Ultrafine-grained (UFG) materials have two major advantages over conventional coarse-grained materials:

(i) they have high strength through the Hall-Petch relationship;

(ii) they have a potential for exhibiting superplastic ductilities at high temperatures.

There are two major procedures for producing metals with UFG structures:

Equal-Channel Angular Pressing (ECAP) A sample is pressed through a die contained within a channel bent through an angle close to 90°.	High Pressure Torsion (HPT) High pressure is applied to a disk together with concurrent torsional straining.

Processing by ECAP is well developed at USC. We have a 150 ton Dake hydraulic press and several dies of different sizes and different angles (e.g. 90° and 110°). We are able to perform ECAP on samples having different diameters and over a range of pressing strain rates. We can also press samples at elevated temperatures. Numerous materials have been pressed using the facilities at USC. Although the emphasis has been on aluminum-based alloys, we have also pressed other materials such as magnesium alloys, copper alloys, pure iron and the Zn-22% Al eutectoid alloy.

Prof. Langdon and Dr. Cheng Xu undertaking an ECAP experiment using the Dake press at USC.

Examples of the Application of ECAP to an Aluminum Alloy:

(i) Homogeneity through the cross-section of the sample after ECAP:

The structure becomes homogeneous with increasing number of passes and at the same time the hardness increases.

(ii) The break-up of precipitates in ECAP:

An Al-7034 alloy in the initial unpressed condition with rod-like precipitates.	The Al-7034 alloy processed by ECAP for 6 passes at 473 K showing evidence for the breaking of the precipitates.

(iii) Very high elongations achieved after ECAP

Superplastic elongations of >1000% are achieved in a spray-cast 7034 aluminum alloy after ECAP at a temperature of 473 K through either 6 passes (6p) or 8 passes (8p): the maximum elongation is displaced to faster strain rates when the number of passes is increased.

Reference: C. Xu, M. Furukawa, Z. Horita and T.G. Langdon, Acta Mater. 51, 6139 (2003).

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