Mechanism Of Martensitic Transformation Research Articles

Surface relief accompanying martensitic transitions in an Fe-Ni-C alloy by atomic-force microscopy and phenomenological theory of martensitic crystallography.

We studied the surface relief induced by martensite in an Fe-Ni-C alloy with atomic-force microscopy (AFM) quantitatively. The phenomenological theory of martensitic crystallography (PTMC) is applied to calculate the surface relief angle and compare with that obtained by AFM, and it seems to conform with the experimental result, indicating a reasonable mechanism of martensitic transformation proposed by PTMC.

Martensitic Phase Transformation in Au–Cu–Zn Alloys Studied by <SUP>63</SUP>Cu NMR

In order to obtain much information about the microscopic mechanism of martensitic transformations, 63 Cu nuclear magnetic resonance (NMR) and relaxation measurements have been performed at temperatures above and below the transformations in Au-Cu-Zn alloys with different compositions and transformation temperatures. The 63 Cu NMR spectra taken from the parent phase with a L2 1 type structure of the alloys comprising less and more Au contents can be decomposed into three and two components, respectively. It implies that the majority of Cu atoms occupy the corner sites, referring to the fundamental bcc lattice, and the minority of Cu atoms the body-centered sites with different Knight shifts. The component spectra reveal different distribution of the Cu atom, changing with Cu content. Although the NMR spectra taken from the martensite phase are too broad to decompose, we can obtain the average Knight shift with respect to the Cu atom distribution. The temperature dependence of the 63 Cu spin-lattice relaxation time follows the Korringa-like behavior in different ways by the alloys. The data on the relaxation rate are discussed in terms of the Knight shift value and the enhancement of electron-electron correlation. The transformation temperature has no clear relation with the NMR parameters.

Martensitic transformation of γ-Fe precipitates in a Cu 1.5 at % Fe Alloy

In the present work, the mechanism of martensitic transformation, the influence of γ-Fe particle size on the martensitic transformation induced by cold working, and the transformation of γ-Fe into α-Fe by thermal treatment alone in a Cu-1.5 at% Fe alloy, was studied using field ion microscopy (FIM) and transmission electron microscopy (TEM). It has been found that γ-Fe precipitates smaller than about 10 nm did not transform martensitically to α-Fe by cold working. Precipitates larger than 10 nm adopted a Kurdjumov-Sachs orientation relationship with the copper matrix; and the martensitically transformed α-Fe precipitates were ellipsoidal in shape, with their major axes being oriented parallel to the 〈1 1 0〉 direction in the matrix. Dislocations were found in the matrix near the vicinity of transformed α-Fe precipitates, giving support to the dislocation cutting mechanism proposed by other workers for the transformation. In thermally aged alloys, no transformation of γ-Fe to α-Fe was observed during the coarsening of γ-Fe precipitates up to sizes as large as about 50 nm. These precipitates still remained coherent or semi-coherent with the copper matrix.

The development of Spangold

The underlying mechanisms of martensitic phase transformations in gold alloys have attracted a great deal of study. Whereas their technological exploitation in shape-memory devices has found only limited application, theSpangold concept introduces a novel application — that of using the transformation to decorate gold ornaments. This article examines the attributes of suitable alloys and the development of a prototype alloy for use in jewellery.

Recent Neutron Scattering Studies on the Premartensitic State of BCC-Based Metals and Alloys

In the present article, we review the recent neutron scattering studies on the anomalous behaviours found in the premartensitic bcc-based metals and alloys, i.e. alkali metals, group-IV metals, β-phase alloys, Ni-base alloys and others such as T1 and group-III metals. Lattice dynamical anomalies appearing on the [110]TA 1 and [111]LA phonon dispersion relations of these materials are discussed from a view point of the martensitic transformation mechanism with referring briefly to theoretical studies. We also consider characteristics of anomalous elastic scatterings associated with the ω phase formation

Mechanism of Isothermal Martensitic Transformation

The mechanism of isothermal martensitic transformation has been studied, using the most typical alloy, Fe-Ni-Mn, of this type of martensitic transformation. The martensitic nucleation rate was investigated by measuring effect of applied stress on the transformation rate. The volume fraction of the martensite transformed was measured in situ by a magnetic detector with the coil covering the specimen. The transformation at liquid nitrogen temperature was studied in detail. The most important feature of observed results is that there is a critical stress level above which the initial transformation rate is drastically increased

Journal. Pseudoelastic Behavior in Ce‐TZP Al2O3 Ceramics

The pseudoelastic behaviour of 12 mol% Ce–TZP + 10 wt% Al2O3 ceramics in the loading–unloading cycles was found for the first time at room temperature, and corresponding reversible tm martensitic transformation mechanism was clearly identified the possibility of reversible ferroelastic domain switching at room temperature was also discussed.

Strain hardening and transformation mechanism of deformation-induced martensite transformation in metastable austenitic stainless steels

Austenitic stainless steels with lower chromium or nickel contents can transform to martensite when they are plastically deformed at sufficiently low temperatures. The correspondingly obtained stress-strain curves show an unusual strain hardening behaviour, e.g. minima and maxima of the strain hardening coefficient dσ/d∅, in contrast with normal materials without transformation. A unified model is proposed on the basis of dislocation theory, thermodynamic and experimental observations, which can explain the strain hardening abnormality and give a quantitative relation between the flow stress and martensite transformation. It is shown that deformation-induced martensite formation closely correlates with the homogeneity and stability of plastic deformation. The martensite transformation is initiated by inhomogeneous deformation. The transformation proceeds by orientated rapid dislocation movement and multiplication. Its onset takes place spontaneously when a sufficiently high stress is reached, rather than cumulatively with increasing strain. The increase in the martensite volume fraction is linearly proportional to the square of the flow stress.

Phase transformations in zones of plastic deformation beneath the surface of impact fractures in steel N32T3

Impact tests are performed on steel N32T3 in the hardened and aged conditions. The mechanism of failure and martensitic transformation in zones of plastic deformation between the surface of fractures obtained over a wide temperature range is studied. It is shown that the failure mechanism for both hardened and aged steel in the stage of crack propagation depends weakly on test temperature. An increase in KCU and KCT impact strength for the steel with an increase in temperature is due to an increase in the work for crack generation and formation at the failure site of zone L whose microrelief is predominantly folded with elongated pits. Two zones of plastic deformation are detected by means of x-ray structural analysis beneath the surface of fractures in the hardened condition and one in the aged condition. It is established that within the limits of a highly deformed microzone for hardened steel the amount of martensite is constant in spite of presence of a deformation gradient, but in aged steel it decreases constantly. It is noted that for correct evaluation of the effect of phase transformations occurring in zones of plastic deformation on impact strength and failure mechanism for the steel it is necessary to consider local heating and the actual phase composition of the steel at the tip of a propagating crack.

X-Ray Diffuse Scattering and Martensitic Transformation in β(Au–Cd)+Cu Alloys

X-ray diffuse scattering measurements have been carried out on β-phase Au-47.5 at%Cd-0.75 at%Cu and Au-49.0 at%Cd-0.75 at%Cu alloys. Five notable features were found in the distribution of diffuse scattering: (i) diffuse sheets extending uniformly parallel to the (111) planes, (ii) diffuse streaks running through the fundamental reflection points in the [1\bar10] directions, (iii) diffuse peaks near q =(1/3)[110] along the [1\bar10] direction, (iv) diffuse peaks near q =(1/3)[112] along the [11\bar2] direction and (v) diffuse peaks near q =(3/4)[110] along the [110] direction. The intensity of these five kinds of anomalous diffuse scattering shows characteristic temperature dependence. The diffuse scattering is discussed in relation to the mechanism of martensitic transformation.

Martensitic transformation and change in the lattice periods of methane in the low-temperature region

The study establishes the generality of the fcc-bct martensitic transformation mechanisms in methane and steels in which the martensite phase is in the form of two alternating types of structural domains (a packet of thin twins {110}〈101〉). This finding made it possible to decribe the crystallogeometric connection between the γ- and α-phases in methane by the Kurdyumov-Sachs orientation relations and to explain the low-temperature lattice anomalies of α-martensite.

The influence of a nano-scale stress region on the austenite-martensite transformation in Ni-Al as observed by CEM and HREM

Two different mechanisms of the stress-induced martensitic phase transformation at the crack tip in body-centered cubic (bcc) structural metals and alloys have been studied by molecular dynamics simulations. For cracks with 〈1 0 0〉 crack fronts, the bcc (B2) to face-centered cubic (fcc) (L10) phase transformation along the Bain stretch occurs. Whereas for cracks with 〈1 1 0〉 crack fronts, either the bcc (B2) to fcc (L10) or the bcc (B2) to hexagonal close-packed (hcp) transformation is the candidate. We have found that the combination of local stress and crystal orientation plays an important role in the mechanism of the martensitic transformation. Thus a simple way to determine the mechanism of the martensitic transformation is developed. The complicated deformation behaviors at the crack tip in bcc iron and B2 NiAl are discussed in terms of this method.

Mechanism of pressure-induced martensitic phase transformations: A molecular-dynamics study.

The pressure-induced martensitic transformation from bcc to hcp iron has been studied in great detail with synchrotron radiation and diamond-anvil cells. The mechanism of the transformation has been postulated to be associated with a sliding of parallel bcc (110) planes combined with a contraction along the [001] direction; the (110) planes becoming the close-packed planes of the hcp structure. The synchrotron experiments support this picture of the transformation. We carry out the computer experiment of pressure-loading a bcc crystal to induce this transformation using molecular dynamics. We present the detailed microscopic motion of the particles during the transformation and show that the mechanism mentioned above for the transformation is confirmed for two different potentials and for two different particle numbers. The reverse transformation shows hysteresis, as is experimentally observed. For the two potentials which give the transformation we find that the final structure is often a close-packed polytype. We never observed a fcc crystal as the final structure. For another potential the final state was a defective crystal with no obvious resemblance to a close-packed structure. Most of our simulations were started by applying pressure to a low-temperature bcc crystal. A study of the sliding of the bcc (110) planes relative to each other suggests that the hcp structure is the preferred structure at low enough temperatures in pressure-induced bcc-to-close-packed structural transformations. For both potentials that give successful transformations the final structure is hcp at low enough temperature. In the phase diagram for iron the hcp structure is the low-temperature structure obtained by compressing bcc iron.

Modulated lattice relaxation in β-based premartensitic phase

Recent X-ray studies on a TiNi(Fe) alloy by Shapiroet al. show that the incommensurability of satellite reflections changes from Brillouin zone to Brillouin zone, giving an overall swirl-like shift pattern in the reciprocal space, which precludes any simple incommensurate wave model. We have succeeded in reproducing the intriguing shift pattern observed, based on two simple assumptions: (i) existence of a dip in TA phonon dispersion atq = 1/3[110] and (ii) existence of embryos of low temperature martensite phase. The proposed features of the premartensitic phase suggest a possible microscopic mechanism of martensite transformation in general as well as of w-phase formation. fa]This paper is based on a presentation made in the symposium “Pretransformation Behavior Related to Displacive Transformations in Alloys” presented at the 1986 annual AIME meeting in New Orleans, March 2–6, 1986, under the auspices of the ASM-MSD Structures Committee.

ジルコニアのマルテンサイト変態機構について

ジルコニアのマルテンサイト変態機構について

Surface relief and the mechanism of a phase transformation

Surface relief of the invariant plane strain type is invariably observed during martensitic transformations. However, some diffusion-controlled transformations also exhibit this effect. The observation of a surface relief effect is therefore no proof for a martensitic transformation mechanism. The same reasoning applies to the formation of groups of variants that accomodate each other's strain. Self-accommodating groups of four variants of martensite plates have been observed in several martensites but they are also found in precipitation reactions. While these observations cannot be used as proof for a martensitic mechanism, neither does the presence of structural ledges in a reaction interface necessitate diffusion. Both these arguments have been used by different sides in a recent controversy (1,2), and the present paper is an attempt to clarify the situation. Starting with the simple example of AlAg, the mechanism of surface relief in a diffusional fcc-hcp transformation and its relation to ledge growth will be examined. Applying the same mechanism to increasingly complex cases the relation to structural ledges, fcc-bcc transformations and finally a general martensite interface will be developed.

‘Modulated lattice relaxation’ in β-based premartensitic phase

Recent X-ray studies on TiNi(Fe) alloy by Shapiro et al. show that incommensurability of satellite reflections changes from Brillouin zone to Brillouin zone, giving overall ‘swirl’ like shift pattern in the reciprocal space which precludes any simple incommensurate wave model. We have succeeded to reproduce the intriguing shift pattern observed, based on two simple assumptions: (i) existence of soft phonons at q = 1 3 [110] (ii) existence of ‘embrios’ of low temperature martensite phase. The proposed features of premartensitic phase suggest a possible microscopic mechanism of martensite transformation in general as well as of ω-phase formation.

The Mechanism of HCP-FCC Martensitic Transformation: A Diffraction Approach

A new approach based on diffraction evidence for the involvement of shear at atomic level is proposed to elucidate the mechanism of martensitic transformation between HCP and FCC structures. This approach, as distinct from the convectional microstructural approach, can be especially useful in situations where the transformation strains are self-accomodating in nature such that there is no observable surface relief effect. The importance of this aproach is highlighted by comparing the theoretically predicted diffraction effects with those experimentally observed on single crystals of ZnS undergoing the HCP to FCC transformation.

The effect of quenching on the martensitic transformation in �1 phase of Au-Cd alloys

The effect of quenching on the martensitic transformation mechanism in β1 Au-Cd alloys has been investigated by measurements of the electrical resistivity and X-ray diffraction. In the case of the Au-47.5 at%Cd alloy, the ζ′2-martensite is the characteristic product under quenching conditions, but it always exists with the equilibrium γ′2-martensite phase. Consequently, the β1 ↔γ′2 and β ↔↔′2transformations occur simultaneously during the heating and cooling cycles. The corresponding resistivity behaviour is very complicated and extremely sensitive to thermal treatments such as quenching temperature and thermal cycling. On the other hand, in the case of the Au-49.0 at%Cd alloy, only the γ1 ↔ζ′2 transformation occurs even when quenched, and the transformation is unaffected structurally by quenching. A distinct resistivity anomaly, which is considered to be due to the disappearance of quenched-in vacancies, is observed in quenched alloys. Some important characteristics of this anomaly are determined. In particular, the quenching effect disappears when the specimen is heated above the temperature at which the resisitivity anomaly begins. This result suggests that the quenched-in vacancies play an essential role in the martensitic transformation process under quenching conditions.

Second All-Union Conference on the Mechanism and Kinetics of Martensitic Transformations

Second All-Union Conference on the Mechanism and Kinetics of Martensitic Transformations