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To gain insight into the factors which control the creep-rupture properties of powder metallurgical nickel-base superalloys at intermediate temperatures (650 to 775°C), a comparative study was conducted on the alloys AF115, modified MAR-M432 (B6) and modified IN100 (MERL76). Creep-rupture properties in these alloys were characterized in terms of the stress and temperature dependence of the secondary creep rate, eS, andrupture time,t R . Within the limited stress ranges used, the stress dependence of both eS andt R at 704°C can be represented by power laws eS andC n andt R =Mσ -p ; whereC,M, n, andp are constants. The stress exponentsn andp are approximately equal for both AF115 and B6 with values of 16 and 7, respectively. In the case of MERL76,n andp are different, with values of 15 and 5, respectively. The apparent activation energies,Q, are 700, 370 and 520 KJ mol-1 for AF115, B6 and MERL76, respectively. For these alloys, long creep-rupture lives are associated with large values ofn andQ. The sig-nificant differences inn andQ values between AF115 and B6 were related to creep re-covery processes for which the lattice misfit between the gamma and the gamma prime was identified to be an important parameter. However, the unequaln andp values in MERL76 compared with those in AF115 and B6, were traced to differences in fracture mode. Failures in AF115 and B6 were initiated at carbide particles at grain boundaries. In contrast, fracture in MERL76 was initiated at grain boundary triple junctions. The rupture lives of AF115 and B6 can be modeled reasonably well by the growth of cavities during secondary creep and propagation of a surface-nucleated crack during the tertiary creep.

Turbine disks for gas turbine engines are made from nickel-base superalloys but can be fabricated by several processing methods. The choice of a specific processing route depends on a variety of interrelated factors which include design property requirements, materials selected, and processing costs. It is shown that disk fabrication by direct hot isostatic consolidation of nickel-base superalloy powders, is the most cost-effective method of production; however, such a manufacturing route must yield a product compatible with the property requirements of the component. Some of the metallurgicalfactors that influence such properties in as-hot isostatically pressed (HIP) material are examined. It is shown that by control of the HIP condition, manipulation of composition and heat treatment can be used to control properties. Discussion centres on the balance between strength and stress rupture properties, and resultsfor low-carbon Astroloy and a modified IN 100 alloy, identified in a recent alloy devel...

The U.S. Bureau of Mines development of the Kroll process1 for the production of titanium metal resulted in the general availability of this material by the early 1950’s. Titanium was immediately hailed as the panacea for all structural applications demanding high strength and low density. Today, perhaps, this view could be regarded as somewhat overoptimistic. However, titanium alloys have been used successfully for a number of years in a wide range of industrial applications.

This program addresses the development of iron aluminide base alloys by rapid solidification and the determination of their potential for use in aircraft turbine engines as alternatives to high chromium steels and nickel-base alloys. The program was divided into three tasks. A process and model development task to define thermomechanical processing requirements for Fe3A1 base alloys, a task to produce and deliver to the Air Force Materials Laboratory a number of alloys produced by the P&W rapidly solidified powder process and a task for the development of austenitic Fe-Mn-A1 alloys with improved elevated temperature properties.

A 2-year program was conducted to explore the potential of NiAl as a high temperature structural material with emphasis on overcoming the low temperature brittleness problem. The inherent ductility characteristics of binary alloys containing 29 a/o Al to 54 a/o Al were studied and it was observed that room temperature ductility was achieved only in alloys with lower amounts of Al. The highest Al level in which room temperature ductility was observed is process dependent, being 35 a/o in the case of the melt spun alloys and 29 a/o for the cast alloys. The brittle to ductile transition temperatures for the higher aluminum alloys (43 a/o - 52 a/o) were found to be in the range 375 c to 625 c, showing no clear trends in terms of the Al content and starting form (castings versus powder). Minor additions of boron significantly raised strength and the brittle to ductile transition temperature. An attempt to improve the ductility of NiAl through modification of the basic deformation behavior was made. Keywords: Nickel aluminide, Ordered alloys, Intermetallic, Ductility, Disk alloys.

The U.S. Bureau of Mines development of the Kroll process1 for the production of titanium metal resulted in the general availability of this material by the early 1950’s. Titanium was immediately hailed as the panacea for all structural applications demanding high strength and low density. Today, perhaps, this view could be regarded as somewhat overoptimistic. However, titanium alloys have been used successfully for a number of years in a wide range of industrial applications.

To understand the role of cobalt in nickel-base superalloys, multicomponent nickel solid solutions with various amounts of cobalt were studied. Alloys A, B, C and D which contain cobalt at 30, 17, 8 and 0, respectively, and constant concentrations of Cr(25), Mo(5), Al(0.5), Ti(0.5), Nb(0.4), Hf(0.1), all in weight percent, were produced by casting. Compressive creep tests of these alloys were conducted at temperatures between 650°C and 810°C. Table 1 shows the minimum creep rates at 704°C at a stress of 120% of their respective yield strengths. It can be seen that the alloy without cobalt (Alloy D) creeps at a rate 20 times faster than Alloy A with 30 wt.% cobalt and the values for Alloys B and C with intermediate cobalt contents fall between Alloys A and D. Thin foil studies have revealed that the differences can be attributed, in part, to (i) change of stacking fault energy with cobalt, (ii) precipitation of fine gamma prime particles during creep.