Materials Science Forum Vols. 738-739

Abstract: Scheme of pseudo elastic cyclic deformation of Ti49,4Ni50,6 wire (Ms=0±2oC) in 18-70оС temperature range included loading up to 4,3% elongation and unloading. It was shown that the parameters of the strain/stress patterns are strongly resulting from numbers of preliminary deformation cycles. Preliminary deformation cycling in the range of 3-4 percent provides stabilization after 10-15 cycles of the whole complex of mechanical properties including 30-50% increase of the stress during unloading, which is necessary to finish of the reverse martensitic transformation. Following cycling stabilizes all mechanical characteristics excluding one. An increase of numbers of preliminary deformation cycles (up to 50) results in monotonous decrease of the energy, which disappears during one cycle.

Abstract: The demand of emerging joining techniques for shape memory alloys (SMA) has become of great importance, as their functional properties, namely shape memory effect (SME) and superelasticity (SE) present unique solutions for state-of-the-art applications. Literature shows that significant efforts have been conducted on laser welding of these alloys, although very limited results concerning mechanical properties are repeatedly achieved. A better understanding of the mechanical behaviour of these welded joints may be got through a detailed analysis of the structural characteristics of the material from the base metal to the weld bead. Such studies have been carried out on a series of Ni-rich Ni-Ti SMA laser welded plates using synchrotron radiation.

Abstract: In order to increase corrosion resistivity of the NiTi alloy the surface is covered by layers. Layers can be made from such as titanium nitrides and/or oxides as well as their mixture. Recently, a glow discharge technique has been applied for coatings formation. However, the deposition process requires to be done at elevated temperature. Therefore, it may have a negative effect on the structure, which is responsible for the shape memory phenomena. The results obtained from studies, done over the influence of the glow discharge nitriding and combination of nitriding and oxidizing process on the structure, the kinetics of martensitic transformation, the one-way shape memory effect and the superelasticity effect of the NiTi alloys are reported. The results showed that during deposition process, curried out at temperatures above 250°C and for time up to 30 minutes, the precipitation of dispersive particles of Ni4Ti3 phase already starts and has a positive effect on the superelasticity phenomena. The applied deposition technique does not affect also negatively the shape memory effect.

Abstract: The ternary Ni47Ti50Co3 alloy was produced using twin roll technique and traditional casting. The bulk alloy was hot rolled and thermally treated. Both alloys showed presence of the reversible martensitic transformation, in which the parent phase transformed to the B19’monoclinic martensite via the R-phase. However, they significantly differed in grains orientation. Thermally treated alloy showed weak fibre texture (35% of total amount), whereas the strip revealed fibre and {100} sheet texture components. In the strip more than 70% of total amount of grains were preferentially oriented. Presence of the texture affected the shape memory effect. In the strip, the two-way shape memory effect was induced after 30 training cycles. In thermally treated bulk, 52% of shape recovery was reached after 100 training cycles.

Abstract: Recently we reported on the development of a composite material exhibiting reversible shape memory effect. A Ti–25Ni–25Cu (at.%) alloy was obtained by the melt spinning technique as amorphous–crystalline ribbons with a thickness of approximately 40 μm. The thickness of the amorphous and crystalline layers (d_а and d_c, respectively) was varied by electrochemical polishing. It has been ascertained that with varying the relationship d_c/d_а the martensite transformation and shape-recovery temperatures do not actually change, while the minimum radius of the ribbon bending decreases from 8.0 mm to 2.4 mm with increasing the relationship d_c/d_а from 0.33 to 1.40. The maximum reversible strain comprises 0.4% at d_c/d_а = 0.82. On the basis of experimental data obtained the phenomenological description, providing an explanation for nature of the phenomena taking place in the rapidly quenched amorphous-crystalline ribbon composite, has been proposed.

Abstract: Vacuum Plasma Spraying has been used to produce coatings onto steel substrates. This work deals with the study of splat morphology of the NiTi alloy sprayed by VPS onto different substrates (aluminium, copper, stainless steel, glass and alumina). All the previous characteristics are discussed in terms of wettability and thermal conductivities regarding the rapid cooling involved in the process. Although identical conditions were used during thermal spraying, a wide variety of splat formations were observed; commonly, slushy or splash/disc splats are formed depending on whether the particles have partially or fully melted. The thermal effusivity of the substrate material, which is a measure of its ability to exchange thermal energy with its surroundings, seems to play an important role promoting more or less spreading. The higher the thermal effusivity is, the more rapidly the splats are cooled, thus starting the solidification before they come to rest and, changing their morphology.

Abstract: In the present study we investigate the effect of ultrasonic treatment on the functional properties of Ti 50,4at.%Ni alloy after the processing in free and pre-deformed conditions. It is shown that the shape memory effect and its realization peculiarities depend on the parameters and conditions of ultrasonic influence. Ultrasonic treatment of preliminary deformed TiNi alloy in pre-deforming conditions promotes the reduction of strain restoring at the subsequent heating through the temperature interval of reverse martensite transformation. I. e. some setting of induced strain occurs during the ultrasonic treatment of material in pre-deforming conditions. In the specimens after ultrasonic treatment, it is observed the extension of shape recovery temperature range in the first thermal cycle, as well as the dependence of strain recovery temperature dynamics on the power of ultrasonic influence. Also it is established that ultrasonic treatment helps to increase the shape memory effect.

Abstract: This investigation has studied the characterization of the thermomechanical behaviour of a TiNiCu shape memory alloy with different heat treatments. This characterization has been done by analyzing martensitic transformation at constant stress tests. The martensitic transformation was analyzed by measuring electrical resistance and strain. The results showed that orientation is reached at different stress levels during each heat treatment; low stress thermomechanical cycling does not affect transformation temperatures and thermomechanical cycling at upper stress levels produces different changes of behaviour depending on the stress level and/or heat treatment.

Abstract: Variation in kinetics of martensitic transformation, in an equiatomic TiNi shape memory alloy, during thermal cycles was investigated. Samples annealed at 500 °C for 1 hour were subjected to repeated thermal cycles, through the temperature range of martensitic transformation. Unusual 4-stage martensitic transformation during cooling after 30 thermal cycles was observed. Moreover, a new unusual phenomenon was found in the preliminary thermal cycled TiNi alloy. It was observed that variation in the highest temperature Th of thermal cycles temperature interval resulted in the redistribution of released heat among four calorimetric peaks, observed on cooling. It was found that if the Th temperature did not exceed 240 °C the variation in kinetics was repeatable, and determined only by the value of Th. It was assumed that the defect structure induced on preliminary thermal cycling changes reversibly on cooling and heating.

Abstract: The present article is dedicated to the study of thermoelastic martensitic transformation in the TiNi equiatomic composition by low-frequency internal friction. Considered the influence of oscillation frequency, strain amplitude and heating rate on the internal friction peak of the martensitic nature.