Martensitic aging

March 29, 2011

Stabilizing internal stress as the thermodynamic factor of martensite aging effects

A Kosogor et al

The symmetry-conforming Landau theory has been developed for the description of interplay between the symmetry of the deformable crystal lattice and the configuration of crystallographic defects in martensitic alloys. For this purpose, the multi-component non-scalar order parameter describing the slow reconfiguration of defects after the deformation of crystal lattice has been introduced within the framework of the Landau theory of martensitic transformations (MTs). Due to the transformational equivalence of the configurational order parameter and strain/stress tensor components, the conception of stabilizing internal stress (SIS), which is linearly related to this parameter, has been formulated. The complete agreement between the developed theory and the symmetry-conforming short-range-order principle formulated by Ren and Otsuka has been established. The effect of stabilizing the product (martensitic) phase after aging has been described by considering the stress–temperature phase diagram, which was constructed by taking into account the time dependence of SIS. The applicability of the theory to the aging effects in the Au–Cd shape memory alloy has been demonstrated. The time dependencies of the experimentally observed slow changes in the MT temperatures, lattice parameters and yield stress values have been derived from the SIS conception.
Highlights

► A martensite aging is accompanied by reconfiguration of the crystal defects system. ► The reconfiguration causes an internal stressing of crystal lattice. ► The internal stressing elevates the martensitic transformation temperature. ► A theory that accounts for the internal stress adequately describes aging effects. ► A quantitative description of aging effects observed in Au–Cd alloys is presented.

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Magnetocrystalline anisotropy

December 26, 2010

Magnetocrystalline anisotropy in Fe-Mn-Ga magnetic shape memory alloy

T Omori et al

Magnetocrystalline anisotropy of the martensite phase was investigated in a single crystal of Fe44Mn28Ga28 shape memory alloy. It was found that the martensite phase of Fe44Mn28Ga28 alloy has a tetragonal distorted L21 with a = 0.5368 nm and c = 0.7081 nm. An approximately single variant was obtained by thermomechanical treatment, and magnetization measurement revealed that the c-axis is the magnetic easy axis. The magnetocrystalline anisotropy constant Ku was determined to be 7.6 × 105 J m-3 at 300 K.

[1] In situ study of nucleation and growth of the irregular α-Al/β-Al5FeSi eutectic by 3-D synchrotron X-ray microtomography

S Terzi et al

In order to better understand the formation of β-Al5FeSi intermetallic plates during solidification of Al–Si casting alloys, an Al–8% Si–4% Cu–0.8% Fe alloy has been studied by in situ microtomography using high-energy X-rays in the synchrotron. After formation of the aluminium dendrites, the β phase forms as an irregular eutectic together with eutectic α-Al. Only four plates were nucleated in the sample, and all nucleated in the very early stage of the eutectic reaction and subsequently developed into complex connected three-dimensional plates. The plates display very rapid lateral growth and slow thickening, which, together with the observation of imprints of dendrites and ridges in the plates, suggest a very weakly coupled eutectic.

[2] Deformation of hierarchically twinned martensite

P Muellner and A H King

Shape-memory alloys deform via the reorganization of a hierarchically twinned microstructure. Twin boundaries themselves present obstacles for twin boundary motion. In spite of a high density of obstacles, twinning stresses of Ni–Mn–Ga Heusler alloys are very low. Neither atomistic nor dislocation-based models account for such low yield stresses. Twinning mechanisms are studied here on a mesoscopic length scale making use of the disclination theory. In a first approach, a strictly periodic twin pattern containing periodic disclination walls with optimally screened stress fields is considered. Strict periodicity implies that the twin microstructure reorganizes homogeneously. In a second approach, a discontinuity of the fraction of secondary twins is introduced and modeled as a disclination dipole. The stress required for nucleation of this discontinuity is larger than the stress required for homogeneous reorganization. However, once the dipole is formed, it can move under a much smaller stress in agreement with experimental findings.

[3] New Interpretation of the Haasen Plot for Solute-strengthened Alloys

W A Curtin

The Haasen plot (inverse activation area 1/Δa versus offset flow stress σ-σs) for solute-strengthened alloys is usually assumed additive, 1/Δa=1/Δas+1/Δaf, with 1/Δafnot, vert, similarβ(σ-σs) due to forest interactions. Experiments often show a slope < β. Here, a model for the dislocation activation enthalpy is proposed that predicts a slope 1/(Δasσs) determined only by solute parameters Δas and σs and not directly connected to forest hardening. This parameter-free prediction agrees well with a wide range of experiments on Al-X alloys at T=78K.

In situ TEM observation of stress-induced martensitic transformations and twinning processes in CuAlNi single crystals

N Zarubova et al

Stress-induced martensitic transformations and twinning processes were studied in thin foils of CuAlNi single crystals strained in situ in a transmission electron microscope. The nucleation and growth of the martensite plates were monitored for three transformation processes known from bulk experiments: (i) the transformation of austenite into 2H martensite at low-stress levels; (ii) the twinning/detwinning processes in 2H martensite; and (iii) the transformation between austenite and 18R martensite at higher stress levels. The morphology of the austenite/martensite habit planes was examined, and the existence of planar interfaces between a single variant of 2H martensite and austenite on the microscopic level was proven.

The role of self-shadowing on growth and scaling laws of faceted polycrystalline thin films

C Ophus et al

We investigate, via both experiment and simulation, the effects of self-shadowing on the growth of faceted polycrystalline thin films. Faceted aluminum thin films were sputtered and the anomalous scaling behaviour of their surfaces was characterized. To understand the causes of this anomalous behavior, growth of faceted thin films was simulated by coupling a level set construction to a ballistic deposition model. The angular distribution function of deposition flux was varied to control the degree of self-shadowing. We show how differing degrees of self-shadowing strongly modify film surface morphologies and compare these results with experimental findings.

Tuning of the martensitic transformation temperature in Cu-Zn thin films by control of zinc vapor pressure during annealing

N Haberkorn et al

We report shape memory effect in Cu-Zn thin films grown by electrodeposition on pyrolytic graphite from pyrophosphate-based electrolytes. Cu-Zn films showing martensitic transformation could only be obtained after the optimization of thermal annealing parameters like annealing temperature, Zn vapour pressure and fast quenching. By means of the control of the Zn vapour pressure during annealing using a bulk reference Cu-Zn alloys the chemical composition of the films could be adjusted and martensitic transformation temperatures of the films could be tuned.

Title: The transformation sequences in the cubic → tetragonal decomposition

Authors: Y. Ni, Y.M. Jin and A.G. Khachaturyan

Source: Acta Materialia, Volume 55, Issue 14, August 2007, Pages 4903-4914

Abstract:

The decomposition of a generic supersaturated binary cubic solid solution into a mixture of cubic and tetragonal phases is investigated by phase field microelasticity modeling and simulations. It is shown that the decomposition in such a system is not necessarily developed by conventional nucleation and growth of the tetragonal phase. There are three temperature and composition ranges where the sequences of transient structures formed are different. The transformation pathways are predicted and the corresponding thermodynamic parameters are identified. In particular, the simulations reveal unusual transformation sequences occurring in the process of isostructural decomposition followed by cubic → tetragonal MT confined within one of the decomposed cubic phases. Mechanisms for the formation of the stress-accommodating multi-domain aggregates of the tetragonal phase and the checkerboard-like structures comprised of parallel rods of cubic and tetragonal phases are discussed.