S. Deville et al Acta Materialia 52(2004)5709-5721 ants, the trace of the junction plane being in a constant transformation completion of the grain cannot proceed location. This behavior is well understood considering anymore, until an external event can provide supple the variants progressively grow into the volume of the mentary stresses to overcome the energy barrier opposed mple, while the junction plane remains in the same to transformation completion. The remaining untrans position(Fig. 2(b). All the transformation strain is formed part is transformed very rapidly. The grain goes again accommodated; the variants are at equilibrium from very partial transformation to transformation during all the growth steps. More complex arrange- completion during the last treatment step ments, like the so-called"L-arrangement[17 can be Though no statistical analysis was performed on the obtained by this mode, as shown in the last micrographs different growth modes, it seems from our experimental f Fig. 2(a). Since the variants grow in a direction op- observations the internal growth mode is the first mod posed to the junction plane, this mode will be called"ex- activated, after an apparent initial incubation period of ternal growth about 60 h, during which no surface transformation at all seemed to be observed. The external growth mode 3.1.3. Needle growth follows, and finally the isolated needle growth appears The last growth mode observed in these experiments Hence, the controlling factors of each growth mode corresponds to the transformation sequence observed in must be different Fig. 3. Again, transformation is very likely nucleating at the surface. An isolated martensite needle is formed and 3. 2. Variants growth kinetics grows in size. As soon as it is formed, it begins to gener ate opposite stresses(back stresses) in the surrounding sing experimental ob ons like in Figs. I(a). matrix, opposing continued transformation of the initial 2(a) and 3, the size of eac nt can be measured as variant. Since no complementary variants that could a function of their age, so the kinetic of variants accommodate the transformation strain are observe growth are obtained for every growth mode. These in the first steps, the back stresses are building up as kinetics are given, respectively, in Figs. 4-6. The width the variant thickens, until the formation of a second is normalized to the width of the fully transformed var variant is triggered by the shear components of the back ants for comparison between the three modes. Growth stresses. This second variant will again grow in size and speeds in each case were measured before normalization the formation of a third variant with this last Differences are observed between the three modes configuration, the added stresses are so high that the though similarities are also found. The simplest case is Fig 3. Observation of consecutive isolated needles growth. Aging steps of 20 h.ants, the trace of the junction plane being in a constant location. This behavior is well understood considering the variants progressively grow into the volume of the sample, while the junction plane remains in the same position (Fig. 2(b)). All the transformation strain is again accommodated; the variants are at equilibrium during all the growth steps. More complex arrange￾ments, like the so-called ‘‘L-arrangement’’ [17] can be obtained by this mode, as shown in the last micrographs of Fig. 2(a). Since the variants grow in a direction op￾posed to the junction plane, this mode will be called ‘‘ex￾ternal growth’’. 3.1.3. Needle growth The last growth mode observed in these experiments corresponds to the transformation sequence observed in Fig. 3. Again, transformation is very likely nucleating at the surface. An isolated martensite needle is formed and grows in size. As soon as it is formed, it begins to gener￾ate opposite stresses (back stresses) in the surrounding matrix, opposing continued transformation of the initial variant. Since no complementary variants that could accommodate the transformation strain are observed in the first steps, the back stresses are building up as the variant thickens, until the formation of a second variant is triggered by the shear components of the back stresses. This second variant will again grow in size and trigger the formation of a third variant. With this last configuration, the added stresses are so high that the transformation completion of the grain cannot proceed anymore, until an external event can provide supple￾mentary stresses to overcome the energy barrier opposed to transformation completion. The remaining untrans￾formed part is transformed very rapidly. The grain goes from very partial transformation to transformation completion during the last treatment step. Though no statistical analysis was performed on the different growth modes, it seems from our experimental observations the internal growth mode is the first mode activated, after an apparent initial incubation period of about 60 h, during which no surface transformation at all seemed to be observed. The external growth mode follows, and finally the isolated needle growth appears. Hence, the controlling factors of each growth mode must be different. 3.2. Variants growth kinetics Using experimental observations like in Figs. 1(a), 2(a) and 3, the size of each variant can be measured as a function of their age, so that the kinetic of variants growth are obtained for every growth mode. These kinetics are given, respectively, in Figs. 4–6. The width is normalized to the width of the fully transformed var￾iants for comparison between the three modes. Growth speeds in each case were measured before normalization. Differences are observed between the three modes, though similarities are also found. The simplest case is Fig. 3. Observation of consecutive isolated needles growth. Aging steps of 20 h. S. Deville et al. / Acta Materialia 52 (2004) 5709–5721 5713