Transformation squence in a Cu-Al-Ni shape memory alloy at elevated temperatures

Precipitation sequences in a Cu-14 pct Al-4 pct Ni (wt pct) shape memory alloy were studied by means of transmission electron diffraction and microscopy as well as X-ray microanalysis techniques. On aging thin foil specimens up to 550 °C in the electron microscope, an as-quenched sample having a mixture of 2H-type and D0₃-type metastable structures transformed to the stable simple cubic γ₂ phase at or above 450 °C. The remaining matrix either showed precipitates of the fcc α-phase on prolonged annealing at 500 to 550 °C for a longer period, or transformed to martensite on cooling below theM _s temperature (~150 °C).     

Stabilization and two-way shape memory effect in Cu-Al-Ni single crystals

The two-way shape memory effect (TWME) induced by stabilization of the martensite phase during aging has been studied in Cu-13.4 Al-4.0 Ni (mass pct) single crystals. The influence of the degree of long-range order on the transformation has been determined by using different heat treatments. The transformation temperatures are strongly influenced by the degree of order in the austenite: annealing from above or below the second neighbor L2₁ ordering temperature changes the M _s by more than 100 °C. It has been established that the diffusion in the austenite as well as in the martensite phase is considerably slower in this alloy than in other Cu-based ones, due to the presence of Ni. The obtained TWME has a similar efficiency as when other more complex thermomechanical trainings are made. In this alloy, the TWME by stabilization is not complete, in contrast to that in Cu-Zn-Al single crystals.     

NiTi合金线材无模拉拔加工过程热力耦合数值模拟

结合所研制的连续式无模拉拔加工设备的实际情况,利用DEFORM有限元软件建立了加工过程的热力耦合有限元分析模型,开展了NiTi合金线材无模拉拔加工过程的热力耦合模拟研究,获得了线材无模拉拔加工过程的温度场和应力场的分布规律,以及冷热源距离、冷却水流量及拉拔速度对成品线径的影响规律.通过实验验证了模拟结果的正确性.     

Effects of strain aging at 373 K in wires of NiTi shape memory alloy

Abstract

Strain and temperature aging for a NiTi shape memory alloy is performed at 373 K, using wires of 2·46 mm and eventually 0·5 mm in diameter. Aging under large strains modifies the hysteresis shape and induces an increase in the maximal stress in the transformation from 600 to 800–1000 MPa (wires of 2·46 mm). On applying a strain up to 7–8%, the effect according to the initial state is close to 50 MPa per month of strain–temperature aging. The results enable efficient NiTi damper application from 233 to 323 K. A partial study of wires of 0·5 mm shows quantitatively different effects.     

镍钛形状记忆合金线坯连续定向凝固温度场的数值模拟

连续定向凝固过程中结晶器的温度分布对固-液界面位置和形状具有重要影响.在建立三维物理模型以及确定材料热物性参数、边界条件与冷却水对流换热系数计算方法的基础上,采用ANSYS有限元软件对不同参数组合条件下镍钛形状记忆合金线坯连续定向凝固的稳态温度场进行了数值模拟.研究结果表明,在所给定的模型及各种参数条件下,镍钛形状记忆合金在结晶器内可以完成凝固过程,且固-液界面呈平直状,具备了进行连续定向凝固制备的基本条件.

     

Development of Fe-based shape memory alloys associated with face-centered cubic-hexagonal close-packed martensitic transformations: Part I. shape memory behavior

As part of a study on newly developed Fe-based shape memory alloys associated with face-centered cubic-hexagonal close-packed (fcc-hcp) martensitic transformations, the shape memory behavior is reported and various physical factors influencing it discussed, based on experimental results from dilatometry and bending tests. It is shown that the alloys studied exhibit a good shape memory effect (SME) without special training. The SME is significantly improved by prestraining at low temperatures. A higher net reversible strain is made available by over-prestraining, although the fully reversible prestrain usually does not exceed 2 pct. Super-elasticity, the two-way shape memory, and a softening effect of the matrix at low temperatures are briefly discussed.     

Observations of aging effects in a Cu-Sn shape memory alloy

A Cu-15.0 at. pct Sn alloy has been chosen as a model alloy for the study of aging effects in copper-based shape memory alloys. Different thermal aging treatments were carried out to determine the effects of both parent phase and martensite aging on the amount of shape recovery and the characteristic transformation temperaturesM _s ,A _s , andA _f . Aging of the martensite reduces both the amount of shape recovery and the extent of the reverse martensite → parent transformation. High martensite heating rates promote complete shape recovery and reverse transformation while the aging occurring during slow heating can inhibit or prohibit both. But irrespective of the martensite heating rate the transformation temperature hysteresis as given by (M _s -A _s ) is large for the Cu-15 pct Sn alloy compared to other shape memory alloys exhibiting thermoelastic behavior. On the other hand, some beneficial effects were noted when the Cu-15 pct Sn alloy was aged in the parent phase condition prior to subsequent transformation to martensite. TheM _s ,A _s , andA _f were lowered following prior parent phase aging, possibly because of a change in long range order, but prior parent phase aging was found to diminish the deleterious effect of martensite aging. Both shape recovery and the extent of the reverse martensite → parent transformation are enhanced by prior parent phase aging. The enhancement is greater the higher the aging temperature or the longer the aging time at a given temperature. J. D. STICE, formerly Research Assistant at the University of Illinois     

Mechanisms of internal friction in a cu-zn-ai shape memory alloy

The internal friction in a Cu-Zn-AI shape memory alloy has been studied in various microstructural states in a wide range of stress amplitude. Simultaneous measurements of the corresponding shape change indicate correlations between internal friction and the deformation mechanisms. It is found that the observed stress-amplitude dependence of the internal friction cannot be represented by a single formalism. Rather, in different ranges of stress amplitude, where the internal friction mechanisms are different, separate formalisms are applicable. The comparison of the experimental results with a previously developed theory is presented.     

Deformation and structure of a shape memory titanium alloy during electroplastic processing

The effect of a high-density pulse electric current passing through a sample during rolling on the deformability, microhardness, and microstructure of a coarse-grained shape memory TiNi alloy is studied. The current is shown to enhance the ductility and to form a spectrum of amorphous and ultrafine-grained structures.     

Some features of the shape memory effect in a porous nickel-titanium material

Conclusions The shape memory effect exhibited by porous materials accounts for the volume memory characterizing porous bodies. Under conditions of a volume stressed state, porous sintered titanium nickelide can be deformed with densification without rupture to high degrees of deformation. The degree of completeness of shape recovery achieved during heating after deformation is less for porous than for nonporous titanium nickelide. This is linked with a heterogeneous character of deformation of a porous body and with the appearance in its volume of local zones characterized by high degrees of deformation, exceeding the reverse deformation reserve of the material, which lower the degree of completeness of shape recovery. With increasing starting porosity, the degree of completeness of shape recovery by titanium nickelide grows (at any given degree of prior macrodeformation). The degree of reversible deformation during the recovery of high-porosity titanium nickelide is not less than that of the intermetallic compound in the nonporous state, and the range of permissible degrees of prior macroscopic deformation for porous materials is much greater, which widens scope for the processing and application of these materials.Translated from Poroshkovaya Metallurgiya, No. 12(228), pp. 41–45, December, 1981.     

Microstructure-mechanical property relationship to copper alloys with shape memory during thermomechanical treatments

The phase transformations during fabrication and aging after cold deformation in three polychrystalline copper alloys of the Cu-Al-Ni system with shape memory effect (SME) were characterized. Some phase transformations were identified with clear repercussion in their mechanical properties during thermomechanical treatments. Around 430 °C, mutual effects of β-phase recrystallization and precipitation of γ₂ and NiAl phases were observed. Close to 600 °C the dissolution of phase α was observed, beginning transformation into β phase process. Brittle phases such as γ₂ and NiAl began to precipitate during a short exposure time at 380 °C, 585 °C, 600 °C, and 700 °C temperatures. The phase transformations were intensified due to the plastic deformation that acted as a driving force for the diffusion processes. The introduction of chemical elements inhibited the grain growth and increased the structural disorder generating an elevation in the hardness property.     

Plastic straining effects on the microstructure of a Ti-rich NiTi shape memory alloy

A Ni-52 at. pct Ti shape memory alloy, cold drawn to 30 pct, was annealed at 1173 K for 1 hour, water quenched, and then subjected to differential scanning calorimetry (DSC). No evidence of the premartensiticR transformation was found during either the forward or the reverse transformation. Microstructurally, it was found that the alloy possessed a relatively large volume fraction (∼0.05) of coarse second-phase brittle particles. These precipitates acted as preferential sites for martensite plate nucleation and gave rise to a “starlike” morphology. The tensile and compressive properties of the alloy in the as-received condition were also investigated. The alloy exhibited relatively good ductility (fracture strain=0.28), which was attributed to its inherent ability to relieve or delay the development of plastic instabilities through rapid strain hardening. In addition, X-ray diffraction (XRD) of deformed specimens indicated the presence of an extraintensity peak corresponding to the B2 phase (110)_B2 when the alloy was plastically deformed in compression. Accordingly, it is suggested that plastic deformation induces the reverse transformation to the B2 phase in highly stressed local regions. Transmission electron microscopy (TEM) of deformed martensite structures showed slip lines probably due to dislocation slip, as well as variant interpenetration. Besides, optical and scanning microscopy of regions adjacent to the fractured surfaces indicated that fine martensite plates and/or “apparent” new grains develop at regions of prior stress intensification (former crack-tip regions) during crack propagation.