YL Zhang et al. Acta Materialia 54(2006)1289-1295 Fig 4. In situ TEM images for stress-induced martensitic transformation in 8Ce-025Y-TZP. The stress-induced martensite forms(marked by an arrow in (a))and grows(b, c) when focusing the electron beam, shrinks (d, e)and disappears(f) when defocusing the electron beam. Fig 3(e)disappeared, implying the occurrence of a reverse whereas it is a non-thermoelastic transformation when transformation. By carefully controlling the electron beam, none of them is satisfied. A semi-thermoelastic martensitic the reversible motion of the t/m interface in the same grain transformation can be identified if the three criteria are was recorded, as shown in Fig. 4 partially satisfied. When switching the electron beam on again, a new The transformation characteristics of the t-m tran monoclinic lath marked by the arrow in Fig 4(a)was intro- formation in Ce-Y-TZP revealed by the in situ TEM duced, and then continuously grew as shown in Figs. 4(b) observations indicated that the reversibility of the motion ind(c). While defocusing, the lath receded and eventually of the interface and the contribution of the stored energy disappeared(Figs. 4(d)f). These results indicate a for reverse transformation indeed occur in the thermal smooth motion of the t/m interface corresponding to ther- stress-induced martensite. However, the critical driving mal stress resulting from the focusing and defocusing of the force for Ce-Y-TZP is m4000 J/mol and the thermal electron beam hysteresis of 8Ce-0 25Y-TZP is As-Ms=437C [35] These values are orders of magnitude differer IScussIon compared to x10 J/mol and 10C of the corresponding values for a typical thermoelastic transformation [33] 5.1. Semi-thermoelastic feature This means that the t-m martensitic transformation in Ce-Y-TZP only partially satisfies the three criteria Based on previous work [29-34], Hsu and co-workers outlined above. Hence, the t-m martensitic transfor [13, 14] suggested the criteria for the thermoelastic transfor- mation in Ce-Y-TZP is suggested as a semi-thermoelastic mation as follows: (1)a small critical driving force and a transformation small hysteresis;(2) the reversibility of the motion of the interface between martensite and parent phases; and(3) 5.2. Reverse martensitic transformation in Ce-Y-TZP the shape strain is accommodated elastically and the store energy in martensite can contribute a part of the driving It is found that the t- m reverse transformation of our force for the reverse transformation. A transformation is in situ TEM observations may occur through two different thermoelastic when all of these three criteria are satisfied, modes: (1) the receding of the transformed monoclinic lathFig. 3(e) disappeared, implying the occurrence of a reverse transformation. By carefully controlling the electron beam, the reversible motion of the t/m interface in the same grain was recorded, as shown in Fig. 4. When switching the electron beam on again, a new monoclinic lath marked by the arrow in Fig. 4(a) was intro￾duced, and then continuously grew as shown in Figs. 4(b) and (c). While defocusing, the lath receded and eventually disappeared (Figs. 4(d)–(f)). These results indicate a smooth motion of the t/m interface corresponding to ther￾mal stress resulting from the focusing and defocusing of the electron beam. 5. Discussion 5.1. Semi-thermoelastic feature Based on previous work [29–34], Hsu and co-workers [13,14] suggested the criteria for the thermoelastic transfor￾mation as follows: (1) a small critical driving force and a small hysteresis; (2) the reversibility of the motion of the interface between martensite and parent phases; and (3) the shape strain is accommodated elastically and the stored energy in martensite can contribute a part of the driving force for the reverse transformation. A transformation is thermoelastic when all of these three criteria are satisfied, whereas it is a non-thermoelastic transformation when none of them is satisfied. A semi-thermoelastic martensitic transformation can be identified if the three criteria are partially satisfied. The transformation characteristics of the t ! m trans￾formation in Ce–Y-TZP revealed by the in situ TEM observations indicated that the reversibility of the motion of the interface and the contribution of the stored energy for reverse transformation indeed occur in the thermal stress-induced martensite. However, the critical driving force for Ce–Y-TZP is
4000 J/mol and the thermal hysteresis of 8Ce–0.25Y-TZP is As Ms = 437 C [35]. These values are orders of magnitude different when compared to
10 J/mol and
10 C of the corresponding values for a typical thermoelastic transformation [33]. This means that the t ! m martensitic transformation in Ce–Y-TZP only partially satisfies the three criteria outlined above. Hence, the t ! m martensitic transfor￾mation in Ce–Y-TZP is suggested as a semi-thermoelastic transformation. 5.2. Reverse martensitic transformation in Ce–Y-TZP It is found that the t ! m reverse transformation of our in situ TEM observations may occur through two different modes: (1) the receding of the transformed monoclinic lath Fig. 4. In situ TEM images for stress-induced martensitic transformation in 8Ce–0.25Y-TZP. The stress-induced martensite forms (marked by an arrow in (a)) and grows (b,c) when focusing the electron beam, shrinks (d,e) and disappears (f) when defocusing the electron beam. Y.L. Zhang et al. / Acta Materialia 54 (2006) 1289–1295 1293