Yuriy Chumlyakov

Compressive response of NiTi single crystals

The deformation of NiTi shape memory single crystals are reported under compression loading for selected crystal orientations and two diAerent Ti3Ni4 precipitate sizes. For the (148) orientation, selected for highest recoverable strains, the peak aging treatment decreased the transformation stress from austenite to martensite. At the same time, peak aging raised the flow stress of both the austenite and martensite compared to the overaged case by increasing the resistance of the material to dislocation motion. The transformation proceeds beyond the stress plateau region and extends until martensite yielding occurs. This results in recoverable strain levels equivalent to the theoretical estimate of 6.4%. The (112) orientation was chosen to produce two variant formations and in this case, the transformation proceeded over an ascend- ing stress-strain curve compared to the nearly plateau response for the (148) case. Since the austenite and martensite yield levels are reached at a smaller strain level in this case, the maximum recoverable strain was limited to 3.5% even though the theoretical estimates are near 5.1%. The theoretical estimates of transformation strains were established for Type I and Type II twinning cases to cover all possible habit plane and twin systems. TEM investigations support that slip in austenite occurs concomitant with increas- ing transformation strains. In the (001) orientation, the unfavorable slip systems for dislocation motion in the austenite inhibit slip and permit recoverable strains similar to the theoretical estimates (nearly 4.2%). The (001) orientation exhibits a continuous increase of flow stress with temperature beyond 360 K unlike any other orientation. The results point out that in order to optimize the material performance, close atten- tion must be paid to the selection of the crystallographic orientation, and the precipitate size through heat treatment. 7 2000 Acta Metallurgica Inc. Published by Elsevier Science Ltd. All rights reserved.

Instrumented micro-indentation of NiTi shape-memory alloys

Abstract We study the instrumented Vickers micro-indentation of single-crystal Ti–50.9 at% Ni shape-memory alloys with systematically varied surface normal orientations ([100], [210], [111] and [221]) and Ti 3 Ni 4 precipitate sizes (0 nm, 10 nm, 50 nm, 100 nm, 300 nm and 500 nm). Based on transmission electron microscopy observations, indentation of solutionized NiTi induces inelastic deformation via dislocation activity and a stress-induced martensitic transformation. The room-temperature hardness, Hv, and recoverable energy, E r , of NiTi are shown to be a maximum for very small precipitate sizes, decrease for intermediate precipitate sizes, and increase for large precipitate sizes. The maximization of Hv and E r at small precipitate sizes (10 nm) is attributed to the relatively high resistance to both dislocation motion and a recoverable stress-induced martensitic transformation. The decreases in Hv and E r at intermediate precipitate sizes (50–300 nm) are attributed to a decrease in the resistance to dislocation motion and a measured increase in the transformation temperatures with respect to the indentation temperature. The increases in Hv and E r at large precipitate sizes (500 nm) are attributed solely to measured decreases in the transformation temperatures with respect to the indentation temperature, since the resistance to dislocation motion remains constant as the precipitates grow from 300 nm to 500 nm. For nearly all heat treatments, the [100] and [221] surfaces demonstrate the highest and lowest values of Hv and E r , respectively, an effect attributed primarily to orientation of favorable slip systems.

On the mechanical behavior of single crystal NiTi shape memory alloys and related polycrystalline phenomenon

Room temperature monotonic and cyclic stress–strain curves for single crystal and polycrystalline NiTi shape memory alloys containing Ti3Ni4 precipitates are presented. The tensile and compressive single crystal results illustrate the importance of crystallographic texture, the unidirectional nature of the martensitic transformation, and martensite detwinning on tension-compression stress–strain asymmetry in polycrystalline NiTi. Moreover, results on the fatigue of NiTi single crystals demonstrate the fundamental characteristics of cyclic deformation in NiTi alloys and the importance of texture on the fatigue of polycrystalline NiTi.

Shape memory and pseudoelastic behavior of 51.5%Ni–Ti single crystals in solutionized and overaged state

Abstract Deformation of nickel rich (51.5%Ni) Ni–Ti single crystals are investigated over a wide range of temperatures (77–440 K) and strain levels in compression as high as 9%. These alloys combine high strength with an unusually wide pseudoelasticity temperature interval (near 200 K) and can be exploited to suit specific applications. The slip deformation in [001] orientation can not occur due to the prevailing slip systems, as confirmed by transmission electron microscopy. Consequently, the [001] orientation exhibited pseudoleastic deformation at temperatures ranging from 77 to 283 K for the solutionized case and 273–440 K for the aged condition respectively. The critical transformation stress levels were in the range 800–1800 MPa for the solutionized case, and 200–1000 MPa for the aged case depending on the temperature and specimen orientation. These stress levels are considerably higher compared to the near equiatomic Ni compositions of these class of alloys. On the other hand, the maximum transformation strains, measured from incremental straining experiments in compression, were lower compared to both the phenomenological theory with Type II twinning and the previous experimental work on 50.8%Ni NiTi crystals. A new theory for compound twinning is introduced with lattice invariant shear as a solution, and relies on the successive austenite phase (B2) to intermediate phase (R) to martensite phase (B 19′) transformation. The compound twinning model predicts lower transformation strains compared to the Type II twinning case lending an explanation of the experimental transformation strain levels.

Cyclic deformation behavior of single crystal NiTi

Abstract Single crystals of NiTi (with 50.8 at.% Ni) were subjected to cyclic loading conditions at room temperature which is above the M s (martensite start) temperature of −30°C. The single crystals exhibited remarkable cyclic hardening under zero to compression strain control experiments. The stress range under strain control increased by as much as a factor of 3 in compression. The increase in stress range is primarily due to the increasing strain hardening modulus. In the tension case, loop shape changes occurred but the increase in stress range is rather small. The fatigue cycling was undertaken with a strain range of 3% which is far below the theoretical transformation strains levels exceeding 6%. The maximum stress levels reached in the experiments are below those that cause martensite slip. Therefore, the stress–strain response is governed by transformation from the austenite to the martensitic phases and the dislocation structure evolution in the austenite domains. Two single crystal orientations [148] and [112] were examined during the experiments with single and double CVP (correspondent variant pair) formations respectively. The strain hardening in compression cases is rather substantial with the stress range in the double CVP case surpassing the single CVP case. Two heat treatments were selected to produce coherent and incoherent precipitates in the microstructure respectively. The influence of the coherent precipitates on the stress–strain response is significant as they lower the transformation stress from austenite to martensite, and at the same time, they raise the flow stress of the austenite and martensite domains leading to higher saturation stresses in fatigue.

The role of intergranular constraint on the stress-induced martensitic transformation in textured polycrystalline NiTi

Abstract The influences of grain boundaries and relative grain misorientations on stress-induced martensitic transformations in NiTi are studied using unique experiments and finite element modeling. Tensile and compressive mechanical tests reveal that polycrystalline NiTi with a dominant fiber texture and single crystal NiTi oriented along the [111] direction exhibit nearly identical stress–strain curves during a stress-induced martensitic transformation. Micro-mechanical finite element simulations of fiber textured polycrystals and single crystals undergoing a multi-variant martensitic transformation confirm the relative indifference of the macroscopic transformation attributes to the presence of grain boundaries. On the microscale, the finite element simulations further reveal that the insensitivity of the transformation to intergranular constraint is linked to the local stress disturbance created by transforming grains. The transformation of grains that are favorably oriented with respect to the loading axis creates local stresses that invariably assist the transformation in neighboring grains, effectively lowering the influence of grain misorientations and boundaries on the macroscopic transformation behavior.

The influence of aging on critical transformation stress levels and martensite start temperatures in NiTi: Part I- Aged microstructure and micro-mechanical modeling

Transmission electron microscopy is used to determine the microstructures of a Ti-50.8 at% Ni alloy given different aging treatments. Two different peak-aging treatments are shown to result in disk shaped semi-coherent Ti{sub 3}Ni{sub 4} precipitates with a diameter ranging from 50 nm to 200 nm depending on the aging temperature. In the peak-aged materials, strong strain fields are clearly visible on TEM micrographs. An Eshelby based model is used to predict the local stress fields due to the differences in the lattice parameters of the precipitates and surrounding matrix. The position dependent local stress fields are then resolved onto the 24 different martensite correspondence variant pairs (CVP`s). It is further demonstrated that due to the unique orientation relationship that exists between the precipitate variants and the martensite CVP`s, the local resolved shear stresses are extremely large on some CVP`s and negligible on others. When the Ni rich NiTi is over-aged, it is found that the precipitates coarsen to approximately 1000 nm, they become in-coherent, and the local stress fields disappear. It is also determined that after over-aging in the average composition of the matrix drops from 50.8 at% Ni to approximately 50.4 at% Ni. In a subsequent paper (part 2)morexa0» the results here are used to explain the dependence of the critical transformation stress levels and martensite start temperatures on the aging treatment.«xa0less

Fracture of precipitated NiTi shape memory alloys

The fracture mechanisms in single crystal and polycrystalline Ti-50.8at%Ni shape memory alloys containing Ti3Ni4 precipitates are studied using the scanning electron microscope (SEM). Aged materials with three different precipitate sizes (50xa0nm, 150xa0nm, and 400xa0nm), which have interfaces ranging from semi-coherent to incoherent, are considered. The mechanisms of material fracture identified in the single crystal NiTi are: 1. Nucleation, growth, and coalescence of voids from the Ti3Ni4 precipitates, 2. Cleavage fracture on {100} and {110} crystallographic planes, 3. Nucleation, growth, and coalescence of voids from fractured Ti-C inclusions. Cleavage and ductile tearing mechanisms also operate in polycrystalline NiTi, however, since the Ti-C inclusions are an artifact of single crystal growth processes, mechanism 3 was not discovered in the polycrystalline materials. Cleavage fracture and ductile tearing are found to act in conjunction, with the relative dominance of one over the other depending on the local precipitate size and concentration. As the Ti3Ni4 precipitate size increases to about 400 nm, the overall fracture is dominated by failure mechanism 1, and the cleavage markings become diffuse. Finally, we assert that the high tensile ductility of drawn NiTi polycrystals is due partially to the fact that drawn bar and wire stock usually have a strong {111} fiber texture. Such a texture promotes the initiation of the transformation at low stresses and concurrently prevents primary cleavage on the {100} or {110} planes.

The Influence of Aging on Critical Transformation Stress Levels and Martensite Start Temperatures in NiTi: Part II—Discussion of Experimental Results

An experimental study was performed to determine the effect of aging on martensitic transformations in NiTi. Polycrystalline and single crystal NiTi ([100], [110], and [111] orientations) were both considered. Stress-induced transformations in polycrystalline NiTi were found to closely resemble transformations in single crystals of the [110] and [111 orientations. Solutionized and over-aged single crystals exhibited a strong orientation dependence of the critical stress required to trigger the transformation, σ cr . The Schmid law was able to accurately predict the orientation dependence of σ cr in the solutionized and over-aged single crystals. Peak-aged single crystals demonstrated a much weaker orientation dependence of σ cr and in general, the Schmid law was not obeyed. By considering the local stress fields outside of the semi-coherent precipitates, the decrease in the orientation dependence of σ cr was accounted for. The martensite start temperatures, M s , in aged single crystal and polycrystalline NiTi were much higher than in solutionized samples. In peak-aged NiTi the increase was primarily attributed to the local stress fields outside the coherent precipitates which create preferential nucleation sites for the martensite. In the over-aged NiTi the increase in M s was primarily attributed to the decrease in the average Ni concentration of the matrix surrounding the coarsened precipitates.

Micro and Macro Deformation of Single Crystal NiTi

We present experimental results on the instrumented Vickers micro-indentation and compression of solutionized Ni-rich NiTi single crystals. The tests are conducted at room temperature where the solutionized Ti-50.9 at percent Ni material is 18 degrees above A f and the solutionized Ti-51.5 at percent Ni material is more than 100 degrees above A f Aside from elastic deformation, it is discovered that dislocation motion and a reversible stress-induced martensitic transformation influence the micro-indentation response of Ti-50.9 at percent Ni, while the micro-indentation of Ti-51.5 at percent Ni only induces irreversible dislocation motion. The effect of the surface normal orientation on material hardness was negligible in the Ti-51.5 at percent Ni and followed trends anticipated by the activation of favorable slip systems in the Ti-50.9 at percent Ni. Compression tests on the identical Ti-50.9 at percent Ni samples revealed deformation by coupled stress-induced martensite and plastic flow, depending on the crystallographic orientation. The trends in hardness with surface normal orientation were not commensurate with the orientation dependence of the uniaxial compressive transformation or yield strength. The ramifications of the results in terms of comparing micro-indentation and macrocompression and the interactions between plasticity and the stress-induced martensitic transformation are discussed.