Rapid Solidification

Rapid solidification (RS) allows extension of solubility limits, production of novel phases, and more refined microstructures than ingot metallurgy (I/M) techniques. The greatly increased chemistry/microstructure "window" can lead to enhanced mechanical and physical properties.

Aluminum. Five families of alloys being explored using the RS approach are the high-strength corrosion-resistant alloys based on traditional 7000-series alloys, low-density aluminum-lithium alloys with increased lithium levels over those possible using the I/M approach, dispersoid-strengthened elevated-temperature alloys based on low-solubility/low-diffusion-rate additions such as the transitional metals (iron, molybdenum, nickel) and rare earth elements (cerium), wear-resistant high-silicon alloys, and recycled alloys (in which conventional ingot processing would lead to excessive segregation)(Table 5).

Table 5 Tensile properties of elevated-temperature P/M RS aluminum alloys at 315 °C (longitudinal)

Alloy

Ultimate tensils strength, MPa

%

Al-Fe-Ce

270

225

7

Al-Fe-Mo

235

210

10

Al-Fe-V-Si

310

300

7

Al-Zr-Cr-Mn

235

215

. . .

Magnesium. Cast magnesium alloys exhibit lower than desirable strength, ductility, and creep behavior, and the nonprotective oxide skin can lead to severe corrosion problems. High-strength, corrosion-resistant magnesium alloys containing rare earth additions (yttrium, neodymium, cerium) have been developed using RS. Mg-Al-Zn-X (X = Si, Y, Nd, or Ce) alloys can exhibit tensile strengths ranging between 450 and 510 MPa. These alloys have strength and ductility combinations equivalent to high-strength aluminum alloys and are five times more corrosion resistant than the most resistant conventional magnesium alloys.

Titanium. The major concentration on RS terminal alloys to date has been to enhance elevated-temperature capability beyond I/M alloy levels (i.e., >700 °C) through dispersion hardening (Ref 2, 3). The additions of erbium and other rare earth elements produce dispersoids that resist coarsening at least up to 800 °C; however, much further optimization is required particularly in increasing the volume fraction of second-phase particles. Alloys containing additions such as iron offer the potential for extremely high-strength levels (>1400 MPa ultimate tensile strength), with possible applications as replacement for steel in components such as landing gears.

Titanium Aluminide Intermetallics. Improved ductility can be achieved by disordering, grain refinement, and deoxidation of the matrix, while good elevated-temperature properties are achieved due to a dispersion of fine, thermodynamically stable, second-phase particles. In combination with the near-net-shape advantages offered by the P/M approach, RS may offer some advantages for the processing of 7 alloys over the I/M approach; presently, the same cannot be said for RS processing of O^-TiaAl alloys.

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