Influence of solidification conditions on the castability of nickel-base superalloy IN792
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I. INTRODUCTION
DIRECTIONAL solidification (DS) to form columnar grain structures has been employed for decades to improve the mechanical properties of Ni-base superalloys. Numerous beneficial effects were demonstrated because of the introduction of a preferred grain orientation with a low modulus parallel to the main stress axis and the elimination of grain boundaries perpendicular to the same stress axis.[1] Unfortunately, the formation of cracks along the longitudinal grain boundaries during the last stages of solidification prevented the application of the technology in certain alloys.[2,3] The phenomenon is known as hot tearing, hot cracking, or solidification cracking in contrast to cold cracking or stress relief cracking, which occurs below the solidus temperature during further cooling. The most common solution for the problem of hot tearing is compositional modification. In the early 1970s, a major breakthrough occurred when the beneficial effect of Hf on the castability of Ni-based superalloys was discovered.[4,5] Hafnium has since become a common ingredient in DS Nibased superalloys.[6] However, a number of detrimental effects of Hf additions were found in later studies: (1) Hf is a reactive element that may cause reactions with the mold and the formation of brittle Hf-containing inclusions; and (2) Hf lowers the incipient melting point and thus the solutionizing temperature, which limits the possibility to homogenize the alloy and to dissolve coarse -precipitates. Consequently, DS alloys containing Hf show little or no rupture strength advantage compared with the same alloy without Hf modification in the equiaxed condition. This is why there is a present trend to develop Hf-free superalloys,[7,8,9] and why the interest in new solutions for the solidification cracking problem has intensified. According to the general view of solidification cracking, hot tears develop in the last stages of the DS process when the volume fraction solidified is high and feeding is impeded. The remaining thin film of liquid along the grain boundaries YIZHOU ZHOU, ANDREAS VOLEK, Ph.D., and ROBERT F. SINGER, Professor, are with the WTM Institute, Department of Materials Science, University of Erlangen–Nürnberg, D-91058 Erlangen, Germany. Contact e-mail: [email protected] Manuscript submitted August 31, 2004. METALLURGICAL AND MATERIALS TRANSACTIONS A
has no sufficient resistance against stresses and strains due to the volume contraction associated with the liquid-to-solid phase transformation in the alloy and the thermal contraction in the solid.[10–13] At the WTM Institute, a number of factors that influence solidification cracking have been studied in the past, such as overall alloy composition,[7,14] content of grain boundary elements such as B and Zr,[15] and grain boundary misorientation.[16] It was found that common explanations based on freezing range or amount of eutectic melt are not able to describe the effects observed. It seemed that grain boundary cohesion in the final stages of solidification is
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