The effect of matrix microstructure on the stress-controlled fatigue behavior of a 2080 Al alloy reinforced with 30 pct SiC particles was investigated. A thermomechanical heat treatment (T8) produced a fine and homogeneous distribution of SЈ precipitates, while a thermal heat treatment (T6) resulted in coarser and inhomogeneously distributed SЈ precipitates. The cyclic and monotonic strength, as well as the cyclic stress-strain response, were found to be significantly affected by the microstructure of the matrix. Because of the finer and more-closely spaced precipitates, the composite given the T8 treatment exhibited higher yield strengths than the T6 materials. Despite its lower yield strength, the T6 matrix composite exhibited higher fatigue resistance than the T8 matrix composite. The cyclic deformation behavior of the composites is compared to monotonic deformation behavior and is explained in terms of microstructural instabilities that cause cyclic hardening or softening. The effect of precipitate spacing and size has a significant effect on fatigue behavior and is discussed. The interactive role of matrix strength and SiC reinforcement on stress within "rogue" inclusions was quantified using a finite-element analysis (FEA) unit-cell model.
1 his paper explores ad\ antages of ponder ~netallurgical (Plh4) processing of Alloj 7 18 using gas atomization and hot isostatic pressing (HIP). To e1,aluate the effect of different processing parameters on HIP PIAM 718. pouders uith lou and standard carbon lelels were produced b j nitrogen and argon atomization. A small amount of Mg mas added to one heat to imestigate the possibilitj of improving elevated temperature properties. The Mg-treatment is a necessitj to obtain good creep properties. L I'he consolidated materials are f~dl! dense and ha\ e verj fine and homogeneous n~icrostructures t j pica1 for fullj dense PIM alloj s. Yield strength lelels range from 140 to 170 ksi at room te~npesature. and from 1 15 to 145 ksi at 1200°F. \I hile tensile ductilities range f o r~n 19 to 15 and 9 to 24%. respectivelj . Different heat treatments mere ebaluated. since the standard heat treatment for cast and l~rought 71 8 led to hea\;4 decoration of prior p o~~d e r particle and grain boundaries. \\hich resulted in 10% ductilitj. especially at 1200°F. Creep rupture properties mere elaluated at 1200°F and 100ksi. Depending on heat treatment. time to rupture Lvas as high as 170 hrs. Higher solution temperatures resulted in n~icrostructures uith less grain boundarq decoration. better d~lctilities and i m p r o~ ed creep properties mith little loss in strength. During solutioning at 1800°F. no grain grouth n a s o b s e r~ ed. and \. e q limited grain grou-th %as obser~ ed during heat treatments at temperatures up to 2200°F. Taking into account the relath el) lou le\ els of precipitate forming elements. the strength levels are still fairlj good. Superalloys 718. 625. 706 and Various Derivatives Edited b! E.A. L~r i i i ThlS ( P h s XIinerals. &letals (9: Matsrials Societ) ). 2001
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