Materials Science and Engineering A239 – 240 (1997) 410 – 418 Evaluation of methods to produce tough Cr 3 Si based composites T.A. Cruse *,J.W. Newkirk Department of Metallurgical Engineering, Uni6ersity of Missouri-Rolla, Rolla, MO 65401 , USA Abstract Severalmethods of improving the toughness of Cr–Cr 3 Si composites produced by powder metallurgy have been examined. Mechanical alloying of Cr-3.92 w/o with 0.5 w/o V showed the best improvement in toughness of this phase. Replacing Cr 3 Si with a (Cr 0.57 , Mo 0.43 ) 3 Si, produced in situ from a mechanically alloyed powder, offered a slightimprovement in toughness. By combining these phases a composite was produced with a toughness greater than 10 MPa m −2 . Other second phase materials were also examined for providing second phase toughening of Cr 3 Si. These include an Fe – Al alloy, Ni 3 Al + B, and 304L stainless steel. At the levelof 25 v/o these materials at present do not appear to offer much improvement in toughness. Cr–Cr 3 Si composites were hotforged to produce a more layered microstructure and refine the microstructure of arc-melted samples. The layered microstructure should be tougher, but has not been tested yet. © 1997 Elsevier Science S.A. Keywords : Cr–Cr 3 Si composites; Second phase toughening; Layered microstructure; Powder metallurgy 1.Introduction The intermetallic compound Cr 3 Si has many desir- able high temperature properties: high melting point, good creep resistance, good oxidation resistance, high stiffness, and a comparatively low density. The major problem with using this material, as with a large num- ber of intermetallics, has been a lack of room tempera- ture toughness [1]. Before Cr 3 Si can be considered for commercial applications, its room temperature tough- ness must be improved. There are several methods by which the toughness of a materialcan be improved.Some possible methods include grain refinement, alloying, reduction of impuri- ties,and the incorporation of a ductile second phase material. Ductile second phase toughening was selected as the primary method to improve the toughness of Cr 3 Si, because it appeared to offer the greatest potential for tougheningwhile maintaininghigh temperature properties.When using thismethod,severalfactors need to be considered; these includethe mechanical properties of both phases and the amount, distribution, size and shape of the second phase [2]. Chromium hasshowed somesuccessas a second phase material to toughen Cr 3 Si; composites of these materialshavetoughnesses superiorto that of pure Cr 3 Si [3 – 7]. The potential for Cr 3 Si as a high tempera- ture material should not be adversely affected by mak- ing a composite with chromium; chromium has both good high temperature properties and good oxidation resistance. Initially arc-melting techniques were used to produce samples. This resulted in composites that were near full density,but with dendriticmicrostructures [3,4].In order to achieve a refined and uniform mi- crostructure powder metallurgy (PM) techniques were used.The toughness values for both processing meth- ods were similar but unfortunately, the PM composites possessed lower densities [5 – 7]. Under equilibrium conditions chromium dissolves up to 3.92 w/o Si during arc-melting or PM processing. This levelof silicon has been observed to reduce the toughness of chromium [7]. The toughness of PM sam- ples of pure chromium and chromium mechanically allowed with 3.92 w/o Si were prepared and tested as notched three point bend bars.The resultsindicated that the toughness of these materials was too low [7] to produce a composite with the desired toughness of 15 MPa m −2 , the accepted minimum for a usable mate- rial. The pure chromium sample had a toughness of only 12.691.4 MPa m −2 , the addition ofthe equi- librium amountof silicon lowered thetoughnessto 7.193.4 MPa m −2 [7]. Another problem with using chromium is that its ductility,DBTT, and toughnessare very sensitive to * Corresponding author. 0921-5093/97/$17.00 © 1997 Elsevier Science S.A. All rights reserved. PII S 0 9 2 1 - 5 0 9 3 ( 9 7 ) 0 0 6 1 1 - 4