Braze Alloy Development for Fast Epitaxial High-Temperature Brazing of Single-Crystalline Nickel-Based Superalloys
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NOWADAYS, turbine components in aircraft engines as well as in stationary gas turbines are frequently made from single-crystalline nickel-based superalloys, since these materials provide good hightemperature durability.[1–6] However, due to the high costs of these components, the repair of damaged parts instead of a complete exchange is of special economical interest. One of the most frequently occurring damages is the formation of surface cracks, which are usually repaired by high-temperature diffusion brazing.[2,3,5,7] This repair technology is characterized by the use of a braze alloy, which is similar to the base material but contains additionally a fast diffusing melting-point depressant, in most cases, boron. As this element diffuses into the base metal during brazing, a joint is formed having almost the same properties as the base metal. It has also been noticed in the past that diffusionbased solidification at elevated temperatures leads to epitaxial growth of the solidification front. Thus, singlecrystalline components can be repaired, reproducing the single-crystalline microstructure in the braze gap.[5,7–11] This is particularly attractive in view of the severe thermomechanical loading of these components.
B. LAUX and S. PIEGERT, PhD Students, and J. RO¨SLER, Head, are with the Institut fu¨r Werkstoffe, Technische Universita¨t Braunschweig, 38106 Braunschweig. Contact e-mail: [email protected] Manuscript submitted July 16, 2007. Article published online November 11, 2008 138—VOLUME 40A, JANUARY 2009
Unfortunately, diffusion is a relatively slow process. Therefore, long hold times are required to complete epitaxial solidification.[2,12–16] For example, epitaxial closure of a 300-lm-wide gap requires hold time of about 50 hours,[12] which is economically not viable. Furthermore, significant coarsening of the c¢ precipitates in the base material can occur as a result of the extended elevated temperature exposure, thus deteriorating the mechanical properties. If the sample is cooled prematurely, the melting-point depressant is repelled from the advancing solidification front due to its poor solubility in the solid phase, without having the change to diffuse into the solid. This enrichment in the liquid phase causes precipitation of brittle secondary phases during solidification, serving as nucleation sites for stray grains. Consequently, a polycrystalline microstructure containing brittle phases results in the center of the braze gap. An alternative procedure, allowing for fast epitaxial closure of braze gaps, is presented here. The essential idea is to replace poorly soluble melting-point depressants, such as boron, by elements that are completely soluble in the nickel matrix. Then, it is not necessary to almost completely remove the melting-point depressant from the braze gap as single-phase solidification is fundamentally ensured. As a result, much faster epitaxial solidification should be possible because extended hold times for diffusion are no longer required. The nickel-manganese system is proposed here as an appr
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