Lecture 27: Mechanical Alloying

PHYS 430/603 material

Laszlo Takacs

UMBC Department of Physics

Mechanical alloying takes place via repeated plastic deformation, fracturing, and cold welding of powder particles in a high-energy ball mill.

It is a method that can produce extremely small grain size (to below 10 nm), metastable phases (both crystalline and amorphous), and high concentration of lattice defects.

The figure below is a very schematic representation of the process in a mixture of two ductile materials. Notice the formation of layers that get randomized later.

The equipment of mechanical alloying

Ball mills produce a mixture of impact and shearing/friction between the balls producing the mixing/alloying needed.

Planetary mills and attritors produce more friction, the dominant form of action in vibratory and shaker mills is impact/ compression.

Available mills range from small laboratory versions to large industrial mills.

Szegvari attritor model 1-ST

Fritsch P-7 planetary mill

SPEX 8000 Mixer Mill50 mm, 20 Hz, vmax≈ 6 m/sFlat-ended vial

Home made vibratory mill4 mm, 50 Hz , vmax≈ 1.3 m/sCylindrical vial

Interior of the milling container of the vibratory mill.

Seeing the powder is difficult as it is a small fraction of the ball mass, typically about 1/10. Some powder coats the balls and the wall of the

container, the rest collects at the bottom, under the balls.

Internal diameter: 3.5 cm

Volume: 36 ml

Balls: 5 pieces of 8-mm and 100 g of 3.5-mm

Industrial tumbler mill and the insider of a typical clinker mill. The clain is often maid that mechanical alloying can be scaled up to industrial quantities, as the necessary equipment is available.

History of mechanical alloying

• Developed to produce oxide dispersion strengthened nickel alloys, INCO Alloys, Gilman and Benjamin, 1968

• Ball milling can produce amorphous alloys, C.C. Koch, UNC, 1983 (top photo)

• Mechanically induced self-sustaining reactions, MSR,

P.G. McCormick, 1989• The first comprehensive book on mechanical alloying,

C Suryanarayana, UCF, 2004 (bottom photo)

• Research on the chemical effects of mechanical milling has been ongoing since the 1950s, but the two areas did not have contact until the 1990s.

The microstructure develops according to the mechanical properties of the components (Koch,1989). The length scale is an important

characteristic of the degree of activation.

Ni-Al, a ductile-ductile system Cu-W, a ductile brittle systemThe only metal-metal showing MSR

D.L. Zhang, 2005

Fe-Zr, a system forming amorphous alloy.

This is an early stages of the process, the lamellae become too thin for viewing by SEM quickly.Mixing takes place by direct mechanical effect and also by enhanced diffusion due to the lattice defects created by milling.

What happens during mechanical alloying?

• Balls and the container wall gets coated with the processed powder, part of the powder remains free. Collisions can both deposit and remove powder from the surface, leading to mixing and homogenization.

• Powder gets caught between colliding balls and a ball and the container wall. Collisions provide impact and shear, they both fracture and cold weld particles.

• Grain size decreases, lattice defects accumulate.• Alloying, solid state reactions take place, both by direct mechanical effect and

defect-enhanced diffusion.• Uniform powder is formed.

• The product of mechanical alloying is a fine powder with particles typically a few time 10µm in diameter, consisting of grains that are as small as 10 nm.

Questions related to the mechanism of mechanical alloying

• The mechanics of the ball mill. Quite complicated if non-central collisions, the rotation of the balls, partial inelasticity of the collisions due to powder coating, etc. are allowed.

• The motion of powder inside the mill, transfer from free particles to coating on the surfaces and back.

• Plastic deformation in the “miniforging” volume between colliding surfaces.

• Defect formation, diffusion, alloying, nucleation of new phases in the particles under mechanical action.

Mechanical alloying is a very complex process that requires understanding on multiple length and time scales.

General aspects• The effect of mechanical action on the thermodynamic behavior.• The energetics of the process.

Delogu and Cocco, 2006

<------Ni asperity impacts a Zr surface.

------>Impact with shear between a Ni and a Zr asperity.

The effect of milling on the grain size of some fcc metals

The grain size and strain effects were separated using the diffraction angle dependence of the line width.

Eckert et al., 1992

MA for 60 hours in a planetary mill under hexane.Notice the limited solubility of Mn in Ti in the equilibrium state.

Zhang et al. 2009

Ni-Fe-Ag mixture milled for 1, 3, and hours in a SPEX 8000 mill

Notice that Fe and Ni form an fcc alloy and up to 7% Ag dissolves in this matrix, based on shift of lattice parameter.

Bennett et al. 1995

Reaction between Fe3O4 and Zn

Pardavi, Takacs, 1995

The Mechanically induced Self-propagating Reactions can occur, if a strongly exothermic chemical reaction (with high adiabatic temperature) is possible in the powder mixture. MSRs consist of three steps:

1. Activation2. Ignition3. Self-sustaining reaction

The usual indication of an MSR is the “jump” of the temperature of the milling container, signaling ignition.

Ti + C (Sassari, Italy) Fe3O4 + Zr 0.4 Sn + 0.4 Zn + S

Ignition of Ti : Si = 5 : 3SPEX 8000 mill, quartz vial, one steel ball, 9.5 mm diameter.

Tad = 2550 KTime from first smoldering spot to maximum brightness: 250 ms

Temperature of the outside surface of the milling vial.

Temperature inside the milling vial close

to ignition.

Monagheddu, Doppiu, Deidda, Cocco, 2003