J. P. Hirth et al. Acta Materialia 54(2006)1917-192 4. Discussion The defect-based TM gives predictions for the martens- ite habit planes in closer agreement with experiment than do the IPS/PTMC theories. Because of the large magnitude of b2 in the present case, differences between the two types of theory are expected [15]. The strains involved in the transformation are large, though, so it may be questioned whether a terrace/disconnection model of the interface applies. An alternative would be a nominally planar inter- Trace of (323) face with large local disregistry, resembling that in a high angle grain boundary or disclination. Electron microscopy Fig.I1.Trace of (323)slip plane intersecting a habit plane. Burgers could only resolve this question by lattice imaging, which vector bL, arbitrarily inclined to the terrace plane, has in-terrace-plane has not yet been achieved in Pu, largely due to its strong omponents bi and b propensity to oxidize Molecular dynamics simulations of the interface would help to resolve this issue. Nevertheless would be expected. Fig 1l shows the trace of the(323) the near parallelism of (111)8//010) and of plane. The Burgers vector would have both b1 and b3 [11018 //[1001 are consistent with a disconnection/terrace components in general. The b3 component would relieve plane structure and encourage us to accept its feasibility the E33 coherency strain. The b1 component would in part A final discussion point concerns the large hysteresis elieve the Eu strain hence fewer disconnections would observed for the reverse ato 8 transformation. This needed to achieve a strain-free habit plane and the and other noteworthy features of the 8 to o' transforma angle o would decrease from 23. 1. Thus, this effect tion are best illustrated with dilatometry observations. would also bring the TM and experiment into closer Fig. 12 shows the results from low-temperature dilatome- agreement as shown by path 2 in Fig. 7. The LID would try of a Pu-1.7 Ga at alloy and its significant contrac- also provide added rotation of the habit plane referred to tion, large transformation hysteresis (227C), and the the aphase. We cannot be more quantitative about the incomplete nature of the transformation(18%). Other LID effects without more details on the slip systems features of this particular experiment include the re-initia namely slip directions for the various slip systems and tion of transformation upon heating and the small steps their critical resolved shear stresses for slip. However, that occur during reversion. The former is attributed to the qualitative trends are consistent with the experimental partial relief of stresses built up during the accommoda results tion of the 20% volume change [1]. The reversion steps onset:-1117C start(RT) reversion end 2: 114.9 reversion end 1: 111.4C 1.isothermal second onset:-949°c reversion onset 93. 1'C end-66.3"C temperature(C) ig. 12. Dilatometry of a Pu-1.7 at Ga alloy through the transformation and reversion. The transformation starts at-l12C, proceeds during cooling 155C, and re-initiates during warming at -95C. The reversion starts at 93C and finishes at 115C, producing a transformation The transformation produced 24 vol%awould be expected. Fig. 11 shows the trace of the ð3 23Þ plane. The Burgers vector would have both b1 and b3 components in general. The b3 component would relieve the e33 coherency strain. The b1 component would in part relieve the e11 strain. Hence, fewer disconnections would be needed to achieve a strain-free habit plane and the angle x would decrease from 23.1. Thus, this effect would also bring the TM and experiment into closer agreement as shown by path 2 in Fig. 7. The LID would also provide added rotation of the habit plane referred to the a0 phase. We cannot be more quantitative about the LID effects without more details on the slip systems, namely slip directions for the various slip systems and their critical resolved shear stresses for slip. However, the qualitative trends are consistent with the experimental results. 4. Discussion The defect-based TM gives predictions for the martens￾ite habit planes in closer agreement with experiment than do the IPS/PTMC theories. Because of the large magnitude of b2 in the present case, differences between the two types of theory are expected [15]. The strains involved in the transformation are large, though, so it may be questioned whether a terrace/disconnection model of the interface applies. An alternative would be a nominally planar inter￾face with large local disregistry, resembling that in a high￾angle grain boundary or disclination. Electron microscopy could only resolve this question by lattice imaging, which has not yet been achieved in Pu, largely due to its strong propensity to oxidize. Molecular dynamics simulations of the interface would help to resolve this issue. Nevertheless, the near parallelism of (1 1 1)d//(0 1 0)a and of ½ 110 d==½100 a are consistent with a disconnection/terrace plane structure and encourage us to accept its feasibility. A final discussion point concerns the large hysteresis observed for the reverse a0 to d transformation. This and other noteworthy features of the d to a0 transforma￾tion are best illustrated with dilatometry observations. Fig. 12 shows the results from low-temperature dilatome￾try of a Pu–1.7 Ga at.% alloy and its significant contrac￾tion, large transformation hysteresis (227 C), and the incomplete nature of the transformation (18%). Other features of this particular experiment include the re-initia￾tion of transformation upon heating and the small steps that occur during reversion. The former is attributed to partial relief of stresses built up during the accommoda￾tion of the 20% volume change [1]. The reversion steps Fig. 11. Trace of ð32 3Þ slip plane intersecting a habit plane. Burgers vector bL, arbitrarily inclined to the terrace plane, has in-terrace-plane components b1 and b2. Fig. 12. Dilatometry of a Pu–1.7 at.% Ga alloy through the transformation and reversion. The transformation starts at 112 C, proceeds during cooling and an isothermal at 155 C, and re-initiates during warming at 95 C. The reversion starts at 93 C and finishes at 115 C, producing a transformation hysteresis of 204 C. The transformation produced 24 vol.% a0 . 1924 J.P. Hirth et al. / Acta Materialia 54 (2006) 1917–1925