JOURNAL OF MATERIALS SCIENCE 37 (2 0 0 2 ) 5209 – 5214 Microstructure and strength of a deformation processed Al-20%Sn in situ composite K. XU ∗ , K. WONGPREEDEE, A. M. RUSSELL Ames Laboratory of the U.S. Department of Energy and Materials Science and Engineering Department, Iowa State University, Ames, IA 50011, USA E-mail: xukai@iastate.edu An Al-20 vol% Sn metal-metal composite was deformation processed by extrusion, swaging, and wire drawing to a total true strain of 7.4, resulting in a microstructure with Sn filaments in an Al matrix. Both the size and spacing of the Sn filaments decreased as deformation processing progressed. The strength of these composites increased exponentially with the reduction in spacing of the Sn filaments. Immediately after deformation, the Sn second phase showed a convoluted, ribbon-shaped filamentary morphology, but the Sn filaments spheroidized during prolonged storage at room temperature. A thermodynamic assessment is presented for this spheroidization phenomenon. C  2002 Kluwer Academic Publishers 1. Introduction If the flow stresses of two metal phases are similar, severe mechanical deformation of a two-phase mi- crostructure of these metals will co-deform the two phases in roughly equal amounts. These Deformation- processed Metal Metal Composites (DMMCs) pos- sess extraordinary mechanical and electrical properties. Since the filamentary microstructure is formed within the composites during deformation, these composites are sometimes called in situ composites. The typical method of making DMMCs is to prepare starting billets by ingot casting or by powder metal- lurgy followed by rolling or by axisymmetric defor- mation such as extrusion, swaging, and wire drawing. Extensive deformation will produce a composite ma- terial with elongated filaments aligned parallel to the deformation direction. Most of the initial studies on DMMCs were con- ducted on composites with a face centered cubic (fcc) matrix containing about 20 volume percent of a body centered cubic (bcc) secondary reinforcing phase. Cu-X (where X is a bcc metal such as Nb, Ta, Cr, W or Fe) systems [1–9] were the most thoroughly studied. Bevk, Harbison, and Bell [1] first reported a Cu-Nb DMMC in the 1970s that was intended to serve as a precur- sor material for production of superconducting wire. A Cu-18 vol%Nb DMMC was made with an ultimate ten- sile strength of 2200 MPa at a deformation true strain of 11.5. This strength level was as high as the best Cu whisker strengths. A deformation true strain para- meter, η = ln( A i / A f ), was used to characterize the de- gree of deformation, where A i and A f are the spec- imen’s initial and final transverse areas respectively. In Cu-X DMMCs, the bcc second phase developed a ∗ Author to whom all the correspondence should be addressed. 〈110〉 fiber texture that limits the deformation of the second phase to plane strain [1, 10]. As a result, the typ- ical microstructure of fcc/bcc DMMCs is a convoluted, ribbon-shaped second phase embedded in the matrix. In addition to Cu-X DMMCs, other DMMCs with hexagonal close packed (hcp) metals were reported in the 1990s. Experiments in the Ti-Y system have exam- ined Ti matrix DMMCs containing 20% Y and 50% Y. Ti-Y was first hot deformed followed by cold defor- mation with intermediate annealing at 890 K. A defor- mation true strain of 7.3 was achieved with this pro- cedure, and an ultimate tensile strength of 950 MPa was reported by Russell et al. [11, 12]. The Y fila- ments had a kinked ribbon-shaped morphology similar to that of the Nb filaments in the Cu-Nb DMMCs. Both Ti and Y adopted a pronounced 〈10 ¯ 10〉 fiber texture along the wire axis, which limited slip to plane strain just as had been observed with the 〈110〉 texture in the bcc phases of the Cu-X DMMCs. Other DMMCs con- taining hcp metals have since been reported, including Mg-Ti [13, 14], Al-Ti [15], and Al-Mg [16]. All hcp second phases showed the same ribbon shaped filamen- tary microstructure due to 〈10 ¯ 10〉 fiber texture of second phases. With the exception of early studies on pearlitic steel [17], all the DMMCs that have been investigated have used metals with fcc, bcc, and/or hcp crystal structures. In this study, an fcc Al matrix DMMC containing body centered tetragonal (bct) Sn as the second phase was produced. Although the material displayed the familiar continuous elongated filamentary microstructure in the wire direction immediately after deformation, the mi- crostructure spheroidized during prolonged storage at room temperature. 0022–2461 C  2002 Kluwer Academic Publishers 5209