Grain boundary cracking during directional solidification of columnar grained nickel base superalloys has been investigated. A quantitative test has been developed that allows a classification of the castability of directionally solidified nickel base superalloys: CM247L C showed excellent castability, while IN792 showed severe cracking. T he cracking was found to originate at a temperature near the solidus temperature (hot tearing). A number of experimental alloys based on the composition of IN792 were tested, with the eVects of variations in hafnium, aluminium/titanium, carbon, boron and zirconium contents being examined. A combination of 0%B and 0•01%Zr shows significant promise. A number of approaches were applied to explain the cracking susceptibility. A relationship was found between a small nil ductility region, obtained by Gleeble testing, and good alloy castability for IN792 and CM247L C. A correlation between the eutectic content of the alloy and its cracking tendency was also found. At low (<5%) and high (>22%) amounts of eutectic, a significant reduction in cracking was apparent, while with intermediate amounts (15%), severe cracking was observed.MST /3958

Low thermal expansion superalloys have been used for a number of years in a variety of applications, including gas turbine engines. The low thermal expansion characteristics of the most widely used class of materials are derived from the ferromagnetic characteristics of Ni, Fe, and Co-based austenitic matrices containing little or no Cr. Over time, a progression of alloy developments ensued, aimed at improving the oxidation resistance and stress accelerated grain boundary oxygen (SAGBO) attack. While notch rupture tests have been wed to screen for the SAGBO phenomenon, a more sensitive measure of this characteristic is the sustained load crack growth test performed in air.This paper describes some final iterations in the development of a new cIass of low expansion superalloys utilizing high Al content and y, y', and p phases in the microstructure. Such alloys provide good general oxidation resistance, and rupture strength and ductility, and varying degrees of crack growth resistance. A number of designed factorial experiments were carried out to optimize 538°C crack growth resistance, yet maintain a balance of other important engineering properties. These experiments included examinations of Ni, Fe, Co, Cr, Nb, and Ti content combined with heat treatment studies. Al content remained essentially fixed on the basis of prior development work. Tests performed included thermal expansion, tensile, tensile and Charpy impact stability, stress rupture, creep, 538°C static crack growth, and microstructural analysis.These studies showed, that for a given heat treatment cycle, a small amount of Cr combined with increased Co content in place of Ni provides a decrease in crack growth rate. Furthermore, the small Cr addition improves salt spray resistance, yet the addition is small enough as not to significantly affect thermal expansion performance. The crack growth rate was also reduced with increased Co content replacing Ni. The final alloy composition was designated INCONEL@ alloy 783.Crack growth rates were affected by heat treatment. Microstructural examinations showed heat treatment affected amounts of globular p phase present after hot working and annealing, and amounts of the phase re-precipitated within grain boundaries or intragranularly. Slower propagation rates correlated with increased volume percent of p phase with lower temperature anneals, or increased amounts of p phase precipitated in grain boundaries after high temperature anneals and "P-aging" at intermediate temperatures. A high temperature anneal was selected for compatibility with high temperature braze cycles without significantly coarsening grain structure. An appropriate p age was determined for good rupture and crack growth properties. Heat treatment studies further showed that higher yield strengths are achieved with treatments incorporating slow cooling within the r' Alloy 783 has been successfully produced as VIM-VAR large diameter forging billet, and hot rolled small rounds and flats. Sustained load crack growth data at 538°C obtained fro...

Due to their low coefficients of thermal expansion, lNCOLOY@ alloys 903, 907 and 909 have contributed to improved efficiency in many aircraft engines designed since the mid '70's. The application of these Cr-free superalloys was, however, limited by inadequate oxygen environment resistance. The alloys were susceptible to both general oxidation and SAGBO (Stress Accelerated Grain Boundary Oxygen) embrittlement. Until just recently development efforts had focused on improving SAGBO resistance, but had ignored general oxidation resistance.* Compositions shown are in weight % unless stated otherwise.

The isothermal time-temperature-transformation behavior of wrought, triple melted (VIM-ESR-VAR) and homogenized INCONEL@ alloy 706 was characterized by X-ray diffraction, SEM/EDX, and optical metallographic techniques.Microstructural features, including phase compositions, morphologies, and crystal structures are discussed. Alloy 706 is an agehardenable superalloy strengthened by 7' and y U precipitation at lower temperatures. The propensityfor the alloyto form q at higher temperatures distinguishes alloy 706T-T-T behavior from that of the well documented INCONEL@ alloy 718. The effect of differing Nb-Ti-Al ratios and Fe content on the physical metallurgy of Nb-Containing superalloys is discussed.

INCONEL® alloy 783 is an oxidation resistant low coefficient of thermal expansion (low CTE) superalloy developed for gas turbine applications. Turbine efficiency can be increased through the use of low CTE shrouds and case components that maintain tight blade tip clearances at different turbine operating temperatures. To achieve low CTE, alloys based on Ni-Fe-Co compositions require Cr content be maintained at low levels. Added Cr lowers the Curie temperature and thereby increases thermal expansion rate over a wider temperature range. The necessary lack of Cr minimizes resistance to both general oxidation and stress accelerated grain boundary oxygen enhanced cracking (SAGBO). Increased amounts of Al in alloys strengthened by γ’ alone also promotes SAGBO. Alloy 783 is the culmination in the development of an alloy system with very high aluminum content that, in addition to forming γ′, causes β aluminide phase precipitation in the austenitic matrix. It was discovered that this type of structure can be processed to resist both SAGBO and general oxidation, while providing low thermal expansion and useful mechanical properties up to 700°C. The high Al content also reduces density to 5% below that of superalloys such as INCONEL alloy 718. Key aspects of the alloy development are presented, including the assessment of SAGBO resistance by evaluating elevated temperature crack growth in air. The alloy, now commercially available, has been successfully fabricated and welded into gas turbine engine components.