Natalia Resnina

Martensitic transformation and physical properties of ‘steel–TiNi’ bimetal composite, produced by explosion welding

The aim of this work is an investigation of structure and martensitic transformation in bimetal composite ‘TiNi–stainless steel’ produced by explosion welding. The results have shown that the mixture of chemical elements is observed in very narrow intervals of 6 µm close to the joint – 2 µm from the TiNi side and 4 µm from the steel one. Micro-hardness distribution in the vicinity of the joint is non-monotonic in the interval of 60 µm. Connection of stainless steel and TiNi plates by explosion welding leads to a dramatic change of martensitic transformation kinetics. Temperatures and the temperature interval of phase transformation increase strongly and heat transformation decreases. Annealing at 500°C for 2 h of bimetal composite decreases the interval of micro-hardness variation and partially recovers kinetics of phase transitions.

Functional properties of ‘Ti50Ni50–Ti49.3Ni50.7’ shape memory composite produced by explosion welding

A bimetal composite, Ti50Ni50–Ti49.3Ni50.7, was produced by explosion welding, causing a Martensitic transformation. The functional properties of these objects were studied. It was found that explosion welding partially depressed the Martensitic transformation; however, a subsequent annealing resulted in the recovery of the kinetics of Martensitic transformations. Moreover, a variation in the annealing temperature allowed the control of a sequence of Martensitic transformations in the Ni-rich layer. The influence of the ratio of the equiatomic TiNi layer thickness to the total thickness of the bimetal composite on the recoverable strain was studied, and it was found that the maximum recoverable strain was observed when the thickness of the equiatomic TiNi layer was approximately equal to 55% of the total thickness of the sample. Functional properties were studied in the bimetal composite using the optimal ratio of the layers thickness. It was found that the value of the recoverable strain depended on the value of the residual strain as well as the sequence of the Martensitic transformations that occurred in the Ni-rich TiNi layer.

Mechanical and functional properties of amorphous–crystalline thin ribbons of Ti50Ni25Cu25 and Ti40.7Hf9.5Ni44.8Cu5 shape memory alloys

Amorphous melt spun ribbons of Ti50Ni25Cu25 and Ti40.7Hf9.5Ni44.8Cu5 alloys were partially crystallized to different volume fractions of the crystalline phase from 0 to 100%. The mechanical and functional properties of amorphous–crystalline thin ribbons were studied. It was shown that the fully amorphous sample of Ti50Ni25Cu25 alloy was deformed elastically up to 2.2% and then it was deformed plastically up to breaking. The fully amorphous sample of Ti40.7Hf9.5Ni44.8Cu5 alloy was deformed elastically up to failure and plastic deformation was not observed. It was found that amorphous–crystalline samples demonstrated the shape memory effect on heating. The maximum recovered strain was 3.3% in fully crystallized Ti50Ni25Cu25 alloy and it attained 5% in Ti40.7Hf9.5Ni44.8Cu5 alloy. The Ti40.7Hf9.5Ni44.8Cu5 alloy demonstrated the two-way shape memory effect if the amorphous and crystalline phases co-existed in the sample.

Recrystallization of nickel upon heating below the temperature of thermoactivated nucleation

The main mechanisms of grain growth upon low-temperature recrystallization of pure nickel (99.98%) with structures of various types formed upon deformation in Bridgman anvils have been studied. A decrease in the amount of the stored energy of deformation at the stage of submicrocrystalline (SMC) structure has been revealed using the method of differential scanning calorimetry. The isothermal annealings with durations of up to 64 h made it possible to show that the low-temperature recrystallization in both the mixed and SMC structures is developed via the growth of separate centers that are formed during deformation. As a result, no homogeneous submicrograin structure is formed in nickel upon low-temperature recrystallization.

Unusual Multistage Martensitic Transformation in TiNi Shape Memory Alloy after Thermal Cycling

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.

Stability of mechanical behavior and work performance in TiNi-based alloys during thermal cycling

Abstract Variations in strain and work output in two Ti-47 at.%Ni-3 at.%Cu and Ti-51.5 at.%Ni alloys during thermal cycling in the mode of fixed stress acting on cooling and heating were studied. It was found that in the Ti50Ni47Cu3 alloy with a low level of yield stress, the values of transformation plasti city and shape memory effect and work output were changing during thermal cycling. It was observed that the mechanical behavior and work performance of Ti-51.5 at.%Ni alloy with a high level of yield stress was stable on thermal cycling irrespective the value of stress acting on cooling and heating. It was found that under the same stress conditions the value of work output in Ti-51.5 at.% Ni alloy was four times lower than in the Ti-47 at.%Ni-3 at.%Cu alloy. Moreover, the accumulated residual strain was approximately 40 times lower than in Ti50Ni47Cu3 alloy with a low level of yield stress.

Alternate stresses and temperature variation as factors of influence of ultrasonic vibration on mechanical and functional properties of shape memory alloys.

It is known that the main factors in a variation in the shape memory alloy properties under insonation are heating of the material and alternate stresses action. In the present work the experimental study of the mechanical behaviour and functional properties of shape memory alloy under the action of alternate stresses and varying temperature was carried out. The data obtained had demonstrated that an increase in temperature of the sample resulted in a decrease or increase in deformation stress depending on the structural state of the TiNi sample. It was shown that in the case of the alloy in the martensitic state, a decrease in stress was observed, and on the other hand, in the austenitic state an increase in stress took place. It was found that action of alternate stresses led to appearance of strain jumps on the strain-temperature curves during cooling and heating the sample through the temperature range of martensitic transformation under the constant stress. The value of the strain jumps depended on the amplitude of alternate stresses and the completeness of martensitic transformation. It was shown that the heat action of ultrasonic vibration to the mechanical behaviour of shape memory alloys was due to the non-monotonic dependence of yield stress on the temperature. The force action of ultrasonic vibration to the functional properties was caused by formation of additional oriented martensite.

Mechanical behaviour and functional properties of porous Ti-45 at. % Ni alloy produced by self-propagating high-temperature synthesis

The mechanical behaviour and shape memory effects were studied in the porous Ti-45.0 at. % Ni alloy produced by self-propagating high-temperature synthesis. It is shown that the porous Ti-45.0 at % Ni alloy is deformed by the same mechanisms as a cast Ti50Ni50 alloy. At low temperatures, the deformation of the porous alloy is realised via martensite reorientation at a low yield limit and by dislocation slip at a high yield limit. At high temperatures (in the austenite B2 phase) the porous Ti-45.0 at % Ni alloy is deformed by the stress-induced martensite at a low yield limit and by dislocation slip at a high yield limit. The pseudoelasticity effect is not found in this alloy, while the transformation plasticity and the shape memory effects are observed on cooling and heating under a constant load. The values of the transformation plasticity, and the shape memory effects, depend linearly on the stress acting on cooling and heating. The temperatures of the martensitic transformation increase linearly when the stress rises up to 80 MPa. The porous Ti-45.0 at % Ni alloy accumulates an irreversible strain on cooling and heating and demonstrates unstable functional behaviour during thermal cycling.

Pseudoelasticity effect in amorphous—crystalline Ti40.7Hf9.5Ni44.8Cu5 shape memory alloy

The pseudoelasticity effect was studied in amorphous–crystalline Ti40.7Hf9.5Ni44.8Cu5 shape-memory alloy with different volume fractions of the crystalline phase. Pseudoelastic behaviour was found in the samples with a volume fraction of the crystalline phase of 60% and above. An increase in the volume fraction of the crystalline phase led to a rise in the strain accumulated due to stress-induced martensite, an increase in the mechanical hysteresis and a decrease in the stress needed for the appearance of the stress-induced martensite. It was found that the dependences of the pseudoelasticity effect parameters on the volume fraction of the crystalline phase were due to the influence of the surface energy on the formation of martensite crystals with preferable orientation. An increase in the volume fraction of the crystalline phase led to an increase in the grain sizes and, as a result, a decrease in the surface energy. It provided an increase in the number of preferably oriented martensite variants appearing in the grain under load.

Mechanical and functional properties of amorphous-crystalline ribbons of Ti40.7Hf9.5Ni44.8Cu5 alloy

The mechanical behavior and functional properties in amorphous-crystalline thin ribbons of Ti40.7Hf9.5Ni44.8Cu5 alloy with a different volume fraction (from 0 to 100%) of crystalline phase were studied. The results obtained showed that a fully amorphous sample was deformed elastically up to 6%. The existence of the crystalline phase in the sample resulting in the deformation was realized by reorientation of martensite at the early stage. The strain accumulated at this stage was fully recoverable during subsequent heating through the temperature range of reverse martensitic transformation. It was found that an increase in the volume fraction of crystalline phase led to a rise in the value of the shape memory effect. This was due to the increase in the volume fraction of the sample deforming by the mechanism of martensite reorientation. It was observed that the amorphous-crystalline Ti40.7Hf9.5Ni44.8Cu5 alloy was deformed by three mechanisms of unelastic deformation: reorientation of martensite crystals, plastic deformation of the amorphous phase, and dislocation slip. It was shown that the change in the deformation mechanism was determined by the volume fraction of crystalline phase and the value of preliminary strain. It was found that the Ti40.7Hf9.5Ni44.8Cu5 alloy exhibited a two-way shape memory effect only if the amorphous and the crystalline phase coexisted in the alloy.