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1.
Microstructure of Ti-48.2 at. Pct Ni shape memory thin films   总被引:1,自引:0,他引:1  
Amorphous thin films of Ti-48.2 at. pct Ni formed by sputtering were annealed at 773 K for 5 minutes, 1 hour, and 10 hours. It was found by transmission electron microscopy (TEM) that the microstructure changes in the sequence of (1) Guinier-Preston (GP) zones for 5 minutes, (2) GP zones and Ti2Ni precipitates for 1 hour, and (3) Ti2Ni precipitates for 10 hours. A high-resolution electron microscope (HREM) revealed that Ti2Ni precipitates have partial coherency with the TiNi matrix.  相似文献   

2.
The composition and structure of Ti-48 at. pct Al alloys with various oxygen contents, quenched from a homogeneous α state, have been studied by means of one-dimensional atom-probe (1DAP) and transmission electron microscopy (TEM) analysis. Two regimes are observed. The change from one regime to the other depends on the global oxygen content. If the oxygen content is lower than 1.2 at. pct, the αγ m massive transformation is involved during the quench. The alloys, hence, exhibit massive γ m-structure regions and regions having a two-phase (α 2+γ) ultrafine lamellar structure. Very thin α 2 plates, saturated with oxygen, are observed in γ m regions. The precipitation of these α 2 plates is promoted by excess oxygen in the γ m structure. Within ultra-fine lamellar-structure regions, oxygen is concentrated in α 2 lamellae (not saturated with oxygen) and is found to be responsible for the high volume fraction of α 2 phase. When the oxygen content is larger than 1.2 at. pct, the massive transformation is suppressed and the ultrafine lamellar structure is only observed in quenched samples. Analysis of the αα 2 chemical ordering in the classical lamellar structure, formed within the (α+γ) dual-phase field, shows that high oxygen contents favor the chemical-ordering reaction of α phase at high temperatures (e.g., 1423 and 1523 K). It has, hence, been inferred that, above 1.2 at. pct O, the αγ m massive transformation is suppressed and replaced by the αα 2+y transformation paths. This article is based on a presentation made in the symposium entitled “Fundamentals of Structural Intermetallics,” presented at the 2002 TMS Annual Meeting, February 21–27, 2002, in Seattle, Washington, under the auspices of the ASM and TMS Joint Committee on Mechanical Behavior of Materials.  相似文献   

3.
The effect of aging on transformation and deformation behavior, i.e., the transformation temperatures, shape memory behavior, and multistage martensitic and R-phase transformations, was investigated for a Ti-50.9 at. pct Ni alloy aged at a low temperature (<600 K) rarely used for practical applications and at a high temperature (>600 K) conventionally used for practical applications. It was found that there are many differences between aging at 473 and 673 K. The martensitic and R-phase transformation temperatures significantly varied depending on aging time and temperature. It is found that two-stage R-phase and multistage martensitic transformations appear in both the specimens aged at 473 and 673 K, respectively. The two-stage R-phase transformation appeared by aging at 473 K over 36 ks, while the multistage martensitic transformation (MSMT) appeared by aging at 673 K in the range of aging times between 1.2 and 36 ks. It is found that the critical stress for slip increases with increasing aging time in specimens aged at 473 K, while that of specimens aged at 673 K increases with increasing aging time until reaching a maximum, then it decreases with a further increase in aging time. It is also found that the critical stress for slip is superior for specimens aged at 473 K than that for specimens aged at 673 K. It was confirmed that dense and fine lenticular precipitates of about 10 nm in length were formed through aging, resulting in superior shape memory characteristics.  相似文献   

4.
5.
The αγ phase transformation during rapid quenching and subsequent isothermal aging has been investigated in a Ti-48 at pct Al alloy. The microstructure changes from a completely massively transformed γ-grain structure to a mixed microstructure of the massively transformed γ grains and the untransformed (meaning massively untransformed) fine α 2/γ lamellae with an increase in the cooling rate from the high-temperature α phase field. Fine γ grains are generated from these fine α 2/γ lamellae by subsequent again at 1323 K. The fine γ grains contain many defects, such as dislocations, microtwins (or stacking faults), domain boundaries, and variants, which are frequently observed in the massive γ grains. This result suggests that the formation mechanism of the fine γ grains during aging is similar to that of the massive γ grains. When the fine γ/γ lamellar sample, which is formed by preliminary aging at a lower temperature (1173 K), is aged at a higher temperature (1323 K), apparent changes in microstructure could not be recognized. This result indicates that the fine γ-grain formation is closely related to the α 2γ phase transformation in the fine α 2/γ lamellae. This article is based on a presentation made in the symposium “Fundamentals of Gamma Titanium Aluminides,” presented at the TMS Annual Meeting, February 10–12, 1997, Orlando, Florida, under the auspices of the ASM/MSD Flow & Fracture and Phase Transformation Committees.  相似文献   

6.
A study of the high-temperature deformation behavior of a binary α 2+γTi-47 at. pct Al-3 at. pct Cr alloy was undertaken. The alloy was produced by induction melting and exhibited a structure of coarse columnar grains oriented in the radial direction. After a solution treatment at 1653 K for 3600 seconds and aging at 1223 K for 28,800 seconds, a nearly lamellar structure was formed. Deformation behavior was investigated by compression-strain-rate-change tests at strain rates ranging from 10−6 to 10−3 s−1 and temperatures ranging from 1073 to 1373 K. This alloy shows at low temperature/high stress a stress exponent of about 5. The deformation behavior is explained in this regimen by a dislocation climb mechanism, which includes a threshold stress. Finally, at the lowest stress levels and highest temperatures of testing, a stress exponent of about 3 is observed, which suggests that deformation is controlled by the viscous glide of dislocations.  相似文献   

7.
Porous Ni-Ti shape memory alloys (SMAs) have been successfully fabricated by the conventional powder sintering and combustion synthesis. In the present study, the microstructure, phase transformation, mechanical property, and stress-strain behavior of porous Ni-Ti SMAs have been investigated.  相似文献   

8.
The effects of mechanical twinning on both the tensile and compressive stress-strain behavior of Cu-4.9 at. pct Sn were investigated at 77, 193, and 298 K using polycrystalline specimens with a mixed <111>-<100> wire texture. In tension, twinning occurred mainly in the <111> structure component while in compression it occurred preferentially in the <100> component in agreement with the relevant Schmid factors for twinning. The volume fraction of twins was larger in the compression than in the tension specimens. The stressstrain curves in tension and compression differed significantly with the compression curves lying below the tensile curves. This difference can be rationalized in terms of the effect of twinning on the stress-strain behavior. The start of twinning is accompanied by a decrease in the work hardening rate, since at low strains twins tend to be parallel in any given region of the structure. At larger strains the tendency for twins to form on intersecting planes should act to increase the flow stress due to grain subdivision. Microhardness data obtained from twinned and untwinned regions of compression specimens support this hypothesis.  相似文献   

9.
The durability of Fe-Co thin films bonded on Ti-6Al-4V was studied as a function of layer thickness at ambient temperature. Interface toughness of the thin films was characterized by indentation and analyzed using an interface fracture model. The critical stresses for interface debonding and the fatigue life response of Ti-6Al-4V with and without Fe-Co thin films were evaluated by three-point bend fatigue at a stress ratio R of 0.1. The results indicated that the critical stress for interface debonding increased with decreasing layer thickness according to a critical energy release rate criterion. The Fe-Co thin films did not alter the fatigue life of the Ti-6Al-4V substrate. The presence of microcracks and interface debonds in the thin films did not affect the functionality of the sensor to detect strain via the inverse magneto-elastic effect. The overall durability of the films was in the range needed for practical application of the film as an embedded sensor.  相似文献   

10.
11.
It is shown that for a Ti-8 wt pct Al alloy aged at a temperature high in the two-phase region (695°C) to precipitate the ordered α2 phase, an increase in oxygen content from 600 ppm to 1200 ppm decreases the fracture strain from 20 to 1 pct elongation at room temperature and slightly increases the yield strength. The fracture mode is changed from dimpled rupture to predominantly cleavage. Further increase in oxygen content to 3000 ppm does not produce significant additional changes in ductility or yield strength. It is demonstrated that oxygen additions alter the position of the α/α + α2 coherent solvus, resulting in formation of coherent α2 in specimens containing ⪞ 1000 ppm oxygen aged at 968 K (695°C). For a given aging time the volume fraction of α2 increases with increasing oxygen up to 1300 wt ppm and then levels off. The changes in mechanical behavior are attributed to the presence of α2. The experimental evidence suggests that oxygen partitions preferentially into α2. Formerly a Graduate Student in the Department of Metallurgy and Materials Science, University of Pennsylvania  相似文献   

12.
Shape memory alloys exhibit superelasticity when they are deformed in a temperature range where the thermoelastic martensite forms on application of a strain. The martensite persists upon removal of the applied strain, and the alloy recovers the original shape on heating over the reverse-transformation temperature after removing the strain. The β-phase CuZnAl alloys have β-type superlattice in the parent case, and M9R or M18R martensites occur on quenching the alloys from the homogenization temperature. The basal plane of martensite is exposed to hexagonal distortion with martensitic transformation as well as the monoclinic distortion in the crystal structure, and splittings are observed in some selected diffraction-peak pairs due to the differences in atom sizes in lattice points. These pairs have a great importance as ordering criteria and satisfy a special relation between Miller indices. The present text reports the variation of the differences in interplane spacings (Δd) between some selected planes upon the further aging at room temperature at which alloys are fully martensitic. The decrease of Δd during the aging implies that the monoclinic distortion decreases. The mass increases are caused by the oxidation upon heating the alloys at high temperatures close to the betatizing temperature at free atmosphere.  相似文献   

13.
The superplastic deformation properties of Ti-6 pct Al-4 pct V and modified alloys containing 1/4 pct, 1/2 pct, 1 pct, and 2 pct of either cobalt or nickel have been investigated in the temperature range 950 to 750 °C. The results show that both cobalt and nickel modified alloys have reduced flow stresses, in comparison with Ti-6 pct Al-4 pct V, the reductions being particularly marked at the lower temperatures and lower strain rates. The results are shown to be consistent with an isostress model for the deformation of (α + β) two-phase alloys in which the varying β volume fractions and differing diffusivities of titanium, cobalt, or nickel in the β phase are taken into account.  相似文献   

14.
The shape of stress-strain curves for a pseudoelastic copper-based shape memory alloy (SMA) has been found to depend strongly on the grain size-to-sample thickness ratio (gs/t). Previous investigators have attributed this effect to reduced grain constraint in coarse-grain samples. The present investigation further analyzes the grain constraint effect by modeling shape memory alloy stress-strain curves. The model results reveal that varying grain constraint can explain the observed grain size effect on stress-strain curves. Furthermore, detailed consideration of the Taylor factor equivalent for Cu-Zn-Al and NiTi shape memory alloys can explain the opposite curvature of polycrystal stress-strain curves for these two materials. Finally, several indices of grain constraint are analyzed for the Cu-Zn-Al alloy examined in the present investigation and for similar alloys used in previous studies. This evaluation reveals that both transformation modulus and transformation stress correlate with gs/t, and each can be used as an index of grain constraint.  相似文献   

15.
This work investigates the fracture behavior of shape memory alloy NiTi (50.7 at. pct Ni) at room temperature. Macroscopic mechanical tests, microscopic in situ observations of tensile fracture processes by scanning electron microscopy (SEM), and detailed analyses of fracture surfaces were carried out. The results reveal that specimens with different thicknesses show various shape memory effects and superelasticities. The main crack with a quasi-cleavage mode that combines cleavage with ductile tearing is initiated at the notch tip and is stress-control-propagated in line with the direction of the maximum normal stress. The microstructure has little effect on the direction of crack propagation, but coarser substructures show lower resistance to the crack propagation. In specimens with various types of notches, various notch acuities present different effects on the crack initiation and propagation and result in different fracture behaviors.  相似文献   

16.
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.  相似文献   

17.
Mechanically synthesized, reproducible polytwins of an In-22.5 at. pct Tl shape memory alloy were studied quantitatively for their deformation behavior due to bending stresses, in the ferroelastic and rubberlike temperature regime, within situ video recordings of the evolving mesostructure. The experimental ratios of the twin modulus, ET, to the elastic modulus, Eel, were two orders of magnitude higher than the theoretically calculated values. Further investigations to reconcile this discrepancy revealed a fine substructure within each domain of the polytwin. The response coefficient of this substructure, rather than the elastic modulus, was found to be the controlling deformation parameter for the polytwin. Theoretically predicted results were modified, which established a close agreement with the experimental results.  相似文献   

18.
Cu基形状记忆合金的阻尼及记忆效应的应用   总被引:1,自引:0,他引:1  
综合回顾了Cu基形状记忆合金的研究发展现状,并概述了其阻尼机制及分类,分析了Cu-Al、Cu-Zn系合金的阻尼能力和影响因素,并介绍了Cu基形状记忆合金的应用,提出了其应用推广前要解决的问题.最后给出了一些应用实例.  相似文献   

19.
Isothermal and nonisothermal aging behaviors of five Cu/(13 to 28)Zn/(3 to 9)A1 (weight percent) shape memory alloys have been investigated. The electrical resistivity during isothermal aging from 180 °C to 340 °C increases with aging time at each aging temperature. The time to reach a constant fraction transformed during the isothermal aging increases with increasing Al content or decreasing Zn content in the Cu-Zn-Al alloys, except alloy D (Cu-18.2Zn-7.2Al). This alloy exhibits a double aging behavior by the earlier formation of the α1 bainite and the subsequent formation of the cubic γ2 phase from the β1 matrix phase. The apparent activation energy of alloy D varies with the fraction transformed by the double aging behavior. However, other alloys A, B, C, and E exhibit isokinetic behavior. The apparent activation energies estimated in the nonisothermal aging from 100 °C to 500 °C are found to be about equal to those obtained in the isothermal aging.  相似文献   

20.
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