Forming magnesium alloys with improved ductility

Abstract

A magnesium alloy comprising up to about one weight percent of cerium may be hot worked to produce an intermediate or final alloy workpiece that exhibits enhanced ductility at room temperature. The addition of a small amount of cerium may affect the magnesium alloy by reducing yield strength, refining grain size, and improving the work hardening behavior. Recrystallization during hot deformation of the rare earth containing magnesium material alters the texture of the alloy and orients the grains in a manner that favors basal slip activity. The alloy thus deforms at room temperature by a combination of twinning and slip mechanisms.

Description

[0001]This application claims priority based on provisional application 60 / 952,018, titled “Forming Magnesium Alloys with Improved Ductility,” filed Jul. 26, 2007 and which is incorporated herein by reference.TECHNICAL FIELD[0002]This invention generally relates to processed magnesium alloy compositions exhibiting improved ductility at room temperature. More specifically, magnesium alloyed with cerium is subjected to high temperature deformation to improve the alloy's formability at room temperature.BACKGROUND OF THE INVENTION[0003]Magnesium is the lightest structural metal. In engineering applications it is alloyed with one or more elements, for example, aluminum, manganese, rare earth metals, lithium, zinc, and silver. Magnesium usually constitutes eighty-five percent by weight or more of these alloys.[0004]The cost of magnesium has decreased dramatically in recent years and magnesium and its alloys have become attractive structural materials for a wide range of applications due i...

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Benefits of technology

[0009]It was observed that the grain size of the extruded cerium-containing magnesium billets was smaller than the grain size of the extruded pure magnesium billet. The room temperature tensile strength of the cerium-containing billet material was decreased and the percent elongation was significantly increased. Elongation at ultimate load in tensile testing is a measure of ductility. As will be seen in data presented below in this specification, the ductility of the hot deformed cerium-containing magnesium alloys was surprisingly increased compared with magnesium and other known magnesium alloys.

[0010]The addition of cerium in amounts up to about one percent by weight is found to enhance the room temperature ductility and workability of magnesium alloys following suitable hot deformation processing. In a specific embodiment, the hot deformation is accomplished by extrusion at billet temperatures of about 350° C. to about 475° C. with extrusion ratios in the range of about 10:1 to about 60:1 at suitable extrusion speeds. During the hot deformation the billets were suitably lubricated with graphite based lubricants or boron nitride, although this is not necessarily required.

[0011]The presence of the small amount of cerium favors the formation of recrystallized grains with their basal planes oriented at 40-50 degrees to the extrusion axis. The magnesium-cerium alloy provides easier basal slippage during subsequent straining along the extrusion axis which is effectively precluded in the extruded rods of pure magnesium. But whatever the straining mechanism, the presence of about 0.2 to about 0.5 weight percent cerium in the hot worked magnesium matrix markedly increased the ability to further shape the extruded bar material at room temperature. And the improvement was realized whether the hot worked material was solid in cross-section

Problems solved by technology

However, the use of magnesium in wrought products like sheet and extrusions has been limited due to the poor workability of magnesium castings and the lower formability and ductility of magnesium in the primary fabricated stage.

At room temperature, pure magnesium is generally characterized by limited ductility as a result of its hexagonal close-packed crystal structure and resulting limited number of active slip systems.

This inherent limitation often discourages widespread use of magnesium in wrought products made from sheets and extrusions because it is diffi

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