AGING EFFECTS IN A CU-12AL-5NI-2MN-1TI SHAPE-MEMORY ALLOY

The isothermal aging effects in an as-quenched Cu-11.88Al-5.06Ni-1.65M n-0.96Ti (wt pct) shape memory alloy at temperatures in the range 250 degrees C to 400 degrees C were investigated. The changes in the state of atomic order and microstructural evolutions were traced by means o f in situ X-ray diffraction and electrical resistivity measurements, a s well as transmission electron microscopy (TEM) and optical observati ons. The kinetics of the aging process, i.e., the temperature and time dependence of the properties including hardness, resistivity, martens itic transformation temperatures, and shape memory capacity were chara cterized, and at least three temperature-dependent aging stages were d istinguished: (1) DO3 or L2(1) atomic reordering, which causes the mar tensitic transformation temperatures to shift upward and leads the M18 R martensite to tend to be a N18R type structure; (2) formation of sol ute-depleted bainite which results in a drastic depression in martensi tic transformation temperatures and loss of the shape memory capacity, accompanied by the atomic disordering in both the remaining parent ph ase and bainite; and (3) precipitation of the equilibrium cu and gamma (2) phases and destruction of the shape memory capacity.