V. J. Toplosky

Mechanical properties of MP35N as a reinforcement material for pulsed magnets

A cobalt multiphase alloy, MP35N, is studied as one of the reinforcement materials for pulsed magnets. The mechanical properties of this alloy at room temperature and 77 K are examined. The cold-rolled and aged MP35N produces a hardness of 5650 MPa and yield strength of 2125 MPa at room temperature. At 77 K, the yield strength reached 2500 MPa and the work hardening rate was higher than that at room temperature. The Youngs modulus increases about 6% upon cooling from 300 to 5 K. Therefore, the increase of the strength at low temperatures is attributed mainly to the increase of the work hardening rate rather than modulus. The potential for further increasing the strength of this alloy is discussed.

Properties of high strength Cu-Nb conductor for pulsed magnet applications

Various tests have been undertaken to study the effects of annealing, testing temperatures and volume fraction of the Cu cladding on the properties of Cu-Nb conductors being developed for pulsed magnet applications. The results demonstrate that short time annealing used for insulation had no significant effect on the tensile property of Cu-Nb conductors. The cryogenic temperatures are beneficial to both the conductivity and mechanical properties of the conductors, especially the tensile strength of the Cu cladding. The wire-drawing fabrications showed that wires of 4 mm/spl times/6 mm cross-section-area with a significant volume fraction of Cu cladding could be obtained, leading to final tensile strengths of up to 1100 MPa at room temperature. The strength is increased by about 20% at 77 K. The 77 K-conductivity is about 4.5 times of the room temperature one. The strengthening mechanisms and resistivity variation of the Cu-Nb composite are discussed and it is argued that the distance between the Nb ribbons plays an important role in the variation of these properties.

Effect of an Aging Heat Treatment on the 4 K Fracture and Fatigue Properties of 316LN and Haynes 242

Since the introduction of the cable‐in‐conduit conductor (CICC) concept, a variety of alloys have been proposed for fabricating the jacket. The jacket provides primary containment of the liquid helium coolant and is typically also the primary structural component for the magnet coils. These functions create requirements for strength, toughness, fatigue crack resistance, and fabricability. When the CICC uses Nb3Sn superconductor, the conduit alloy must retain good mechanical properties after exposure to the superconductor’s reaction heat treatment. Here we present data from cryogenic fracture toughness and fatigue crack growth rate tests on 316LN and a Cr‐Mo‐Ni base super‐alloy (Haynes 242) at 4 K before and after the exposure to the heat treatment. These alloys are presently being considered as candidates for use in the next‐generation series connected hybrid magnet for the NHMFL. Both of the alloys are found to have adequate fatigue and fracture properties for the CICC application while the superalloy ha...

Impacts of Heat Treatment on Properties and Microstructure of Cu16at%Ag Conductors

CuAg in situ composite conductors are used as conductors for Florida Bitter magnets and potential candidates for pulsed magnets in the National High Magnetic Field Laboratory, and are likely exposed to temperatures higher than ambient during operations in magnets. The conductors are fabricated by cold rolling that introduces lattice distortions and high densities of interfaces in a unit volume. High temperature exposure by the conductors may affect the characteristics of the lattice distortions and the interfaces. The lattice distortion and density of the interface affects the mechanical properties of the conductors, such as the tensile and yield strength, as well as the electrical conductivity of the composites. Understanding the performance of the conductors after they are exposed to high temperature heat treatments helps one to make good use of them for magnets and to manufacture conductors to meet the requirements of the magnets, particularly when the magnetic stress reaches the limit of the mechanical strength of the conductors. The goal of this research is to understand the microstructure evolution of the Cu16at%Ag after the high temperature heat treatment and to relate such microstructural features to mechanical tensile strength and electrical conductivities.

EFFECTS OF WINDING STRAIN AND HEAT TREATMENT ON PROPERTIES OF 316 LN AND HAYNES 242

The outer coils of the hybrid magnets at the NHMFL are superconducting magnet and use Cable‐in‐Conduit‐Conductor (CICC) technology. This technology requires us to wind the coils before the Nb3Sn heat treatment is undertaken. The winding introduces both tensile and compressive stresses to the conduit alloys. The subsequent heat treatment has to be done when the conduit alloys are under the pre‐stress. We have simulated the conduit heat treatments with the alloys under various stress levels, and undertaken tensile tests at 4 K and microstructure examinations. The results indicate that the pre‐stress before the heat treatment influences the microstructure and therefore tensile test properties of the conduit alloys at 4 K. The tensile test property changes are related to the grain boundary precipitation variation introduced by pre‐stress.

Fatigue Property Examinations of Conductors for Pulsed Magnets

Cu matrix composites are used as conductors for pulsed magnets that have potential to reach 100 T. The conductors are fabricated by cold drawing that introduces high densities of dislocations or interfaces and internal stress. The density of the dislocation and the interface affects the mechanical properties of the conductors, such as the tensile strength and fatigue endurance at 77 K of the composites. Understanding the performance of the conductors under cyclic loading, i.e. fatigue properties, helps one to make good use of them for pulsed magnets and to manufacture conductors to meet the requirements of the magnets, particularly when the magnetic stress reaches the limit of the mechanical strength of the conductors. The goal of this research is to understand the fatigue properties of a Cu-0.085wt%Ag conductor and to relate such properties to mechanical tensile strength, dislocation densities and interface structures. The fatigue test loading is either in stress-controlled or strain-controlled mode. This work sheds a new light on the correlation between the tensile and fatigue properties at 77 K by consideration of dislocation densities and precipitate in particle strengthened conductors.

MECHANICAL PROPERTIES OF MODIFIED JK2LB for Nb3Sn CICC APPLICATIONS

Since the introduction of the cable‐in‐conduit conductor (CICC) concept, a variety of alloys have been proposed for fabricating the conduit in high field magnets. The conduit provides containment of the liquid helium coolant and is typically also the primary structural component for the magnet coils. These functions create requirements for strength, toughness, fatigue crack resistance, and fabricability. When the CICC uses Nb3Sn superconductor, the conduit alloy must retain good mechanical properties after exposure to the superconductor’s reaction heat treatment. Here we present data from cryogenic tensile, fracture toughness, fatigue crack growth rate, and axial fatigue tests for a modified heat of JK2LB, before and after the exposure to the reaction heat treatment. The alloy is presently being considered as a candidate for use in ITER Central Solenoid (CS) Coils. The direct comparison of the data from the comprehensive test program with earlier versions of JK2LB and another CICC candidate alloy (modifie...

Cold work study on a 316LN modified alloy for the ITER TF coil conduit

In cable-in-conduit conductor (CICC) magnets, such as the ITER TF coils, the conduit is the primary structural component. This function creates requirements for 4 K strength, toughness, fatigue crack resistance, and ductility after exposure to the superconductors reaction heat treatment. The tensile ductility of a steel is a quality factor related to fatigue and fracture resistance that can be evaluated more economically with tensile tests rather than fatigue and fracture tests. Here we subject 316LN modified base metal and welds to a range of cold work from 0% to 20% and a subsequent Nb3Sn reaction heat treatment to evaluate the effects on the tensile properties. With the addition of cold work, the 4 K yield strength increases while tensile elongation decreases in both the base metal and weld. The results are compared to previously published data on the same alloy to evaluate the use of tensile ductility parameters as a materials qualification specification in magnet design.

FRACTURE TOUGHNESS MEASUREMENTS AND ASSESSMENT OF THIN WALLED CONDUIT ALLOYS IN A CICC APPLICATION

The Series-Connected Hybrid Magnets under construction at the NHMFL use Cable-in-Conduct-Conductor (CICC) technology. The 4 K mechanical properties of the conduit are extremely important to the performance and reliability of the magnets. We have measured tensile and fracture toughness of two candidate conduit alloys (Haynes 242 and modified 316LN) in various metallurgical states, with emphasis on the final state of production. To assess the material in its final production state, non-standard specimens are removed directly from the round-corner rectangular conduit and tested after exposure to a simulated Nb{sub 3}Sn reaction heat treatment. Non-standard middle-tension (MT) fracture toughness specimens enable toughness evaluation of the base metal, welds and weld/base transitional region in the as-fabricated conduit with final dimensions not suitable for conventional fracture toughness specimens. Although fracture toughness tests of the thin walled conduit fail to meet ASTM test validity requirements they provide a qualitative evaluation and estimate of the fracture toughness of the conduit and the welds.

Microstructure and Cryogenic Mechanical Properties of a 316L Plate and Its Weldments

This paper reports a study to relate microstructure to cryogenic mechanical properties of a 316L plate and its weldments. Both the tensile and fracture toughness test values at 4 K are governed by microstructure that is influenced by the thermo‐mechanical treatment of the materials. The mechanical properties are better when the loading direction is parallel to the rolling directions. 77 K Charpy impact values show much stronger dependence on the orientations than 4 K fracture toughness and tensile test values. This indicates that the anisotropy in microstructure results in much higher anisotropy in fracture mechanisms in dynamic mode than in static mode. Therefore, care has to be taken when one relates the 77 K Charpy impact strength to 4 K fracture toughness. Stress relieve in weldment enhances the fracture toughness and yield strength, but reduces the strain‐hardening rate and ultimate tensile strength.