Grain refinement of alloys using magnetic field processing

Abstract

A method for refining the grain size of alloys which undergo ferromagnetic to paramagnetic phase transformation and an alloy produced therefrom. By subjecting the alloy to a timed application of a strong magnetic field, the temperature of phase boundaries can be shifted enabling phase transformations at lower temperatures.

Description

[0001] This application claims the benefit of U.S. Provisional Application 60 / 340,311 filed Dec. 14, 2001FIELD OF THE INVENTION [0002] The present invention relates to the production of refined grain structures in structural alloys. The refined grain structures are useful in designing superior structural alloys with step-out combination of mechanical properties such as strength and toughness. The invention includes the application of a high strength magnetic field to shift the phase boundaries of alloys and thereby induce phase transformation. The method includes the alternate application and cessation or decrease in strength of such magnetic field and the attendant rapid forward and reverse phase transformation leading to progressive refinement of the initial coarse grain structure of the alloy into fine equiaxial grains. Equiaxial or equiaxed grains or crystallites have approximately equal dimensions in the three coordinate directions. BACKGROUND OF THE INVENTION [0003] Increasing...

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Benefits of technology

[0016] The invention includes a method for refining the grain size by applying a magnetic field in alloys to reversibly induce phase transitions between ferromagnetic and paramagnetic phases. Other magnetic phases are envisioned but less preferred. This phase transformation can be induced by changes in application of a magnetic field with or without a change in temperature. This invention is based on the effect of a magnetic field fundamentally lowering the free energies and enhancing the thermodynamic stability of the ferromagnetic phase(s), resulting in shifting of the phase boundaries. For this invention the two phases (e.g., ferromagnetic and paramagnetic phases) have different chemistries and/or preferably different crystalline structures and transition from one phase to the other phase requires a chemistry (e.g., precipitates) and/or crystalline structure change. The magnetic field is applied and ceased or decreased for one or more cycles to obtain the desired equiaxed grain size. The number of cycles is preferably less than 100, more preferably less than 10, even more preferably less than 5. The time between cycles is preferably about the same as the time the magnetic field is applied, but can be up to 10 times shorter or greater. Ramping time during increasing or decreasing the magnetic field is preferably minimized. Ramp up and ramp down times for 5%←→95% of the peak magnetic

Problems solved by technology

U.S. Pat. No. 3,723,194 proposes rapidly heating a material from its initial α state to a temperature inside the α+γ dual phase region, thus inducing instability that provides superplasticity.

The limitation with current methods for grain refining is concerned with the conflicting requirements for efficient and uniform grain refinement: high nucleation rat

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