The iron-manganese TWIP steels, which contain 17-20% of manganese, derive their exceptional properties from a specific strengthening mechanism: twinning. The steels are fully austenitic and nonmagnetic, with no phase transformation. The formation of mechanical twins during deformation generates high strain hardening, preventing necking and thus maintaining a very high strain capacity.
Generally, the metal continues to strain-harden all the way up to fracture, so that the stress required to produce further deformation should also increase. If the true stress, based on the actual cross-sectional area of the specimen, is used, it is found that the stress-strain curve increases continuously up to fracture. If the strain measurement is also based on instantaneous measurements, the curve, which is obtained, is known as a true-stress-true-strain curve.
The engineering tension test is widely used to provide basic design information on the strength of materials and as an acceptance test for the specification of materials. The parameters, which are used to describe the engineering stress-strain curve of a metal, are the tensile strength, yield strength or yield point, percent elongation, and reduction of area.
The main groups of aluminum alloys which are the most often used in practice besides technically pure aluminum are AlMn, AlMg, AlMgMn, AlMgSi, AlZnMg, and AlZnMgCu alloys. These are wrought alloys which are shaped into products by rolling, extrusion, and forging. Each of the mentioned groups consists of numerous subgroups, depending on amounts of main and additional alloying elements, and they have tensile strength values varying in a wide range from 70 to 600 MPa.