tailieunhanh - Kinetics of Materials - R. Balluff_ S. Allen_ W. Carter (Wiley_ 2005) Episode 14

Tham khảo tài liệu 'kinetics of materials - r. balluff_ s. allen_ w. carter (wiley_ 2005) episode 14', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | 574 CHAPTER 24 MARTENSITIC TRANSFORMATIONS When the shape change is relatively large the parent phase will no longer be able to accommodate the inclusion elastically and anticoherency lattice dislocations will be generated to relieve the long-range stress field and reduce the elastic energy. Plastic deformation will therefore occur in the parent phase and anticoherency dislocations will be added to the interface. These dislocations will generally tend to reduce the mobility of the interface. Because martensite interfaces can be represented as arrays of dislocations the velocity with which they move will generally be controlled by the same factors that control the rate of glide motion of crystal dislocations. As discussed in Section these include dissipative drag due to phonons and free electrons and interactions with a large variety of different types of crystal imperfections which hinder their glide motion. When the martensite forms as enclosed platelets as in Fig. additional work must also be done to produce the increase in interfacial area that occurs as the platelets grow. An extensive discussion of the factors involved in the motion of martensite interfaces has been given by Olson and Cohen 9 . As pointed out in Section there is no clear evidence for the supersonic motion of martensite interfaces. However velocities on the order of the speed of sound can be achieved in the presence of large driving forces. NUCLEATION OF MARTENSITE The homogeneous nucleation of martensite in typical solids is too slow by many orders of magnitude to account for observed results. Calculations of typical values of AỊỷc using the classical nucleation model of Section see Exercise yield values greatly exceeding 76 kT. Furthermore nearly all martensitic transformations commence at very sparsely distributed sites. Small-particle experiments 14 have yielded typical nucleation densities on the order of one nucleation event per 50 zm diameter Fe-Ni .