18R Martensite Research Articles

Structure changes during pseudoelastic deformation of CuAlMn single crystals

Structure changes during pseudoelastic deformation of CuAlMn single crystals were investigated using in situ optical and high-voltage electron microscopy (HVEM). Several crystal orientations were investigated, from an irrational to 〈100〉 and 〈110〉 tensile axis and plane orientations in the case of thin foils. The composition of the alloy was chosen such as to obtain superelastic behavior at room temperature. Optical microstructures allowed to identify parallel plates at the beginning of the stress plateau, the number of which increased with strain. The stress/temperature phase diagram was established within the range of existence of γ ′ 1 and β ′ 1. During in situ HVEM deformation in the 〈100〉 direction of γ ′ 1 plates nucleated on pre-existing ones. At later deformation stages 18R martensite was formed in stacks of narrow needles. The following crystallographic relationship was observed: [001] β 1∥[010] β ′ 1, γ ′ 1 and [110] β 1∥[001] β ′ 1, γ ′ 1. A small permanent deformation observed after stress release was connected with the presence of residual martensite of a high random stacking fault density and consisting often of α ′ 1, γ ′ 1 and β ′ 1 martensite layers. During deformation in the 〈110〉 direction a larger permanent deformation and a density of dislocations was observed.

Structural Defects in the 18R Martensite of CuZnAi Shape Memory Alloys

ABSTRACTThe lattice relaxation at the type I faults in the 18R martensite was studied by high resolution TEM. Local atomic plane rotations and small changes of the interplanar distances were observed over a distance of 1.5 nm. It is shown that this distorted region can be rationalized as an intermediate structure between the martensite and the second variant of the parent phase.

Stabilization of Cu-Zn-Al 18R martensite by 2 MeV proton irradiation

Abstract Irradiation experiments with 2 MeV protons were carried out in Cu-Zn-Al 18R martensite. It was found that the enhanced diffusion brought about by the irradiation caused the stabilization of the 18R martensite. This behaviour is similar to that reported previously for electron irradiation. The stabilization kinetics were studied as a function of the displacement rate k 0. The time necessary to obtain a given degree of stabilization was found to depend on k 0 −0·6. By comparison with previous electron irradiation experiments, an efficiency of about 0·26 was obtained for the production of freely migrating defects by 2 MeV proton irradiation relative to 2·6 MeV electron irradiation. The kinetics of stabilization under proton and electron irradiation could be described by a simple model based on rate equations.

Influence of Grain Size and Ordering on the Two Way Shape Memory Effect in CuAlMn Alloys

Two way shape memory effect (TWSME) in CuAlMn alloys was studied in samples of various grain sizes and a degrees of order. The training was performed by cooling the samples under tensile stress below M f and heating above A f after unloading. It was found that investigated alloys showed well developed TWSME already 10 cycle s.Increasing of the training stress caused broadening of the transformation hysteresis. Training treatment causes formation of dislocation stacks of increased density at higher training stress. Most of dislocations possess a mixed character of Burgers vector (u = , b = ), however some dislocations of b = were observed in g.b contrast experiments. At higher training stress relatively large amount of stabilised 2H or 18R martensite was found which is most probably related to the training effect.

Bulk Defects in Pseudoelastically Cycled Cu-Zn-Al Single-Crystals

The pseudoelastic behavior, in Cu-Zn-Al alloys cycled between the bcc phase (β) and the 18R martensite structure was analyzed for tests performed within the range [-196 °C - 50 °C]. The role of the temperature, in these fatigue experiments, is pointed out by a different evolution of the σ-e curves with cycling, which are related with changes in the nature and density of the defects formed in the bulk.

Stability, stabilization and lattice parameters in CuZnAl martensites

Lattice parameters have been measured in 18R and 6R martensite without and with stabilization. It is shown that 6R is basically cubic, deviations occurring due to remaining traces of 18R. The orthorhombic distortion in 18R depends on the long range order energy inherited from the B2 phase, which generally is not perfect and can be modified by heat treatments in the β phase. Stabilization of 18R martensite can be quantitatively accounted for by the disordering of CuZn pairs.

Martensite ordering and stabilization in copper based shape memory alloys

Stabilization behavior of the martensite in shape memory CuZnAl and CuAlMn alloys has been studied by x-ray measurements and electron microscopy. In these alloys influenced by both quenching and post-quench heat treatments, the degree of stabilization depends on quenching conditions. The stabilization process is mainly due to structural change and atomic arrangement in the martensitic lattice inherited from the parent phase. From x-ray results, it is suggested that stabilization and loss of memory are directly related to disordering in a martensitic state. In the present study, it was concluded that the spacing differences (Δd) between some selected pairs of diffraction planes of type (h 1k 1l) and (h 2k 2l) satisfying the relation sol(h 1 2 − h 2 2) 3 = (k 2 2 − k 1 2) n , with n = 1 for 9R and n = 4 for 18R martensites reflect the degree of ordering in martensite, in addition to the lattice distortion parameters defined as φ = ¦sin 2θ 202 − sin 2θ 122¦ and φ′ = ¦sin 2θ 320 − sin 2θ 040¦ .

Kinetics of the fcc to hcp phase transformation and the formation of martensite in pure cobalt

The experimental data on the kinetics of the fcc to hcp phase transformation in pure cobalt have been summarized (in Figs. 1 and 2) and analyzed in conjunction with the recent finding of a metastable 9R martensite structure by Kajiwara et al. [12,13]. From the observed transformation kinetics, it is predicted that there should exist another martensitic structure for cobalt in addition to the ε martensite and the 9R (ε″) martensite, i.e., there are three kinds of martensite. Each martensite has a separate M s temperature, which is identified as 390 ± 5 °C (ε), 370 ± 8 °C (ε′) and 330 ±15 °C (ε″), respectively. The absence of this unidentified ε′ martensite in the ultra-fine particles of Kajiwara et al. does not exclude its existence in other forms of pure cobalt. Grain size and impurity effects on the transformation kinetics are also discussed and used to explain the experimental observations in both bulk and ultra-fine particle cobalt.

Interatomic potentials for B2 NiAl and martensitic phases

Interatomic potentials of the embedded atom type were developed for the Ni-Al system by empirical fitting to the properties of B2 NiAl and Ni5Al3. Consideration was also given to the properties of L12 Ni3Al as well as the martensitic L10 phase. The B2 phase is predicted as the stable phase for the equi-atomic composition. The potentials also predict the stability of the 3R martensitic structure with respect to the B2 phase for 62.5% Ni alloys. The globally stable phase for this composition is the Ni5Al3 structure. The predicted lattice parameters and tetragonality ratios for NiSAl5 and 3R martensite are very close to experimental values. The structure and energy of various defects was calculated using the new potentials and the results compared with those given by other potentials in the literature.

Pseudoelastic Deformation of CuAlMn Single Crystals

Martensitic transformation associated with a tensile deformation of CuAlMn single crystals in [100] direction was studied using in-situ transmission electron microscopy. Stress/strain curves obtained during pseudoelastic deformation of microscopic samples were slightly different from bulk ones due to martensite formation in the stage of stress increase and a permanent deformation during the first cycle. Individual needles of 2H martensite were formed already in the early deformation stage. They were growing during stress increase, joining together into large plates. At later deformation stages 18R martensite was formed usualy in stacks of single plates. Following crystallographic relationship between parent and martensitic phases was observed: [001]β 1 ∥[010]γ',β' and [110]β 1 ∥[001]γ',β'. At a high stress concentration near crack tips, α' martensite of 3R lattice was observed. Identification of martensite structure was very often not possible there, due to an extremely high random stacking faults density

Transmission electron microscopy studies of the crystallography of B.C.C./18R martensite in FeMnAlC

The 18R martensite phase was observed in the b.c.c. matrix of an Fe−25.8 wt% Mn−7.4 wt% Al−0.11 wt% C alloy, after cooling from high temperature. The crystal structure is determined from the selected area diffraction (SAD) and high resolution imaging techniques. The 18R martensite has an orthorhombic lattice with lattice constants a = 0.448 nm, b = 0.259 nm and c = 3.865 nm and is best described as an 18R(4 2 ) 3 rather than an 18R(5 1 ) 3 structure. [The expression of 18R(4 2 ) 3 and 18R(5 1 ) 3 comes from the combined notation of Ramsdell and Zhdanov [Nishiyama, Martensitic Transformation, p. 75 (1978)].] The stacking fault density relative to the 18R(4 2 ) 3 structure is determined from high resolution imaging to be 0.096. Martensite crystallographic parameters such as orientation relationship, habit plane, shape strain direction and magnitude of lattice invariant shear are calculated using the CRAB theory, in good agreement with experimental observations. The Burgers vector of anticoherency dislocations and the shape strain direction were confirmed using the two beam technique and computer image simulation. The observed interfacial dislocation structure and choice of lattice correspondence are discussed in terms of near-CSL concepts.

Effect of aging on the rubber-like behavior in Cu-Zn-Al martensites

Martensites in several shape memory alloys exhibit peculiar pseudoelastic behavior, referred to as twinning pseudoelasticity or rubber-like behavior. By loading a sample in the martensite state, internal twin boundaries, or intervariant boundaries, move so as to increase the volume fraction of variants more favorable oriented to relieve the applied stress. On unloading, those boundaries tend to return to their original positions; accordingly, the sample restores it original shape. It has been recognized that the rubber effect appears only in aged martensites. When a martensite is deformed immediately after its formation, the boundary motion is not reversible; imposed strain remains after unloading. Unlike the case of superelasticity, which is another class of pseudoelastic behavior in shape memory alloys, the rubber effects does not involve any phase transformation. Deformation is undertaken merely by the reorientation of variants of a single martensite phase. While the free energy difference between two participant phases gives rise to the restoring force in superelasticity, there is, apparently, no such counterpart in the rubber-like behavior. The original of the restoring force in the rubber effect has long been a subject of controversy. In the present study, the authors have investigated the effects of room temperature aging on themore » stress-strain behavior quantitatively in both 9R and 18R martensites of Cu-Zn-Al alloys. In addition, changes in the reverse transformation temperature by aging have been measured. Since aging is known to significantly affect the reverse transformation behavior as well, they have intended to examine a correlation between the rubber-like behavior and the stabilization of martensites.« less

110]TA1Phonon Branch and Anomalous 2/3[110] Elastic Peak in Heusler and B2 Phases of a AuCuZn2Alloy

The present study on the [110]TA 1 phonon dispersion relation in a AuCuZn 2 alloy reveals that the lattice instability towards the 18R martensite is characteristic of only the Heusler phase existing stably below about 670 K, which is the disordering temperature to the B2-phase. Anomalous peak appearing near ζ=2/3[110] in the Heusler-phase is also examined by neutron elastic scattering experiments in a wide range of temperature. It is evident that such a peak has no relation to the 2/3-dip on the [110]TA 1 phonon branch observed in the Heusler-phase but it is originated from vacancies existing inherently and regularly in the B2-lattice.

New description of long period stacking order structures of martensites in β-phase alloys

The martensitic transformations in {beta}-phase alloys have attracted much attention for many years, since they are associated with the unique shape memory and superelasticity effects, and the lattice instability or unusually soft elastic constant c{prime}[=c11-c12/2]. Besides, the structural changes are all similar to each other; i.e. ordered BCC parent (B2 type or DO{sub 3} type) structures transform to long period stacking order structures, with 2-dimensionally close-packed planes. Furthermore, these alloys in a single crystal state even exhibit martensite-to-martensite transformation among the long period stacking order structures. These structural changes became clear after the crystal structure determination of {beta}{sub 1}{prime} martensite in a Cu-Al alloy near the Cu{sub 3}Al composition by Nishiyama and Kajiwara. The authors discuss the structural change following their scheme. The structure of the martensite in {beta}-phase alloys was extensively re-examined. As a result, the displacements of stacking positions from the ideal a/3 positions and the monoclinicity of 18R(9R) martensites were confirmed in most of the alloys examined, such as Cu-Zn-Ga, Cu-Zn-Al, Au-Cu-Zn, and Cu-Al-Ni alloys. Thus, they can conclude that the structures of the martensites in {beta}-phase alloys are intrinsically monoclinic except for 2H martensites.

On the formation of 2H stacking sequence in 18R martensite plates in a precipitate containing CuAlNiTiMn alloy

Basal planes stacking sequence variation in M18R martensite of CuAlNiTiMn alloy have been studied by means of electron diffraction and lattice imaging. Samples subjected to two different homogenisation temperatures (1173 and 973K) and two different cooling rates—water quenched and air cooled were analysed. The observed changes in stacking sequence have been attributed mainly to stress accomodation around XL and XS precipitates. Very small XSS precipitates of the same phase have been observed and their high influence on the stacking sequence of martensite plates have been explained by means of a new “dislocation” mechanism related to their coherency loss into the matrix.

Stabilisation of martensite in CuZnSi

In many copper based shape memory alloys, the retransformation of martensite to the parent occurs at a higher temperature, immediately after quenching through the transformation and/or aging in the martensitic condition when compared to retransformation in continued cycles performed without any delay. This effect termed stabilization, has been used to raise, temporarily, the retransformation temperature of the martensite in Cu-Zn-Si alloys, notably with compositions which transform to martensite well below ambient temperature. This has been achieved by heating the sample, with the martensite microstructure, very slowly, at a rate of about 0.25[degree]/min. There are few reports to date on the mechanism of stabilization of Cu-Zn-Si martensite. These reports cite all the reasons mentioned above as possible causes for the stabilization. Moreover, martensite boundary pinning by vacancies and narrow lamellae of 3R phase within the 9R martensite have been claimed as fairly important contributors to stabilization. The present work was undertaken to review these earlier conclusions and has been carried out in alloys that transform nominally at sub-zero temperatures. The studies are restricted to the first 10[degree] or 15[degree] of stabilization.

Microstructure and characteristics of copper-aluminium-nickel shape memory effect alloys prepared by mechanical alloying: T. Kaneyoshi et al (Hyogo Inst of Industrial Research, Kobe, Japan), J Japan Inst of Metals, Vol 56, No 5, 1992, 517–523. (In Japanese)

Stabilization behavior of the martensite in shape memory CuZnAl and CuAlMn alloys has been studied by x-ray measurements and electron microscopy. In these alloys influenced by both quenching and post-quench heat treatments, the degree of stabilization depends on quenching conditions. The stabilization process is mainly due to structural change and atomic arrangement in the martensitic lattice inherited from the parent phase. From x-ray results, it is suggested that stabilization and loss of memory are directly related to disordering in a martensitic state. In the present study, it was concluded that the spacing differences (Δd) between some selected pairs of diffraction planes of type (h1k1l) and (h2k2l) satisfying the relation sol(h12 − h22)3 = (k22 − k12)n, with n = 1 for 9R and n = 4 for 18R martensites reflect the degree of ordering in martensite, in addition to the lattice distortion parameters defined as φ = ¦sin2θ202 − sin2θ122¦ and φ′ = ¦sin2θ320 − sin2θ040¦.

Transformation to Ni5Al3 in a 63.0 At. Pct Ni-Al alloy

Microstructures in a 63 at. pct Ni-Al alloy, produced by a powder metallurgy process, have been investigated in detail in as-quenched and aged (823 to 923 K) conditions. The parent L10 martensite plus B2 NiAl microstructure in the as-quenched state transformed nearly completely to the orthorhombic Ni5Al3 phase upon aging at 823 K for 720 hours. The volume fraction of Ni5Al3 formed as a function of aging time at 823 K was observed to obey cellular reaction kinetic behavior. The specimens aged at 823 K for short times indicated that nucleation of the Ni5Al3 phase occurred preferentially at grain boundaries. Transmission electron microscopy (TEM) observations of short-time aged specimens revealed a complex microstructure consisting of shortrange ordered domains of Ni2Al in a matrix of 7R martensite, in addition to new variants of 3R martensite and Ni5Al3. Aging at 873 and 923 K for 720 hours produced a stable two-phase microstructure consisting of NiAl and Ni5Al3. A quantitative phase analysis was carried out to calculate the (NiAl + Ni5Al3)/Ni5Al3 phase boundary locations. The measured lattice parameters of Ni5Al3 formed at 823, 873, and 923 K indicated an increase in tetragonality of the phase with increasing nickel content.

The martensitic phases and their stability in CuZn and CuZnAl alloys—I. The transformation between the high temperature β phase and the 18R martensite

The stability between the high temperature β phase and the orthorhombic martensite in CuZn and CuZnAl single crystals is analyzed. Experimental results for high electron concentrations ( e/ a > 1.50) and data from the literature for lower e/ a are used, covering a wide range of compositions. Numerical values for the differences of entropy and enthalpy are reported. Also, the contribution of L2 1 order to the phase stability is evaluated and an estimation of pair interchange energies is obtained.

The martensitic phases and their stability in CuZn and CuZnAl alloys—IV. The influence of lattice parameter changes and evaluation of phase stabilities

The lattice parameters have been measured for 6R, 18R and 2H CuZnAl martensites. Whereas 6R is nearly cubic f.c.c., the 18R and 2H are strongly distorted. The relationship between the M S temperature and the distortion is the same for stabilized and nonstabilized martensite. On the basis of the experimental results the stabilities of the different phases are discussed. The enthalpy differences between disordered f.c.c. and b.c.c. agree well with first principle calculations. The lattice distortion leads to an increase in the stability of 18R, and to a reduction of the composition dependence of M S.