermediate-temperature (600 to 850 8C) aerospace and power-generation applications because they offer significant weight savings over today's nickel-based superalloys. [4] Furthermore, TiAl alloys possess a unique combination of high specific strength, high elastic modulus, and excellent antioxidation capabilities. [5] However, the limited roomtemperature ductility and the decreased high-temperature strength are the most-significant impediments to broadening their practical applications. [6] In order to improve their ductility, recent research efforts have focused on the preparation of ultrafine grained nanocrystalline TiAl. [7] On the other hand, the synthesis of ceramic reinforced TiAl-matrix nanocomposites is an important consideration in improving their high-temperature performance. In particular, the preparation of in situ particulate-reinforced nanocomposites is regarded as the most-promising method. [8] The ultrafine particulates that are formed in situ are thermally stable and possess coherent and compatible interfaces with the matrix, thereby assuring that the composite system has enough strength to transfer stress.MA provides us with an innovative process of the in situ preparation of ceramic-particulate-reinforced nanocomposites materials. Ultrafine, nanometer-scaled grain sizes coupled with a uniform distribution of the reinforcing phases are expected to improve the obtainable mechanical properties of composites prepared by the MA route. Nevertheless, MA is a complex process and accordingly involves a large degree of uncertainty in obtaining the desired phases and microstructures. Significant experimental research is still required to study how the phases, microstructures, and compositions of MA-processed powder are evolved during ball milling. A theoretical understanding of the formation mechanisms behind the microstructural development of milled powders is also regarded as necessary.In this work, TiC/Ti-Al nanocomposite powders were synthesized in situ by MA of a mixture consisting of elemental Ti, Al, and graphite powder, with a nominal composition of Ti 50 Al 25 C 25 . The phase, microstructure, and composition transformations of the milled powders were characterized and reasonable mechanisms for the powder formation during the reactive MA process are proposed. Results and DiscussionThe X-ray diffraction (XRD) spectra of the starting elemental-powder mixture and the milled powders at different MA ti...

Structural evolution and formation mechanisms of TiC/Ti nanocomposites prepared by high-energy mechanical alloying

“…In this study, the TiC reinforcing phase is exteriorly added and the ball milled TiC/Ti nanocomposite powder experiences the above-mentioned structural evolution (figures 4 and 10) due to the competitive predominance between the cold-welding and fracturing mechanisms. On the other hand, our previous work on high-energy MA of a Ti-Al-graphite elemental powder mixture reveals that the in situ formed TiC particulate reinforced Ti(Al) solid solution matrix nanocomposite powder can be produced [16]. In this instance, the competition between cold welding and fracturing of particles is also found to account for the microstructural alteration of the milled powders.…”

“…For the TiC-Ti system as investigated, a successive hcp → fcc → amorphous phase transformation of the Ti constituent is detected during milling and the dominant physical mechanisms of such a transformation have been explained in detail in section 4.1. Differently, for the Ti-Al-graphite in situ synthesis system, the TiC phase is produced through an abrupt, exothermic and self-sustaining reaction between Ti and C elements and, meanwhile, the Ti(Al) solid solution is formed by the gradual and progressive solution of Al into Ti lattice [16]. Thus, for different powder systems (even if consisting of some similar constituents), the dominant physical and chemical mechanisms behind the microstructural developments of the milled powders may vary significantly and, thus, should be carefully determined.…”

“…In this respect, high-energy mechanical alloying (MA) is a well-developed process for dispersing nanocrystalline reinforcing particulates more uniformly in a metal matrix [8][9][10][11][12][13][14][15]. An ultrafine nanocrystalline nature coupled with a homogeneous distribution of the reinforcing particulates is expected to improve the obtainable mechanical properties of nanocomposites prepared by the MA route [16]. MA, as a nonequilibrium, low temperature and solid-state powder treatment process, involves repeated cold welding, fracturing and rewelding of powder particles in a high-energy ball mill [17][18][19][20][21].…”

In Situ Synthesis of Ti5Si3 Matrix Nanocomposites Reinforced with Nanoparticles by High‐Energy Mechanical Alloying

“…The composite powder being milled experiences heavy macroscopic and/or microscopic deformation due to the continuous action of ball‐powder‐ball collisions, providing a high possibility for the formation of nanostructures in both metallic matrix and ceramic reinforcement. An ultrafine nanocrystalline microstructure coupled with a homogeneous distribution of the reinforcing particulates is expected to improve the final mechanical properties of nanocomposites prepared by MA route 18–20. MA processing of in situ composites is essentially regarded as a mechanochemical process, which combines high‐energy ball milling and inherent chemical reactions.…”