108 New Trends and Developments in Automotive System Engineering volume fraction does not represent a large portion of the total volume This observation seems to be in agreement with recent constitutive models of TWIP steels The SFE plays an essential role in the occurrence of the TWIP effect Although the role of deformationinduced twins will be mainly be discussed in the following paragraphs, it must not be forgotten that the rate of dislocation accumulation will automatically increase when an alloy has a low SFE, independently of twin formation, as the larger dissociation width will more effectively reduce the cross-slip and result in a higher rate of dislocation accumulation As shown in figure 9, it is essential for the occurrence of the strain-induced twinning that the SFE be within a very specific range to observe mechanical twin formation A very low SFE results in the strain-induced transformation to either α’ or ε martensite A low SFE, i.e less that