Martensite In Cu Research Articles

Deformation twinning with different twin-boundary mobility in 2H martensite in Cu–Ni–Al shape memory alloy

Three possible twinning modes, Type I, Type II, and compound, as well as corresponding twin boundaries in 2H martensite of Cu69.4Ni3.4Al27.2 single crystal, were studied by X-ray diffraction (XRD) and high-resolution transmission electron microscopy (HRTEM). The results are discussed with regard to the sharply different twinning stress or twin-boundary mobility. In self-accommodated martensite, all three modes not only coexist but are crystallographically coupled. The compound twin boundary is a coherent coplanar mirror plane with the smallest twinning shear. The Type II twin boundary is also a coherent strain-free interface. The high index rational approximation of Type II twin boundary was determined by trace analysis with the help of stereographic projection. The approximation is in good agreement with irrational indices calculated from elastic continuum. The Type I boundary is the most complex interface associated with high stress and high density of stacking faults inside the twin bands. Using HRTEM, two different stressed boundaries of Type I were confirmed. The intrinsic twinning mode in Cu-Ni-Al alloy is the compound twinning. In compression, the Type II twinning is the major deformation twinning mode. During deformation the Type I twins are eliminated, leaving Type II twinning bands with compound twins. The observed differences between the atomic structure of different twin boundaries can contribute significantly to the sharp differences in twinning stress.

Mechanical Stabilization of Martensite in Cu–Ni–Al Single Crystal and Unconventional Way to Detect It

The microstructures and transformation behaviour of self-accommodated and mechanically stabilized martensite of Cu69.4Ni3.4Al27.2 (at.%) single crystal were investigated by optical microscopy, transmission electron microscopy (TEM), X-ray diffraction (XRD) and magnetometry. XRD and TEM analyses showed the presence of both 2H and 18R phases in self-accommodated martensite. The mechanical compression of martensite (≈ 100 MPa) increased markedly the transformation temperature to austenite, i.e. resulting in significant mechanical stabilization of martensite. The 18R phase disappeared after the compression. This reveals the important role of 18R phase in transformation behaviour of Cu–Ni–Al alloy. Furthermore, we demonstrate that thermo-magnetic measurement is suitable method to analyse the martensitic transformation even for diamagnetic material.

HYPERSTABILIZATION OF MARTENSITES

The effect of hyperstabilization of martensite implies that the reverse martensitic transformation proceeds in two well separated stages. Namely, a small fraction of martensite (of the order of 10%) retransforms upon heating into the parent phase over a temperature range slightly higher than the nominal reverse transformation temperature, whereas the rest of the martensite retransforms through a re-nucleation of fine lamellae of the parent phase. The renucleation stage of the transformation is well defined and requires strong overheating of the order of 300 K with respect to the nominal transformation. In this letter, the results are discussed of a study of the hyperstabilization effect in different martensitic structures: faulted [Formula: see text] martensite in Cu–Al–Be system and twinned martensite in ferromagnetic Ni–Fe–Ga crystals by means of differential scanning calorimetry, transmission electron microscopy and internal friction. The conclusion has been drawn that hyperstabilization implies a severe blocking of the motion of interphase boundaries during the reverse transformation, which can be produced either due to a high concentration of highly mobile quenched-in defects ("sweeping" of defects during the reverse transformation) or due to a creation of obstacles by preliminary plastic deformation. The former mechanism requires very intense diffusion of quenched-in defects assisted by dislocations/interfaces, which has been confirmed by internal friction studies. It has been shown that the renucleation stage, which occurs at around 600 K for different alloys, is preceded by a relaxation internal friction peak. A possible role of this relaxation in renucleation of the parent phase is discussed.

The refinement treatment of martensite in Cu–11.38 wt.%Al–0.43 wt.%Be shape memory alloys

The refinement treatment of martensite in Cu–11.38 wt.%Al–0.43 wt.%Be (Cu–22.71 at.%Al–2.57 at.%Be) shape memory alloy was investigated in this work. The results show that the aging treatment at a temperature near the eutectoid temperature caused numbers of oval-like renucleated parent phase in the matrix. Following the TEM-EDX analysis, a concentration gradient of Al content is caused because the Al-rich renucleated phase formed in the matrix. Therefore, the occurrence immediately provides potential sites for heterogeneous nucleation. When a large number of martensites grow simultaneously, mutual collision and obstruction occur, creating a cluster microstructure of fine martensite and improving the recovery ability of the alloy.

Defect-assisted diffusion and kinetic stabilisation in Cu–Al–Be [formula omitted] martensite

An acoustic technique operating at a frequency of about 100 kHz and strain amplitudes of 10 −7 to 2 × 10 −4 is employed to investigate diffusion mobility of point-like defects in the β ′ 1 martensitic phase of a Cu–10.4 wt.% Al–0.47 wt.% Be single crystal at temperatures down to 9 K. Diffusion of the quenched-in point-like defects in the Cu–Al–Be martensite is detected by amplitude-dependent anelasticity at much lower temperatures than for other Cu-based martensitic alloys (Cu–Al, Cu–Al–Ni, Cu–Zn–Al). Strain amplitude dependence of the internal friction and Young's modulus defect indicates that the formation of pinning atmospheres at temperatures up to 200 K is restricted to the regions swept up by oscillating partial dislocations/interfaces and, therefore, should be attributed to the efficient dislocation/interface-assisted diffusion of point-like defects. Observation of the intense defect-assisted diffusion in the Cu–Al–Be martensite accounts for the extremely strong “kinetic” stabilisation of martensite in Cu–Al–Be, which is evident from calorimetry data and is much less intense in other Cu-based martensites.

Experimental and theoretical study of mechanical stabilization of martensite in Cu–Al–Ni single crystals

A direct correlation between the hysteresis of the partial stress–strain ( σ– ɛ) cycles corresponding to the β → γ′ martensitic transformation of a 82.5%Cu–13.5%Al–4.0%Ni (wt.%) single crystal and the induced amount of mechanical stabilization (MS) determined by calorimetry has been established. The results show a good agreement with the coefficient d σ/d T typically obtained from stress-inducing the transformation at different temperatures. In addition, the experimental σ– ɛ curves have been quantitatively reproduced by a statistical model of superelasticity. In partial cycles, a unique probability distribution function (probabilities of martensite appearance/disappearance) can be used for a correct description of all the forward cycles while different distributions, which depend on the maximal strain values achieved during the loading branch, have to be considered for the reverse cycles. Conversely, no mechanical stabilization of martensite has been observed during the β → β′ transformation of 75.0%Cu–17.4%Zn–7.6%Al (wt.%) single crystal in tension mode and, in this case, the forward and reverse partial transformations have been successfully modelled using the same probability distribution function. Thus, the asymmetry in the forward and reverse probability distributions needed for the β ↔ γ′ transformation in Cu–Al–Ni explicitly manifests the immediate stabilization of the mechanically induced martensite.

Stability and stabilization of 2H martensite in Cu–Zn–Al single crystals

The stabilization of the 2H martensitic phase in Cu–Zn–Al single crystals with an electron concentration e/a = 1.53 was investigated. This orthorhombic 2H martensite was first induced from the cubic β phase by the direct β → 2H or the indirect β → 18R → 2H transformations. On loading the 2H martensite, a transition without hysteresis is observed at a stress which was denoted σT1. It was found that this stress is associated with a change in the behaviour of the 2H martensite. A high stabilization of the 2H martensite, around 300 K, is only obtained if an ageing is performed at a stress above σT1. Additionally, the stresses of the transformation to another martensitic phase, called 18R2, were found to be constant when the value of σT1 is below the retransformation stress. The 2H martensite and its behaviour on ageing were studied by dilatometry, calorimetry, mechanical testing, optical microscopy and transmission electron microscopy (TEM). Models accounting for the stabilization of the 2H martensite on ageing are proposed.

The influence of small impurity contents on the diffusion controlled stabilization of Cu–Zn–Al martensitic single crystals

The stabilization of the 18 R stress induced martensite in Cu–Zn–Al single crystals has been measured as a function of ageing time in differently prepared crystals. It is concluded that only small impurity contents of a concentration similar to that of the quenched-in vacancies can affect notably the stabilization kinetics.

A study of the two-way shape memory effect in Cu–Zn–Al alloys by the thermomechanical cycling method

We have examined the two-way shape memory effect (TWME) in Cu–Zn–Al alloys with a novel thermomechanical cycling method: bending the alloy strips around a cylinder mould and by using a constrained heating/cooling technique. The strength of TWME was affected by the number of training cycles and by the constrained heating temperature. We found that the presence of the β-phase is a crucial factor in generating TWME. After the thermomechanical cycling treatment, we observed the new martensite in addition to the existing martensite in Cu–Zn–Al alloys.

Electron irradiation induced stabilisation in Cu–Zn–Al–Ni alloys

It is shown that stabilisation of the 18R martensite in Cu–Zn–Al–Ni can be induced by electron irradiation followed by annealing. The results are discussed with reference to the stabilisation of the 18R martensite induced by quenching and ageing in a similar alloy, and by irradiation in Cu–Zn–Al alloys.

Vacancies and stabilisation in martensitic Cu–Zn–Al single crystals

The stabilisation of the 18R stress induced martensite in Cu–Zn–Al single crystals has been measured as a function of the ageing time and the previous thermal treatment in the high temperature β phase. It is found that the initial and final stages of the stabilisation present a different behavior, and only the latter is related to the quenched-in vacancy concentration. It is confirmed that the martensite does not reach its thermodynamical equilibrium state, although the vacancies remain mobile. It is suggested that the evolution of the martensite can be rationalised by the combined action of mobile vacancies and precursors of the γ equilibrium phase.

Compression-induced hexagonal martensite in Cu–Zn–Al

The martensitic transformations occurring in β Cu–8.54at.%Zn–21.75at.%Al single crystals with e/a=1.52 are studied by compressive pseudoelastic cycles, optical and electronic microscopy. A change between serrated to smooth transformation behaviour is observed at T–Ms around 20 K. Once a critical volume of 18R martensite has formed, 2H martensite is induced in a β-18R mixture at temperatures between Ms and T–Ms=71 K. The pseudoelastic cycles exhibit high stress hysteresis and the 2H phase is retained on unloading. The experimental habit plane of the 2H martensite matches with that calculated from the phenomenological theory. Two 2H variants are mainly formed, which are comparable with those induced by tension from 18R single crystals.

The effect of post-quench aging on stabilization of martensite in Cu–Zn–Al and Cu–Zn–Al–Ti–B shape memory alloys

Abstract The effect of post-quench aging in polycrystalline β Cu–Zn–Al and Cu–Zn–Al–Ti–B on the martensite stabilization has been studied. The samples were heat treated at 1073 K and subsequently quenched into water at 373 K. After the quench, the samples were aged in the β phase, 373 K≤ T ≤ 403 K, during 300 s ≤ t β ≤10 5 s. The stabilization tests were performed by applying compressive stresses at temperatures above the reverse transformation temperature, A f followed by martensite aging under constant strain. When unloading the sample, it recovers its original shape but the reverse transformation stress is shifted to lower values. The magnitude of the stress shift, a measure of martensite stabilization, is a function of the thermal treatment and aging time. The process continues until a saturation is reached. It was found that the stress shift saturation value depends on the temperature and the aging time in the β phase. The results show that the stabilization depends on the vacancy concentration and that dislocations can not be the dominating type of vacancy sinks in martensite.

Stacking Disorder in Martensite of Cu-Zn Alloy

The faulting in stacking sequence of close-packed layers of the martensite in Cu–38.6 at. %Zn alloy has been studied by electron diffraction, utilizing the selected area diffraction technique of electron microscopy. Observed diffraction spots were shifted and diffused owing to the stacking faults. The shifts of sports were analyzed by the Kakinoki-Komura theory with Reichweite s =3, and the stacking fault probabilities α were found to be between 0.13 and 0.43, α being a fault parameter representing the cubic type stacking faults. The result is compared with the case of the Cu-Al martensite: the predominant type of stacking faults is cubic type for the Cu-Zn martensite while hexagonal type for the Cu-Al martensite. This difference is explained by the phenomenological theory of martensitic transformation.

On the Tetragonality of CuAu I Type Superlattice Structure of Strain-Induced Martensite in Cu–Zn Alloy

On the Tetragonality of CuAu I Type Superlattice Structure of Strain-Induced Martensite in Cu–Zn Alloy