Mechanical properties of conventionally cast, directionally solidified, and single-crystal superalloys

Abstract

In this paper the authors compare the creep and low-cycle fatigue properties of conventional, directionally solidified, and single-crystal castings produced from nickel-base superalloys. A brief historical review describes the reasons for the evolution from wrought to cast product through directionally solidified to modern single-crystal ('monocrystal') castings. The influence of microstructural variations produced by the casting conditions, such as porosity and grain size, on creep and low-cycle fatigue properties are illustrated. The important aspects of postsolidification heat treatment, hot isostatic pressing, and the damaging effects of impurities are described for conventional castings. The results of controlling the microstructures produced by directional solidification especially by high temperature gradient solidification are demonstrated by comparing the creep properties of directionally solidified materials with those of the conventionally cast alloys in long-term tests. The creep and low-cycle fatigue properties depend on the stress direction relative to the crystallographic directions of the material for both directionally solidified and single-crystal castings. For single crystals, individual alloys show variable dependences of properties on the crystallographic directions. Directionally solidified materials show advantages in thin sections and are less sensitive to the effects of impurities compared to conventional castings. MST/329

New vision of convection induced freckle formation theory in nickel-based superalloys by electron microscopy

“…Nickel-based superalloys are ideal for these applications thanks to their extraordinary properties, such as high-temperature strength, creep, and fatigue characteristics [1][2][3] . However, a series of casting defects formed during the directional solidification seriously affect the service performance [4,5] . For example, freckles are one of the common casting defects in heavyduty gas turbine (HDGT) blades, wherein equiaxed crystal chains with a random orientation grow on the blade surface along the solidification direction, which introduces transverse grain boundaries into alloys [6,7] .…”

Issues in Processing by the Liquid-Sn Assisted Directional Solidification Technique

“…This has motivated a number of investigations on new high gradient solidification processes. [4][5][6][10][11][12][13][14][15][16] Liquid metal cooling (LMC) is a process which was originally developed in the 1970s to achieve the high gradient conditions required to cast DS eutectic alloys. [17,18] During the LMC process, cooling is achieved by direct withdrawal of the molten superalloy, contained in an investment mold, from a heater into a container of low melting temperature metal coolant.…”

"Direkte Bewertung der Kriechfestigkeit einer gerichtet erstarrten Turbinenschaufel mittels Minikriechproben

“…2a shows the large DS cast gas turbine blade which is investigated in the present study. In the long`100b direction of the elongated grains, DS alloys possess high creep strength and low elastic moduli [14,15]. One reason for the good creep strength in this direction is that there is a low density of grain boundary segments perpendicular to the axis of the applied stress, where creep cavities are known to nucleate and grow [e. g. 6,8].…”