tailieunhanh - New Trends and Developments in Automotive System Engineering Part 4

Tham khảo tài liệu 'new trends and developments in automotive system engineering part 4', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | 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 deformation-induced 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 a or g martensite. A low SFE . less that 20mJ m2 favors the Y -g transformation. As the SFE is an essential parameter there has been a considerable interest in determining its value for TWIP steels. There is still considerable uncertainty about the exact value of the SFE in Mn alloys and whereas the theoretical evaluations agree on the range there is still considerable scatter in the reported SFE values. There are currently no experimental SFE available for most TWIP alloy systems but a considerable number of theoretical calculations are available in the literature. From a theoretical point of view the SFE is proportional to the . and . free energies difference AGY-g. Interfacial energy AGY-gsurface and magnetic energy contribution AGY-gmagnetic to the stacking fault energy need to be taken into account as they may have a significant influence 1 Y AGYT AGf AG ic 8 V The interfacial energy can be taken as the coherent twin boundary energy and the energy of the twinning dislocations. The high value of the stacking fault energy at the . .

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