Investigação das características de superplasticidade de um aço do sistema Fe-Mn-Al

Abstract

The alloys of Fe-Mn-Al system have been rather studied in last few years indicating good potential for application, with the benefit of exhibiting 10 to 15% less density, coupled to good performance related to its mechanical properties and strength at room and cryogenic temperatures, being an alternative for replacing some alloys of the Fe-Cr-Ni system, in certain situations. However, the literature presents a very limited amount of information about the mechanical properties of these materials at high temperatures, as in the case of hot tensile and creep testing. The superplasticity phenomenon remains as an entirely unexplored issue for this kind of steel. This work was planned with the objective of undertaking for the first time a systematic study on the superplastic behavior of an austenitic steel of this system, with chemical composition Fe 24.5Mn 6.5Al 1.5Si 1.1C (weight %). The material was prepared by different termomechanical processing routes, so as to obtain sheets with thickness around 1 mm, having a fine grained equiaxial, dual phase austenite / ferrite structure, with grain size around 3 µm. The material was submitted to both tensile testing ( at constant crosshead speed ) and creep testing ( at constant load ), on a temperature range from 600 to 1000oC, and strain rates raging from 10-6 to 1 s-1. A set of tensile tests was carried out monotonically using separate specimens until rupture under different combinations of crosshead speed and temperature. In another set the specimens were subjected to a sequence of crosshead speed changes at the ultimate tensile stress level, at a certain temperature. In both case the parameter m (strain rate sensitivity exponent) could be determined. The creep experiments enabled obtaining parameters like n (the stress exponent), Qf (the apparent activation creep energy) and ó0 (the threshold stress). Values of åf (maximum elongation observed at rupture) could be also obtained from both the tensile test and creep tests. The results from both procedures, i.e. tensile and the creep testing, were compared together showing good agreement with each other. The largest values of åf (around 660 %) associated to the largest m values (around 0,54) were observed at 800°C, for strain rates in the range from 10-4 to 10-3 s-1 ( in the case of tensile testing) and applied stress in the range from 20 to 50 MPa ( in the case of creep testing). Tensile tests carried at constant strain rate and creep tests carried out at constant stress allowed even greater values of these parameters to be achieved, at the maximum strain rate sensitivity region. A tensile test performed at constant Ý = 2,47 x 10-4 s-1 produced a maximum elongation at rupture åf = 750% and a creep test with constant σ = 30 MPa a maximum elongation åf = 737 % (without rupture of the specimen). Measurements of activation energy from creep tests indicated that the Fe-Mn- Al alloy exhibits values around that of grain boundary self diffusion of Fe in Austenite, in agreement with which is expected from the superplastic flow process. The analysis of creep test results also revealed that the material presents a Threshold Stress (óo ) around 6 7 MPa from 700 to 800ºC, and óo . 30 MPa at 900ºC. In this way, all the data from the variation of strain rate with stress ( Norton diagram ) could be rationalized by a single stress exponent n = 2, which is typical of the grain boundary sliding mechanism. Metallographic observation by optical microscopy on specimens from both the tensile and creep tests indicated that the grain size structure remains equiaxial and essentially stable even after the great levels of deformation attained at the various temperatures, which confirms the superplastic behavior condition achieved in this work.