Tikalon Blog by Dev Gualtieri

Cold Work on the Nanoscale

January 2, 2017

My first metallurgy lesson came early in life as I was bending aluminum to make chassis for electronic circuitry. I started building electronic circuits in elementary school using components harvested from old television receivers. The receivers of that period, and my circuits, were built from vacuum tubes. I could build an audio amplifier from just four or five vacuum tubes. Soldering wires was the easy part. The hard part was machining holes in an aluminum chassis to mount the tube sockets, transformers, and controls.

Transformers typically required a large square hole, and my only tool for making these was an electric drill. I would drill a chain of small holes near the perimeter of the square, and cut a path through the line of holes using a small file, a tedious task. Eventually, I discovered that I only needed to cut three sides of the square. Repeatedly bending the aluminum back and forth would easily break off the final side. I had discovered work hardening.

As everyone familiar with cooking foil and beverage cans knows, aluminum is a soft metal. It's easily created in its pure elemental form by electrolytic reduction of aluminum oxide dissolved in a molten solution by a modified Hall process.[1] Pure metals, as cast and distinguished from alloys, are soft. The repeated bending (cold-working) forms dislocations in the crystal lattice that change the aluminum from a ductile to a hard and brittle material.

Fig. 1 of US Patent No. 400,664, 'Process of reducing aluminium from its fluoride salts by electrolysis,' by Charles M. Hall (April 2, 1889)

The inventive part of the electrolytic extraction of aluminum was finding a way to put it into a solution. Charles Hall discovered that alumina would dissolve in molten fluorides.

(Fig. 1 of US Patent No. 400,664, "Process of reducing aluminium from its fluoride salts by electrolysis," by Charles M. Hall, April 2, 1889.)

Forging and shot peening are two of the more useful applications of work hardening. In forging, a component of near-net shape is hammered into a die to bring it to its desired shape. The forging process hardens the material by cold working, but forging is often done at higher temperature to keep the material ductile. A video of cold forging of aluminum can be found in ref. 2.[2]

Peening is the process of hammering the surface of a metal to increase the surface hardness. The process of hammer peening was used by blacksmiths for centuries without an understanding of why it works. Some readers may have seen a ball peen hammer, but wondered about its purpose (see photograph). Shot peening is a more rapid process that pelts a surface with a stream of hard beads to provide the work hardening.

Ball peen hammer.

My maternal grandfather, who was a machinist, had one of these, and I wondered why it had such a strange shape.

(Via Wikimedia Commons.)

Surprisingly, shot peening was only invented about a century ago, probably because there wasn't enough demand for a rapid peening process before mass production of such items as automobiles. Shot peening not only hardens the surface, but it puts the surface in compression, and this compressive force prevents surface cracks from growing, so it will increase fatigue life. Shot peening is mostly used in the production of gears, crankshafts and their associated cams, and turbine blades.

The shot used is peening is chosen to be much harder than the peened material, but what about the complementary system of soft shot against a hard target? That's the approach taken by materials scientists and mechanical engineers at Rice University (Houston, Texas) and the University of Massachusetts, Amherst, as a way to modify the mechanical properties of silver nanoparticles.[3-5] The results of their research have been published as a recent article in Science.[3] The research was led by Rice materials scientist, Edwin Thomas, and the lead author of the paper is Ramathasan Thevamaran, a Rice postdoc.[4]

The silver particles used for these experiments needed to be as perfect as possible, so they were synthesized as single crystals by bottom seeded crystal growth.[4] The resulting nanoscale cubes were about 1.4 micrometers on a side, and they were shot at a hard silica target at about 400 meters per second using a laser technique.[3] This laser-induced projectile impact test (LIPIT) was developed at Rice University in 2012 for experiments on polymer and graphene film materials.[5-6]

The LIPIT system works by rapid laser heating of a thin gold film beneath a polymer film on which the cubes are placed. The laser power vaporizes the gold film, and the resulting expansion of the polymer film launches the nanocubes.[4] The laser apparatus was designed to accurately deposit the cubes at a desired orientation so the affect of impacts at every angle could be determined.[4] The silver cubes hit the silica surface at supersonic velocity, transforming their momentum into a huge deformation energy over a half millimeter travel distance.

Single crystal silver cubes, such as the one on the left, are deformed on impact with a hard silica surface (right). (Still images from a YouTube Video by Rice University.)[7]

The cubes were initially at room temperature, but impact increased the temperature by about 350 degrees Fahrenheit, a temperature which allowed dynamic recrystallization of the silver.[4] Says Thomas,

"The high-velocity impact generates very high pressure that far exceeds the material's strength... This leads to high plasticity at the impact side of the cube while the top region retains its initial structure, ultimately creating a grain-size gradient along its height."[4]

The resulting high strain rates, strain gradients, and recrystallization from these impacts created a gradient nano-grained structure from the nearly defect-free single-crystal nanocubes.[3-4] The gradient was at least an order of magnitude greater than what has been produced by other techniques.[4] Electron microscope analysis of the cubes eight days after impact showed that the gradient structure was still extant.[4]

Image showing grain boundary lines in the strain gradient of a silver nanocube. (Still image from a YouTube Video by Rice University.[7])

Strain gradients such as those produced can create ductile and tough metals.[3] Such a processing technique might give materials with high strength that are less susceptible to brittle fracture.[4] The technique might be used to enhance cold spray processing.[4]

Cold Work on the Nanoscale

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