J. P. Hirth et al. 1 Acta Materialia 54(2006)1917-1925 Table l t. Ga Pu ator Average bond length No short bonds No long bonds 3. 3.223.432. 3.223.432.78 3.403.213.17 345364)2.663.4 3.45364)2.663.4 2.602.62 8399 3.46 3.393.37 3.393.37 3 3.533.383.16 31 3.22 3.22 3.533.31 3.43 3.383.232.80 2.783.48 Short bonds(<2.80 A)are in italic type. Bonds extending into an adjacent layer are in bold type and occur in pairs(because of the mirror plane). Bond lengths up to 3.73 A are given; those not normally considered to be part of the 12-fold coordination are in parentheses. The 12 nearest neighbors are used to determine the average bond length nship at the near planar center of the broad faces of the structure, are the in-plane displacement vectors that would plate. produce the transformation shear in the a phase. The vec- tors are [1021,,24[502]a, and 24[7021 and correspond to 2. Transformation model the three 1(1010) Shockley partial Burgers vectors in the hcp pseudostructure. Electron microscopy [7]indicates that The first step in the TM is to select a terrace plane. On [100] is the actual line direction of the disconnections. The the basis of the pseudostructure proposed by Crocker [5] misfit of the average along these lines with the and the observations of Olsen [6], we select(010)o// close-packed directions in 8 is least for the [302 , direction (1 11) as the coherent reference terrace plane. The However, if one studies the spacings of atoms along the (010)a plane is shown in Fig. 4. While only the terrace transformation shear directions, one notes that there is lit- plane is needed in the TM, this choice is near to that in tle variation for the [102] direction whereas the variation the lattice correspondence in the IPS approaches [7-9]. is large for both the [502 and 702 directions. This indi The(0 10)a plane is then related to the (0001) hep plane cates that the shuffles entailed in completing the transfo in the pseudostructure. The [100],[102], and [302 mation from the hep pseudostructure to the a' phase are directions in Fig. 4 correspond to the close-packed minimized for the [1 02] case and this is postulated to be 120)directions in the hcp(0001) plane. One of these the reason for the system to select the [100] disconnection directions must be related to the disconnection line direc- line on. The vectors shown in Fig 4, each normal to one of Hence, the configuration of the transformation discon the directions that would be close-packed in the pseudo- nections is that illustrated in Fig. 5. The terrace plane istionship at the near planar center of the broad faces of the plate. 2. Transformation model The first step in the TM is to select a terrace plane. On the basis of the pseudostructure proposed by Crocker [5] and the observations of Olsen [6], we select (0 1 0)a// (1 1 1)d as the coherent reference terrace plane. The (0 1 0)a plane is shown in Fig. 4. While only the terrace plane is needed in the TM, this choice is near to that in the lattice correspondence in the IPS approaches [7–9]. The (0 1 0)a plane is then related to the (0 0 0 1) hcp plane in the pseudostructure. The [1 0 0]a, ½102 a, and ½ 3 02 a directions in Fig. 4 correspond to the close-packed h1 12 0i directions in the hcp (0 0 0 1) plane. One of these directions must be related to the disconnection line direc￾tion. The vectors shown in Fig. 4, each normal to one of the directions that would be close-packed in the pseudo￾structure, are the in-plane displacement vectors that would produce the transformation shear in the a0 phase. The vec￾tors are 1 12 ½102 a, 1 24 ½5 02 a, and 1 24 ½7 02 a and correspond to the three 1 3 h1 01 0i Shockley partial Burgers vectors in the hcp pseudostructure. Electron microscopy [7] indicates that [1 0 0]a is the actual line direction of the disconnections. The misfit of the average spacing along these lines with the close-packed directions in d is least for the ½3 02 a direction. However, if one studies the spacings of atoms along the transformation shear directions, one notes that there is lit￾tle variation for the [1 0 2]a direction whereas the variation is large for both the ½5 02 a and ½7 02 a directions. This indi￾cates that the shuffles entailed in completing the transfor￾mation from the hcp pseudostructure to the a0 phase are minimized for the [1 0 2]a case and this is postulated to be the reason for the system to select the [1 0 0]a disconnection line. Hence, the configuration of the transformation discon￾nections is that illustrated in Fig. 5. The terrace plane is Table 1 Bond lengths (A˚ ) in Pu–1.7 at.% Ga Pu atom number 1 2 3 4 5 6 7 8 Average bond length No. short bonds No. long bonds 1 2.61 2.60 3.43 2.58 3.73 3.12 5 7 3.61 3.22 3.43 2.78 3.22 3.43 2.78 2 2.61 2.62 3.39 3.40 3.21 3.17 4 10 3.61 3.45 (3.64) 2.66 3.48 3.45 (3.64) 2.66 3.48 3 2.60 2.62 3.28 3.26 3.67 3.16 4 8 3.43 3.39 2.68 3.46 3.39 2.68 3.46 4 3.43 3.39 3.28 2.59 2.64 3.14 4 10 3.43 3.39 3.37 (3.34) 2.76 3.39 3.37 (3.34) 2.76 5 3.26 2.59 2.73 3.53 3.38 3.16 4 10 3.53 3.45 2.68 3.34 (3.53) 3.45 2.68 3.34 (3.53) 6 3.67 2.64 2.73 3.31 3.23 3.17 4 10 3.53 3.22 (3.64) 3.46 2.76 3.22 (3.64) 3.46 2.76 7 2.58 3.40 3.53 3.31 2.80 3.17 4 10 3.45 3.43 2.66 3.42 (3.51) 3.43 2.66 3.42 (3.51) 8 3.73 3.21 3.38 3.23 2.80 3.28 3 11 3.45 2.78 3.48 (3.51) 3.53 2.78 3.48 (3.51) 3.53 Short bonds (62.80 A˚ ) are in italic type. Bonds extending into an adjacent layer are in bold type and occur in pairs (because of the mirror plane). Bond lengths up to 3.73 A˚ are given; those not normally considered to be part of the 12-fold coordination are in parentheses. The 12 nearest neighbors are used to determine the average bond length. J.P. Hirth et al. / Acta Materialia 54 (2006) 1917–1925 1919