Takehiko Kikuchi

Atomic force microscope study of stress-induced martensite formation and its reverse transformation in a thermomechanically treated Fe–Mn–Si–Cr–Ni alloy

Abstract Consecutive observations of the stress-induced martensite formation and its reversion by atomic force microscopy have been carried out for the fcc/hcp transformation in the thermomechanically treated sample of an Fe–Mn–Si–Cr–Ni shape memory alloy. It is found that thin martensite plates of 0.1–0.2 μm thickness, which are the same martensite variant on the same habit plane, are formed one after another at the immediate neighbor of the existing martensite plate. These martensite plates make a group of several plates within the width of 1–2 μm. This formation mode of martensite is compared with those martensite plates observed by high resolution microscopy and optical microscopy and it is concluded that the basic mode of the stress-induced transformation is that each martensite plate is induced to relieve the shape strain of the existing martensite plate for all the observed magnifications ranging from several hundreds to several millions. The first martensite plate formation is presumed to occur at the pre-existing stacking fault in austenite. In the reverse transformation on heating, it is likely that each martensite plate is reverse-transformed one after another by reverse movement of the Shockley partial dislocations residing at the tip of the plate. This seems to be true for every range of the observable magnification from namometres to microns. Such a reverse transformation mode ensures a good shape memory effect in Fe–Mn–Si-based shape memory alloys.

The pseudoelastic behavior of Fe-Mn-Si-based shape memory alloys containing Nb and C

The pseudoelastic behavior of an Fe?28Mn?6Si?5Cr?0.53Nb?0.06C (mass%) alloy with respect to the thermomechanical treatment of pre-warm-rolling and subsequent ageing is investigated. A partial pseudoelasticity appears between As and Md, where As is the reverse martensitic transformation start temperature and Md is the temperature above which the martensitic transformation cannot be induced by stress. It can be reasonably interpreted as transformation pseudoelasticity, like for thermoelastic shape memory alloys such as Ti?Ni-based alloys and Cu-based alloys. The thermomechanical treatment is effective in enhancing the pseudoelasticity by increasing the Md temperature. It is also found that the alloy exhibits a partial pseudoelasticity below As when the sample is deformed into a two-phase state: fcc and hcp. A conceivable reason for the driving force of the reverse transformation (from hcp to fcc) on unloading is the back-stress experienced by Shockley partial dislocations residing at the tip of the martensite plates, which originates from NbC precipitates. Perfect pseudoelasticity is also obtained when the specimen is subjected to cyclic loading.

Internal Friction of Fe-Mn-Si-Based Shape Memory Alloys Containing Nb and C and Their Application as a Seismic Damping Material

The damping behavior of an Fe-28Mn-6Si-5Cr-0.5NbC (mass%) shape memory alloy was measured by low cycle fatigue tests during tension-compression loadings. A remarkable damping capacity was observed above the strain amplitude of 0.1%, and the specific damping capacity (SDC) parameter reached saturation at ~ 80% above 0.4%. The reversible motion of the γ/ε interfaces is considered to dominate the cyclic deformation behavior, while the work hardening during tension-compression loading is negligible. These characteristics are favorable for seismic damping devices that protect civil structures from earthquakes.

Continuous transition of deformation mode in Fe-30Mn-5Si-lAl alloy

Deformation modes at various stages of plastic deformation have been investigated at identical locations in an Fe-30Mn-5Si-1Al (mass%) alloy specimen, which exhibits a good shape memory effect associated with FCC(γ)→HCP(e) martensitic transformation and relatively high ductility caused by deformation twinning. The surface relief caused by γ→e martensitic transformation, deformation twinning and slip band formation has been analyzed by measuring surface tilt angles corresponding to each deformation mode by atomic force microscopy. Although e-martensitic transformation is the dominant deformation mode in the early deformation stage, a part of the e-martensite plates changes to deformation twins with the increase of deformation volume. Slip deformation also occurs inside the same region under excessive strain. The continuous transition of these deformation modes also occurs in the other grains in the same order: e martensite→deformation twins→slip bands.Deformation modes at various stages of plastic deformation have been investigated at identical locations in an Fe-30Mn-5Si-1Al (mass%) alloy specimen, which exhibits a good shape memory effect associated with FCC(γ)→HCP(e) martensitic transformation and relatively high ductility caused by deformation twinning. The surface relief caused by γ→e martensitic transformation, deformation twinning and slip band formation has been analyzed by measuring surface tilt angles corresponding to each deformation mode by atomic force microscopy. Although e-martensitic transformation is the dominant deformation mode in the early deformation stage, a part of the e-martensite plates changes to deformation twins with the increase of deformation volume. Slip deformation also occurs inside the same region under excessive strain. The continuous transition of these deformation modes also occurs in the other grains in the same order: e martensite→deformation twins→slip bands.

Continuous Transition of Deformation Mode in Fe-30Mn-5Si-1Al Alloy

Deformation modes at various stages of plastic deformation have been investigated at identical locations in an Fe-30Mn-5Si-1Al (mass%) alloy specimen, which exhibits a good shape memory effect associated with FCC(γ)→HCP(e) martensitic transformation and relatively high ductility caused by deformation twinning. The surface relief caused by γ→e martensitic transformation, deformation twinning and slip band formation has been analyzed by measuring surface tilt angles corresponding to each deformation mode by atomic force microscopy. Although e-martensitic transformation is the dominant deformation mode in the early deformation stage, a part of the e-martensite plates changes to deformation twins with the increase of deformation volume. Slip deformation also occurs inside the same region under excessive strain. The continuous transition of these deformation modes also occurs in the other grains in the same order: e martensite→deformation twins→slip bands.Deformation modes at various stages of plastic deformation have been investigated at identical locations in an Fe-30Mn-5Si-1Al (mass%) alloy specimen, which exhibits a good shape memory effect associated with FCC(γ)→HCP(e) martensitic transformation and relatively high ductility caused by deformation twinning. The surface relief caused by γ→e martensitic transformation, deformation twinning and slip band formation has been analyzed by measuring surface tilt angles corresponding to each deformation mode by atomic force microscopy. Although e-martensitic transformation is the dominant deformation mode in the early deformation stage, a part of the e-martensite plates changes to deformation twins with the increase of deformation volume. Slip deformation also occurs inside the same region under excessive strain. The continuous transition of these deformation modes also occurs in the other grains in the same order: e martensite→deformation twins→slip bands.

Mechanism of improvement of shape memory effect by ausforming in an Fe-28Mn-6Si-5Cr shape memory alloy

Ausforming treatment can improve the shape memory effect of Fe-Mn-Si based shape memory alloys, however, the mechanism of this improvement is not so clear. In this paper, the influence of ausforming treatment on stress-induced martensitic transformation and its reverse transformation in an Fe-28Mn-6Si-5Cr shape memory alloy has been studied using atomic force microscope and TEM aiming to clarify the origin of this improvement. It was found that the ausforming treatment at 970K by 9 percent pre-straining, which is the optimum condition for improving shape memory recovery, can introduce many uniformly distributed stacking faults on the same slip plane in austenite. When an external stress is applied to such an ausformed specimen for shape change, uniformly distributed martensite bands with the same variant are produced in a grain due to the assistance of those preexisted stacking faults. When being heated over Af, these martensite bands are nearly completely reverse-transformed to parent phase through the same atomic pass as for the forward transformation, so a nearly perfect shape memory effect is obtained.