Martensitic transformations of Cu-Al-Ni single crystals in tension/compression

Abstract

Abstract. Cu-AI-Ni alloys, similarly as other Cu-base shape memory alloys, transform into more martensitic structures c^' (6R), Pi' (18R) and y x ' (2H), depending on the temperature, stress, load axis orientation, sense of loading and composition. The transformation stress-temperature conditions at which individual transitions take place are beneficially represented in so called non-equilibrium stress-temperature phase diagrams. On the basis of the <x-T diagrams, complex history dependent thermomechanical behaviors of SMA single crystals undergoing sequentially multiple solid state transitions can be easily understood and predicted. Since chemical composition of the alloy crystals affects mainly the equilibrium transformation temperatures, T 0i and only slightly the slopes of the transformation lines in the a-T diagrams, the diagrams mainly shift in the temperature range (over -200K) with the compositional variations. The shape of the diagrams, however, may change significantly when the To shifts for individual transitions are different. Knowledge of the compositional dependence of CT-T diagrams would be beneficial for the development of shape memory alloys with specific required thermomechanical properties. The aim of the present work is experimental investigation of the martensitic transformations and construction of the cr-T diagram for Cu-AI-Ni alloy with lower Al content (To>363K) and comparison with our previous results obtained on alloys with higher Al content (T 0 <263K).

In-situTEM study of stress-induced transformations in CuAlNi

“…Let us compare briefly the transformation behavior of strained thin foils with the transformation responses of bulk single crystals, expressed conventionally in the form of -T phase diagrams [2,[4][5][6]. We shall compare our observations with the results by Otsuka et al [2] who tensile strained CuAlNi single crystals of a composition and orientation very similar to our foils.…”

“…In the presence of an external stress, multistage thermoelastic behavior is observed reflecting variety of successive austenite-to-martensite and martensite-to-martensite transitions [2][3][4][5][6]. Beside 1 and 1 martensites, also 1 structure was reported in CuAlNi single crystals subjected to tensile stress in the [100] 1 direction [4,6].…”

“…In the presence of an external stress, multistage thermoelastic behavior is observed reflecting variety of successive austenite-to-martensite and martensite-to-martensite transitions [2][3][4][5][6]. Beside 1 and 1 martensites, also 1 structure was reported in CuAlNi single crystals subjected to tensile stress in the [100] 1 direction [4,6]. In-situ light microscopy and X-ray diffraction were utilized to follow the morphological changes associated with successive martensitic transformations under stress and the resulting crystal structures [2][3][4].…”

Phase stability of CuAlMn shape memory alloys

“…Similar history dependent phenomena are frequently encountered in SMA single crystal experiments. Recently, they have been observed on CuAlNi [16,19]. When the crystals were deformed in compression in the low temperature state and unloaded, the reverse transformation temperature shifted significantly upwards.…”

“…These experiments were limited to tension, except for a paper [9] where cyclic tests were performed, yet at temperatures only above A f . Our previous studies on other Cu-base SMA have clearly demonstrated a strong dependence of their thermoelastic behaviour on the sense of loading (tension-compression) and transformation history [12][13][14][15][16]. The knowledge of these effects was employed in modelling SMA polycrystals [17,18].…”