The stress-corrosion cracking behavior of high-strength aluminum powder metallurgy alloys
- PDF / 4,755,380 Bytes
- 15 Pages / 594 x 774 pts Page_size
- 14 Downloads / 264 Views
I.
INTRODUCTION
ALUMINUM powder metallurgy (P/M) technology enables the refinement of microstructural features in alloys, which can often result in improved properties. Aluminum P/M alloy development has proceeded for over twenty years with a major goal to produce alloys having improved combinations of strength and stress-corrosion cracking (SCC) resistance. Two basic techniques have been used in A1 P/M alloy development--rapid solidification (RS) and mechanical attrition.~'2~3 In RS, small particles of liquid metal are rapidly cooled, thereby refining dendrite and cell size and suppressing phase changes that can lead to segregation. Various RS processes such as atomizationy '6 splat cooling, 5'6'7and molten metal spinning techniques s have been developed and are described elsewhere. In the mechanical attrition process, aluminum powder, and at times other powders, is ball milled to control powder size and shape as well as to introduce strengthening features such as oxides from the powder surfaces.~ The production of sintered aluminum powder, or SAP,9't~ was the first such process and SAP alloys were produced having attractive elevated-temperature properties, but limited ductility. Mechanical alloying ~2A3'~4and reaction milling ~SA6A7are two industrial processes that improve SAP technology. The two most commercially advanced RS alloys are 7091 and 7090,18'~9'2~whose powders are made by air atomization. These alloys are 7xxx-type alloys - - A1-Zn-Mg-Cu-- and are more heavily alloyed than similar ingot metallurgy (I/M) alloys, and also contain cobalt. Cobalt is not commercially added to I/M alloys because it has a very low solubility in solid aluminum and results in coarse segregation during casting. Alloy 7091 is essentially a highly alloyed, *IN is a trademark of the INCO family of companies. J.R. PICKENS and L. CHRISTODOULOU are with Martin Marietta Corporation, Martin Marietta Laboratories, 1450 South Rolling Road, Baltimore, MD 21227. Manuscript submitted September 12, 1984. METALLURGICALTRANSACTIONS A
7075-type alloy containing 0.4 wt pct Co. Alloy 7090 is more alloyed than 7091 in Zn content and contains 1.6 wt pct Co. Both 7091 and 7090 are available on a limited production basis in a variety of mill product forms. Of the new generation of mechanically attrited alloys, IN 905221'22'23 was the first to reach the pilot production stage. It is a non-heat-treatable A1-4 wt pct Mg alloy, similar in Mg content to I/M 5083, and obtains its high strength by the superimposition of several strengthening mechanisms: oxide dispersion strengthening, carbide dispersion strengthening, Mg solid solution strengthening, ultra-fine grain size strengthening, and substructural strengthening. All three alloys are reported to have higher SCC resistance than similar I/M alloys. 19.2o.21For example, numerous comparisons between RS alloys 7090 and 7091 (or their experimental precursors MA 67 and MA 87, respectively) and I/M alloys such as 7075, in stress/time-to-failure (ttf) tests in saline environments, have demonstrated the supe
Data Loading...
