Doctor of Philosophy
Department of Materials Engineering
Zhu, Jian Hua, The effect of rapid solidification on martensitic transformation in a copper based shape memory alloy, Doctor of Philosophy thesis, Department of Materials Engineering, University of Wollongong, 1994. http://ro.uow.edu.au/theses/1524
This thesis reports an investigation into the martensitic transformation in a commercial Cu-ll.88Al-5.60Ni-2.01Mn-l.01Ti (wt.%) shape memory alloy (CANTIM 125). Thin rapidly solidified ribbons of this alloy were produced using the technique of planar flow casting. Substantial refinement of grain size and reduction in brittleness were achieved with the formation of the martensitic phase at a cooling rate greater than lxlO5 K/sec. The effects of rapid solidification on the martensitic transformation were compared with the transformation in bulk material conventionally produced.
The study consisted mainly of six parts: 1) transformation behaviour; 2) metallographic features; 3) determination of crystal structures and lattice parameters; 4) crystallographic analysis using the Bowles-Mackenzie theory; 5) microstructural observations by TEM; and 6) mechanical properties.
It was found that rapid solidification of CANTIM 125 shape memory alloy resulted in the thermoelastic transformation temperatures being substantially depressed due mainly to the increased matrix solute content, together with a decreased degree of order and refinement of grain size. However, aging at 300 °C and annealing at 900 °C after rapid solidification led to the precipitation of X-phase which depleted Al from the matrix and significantly raised the transformation temperatures. The growth of the parent grains and increasing degree of order also contributed minor effects.
The transformation temperatures of ribbon annealed at 900 °C were higher than those of the bulk alloy undergoing similar heat treatment despite the grain size of the ribbon being two orders of magnitude smaller than that of the bulk alloy. Therefore, the martensitic transformation temperatures in CANTIM 125 alloy are highly sensitive to thermal processing, including the initial quench rate and subsequent heat treatment. It was also found that a progressive loss of transformation reversibility in ribbon alloy annealed at 900 °C occurred after a small number of DSC thermal cycles due to the formation of incoherent intragranular precipitates of X-phase. There was a strong correlation between the X-phase precipitation and the loss of transformation reversibility. The X-phase precipitates retard the initial reverse transformation sufficiently to allow the onset of decomposition by tempering. The subsequent formation of martensite on cooling is at temperatures high enough to allow further decomposition by precipitation of X-phase within the martensite. It is thus concluded that the thermoelastic martensitic transformation and associated shape memory effect in CANTIM 125 alloy are highly sensitive to the state of X-phase precipitation. It is also inferred that the decomposition by precipitation within the martensite or during reverse thermoelastic martensitic transformation can increase the stabilization effect.
The dominant product phase in melt spun ribbons can be either M18R or M9R martensite depending on the cooling rate, with the transformation temperatures for M9R martensite being lower than those for M18R martensite. These two martensites are formed from DO3 and B2 ordered parent phases, implying two different transformation sequences: β (A2) -> B2 -> D03 -> martensite (M18R); and β (A2) -> B2 -> martensite (M9R). The second transformation sequence is a result of suppression of the B2 -> DO3 ordering transition.
On the basis of lattice parameters of the M18R martensite and DO3 parent phase, the crystallographic features for DO3 -> Ml8R martensitic transformation in rapidly solidified ribbon, annealed ribbon, and bulk alloy have been theoretically predicted using the Bowles-Mackenzie theory. These calculations show that rapid solidification results in decreases in the magnitudes of the shape strain, m1, and the lattice invariant shear, m2, but that the volume change is relatively large compared with that for bulk alloy. Despite the increased volume change, thermoelastic transformation is maintained in the rapidly solidified alloy, probably because of the substantial refinement in martensite plate size, and the small values of m2 and the shear component of m1.
The ductility of the rapidly solidified CANTIM 125 alloy was markedly improved due to the suppression of XL-phase precipitates, significant grain refinement which restricts crack development, and the change from " plane strain" to " plane stress" conditions when the deformation zone decreases to the size of the plate thickness in ribbon samples. Different morphologies for the fracture surfaces of bend test specimens were evident in the bulk and ribbon samples. The bulk alloy was characterized by brittle cleavage failure, whereas a combination of void coalescence and quasi-cleavage was exhibited by the ribbon alloy, indicating a transition from brittle to ductile failure
The research has clarified the effects of rapid solidification in the martensitic transformation of CANTIM 125 alloy, with the main findings being: 1) significant lowering of transformation temperatures; 2) reduced ordered domain size and changed type of order; 3) suppression of β2 (B2) -> β1 (DO3) transition and consequent change in the type of martensite from M18R to M9R; 4) suppression of the formation or precipitation of non-rnartensitic phases such as X-phase; 5) significant refinement in grain size; and 6) improved ductility of ribbons.
The last point together with sensitive controllability of transformation temperatures thermal treatment reflects potential for commercial use as a shape memory strip alloys.