FeNiMnAl alloys show a wide range of microstructures and mechanical properties, but have been little explored. Studies on four different types of microstructures in this alloy system will beoutlined: 1) ultrafine microstructures (5-50 nm), present in Fe30
,which consist of (Fe, Mn)-rich B2-ordered (ordered b.c.c.) and (Ni, Al)-rich L21
-ordered (Heusler) phases, and in Fe30
,which consist of (Ni, Al)-rich B2 and (Fe, Mn)-rich b.c.c. phases, with the phases aligned along <100>; 2) fine microstructures (50-70 nm), present in Fe30
, and Fe28
, which consist of alternating (Fe,Mn)-rich f.c.c and (Ni, Al)-rich B2-ordered plates with an orientation relationship close to f.c.c.(002)//B2(002); f.c.c.(011)//B2(001); 3) coarser (0.5-1.5µm) lamellar microstructures observed in alloys with a lower aluminum content, such as Fe30
, that consistof alternating (Fe,Mn)-rich f.c.c and (Ni, Al)-rich B2-ordered phases with a Kurdjumov-Sachs orientation relationship between the phases; and 4) high-entropy Fe40.4
alloys. The microstructures and mechanical properties in these alloys have been determined as a function of annealing time, testing temperature and strainrate. Some of the unusual mechanical behavior that has been observed will be emphasized. This research was supported by the US Department of Energy (DOE), Office of Basic Energy Sciences (grant DE-FG02-07ER46392).