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Fig. 10. Fe-55Mn-3Al-3Si mass% TWIP-steel: Yield stress R p0.2 , tensile strength R m , uniform elongation e un and total elongation e f as functions of test temperature.

High-strength TRIPLEX light-weight steels of the generic composition Fe-xMn-yAI-zC contain 18 -28 % manganese, 9 -12 % aluminium, and 0.7 -1.2 % C (in mass %). The microstructure is composed of an austenitic y-Fe(Mn, AI, C) solid solution matrix possessing a fine dispersion of nano size x-carbldes (Fe,Mnh AIC 1x and u-Fe(AI, Mn) ferrite of varying volume fractions. The calculated Gibbs free energy of the phase transformation Ylcc -+ Ehcp amounts to AGY~' = 1757 J/mol and the stacking fault energy was determined to rSF = 110 mJ/m 2 • This indicates that the austenite is very stable and no strain induced a-martensite will be formed. Mechanical twinning is almost inhibited during plastic deformation. The TRIPLEX steels exhibit low density of 6.5 to 7 g/cm 3 and superior mechanical properties, such as high strength of 700 to 1100 MPa and total elongations up to 60 % and more. The specific energy absorption achieved at high strain rates of 10 3 s' is about 0.43 J/mm 3 . TEM investigations revealed clearly that homogeneous shear band formation accompanied by dislocation glide occurred in deformed tensile samples. The dominant deformation mechanism of these steels is shear band induced plasticity -SIP effect-sustained by the uniform arrangement of nano size K-carbides coherent to the austenitic matrix. The high flow stresses and tensile strengths are caused by effective solid solution hardening and superimposed dispersion strengthening.

Abstract. Deformation twinning, martensitic phase transformation and mechanical properties of austenitic wt%Mn alloys with additions of aluminium and silicon have been investigated. Tensile tests were carried out at different strain rates and temperatures. The formation of twins, a'-and c-martensite during plastic deformation was analysed by optical microscopy, X-ray diffraction, scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The stacking fault energy y h and the free energy AG"+E for the y +~ phase transformation were calculated using the regular solution model. It is known that additions of aluminium increase yfc, and therefore strongly suppress the y+c transformation while silicon decrease yf,, and sustains the y-E transformation. The y-+c phase transformation takes place in alloys with yk, 5 20 mllm2. The stacking fault energy of the Fe-25Mn-3Si-3A1 alloy was calculated as a function of temperature and related with microstructural changes of the strained sample at different temperatures. These steels with reduced density of about 7,3 g/cm-3 combine high tensile ductility up to 80 % at high strain rates with true tensile strength of about 1000 MPa. The excellent plasticity induced by twinning and additional phase transformation up to extreniely high strain rates of about E = lo3 s-' results in an extraordinary shock resistence and enables deep drawing and backward extrusion operations of parts with complex shapes and high production rates.

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A nickel alloy of a composition similar to that of the nickel based superalloy Inconel alloy 718 (IN718) was produced with the electron beam melting (EBM) process developed by Arcam AB. The microstructures of the as processed and heat treated material are similar to that of conventionally produced IN718, except that the EBM material showed some porosity and the δ phase did not dissolve during the solution heat treatment because the temperature of 1000°C apparently was too low. Mechanical testing of the layer structured material, parallel and perpendicular to the built layers, revealed sufficient strength in both directions. However, it showed only limited elongation when tested perpendicular to the built layers due to local agglomerations of pores. Otherwise, data for the hardness, Young's modulus, 0·2 yield tensile strength and ultimate tensile strength match those recommended for IN718.