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At least 55 records · Page 3
All sourcesNASA NTRSDOE OSTI
NASA NTRS2011

Determining Recoverable and Irrecoverable Contributions to Accumulated Strain in a NiTiPd High-Temperature Shape Memory Alloy During Thermomechanical Cycling

When Ni(29.5)Ti(50.5)Pd30 shape memory alloy is thermally cycled under stress, significant strain can accumulate due to elasticity, remnant oriented martensite and plasticity. The strain due to remnant martensite can be recovered by further thermal cycling under 0 MPa until the original transformation-induced volume change and martensite coefficient of thermal expansion are obtained. Using this technique, it was determined that the 8.15% total accumulated strain after cycling under 200 MPa consisted of 0.38%, 3.97% and 3.87% for elasticity, remnant oriented martensite and creep/plasticity, respectively.

Monroe, J. A.↗
NASA NTRS1982

Analytical electron microscope study of eight ataxites

Optical and electron optical (SEM, TEM, AEM) techniques were employed to investigate the fine structure of eight ataxite-iron meteorites. Structural studies indicated that the ataxites can be divided into two groups: a Widmanstaetten decomposition group and a martensite decomposition group. The Widmanstaetten decomposition group has a Type I plessite microstructure and the central taenite regions contain highly dislocated lath martensite. The steep M shaped Ni gradients in the taenite are consistent with the fast cooling rates, of not less than 500 C/my, observed for this group. The martensite decomposition group has a Type III plessite microstructure and contains all the chemical group IVB ataxites. The maximum taenite Ni contents vary from 47.5 to 52.7 wt % and are consistent with slow cooling to low temperatures of not greater than 350 C at cooling rates of not greater than 25 C/my.

Novotny, P. M.↗
NASA NTRS1994

Microstructurally based variations on the dwell fatgue life of titanium alloy IMI 834

An experimental study was undertaken to determine the role of microstructure on the fatigue life reduction observed in titanium alloy IMI 834 under dwell loading conditions. The wave forms compared were a trapezoid with 15 and 30 second hold times at the maximum test load and a baseline, 10 Hertz, haversine. The stress ratio for both loading wave forms was 0.10. The fatigue loading of each specimen was conducted in a vacuum within a scanning electron microscope chamber which minimized the possibility that the laboratory environment would adversely affect the material behavior. Two microstructural conditions were investigated in the experimental program. The first involved standard 'disk' material with equiaxed alpha in a transformed beta matrix. The second material was cut from the same disk forging as the first but was heat treated to obtain a martensitic alpha prime microstructure. Tensile tests were performed prior to the onset of the fatigue loading portion of the study, and it was determined that the yield strengths of the specimens from both material conditions were within ten percent. The maximum fatigue loads were chosen to be 72 percent of the average yield strength for both materials as determined from the tensile tests. It was found that the cycles to failure from the 10 Hertz loading wave form were reduced by a factor of approximately five when the loading was changed to the trapezoidal wave form for the standard 'disk' material. The fatigue life reduction for the martensitic structure under identical test conditions was approximately 1.75. The improvement observed with the martensitic structure also was accompanied by an increase in overall fatigue life for the wave forms tested. This paper will review the results and conclusions of this effort.

Thomsen, Mark L.↗
NASA NTRS1995

Micromechanics of composites with shape memory alloy fibers in uniform thermal fields

Analytical procedures are developed for a composite system consisting of shape memory alloy fibers within an elastic matrix subject to uniform temperature fluctuations. Micromechanics for the calculation of the equivalent properties of the composite are presented by extending the multi-cell model to incorporate shape memory alloy fibers. A three phase concentric cylinder model is developed for the analysis of local stresses which includes the fiber, the matrix, and the surrounding homogenized composite. The solution addresses the complexities induced by the nonlinear dependence of the in-situ martensite fraction of the fibers to the local stresses and temperature, and the local stresses developed from interactions between the fibers and matrix during the martensitic and reverse phase transformations. Results are presented for a nitinol/epoxy composite. The applications illustrate the response of the composite in isothermal longitudinal loading and unloading, and in temperature induced actuation. The local stresses developed in the composite under various stages of the martensitic and reverse phase transformation are also shown.

Birman, Victor↗
NASA NTRS2005

Potential High-Temperature Shape-Memory-Alloy Actuator Material Identified

Shape-memory alloys are unique "smart materials" that can be used in a wide variety of adaptive or "intelligent" components. Because of a martensitic solid-state phase transformation in these materials, they can display rather unusual mechanical properties including shape-memory behavior. This phenomenon occurs when the material is deformed at low temperatures (below the martensite finish temperature, Mf) and then heated through the martensite-to-austenite phase transformation. As the material is heated to the austenite finish temperature Af, it is able to recover its predeformed shape. If a bias is applied to the material as it tries to recover its original shape, work can be extracted from the shape-memory alloy as it transforms. Therefore, shape-memory alloys are being considered for compact solid-state actuation devices to replace hydraulic, pneumatic, or motor-driven systems.

Noebe, Ronald D.↗
NASA NTRS2007

Correlation between Mechanical Behavior and Actuator-type Performance of Ni-Ti-Pd High-temperature Shape Memory Alloys

High-temperature shape memory alloys in the NiTiPd system are being investigated as lower cost alternatives to NiTiPt alloys for use in compact solid-state actuators for the aerospace, automotive, and power generation industries. A range of ternary NiTiPd alloys containing 15 to 46 at.% Pd has been processed and actuator mimicking tests (thermal cycling under load) were used to measure transformation temperatures, work behavior, and dimensional stability. With increasing Pd content, the work output of the material decreased, while the amount of permanent strain resulting from each load-biased thermal cycle increased. Monotonic isothermal tension testing of the high-temperature austenite and low temperature martensite phases was used to partially explain these behaviors, where a mismatch in yield strength between the austenite and martensite phases was observed at high Pd levels. Moreover, to further understand the source of the permanent strain at lower Pd levels, strain recovery tests were conducted to determine the onset of plastic deformation in the martensite phase. Consequently, the work behavior and dimensional stability during thermal cycling under load of the various NiTiPd alloys is discussed in relation to the deformation behavior of the materials as revealed by the strain recovery and monotonic tension tests.

Bigelow, Glen S.↗
NASA NTRS2001

Variable area nozzle for gas turbine engines driven by shape memory alloy actuators

A gas turbine engine includes a variable area nozzle having a plurality of flaps. The flaps are actuated by a plurality of actuating mechanisms driven by shape memory alloy (SMA) actuators to vary fan exist nozzle area. The SMA actuator has a deformed shape in its martensitic state and a parent shape in its austenitic state. The SMA actuator is heated to transform from martensitic state to austenitic state generating a force output to actuate the flaps. The variable area nozzle also includes a plurality of return mechanisms deforming the SMA actuator when the SMA actuator is in its martensitic state.

Rey, Nancy M.↗
NASA NTRS2012

Deformation and Phase Transformation Processes in Polycrystalline NiTi and NiTiHf High Temperature Shape Memory Alloys

The deformation and transformation mechanisms of polycrystalline Ni49.9Ti50.1 and Ni50.3Ti29.7Hf20 (in at.%) shape memory alloys were investigated by combined experimental and modeling efforts aided by an in situ neutron diffraction technique at stress and temperature. The thermomechanical response of the low temperature martensite, the high temperature austenite phases, and changes between these two states during thermomechanical cycling were probed and reported. In the cubic austenite phase, stress-induced martensite, deformation twinning and slip processes were observed which helped in constructing a deformation map that contained the limits over which each of the identified mechanisms was dominant. Deformation of the monoclinic martensitic phase was also investigated where the microstructural changes (texture, lattice strains, and phase fractions) during room-temperature deformation and subsequent thermal cycling were compared to the bulk macroscopic response. When cycling between these two phases, the evolution of inelastic strains, along with the shape setting procedures were examined and used for the optimization of the transformation properties as a function of deformation levels and temperatures. Finally, this work was extended to the development of multiaxial capabilities at elevated temperatures for the in situ neutron diffraction measurements of shape memory alloys on the VULCAN Diffractometer at Oak Ridge National Laboratory.

Benafan, Othmane↗
NASA NTRS2013

Low Temperature Creep of Hot-Extruded Near-Stoichiometric NiTi Shape Memory Alloy: Isothermal Creep - Part I

This two-part paper is the first published report on the long term, low temperature creep of hot-extruded near-stoichiometric NiTi. Constant load tensile creep tests were conducted on hot-extruded near-stoichiometric NiTi at 300, 373 and 473 K under initial applied stresses varying between 200 and 350 MPa as long as 15 months. These temperatures corresponded to the martensitic, two-phase and austenitic phase regions, respectively. Normal primary creep lasting several months was observed under all conditions indicating dislocation activity. Although steady-state creep was not observed under these conditions, the estimated creep rates varied between 10(exp -10) and 10(exp -9)/s. The creep behavior of the two phases showed significant differences. The martensitic phase exhibited a large strain on loading followed by a primary creep region accumulating a small amount of strain over a period of several months. The loading strain was attributed to the detwinning of the martensitic phase whereas the subsequent strain accumulation was attributed to dislocation glide-controlled creep. An "incubation period" was observed before the occurrence of detwinning. In contrast, the austenitic phase exhibited a relatively smaller loading strain followed by a primary creep region, where the creep strain continued to increase over several months. It is concluded that the creep of the austenitic phase occurs by a dislocation glide-controlled creep mechanism as well as by the nucleation and growth of deformation twins.

Raj, S. V.↗
NASA NTRS2013

Low Temperature Creep of Hot-Extruded Near-Stoichiometric NiTi Shape Memory Alloy: Effect of Thermal Cycling - Part 2

This paper is the first report on the effect prior low temperature creep on the thermal cycling behavior of NiTi. The isothermal low temperature creep behavior of near-stoichiometric NiTi between 300 and 473 K was discussed in Part I. The effect of temperature cycling on its creep behavior is reported in the present paper (Part II). Temperature cycling tests were conducted between either 300 or 373 K and 473 K under a constant applied stress of either 250 or 350 MPa with hold times lasting at each temperature varying between 300 and 700 h. Each specimen was pre-crept either at 300 or at 473 K for several months under an identical applied stress as that used in the subsequent thermal cycling tests. Irrespective of the initial pre-crept microstructures, the specimens exhibited a considerable increase in strain with each thermal cycle so that the total strain continued to build-up to 15 to 20 percent after only 5 cycles. Creep strains were immeasurably small during the hold periods. It is demonstrated that the strains in the austenite and martensite are linearly correlated. Interestingly, the differential irrecoverable strain, in the material measured in either phase decreases with increasing number of cycles, similar to the well-known Manson-Coffin relation in low cycle fatigue. Both phases are shown to undergo strain hardening due to the development of residual stresses. Plots of true creep rate against absolute temperature showed distinct peaks and valleys during the cool-down and heat-up portions of the thermal cycles, respectively. Transformation temperatures determined from the creep data revealed that the austenitic start and finish temperatures were more sensitive to the pre-crept martensitic phase than to the pre-crept austenitic phase. The results are discussed in terms of a phenomenological model, where it is suggested that thermal cycling between the austenitic and martensitic phase temperatures or vice versa results in the deformation of the austenite and a corresponding development of a back stress due to a significant increase in the dislocation density during thermal cycling.

Raj, S. V.↗
NASA NTRS2013

Macroscopic and Microstructural Aspects of the Transformation Behavior in a Polycrystalline NiTi Shape Memory Alloy

The mechanical and microstructural behavior of a polycrystalline Ni(49.9)Ti(50.1) (at.%) shape memory alloy was investigated as a function of temperature around the transformation regime. The bulk macroscopic responses, measured using ex situ tensile deformation and impulse excitation tests, were compared to the microstructural evolution captured using in situ neutron diffraction. The onset stress for inelastic deformation and dynamic Young's modulus were found to decrease with temperature, in the martensite regime, reaching a significant minimum at approximately 80 C followed by an increase in both properties, attributed to the martensite to austenite transformation. The initial decrease in material compliance during heating affected the ease with which martensite reorientation and detwinning could occur, ultimately impacting the stress for inelastic deformation prior to the start of the reverse transformation.

Benafan, Othmane↗
NASA NTRS1974

The role of nickel content and the magnetic remanence in iron-nickel alloys of lunar composition

Lunar samples are magnetic primarily due to the body centered cubic (BCC) iron and iron-nickel alloys they contain. Presented for the first time are results which demonstrate that the magnitude of the martensitic thermal remanence (MTRM) induced on quenching iron-nickel alloy in the geomagnetic field depends on the nickel content of the alloy. High magnetic stability is due to the increasing dislocation density and increasingly complex microstructures associated with increasing nickel content in the alloys. The results agree with the mechanical and structural properties of the alloys. The characteristic quench martensite microstructure observed on metallographic examination provides a recognition criterion for the MTRM mechanism. These results are important for lunar and meteoritic research intending to ascertain the paleofield responsible for the observed remanent magnetization.-

Wasilewski, P.↗
NASA NTRS1976

Phase transformation and stabilization of a high strength austenite

An investigation of the phase transformation and the austenite stabilization in a high strength austenite has been made. An Fe-29Ni-4.3Ti austenite age-hardened by gamma-prime (Ni3Ti) precipitates showed a further increase of strength after martensitic and reverse martensitic phase transformations. The stability of ausaged austenite as well as ausaged and transformation-strengthened austenite was improved significantly through an isothermal treatment at 500 C. The Ms temperature of the strengthened austenite was restored to nearly that of annealed austenite while the austenite was hardened to R(C) 41 through precipitation and phase transformations. The observed austenite stabilization is attributed to the formation of GP zones or short-range order of less than about 10A in size.

Jin, S.↗
NASA NTRS1978

Fractographic analysis of gaseous hydrogen induced cracking in 18Ni maraging steel

Electron microscope fractographic analysis supplemented an extensive study of the kinetics of gaseous hydrogen assisted cracking in 18Ni maraging steel. Temperature determined the crack path morphology in each steel which, in turn, was directly related to the temperature dependence of the crack growth rate. Crack growth in the low temperature regime proceeded along prior austenite grain boundaries. Increasing the temperature above a critical value produced a continuously increasing proportion of transgranular quasi-cleavage associated with lath martensite boundaries. The amount of transgranular cracking was qualitatively correlated with the degree of temperature-induced deviation from Arrhenius behavior. Fractographic observations are interpreted in terms of hypothesized mechanisms for gaseous hydrogen embrittlement. It is concluded that hydrogen segregation to prior austenite and lath martensite boundaries must be considered as a significant factor in developing mechanisms for gaseous embrittlement of high strength steels.

Gangloff, R. P.↗
NASA NTRS1979

The improvement of cryogenic mechanical properties of Fe-12 Mn and Fe-8 Mn alloy steels through thermal/mechanical treatments

An investigation has been made to improve the low temperature mechanical properties of Fe-8Mn and Fe-12Mn-0.2 Ti alloy steels. A reversion annealing heat treatment in the two-phase (alpha + gamma) region following cold working has been identified as an effective treatment. In an Fe-12Mn-0.2Ti alloy a promising combination of low temperature (-196 C) fracture toughness and yield strength was obtained by this method. The improvement of properties was attributed to the refinement of grain size and to the introduction of a uniform distribution of retained austenite (gamma). It was also shown that an Fe-8Mn steel could be grain-refined by a purely thermal treatment because of its dislocated alpha-prime martensitic structure and absence of epsilon martensite. As a result, a significant reduction of ductile to brittle transition temperature was obtained.

Hwang, S. K.↗
NASA NTRS1990

Mechanism of the gamma-beta phase transformation of Mg2SiO4 at high temperature and pressure

The results of experiments on the phase transformation of Mg2SiO4 olivine at 15 GPa pressure in a multianvil cell are reported. At this pressure and a temperature of 900 C, early formed metastable gamma-spinel transforms partially to the beta-phase. The observed microstructures, which are similar to those in shocked meteorites, show that the gamma-to-beta transformation can occur either by diffusion-controlled growth or by a martensitic mechanism, depending on how far the pressure-temperature conditions deviate from their values at phase equilibrium. The results suggest that the diffusion-controlled mechanism is most likely to operate at the beta/gamma phase boundary in the mantle, but martensitic beta-to-gamma transformation might occur in subduction zones and could reduce the shear strength of the subducting slab.

Rubie, D. C.↗
NASA NTRS1992

An APFIM/AEM Study of Phase Decompositions in Fe-Ni Alloys at Low Temperatures

A combined atom probe field ion microscopy and analytical electron microscopy characterization has been performed on laboratory aged martensitic and austenitic specimens of FeNi and FeNiP alloys. These techniques revealed that the martensitic 24.1 and 28.6 at.% Ni alloys decomposed during aging for 1 year at 300 C to form face centered cubic precipitates of approx. 56 at.% Ni in a body centered cubic matrix containing approx. 20 at.% Ni. Some thin platelets were observed in the field ion micrographs of the austenitic Fe-42.9 at. % Ni alloy and the Fe-43.2 at.% Ni-0.44 at.% P alloy after aging at 400 and 350 C. Atom probe analysis revealed phosphorus clustering in the ternary alloy aged at 300 C.

Zhang, J.↗
NASA NTRS2006

Properties of a Ni(sub 19.5)Pd(sub 30)Ti(sub 50.5) high-temperature shape memory alloy in tension and compression

Potential applications involving high-temperature shape memory alloys have been growing in recent years. Even in those cases where promising new alloys have been identified, the knowledge base for such materials contains gaps crucial to their maturation and implementation in actuator and other applications. We begin to address this issue by characterizing the mechanical behavior of a Ni19.5Pd30Ti50.5 high-temperature shape memory alloy in both uniaxial tension and compression at various temperatures. Differences in the isothermal uniaxial deformation behavior were most notable at test temperatures below the martensite finish temperature. The elastic modulus of the material was very dependent on strain level; therefore, dynamic Young#s Modulus was determined as a function of temperature by an impulse excitation technique. More importantly, the performance of a thermally activated actuator material is dependent on the work output of the alloy. Consequently, the strain-temperature response of the Ni19.5Pd30Ti50.5 alloy under various loads was determined in both tension and compression and the specific work output calculated and compared in both loading conditions. It was found that the transformation strain and thus, the specific work output were similar regardless of the loading condition. Also, in both tension and compression, the strain-temperature loops determined under constant load conditions did not close due to the fact that the transformation strain during cooling was always larger than the transformation strain during heating. This was apparently the result of permanent plastic deformation of the martensite phase with each cycle. Consequently, before this alloy can be used under cyclic actuation conditions, modification of the microstructure or composition would be required to increase the resistance of the alloy to plastic deformation by slip.

Noebe, Ronald↗