In-Situ TEM Observation of Transformations In Ti50-Ni40-Cu10 Alloy

Alloys from the system Ni-Ti-Cu, which are usually prepared by alloying by Cu to the expense of Ni in volumes up to approx. 30 at. %, show shape memory phenomena, similarly as binary Ni-Ti alloys. Addition of Cu also reduces high influence of composition on transformation temperature, transformation hysteresis and flow stress in martensitic state. Amount of added copper also influences significantly manner of transformation. Addition of copper causes forming of martensite B19. In dependence on running transformations in respect to the copper contents the following two cases can occur 1) if the Cu contents is less than 5 at. %, there occurs a transformation as in case of binary alloys (Ni-Ti). It means that single stage transformation B2→B19´ occurs. 2) if the Cu contents in the alloy is higher than approx. 7 at. %, there occurs high-temperature transformation of B2 phase to martensite B19 and then to martensite B19´. Transformation sequence can be therefore expressed as B2→B19→B19´. Experimental alloy was prepared by melting in a graphite crucible in high-frequency induction vacuum furnace. After metallurgical preparation of the alloy it was formed – by combinations of swaging at the temperature of 850°C followed by drawing. Samples for TEM were taken from the component volume by electro-spark cutting. Standard holders of TEM foils were designed in such a way that they supported a foil in the form of target with diameter of approx. 3 mm. The specimens were in the form of wire with diameter 2.3 mm. This article deals with observation of micro-structural characteristics of Ni-Ti-Cu shape memory alloy by transmission electron microscopy.