I. Karaman

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.

Competing mechanisms and modeling of deformation in austenitic stainless steel single crystals with and without nitrogen

Abstract The stress–strain behavior of low stacking fault energy AISI 316L austenitic stainless steel (SS) (Fe, 17 Cr, 12 Ni, 2 Mn, and 0.75 Si in wt pct, %) single crystals was studied for selected crystallographic orientations ( [ 1 11] , [001], and [ 1 23] ) under tension. Nitrogen (0.4 wt%) was added to the [ 1 11] , [001] and [011] crystals. The monotonic deformation of 316L SS was presented with and without nitrogen. The overall stress–strain response was strongly dependent on the crystallographic orientation. Transmission electron microscopy demonstrated for the first time that twinning was present in the [ 1 11] orientation of the nitrogen free 316L SS at very low strains (3%) and in the [ 1 23] and [001] orientations at moderate strains (∼10%) as opposed to what is expected from classical twinning theory. Twinning boundaries led to a very high strain hardening coefficient by restraining the dislocation mean free path. The nitrogen addition at the present level caused the following significant changes in the stress–strain response: (1) a considerable increase in the critical resolved shear stresses leading to a deviation from Schmid Law (2) suppression of twinning although planar slip was evident (3) changes in the deformation mechanisms and (4) a decrease in strain hardening coefficients. Most of these differences stemmed from the non-monotonous change in the stacking fault energy with nitrogen concentration and the role of short-range order. A unique strain hardening approach was introduced into a viscoplastic self-consistent (VPSC) formulation. The strain hardening formulation incorporates length scales associated with spacing between twin lamellae (or grain size and dislocation cell size) as well as statistical dislocation storage and dynamic recovery. The simulations correctly predicted the stress–strain response of both nitrogen free and nitrogen alloyed 316L SS single crystals.

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.

Energy harvesting using martensite variant reorientation mechanism in a NiMnGa magnetic shape memory alloy

Magnetic shape memory alloys demonstrate significant potential for harvesting waste mechanical energy utilizing the Villari effect. In this study, a few milliwatts of power output are achieved taking advantage of martensite variant reorientation mechanism in Ni51.1Mn24Ga24.9 single crystals under slowly fluctuating loads (10Hz) without optimization in the power conversion unit. Effects of applied strain range, bias magnetic field, and loading frequency on the voltage output are revealed. Anticipated power outputs under moderate frequencies are predicted showing that the power outputs higher than 1W are feasible.

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.

Recoverable stress-induced martensitic transformation in a ferromagnetic CoNiAl alloy

Abstract The stress-induced martensitic transformation characteristics of a new CoNiAl alloy were investigated under compression. Pseudoelasticity, stages of transformation, and thermal cycling under constant stress were revealed. The present CoNiAl alloy is a candidate material not only for magnetic but also for conventional and high-temperature shape memory alloy applications.

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.

Consolidation of amorphous copper based powder by equal channel angular extrusion

Cu50Ti32Zr12Ni5Si1 gas-atomized powder was consolidated by equal channel angular extrusion (ECAE). Powder was vacuum encapsulated in copper cans and extruded at a temperature above the glass transition temperature (Tg), but below the crystallization temperature (Tx). Five samples were subjected to one extrusion pass, each with a different temperature and extrusion rate. Microstructure, thermal stability, X-ray diffraction measurements and hardness of the ECAE consolidates were examined and compared with those of the initial powder and with a conventional extrusion (CE) consolidate. All consolidates exhibit a supercooled liquid region slightly narrower than that of the starting powder. No significant crystallization peaks are observed in XRD measurements; however, changes in peak shape and the total enthalpy of crystallization in differential scanning calorimetry measurements are attributed to nanocrystallization that is not easily detected by these methods. Greater microhardness values in ECAE consolidates in comparison with the starting powder also support the probability of nanocrystallization. The brittle behavior exhibited by all consolidates is attributed to an initial high oxygen contamination of the powder (∼2000 ppm) and the possibility of crystallization due to long exposure to temperatures above Tg during consolidation. Microstructural examination of the ECAE consolidates shows significant shear deformation of the particles with one ECAE pass. The results of the present study encourage further work on the fabrication of bulk metallic glass from powder by ECAE consolidation.

Transformation behaviour and unusual twinning in a NiTi shape memory alloy ausformed using equal channel angular extrusion

In this study, the microstructure and phase transformation behaviour of Ti–50.8 at.% Ni alloy severely deformed using equal channel angular extrusion (ECAE) were investigated. The aim of the study was to reveal the effect of severe plastic deformation on the interplay between plastic deformation via dislocation slip and twinning and forward and reverse martensitic transformation. The samples were processed at room temperature, i.e. slightly above the austenite finish temperature, and at 450°C, i.e. well above the austenite finish temperature. Transformation temperatures and multiple step transformation after processing and after low-temperature annealing were studied. The unique findings were: (1) the observation of a mixture of heavily deformed B2 (austenite) and B19′ (martensite) phases in the samples processed at room temperature, although martensite stabilization was expected; (2) the observation of highly organized, twin-related nanograins in the B2 phase of the samples deformed at room temperature, which was attributed to the (SIM, stress-induced martensitic transformation; SPD, severe plastic deformation) transformation sequence; and (3) the simultaneous observation of B2 austenite and strain-induced B19′ martensite in the samples deformed at 450°C. Strain-induced martensite in NiTi alloys is reported for the first time. The formation of well-organized, twin-related nanograins via severe plastic deformation opens a new opportunity for twinning-induced grain boundary engineering in NiTi alloys, which is believed to improve the cyclic stability and fatigue resistance of these alloys.

Extrinsic stacking faults and twinning in Hadfield manganese steel single crystals

University of Paderborn, Lehrstuhl fu¨r Werkstoffkunde, D-33095 Paderborn, Germany(Received June 26, 2000)(Accepted in revised form August 21, 2000)Keywords: Twinning; Austenite; Solid solution strengthening; Dislocation mobility; Extrinsicstacking faultsIntroductionExtrinsic stacking faults (SFs) have not been commonly observed in fcc materials. In some specialoccasions, pairs of extrinsic-intrinsic SFs are reported in the low stacking fault energy (SFE) copper andsilver alloys [1]. However, extrinsic SFs can form in elemental and compound semiconductors withprior multiple slip activation [2]. The basic driving factors for the extrinsic extension of partialdislocations in these materials are: a) the different mobility of partial dislocations because of theinherent crystal structure [3], b) lower extrinsic stacking fault energy [4], c) high applied stress levelsbecause of predeformation and low temperatures. The purpose of this work is to demonstrate extrinsicSFs in certain crystallographic orientations in a low SFE steel.Hadfield steel is an fcc material (g