Modeling mechanical alloying: Advances and challenges

Abstract

While mechanical alloying is a commercial entity for oxide-dispersion-strengthened superalloys, its application to other systems has run into a number of scientific and commercial barriers. In part, this is due to the inadequate scientific underpinning. This article reviews the status of the modeling of the mechanical alloying processes and suggests an approach to improving current knowledge of the process. INTRODUCTIONMechanical alloying (MA) presents a unique means for producing a wide variety of far-from-equilibrium structures and phases possessing unusual mechanical and physical properties. The attributes ofMA include the production of a fine dispersion of second-phase particles, the extension of solubility limits, the refinement of the matrix microstructure down to the nanometer range, the synthesis of novel crystalline phases, the development of amorphous phases, and the possibility of alloying difficult-toalloy elements.The demand for further progress in MA necessitates the development of mathematical models of transformations in particles undergoing repetitive plastic deformation/cold welding. A brief overview of existing models is presented here, and the difficulties arising from the far-from-equilibrium attributes of the MA process are outlined. Further development of the macrokinetic description of solid-state diffusion and phase transformations under the conditions of continuous-defect generation due to plastic strain will bring together the mechanistic and atomistic approaches and permit modeling of metastable phase formation phenomena observed in MA (e.g., solid-solubility extension, nanograins, amorphous phases, etc.). STATUS OF MA MODELINGSince the first experimental results were reported, 1 and especially after the discovery of metastable phase formation in the course of ball milling,2-4 MA has attracted considerable attention from materials scientists, not only due to the unique properties of the products and promising commercial applications but also because of the unusual physical phenomena involved. An increasing demand for the adjustment of the properties of novel materials produced by 36 MA to particular applications and the improvement of the MA regimes and apparatus designs for commercial scaling necessitates the development of mathematical models of MA. Modeling and computer simulation are based on theoretical concepts and a formal mathematical description of the process. However, theoretical works in this area are not as abundant as experimental research (see reviews by Johnson,s Weeber and Bakker,6 Koch/ Ma and Atzmon 8 and others).Three typical mechanisms of metastable phase formation during MA have been observed: 6 • The gradual disordering of crystal lattice and eventual amorphization. A similar mechanism (but without the crystal-to-amorphous transition) can occur in immiscible systems (e.g., Fe-Cu), resulting in the formation of supersaturated solid solutions during MA.8 • A coexistence of crystalline and amorphous phases and the gradual growth of the latter. This implies the exist...

Hyperchaotic qualities of the ball motion in a ball milling device

“…However, fundamental problems in quantifying the MA process still exist. 3,4 Mechanistic aspects of the process and their links with the intensity of the mechanical treatment still have to be clarified. The major difficulty is represented by the complexity of the milling dynamics preventing, for a long time, any accurate measurement of the fundamental milling parameters such as collision frequency and velocity.…”

Onset of chaotic dynamics in a ball mill: Attractors merging and crisis induced intermittency

“…4 After about 30 years, however, many initial expectations on possible industrial application of mechanical treatment have been disappointed. 5 Different reasons have been invoked to explain such a situation.…”

Numerical simulations of atomic-scale disordering processes at impact between two rough crystalline surfaces

“…10 Such impact velocity roughly reproduces the average impact velocity in ball mills. [6][7][8]34 In some cases a tangential force F t was also applied along the x Cartesian direction to simulate a shear stress 10 and obtain a couple of sliding. Correspondingly, opposite center-ofmass velocities of about 5 m s −1 were given to the Ni and Zr systems.…”

“…[4][5][6] Such behavior underlies the mechanochemistry as well as the modern techniques of powder metallurgy, [4][5][6] which generally rely upon the repeated mechanical loading of powder particles trapped between colliding surfaces and the related occurrence of local shear events on the microscopic scale. [6][7][8] The localization of considerable amounts of energy in relatively small volumes of solid phase determines the accumulation of strain and the nucleation of defects. [9][10][11][12][13][14][15][16][17][18][19][20] Fast local processes of mass transport and mixing occur assisted by complex aggregates of atoms with defective coordination.…”

“…19,20 Despite the deeper insight provided by these mechanistic scenarios [9][10][11][12][13][14][15][16][17][18][19][20] respect to the ones based on thermal diffusion, [21][22][23][24][25][26] the intimate mechanisms governing the mechanochemical transformations are still a matter of debate. 6,7 This work attempts to clarify the role of localized mechanical stresses and of the associated excited states 6,7,[9][10][11][12][13][14][15][16][17][18][19][20] by focusing on the early stages of the collision between two semicrystals of different metals. To this end, the behavior of semicrystals with surfaces of different roughness was studied under different impact conditions by means of molecular dynamics simulations.…”