Effect of chromium on properties of Fe 3 Al
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I. INTRODUCTION The iron aluminide Fe3Al occurs over the range = 2 4 32 at. % aluminum. It exists in the ordered DO3 structure up to =540 °C, an imperfect ordered B2 structure between 540 and 760 °C, and is disordered above 760 °C. Cast or recrystallized Fe3Al shows a tensile ductility of < 5 % at room temperature, but >20% at temperatures above 200 °C, indicating a brittle-to-ductile transition, as noted for many long-range ordered systems.1 Earlier studies found that increasing the purity of Fe3Al shifted the failure mode at room temperature from intergranular to transgranular cleavage,2'3 with an accompanying small increase in ductility. Over the past 30-40 years, many studies have been conducted on the effect of alloying elements on mechanical properties of Fe3Al, in an attempt to produce alloys with both improved room temperature ductility and high temperature strength.3"7 However, only limited success has been achieved. Slight improvements in ductility have been produced by grain size refinement using powder metallurgy methods4 or precipitation- or dispersion-strengthening.5'6 Increased strengths at temperatures above 500 °C have resulted from solid-solution additions of certain elements or from precipitation-hardening effects.6'7 Beneficial grain boundary segregants have not been found. Few studies have addressed the effects of chromium addition on the metallurgical properties of Fe 3 Al. Culbertson and Kortovich8 reported results involving the addition of certain elements, including chromium, on mechanical properties, workability, and oxidation resistance of Fe3Al. Their alloys were prepared using powder metallurgy processing procedures, as well as isothermal forging. At 600 °C significant increases in yield strength were produced by additions of Si, Ta, Mo, Nb, or Cr. However, only an alloy containing additions of 3.3% Cr and 0.2% Mo (their base alloy was Fe-23.4% Al) exhibited room temperature ductility comparable to the 5% ductility of Fe3Al. Mendiratta and Lipsitt used the same specimens to study the effect of ternary additions upon the DO3 domain structures and on the DO3-to-B2 transition temperature Tc.9 1156
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They found that chromium additions had no effect on the size or character of the DO3 antiphase domains (APD's) but did tend to increase Tc. Recently, work by Diehm et al.10 indicated a decrease in compression yield strengths with addition of up to 8% chromium to alloys with the DO3 structure which also contained Hf, Mo, and Ti in combined additions of 3-8.5%. Likewise our recent studies indicate a slight decrease in room temperature tensile yield strength, accompanied by an increase in ductility, with additions of 2-6% Cr to Fe 3 Al." In particular, our earlier alloy development studies have shown that chromium is the most effective element for improving room temperature ductility.11"13 Our preliminary studies indicated that the addition of chromium resulted in a change in fracture mode from transgranular cleavage to a mixed intergranular-transgranular cleavage fra
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