Publications In association with the MIT Plasma Fusion Center

Background Information:
Our group was working on the development and characterization of properties of a new alloy, Incoloy Alloy 908. This alloy was being developed for use as a sheath material to contain and support superconducting cable and its liquid helium coolant. The application: powerful magnet systems for nuclear fusion reactor systems under the ITER (International Thermonuclear Experimental Reactor) project. Incoloy alloy 908 was optimized for:
> High strength given the constraints of the heat treatment program required to react the superconductor material.
> Low thermal expansion (contraction) coefficient to minimize stresses in the brittle superconductor material when the magnet system is cooled from the heat treatment temperature to the liquid helium operating temperature.

My work was to help in the characterization of the alloy. Some of what I did included:
> Relating cold work processing to hardness and tensile strength.
> Characterization of stress corrosion cracking during heat treatment, development of grips.
> Development a high-cycle fatigue test program, executing tests, and presentation of test results.
> Compilation of a comprehensive data handbook including all non-proprietary properties and test data

INCOLOY 908 DATABASE REPORT ON PROCESS - STRUCTURE - PROPERTY RELATIONSHIP
L.S. Toma, I.S. Hwang, M.M. Steeves

Incoloy 908 is a nickel-iron base superalloy with a coefficient of expansion (COE) and mechanical properties that have been optimized for use in Nb₃Sn superconducting magnets. It has been proposed for use as a conduit material for the International Thermonuclear Experimental Reactor (ITER) magnets. The relationship between manufacturing processes, microstructures and mechanical properties of Incoloy 908 are characterized in support of the magnet fabrication and quality control. This report presents microhardness, microstructure, and yield and ultimate tensile strengths as functions of thermomechanical process variables including heat treatment, annealing and cold work for laboratory prepared Incoloy 908 specimens. Empirical correlations have been developed for the microhardness at room temperature and tensile strength at room temperature and at 4K. These results may be used for manufacturing quality control or for design.
(May 1993, MIT Plasma Fusion Center Report #PFC/RR-93-2)

THERMOMECHANICAL PROCESS EFFECTS ON HARDNESS AND GRAIN SIZE IN INCOLOY® ALLOY 908*
L.S. Toma, I.S. Hwang, M.M. Steeves, and R.N. Randall

The relationship between thermomechanical processing, hardness, grain size and mechanical properties has been studied for Incoloy alloy 908, a nickel-iron base superalloy developed for use as a conduit material in Nb₃Sn Cable-In-Conduit Conductors (CICC). The alloy has thermal coefficient of expansion properties tailored for Nb₃Sn, minimizing compressive strain in the superconductor due to cooldown and the associated reduction in critical properties (upper critical field, critical temperature, and critical current). Alloy 908 also has mechanical properties at cryogenic temperatures that compare favorably with other materials. It is therefore one of the candidate materials for use in the magnets of the International Thermonuclear Experimental Reactor (ITER).
(submitted July 1993, Advances in Cryogenic Engineering Materials)

STRESS ACCELERATED GRAIN BOUNDARY OXIDATION OF INCOLOY ALLOY 908 IN HIGH TEMPERATURE OXYGENOUS ATMOSPHERES
M.M. Morra, S. Nicol, L.S. Toma, I.S. Hwang, M.M. Steeves, R.G. Ballinger

A long recognized limitation in the use and fabrication of low coefficient of thermal expansion iron-nickel base (LCIN) superalloys is their extreme susceptibility to intergranular cracking at high temperature in air, termed stress accelerated grain boundary oxidation (SAGBO). As the name indicates, SAGBO is a synergistic effect of stress, residual or imposed, temperature, and the presence of oxygen.
Interest in SAGBO from a cryogenic materials viewpoint is linked to the development of Incoloy alloy 908 as a Nb₃Sn Cable-In-Conduit-Conductor (CICC) magnet sheathing material. Alloy 908 is a low coefficient of thermal expansion iron-nickel base superalloy containing 4 wt% chromium. The key characteristic of SAGBO, of importance to the fabrication and heat treatment of Nb₃Sn CICC magnets, is the intergranular cracking of stressed components at high temperature in oxygen containing environments. Depending upon both the amount of residual 'fabrication' stress present in the magnet conduit before heat treatment and the oxygen concentration of the furnace environment, this can result in cracking of the conduit during heat treatment in the 550°C to 800°C temperature range. Experience has shown that a magnet constructed of alloy 908 can be successfully heat treated in vacuum. For very large scale magnets such as those of the International Thermonuclear Experimental Reactor (ITER) heat treatment in an argon atmosphere may be considered for economic reasons. If an inert gas atmosphere is to be considered, then the tolerable oxygen impurity level must be determined. The purpose of this paper is to present preliminary results of an ongoing development program for alloy 908.
(submitted July 1993, Advances in Cryogenic Engineering Materials)

U.S. CONDUCTOR R&D AND SMALL SCALE EXPERIMENTS
Minervini, Steeves, Montgomery, Randall, Takayasu, Gung, Jeong, Ballinger,
Hwang, Morra, Jang, Toma, Hrycaj, Guss, Ferri, Ahmed, Long, Hall, Reisner, Johnson

During the Engineering Design Activity (EDA) of the ITER program a major effort is being devoted to conductor R&D for the ITER magnets systems. This program includes all aspects of cable-in-conduit-conductor (CICC) manufacturing development such as superconducting strand, large multistage cables, and fabrication of thick-wall and thin-wall conduits. It also includes an extensive program of small scale laboratory measurements and experiments designed to predict the full-size conductor and magnet performance, and to quantify elements of the conductor design guidelines. A description of the manufacturing development and experimental program is given, including experiments for determining AC losses in the superconductor, CICC stability under DC and fast ramped conditions of field and current, and development of novel quench detection techniques.
(submitted October 1993, IEEE/NPSS Proceedings on Plasma Physics)

INCOLOY ALLOY 908 DATA HANDBOOK
L.S. Toma, M.M. Steeves, R.P. Reed

Incoloy alloy 908 is a nickel-iron base superalloy with a face-centered-cubic (FCC) austenitic gamma matrix hardened by precipitation of ordered intermetallic and carbide precipitates. As alloy 908 is a candidate for use in large-scale superconducting magnets, a comprehensive database of the properties of the alloy will be needed for the design and operation of these magnets. This report includes all currently available data on mechanical, elastic, thermal, electrical, and Stress-Accelerated-Grain-Boundary-Oxidation (SAGBO) properties for alloy 908, and defines what data and testing programs are needed.
(March 1994, MIT Plasma Fusion Center Report #PFC/RR-94-2)

INCOLOY ALLOY 908 HIGH CYCLE FATIGUE TEST REPORT
L.S. Toma, M.M. Steeves

Incoloy alloy 908 has been tested in reversed and rotating bending fatigue. Data are presented as S-N curves of cycles to failure at given stress levels. Tests include both extruded conduit and plate material in the annealed, cold worked and aged conditions, intended to simulate the processing conditions of a Nb₃Sn-based cable-in-conduit-conductor. Results of room temperature and 77 K tests are compared with available 7 K data. At room temperature, Incoloy alloy 908 has an endurance limit of about 310 [MPa] (2.1 ksi). 77 K endurance limit is approximately 482 [MPa] (3.3 ksi).
(August 1994, MIT Plasma Fusion Center Report)

Incoloy Alloy 908 High Cycle Fatigue Test Report
J. Feng, L.S. Toma, C.H. Jang, M.M. Steeves

Incoloy Alloy 908 is a candidate conduit material for NB₃Sn cable-in-conduit superconductors. The conduit is expected to experience cyclic loads at 4 K. Fatigue fracture of the conduit is one possible failure mode. So far, fatigue life has been estimated from fatigue crack growth data, which provide conservative results. The more traditional practice of life estimation using S-N curves has not been done for alloy 908 due to a lack of data at room and cryogenic temperatures. This paper presents a series of fatigue test results in response to this need. Tests were performed in reversed bending, rotating bending and uniaxial fatigue machines. The test matrix included different heat treatments, two load ratios (R=-1 and 0.1), two temperatures (298 K and 77 K) and two orientations (longitudinal and transverse). As expected, there is a semi-log linear relation between the applied stress and fatigue life above an applied stress (e.g., 310 MPa for tests at 298 K and R=-1). Below this stress the curves show an endurance limit. The aged and cold-worked materials have longer fatigue lives and higher endurance limits than the others. Different orientations have no effect on life. Cryogenic temperature results in a much higher fatigue life than room temperature. A higher tensile mean stress gives shorter fatigue life. It was also found that the fatigue lives of the reversed bending specimens were of the same order as those of the uniaxial test specimens, but were only half the lives of the rotating bending specimens for given stresses. A sample application of the S-N data is discussed.
(submitted 1995, Advances in Cryogenic Engineering Materials)

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Updated: 4 February 1997