Microstructural and Fractographic Analysis of CuAlNi Shape Memory Alloy before and after Heat Treatment

Abstract

The paper presents comparison of microstructure and fracture surface morphology of the CuAlNi shape memory alloy (SMA) after different heat treatment procedures. The investigation was performed on samples in as-cast state and heat treated states (solution annealing at temperatures of 850 °C / 60’ / H2O and 920 °C / 60’ / H2O along with tempering at two different temperature 150 °C / 60’ / H2O and 300 °C / 60’ / H2O). The microstructure of the samples was examined by optical (OM) and scanning electron microscope (SEM) equipped with device for EDS analysis. The obtained fracture surfaces were examined by SEM. Optical and scanning electron microscopy showed martensitic microstructure in all investigated samples. However, the fractographic analysis of samples after tensile testing reveals significant changes in fracture mechanism. In both solution annealed states the results shows transgranular type of fracture, but after tempering at two different temperatures the difference is obvious. After tempering at 150 °C, along with transgranular type of fracture appear some areas with intergranular type of fracture. After tempering at 300 °C, fracture surface reveals completely intergranular type of fracture.

Microstructural and Cavitation Erosion Behavior of the CuAlNi Shape Memory Alloy

“…It is known that CuAlNi shape memory alloys undergo a single transformation (β→β 1 or β→γ 1 ) or a mixed transformation (β→β 1 + γ 1 ), which depends on the alloy's chemical composition [35]. The previous works [36] confirmed that, after heat treatment in CuAlNi microstructure, along with β 1 martensite, γ 1 martensite also appears, while in the casted state, only β 1 martensite exists. This indicates that the different types of martensite affect alloys' cavitation erosion resistance.…”

“…The previous work confirmed that, after heat treatment in CuAlNi shape memory alloy, along with β 1 martensite, γ 1 martensite also appears in the microstructure. CuAlNi shape memory alloys undergo a single transformation (β→β 1 or β→γ 1 ) or a mixed transformation (β→β 1 + γ 1 ), which depends on alloys' chemical composition [36]. It can be concluded that the morphology of different types of martensite affects alloys' cavitation erosion resistance.…”

The Effect of Heat Treatment on Damping Capacity and Mechanical Properties of CuAlNi Shape Memory Alloy

“…Although the mechanical properties notably decreased after heat treatment, the development of favorable martensite phases produced better damping properties. Two types of martensite can be found in this alloy, β 1′ and γ 1′ , respectively [ 26 ]. Appearance of γ 1′ martensite improves the damping capacity of the alloy due to movement of twin boundaries in the martensite phase and higher hysteresis in comparison to β 1′ martensite [ 3 ].…”

“…After solution annealing, the appearance of the intergranular type of fracture was visible, and after tempering at 300 °C/60′/H 2 O, an almost completely intergranular type of fracture appeared. The degradation of fracture mechanisms, or shift from a transgranular type of fracture to intergranular type after heat treatment, can be explained by the alloy’s high elastic anisotropy [ 28 , 29 ] and large grain size [ 30 ]. Stress level concentration is high at large grain boundaries and nucleation of the cracks appears specifically in these places due to low cohesion.…”

“…After solution annealing, one sample was subjected to tempering at 300 °C for 60 min following water quenching. The samples for microstructural observation were cut off in the shape of a cylinder, placed in conductive mass, ground, polished, and etched by the procedure explained in our previous work [ 14 ]. In order to reveal microstructural constituents, microstructural analysis after etching was performed using a light microscope (LM) OLYMPUS GX 51 with a digital camera and scanning electron microscope TESCAN VEGA TS 5136 MM (SEM) equipped with energy dispersive spectrometry (EDS).…”