Electron microscopy observations of twin-twin intersections in a particle hardened copper-titanium alloy

The authors (1) have recently reported electron microscopy (CTEM and HREM) observations of mechanical twinning in age hardened copper-titanium alloys containing a fine dispersion of the metastable Cu,Ti (beta'; D1(a)) phase ( approximate to 15 vol%). These particle hardened alloys are known to twin p rofusely (2,3,4) and it has been suggested that single crystals of Cu-Ti-Al alloys containing coherent Cu4Ti (D1(a)) particles yield by twinning at th e onset of plastic flow. In the previous paper by the authors emphasis was directed at characterizing the nature of the coherent twin/matrix interface s; G-ledges and C-ledges as well as pure ledges were identified (1,5). Furt hermore, it was found that the Cu4Ti (beta') particles were not imaged in H REM probably because of their large extinction distances. More recently, th e work has focused on the nature of twin/twin intersections and shear accom modation. Indeed, there has been an explosion of interest in the nature of the mechanisms controlling plastic flow in systems where there is an interp lay between slip and deformation twinning (6,7,8,9,10,11). A conventional approach to the analysis of slip/twin and twin/twin interact ions in f.c.c. materials is based on the methods developed by Sleeswyk and Verbraak (12) for the b.c.c. structure. It is assumed in this first approxi mation that the net Burgers vector must be conserved during the incorporati on of incident slip or twinning in the obstacle twin. Also, in this analysi s the relaxation plane (the plane on which the slip or twinning shear is tr ansferred through the obstacle twin) is assumed to contain the intersection axis of the incident slip/twinning plane and the plane of the coherent twi n boundary (composition plane) of the obstacle twin. According to the last geometrical constraint, if the mechanism of the shear transfer through the obstacle twin is restricted to the conventional slip or twinning systems in f.c.c., there is only one possible relaxation plane in the twin obstacle, i.e. a (111) plane in the obstacle twin symmetrical to the incident (111) s lip/twin plane in the matrix. Based on this approach, a number of dislocati on reactions for accommodation of the incident twinning by secondary twinni ng or slip within an obstacle twin has been discussed (13,14,15). Clearly, the geometry requires b = b' + Delta b where b and b' are the shear vectors in the matrix and obstacle twin, respectively, and Delta b is an interfaci al or boundary reaction product. The dislocations with Burgers vector ab ar e generally considered to be glissile Shockley or unit dislocations. The tr ansfer mechanisms are energetically unfavorable, but Remy (15) concludes th at transfer by slip is the more probable mechanism; Mahajan and Chin (13) i n their work on Co-Fe single crystals indicate that both slip and secondary twinning are observed. Saito (2) in early work on the deformation twinning of particle hardened Cu-Ti alloys has reported accommodation by secondary twinning. In this short paper conventional (CTEM) and high-resolution (HREM) electron microscopy observations of twin/twin intersections in the precipitation ha rdened two-phase copper-titanium alloys are reported. These results will su ggest that the mechanisms governing shear accommodation in obstacle twins r emain to be elucidated.