Fe Mn Si-based Alloys Research Articles

Reverse ε→γ transformations of temperature-induced and stress-induced martensites in Fe–Mn–Si alloys with shape memory effect

In this work we argue that the reverse martensitic transformation (MT) of the stress-induced martensite (SIM) of Fe–Mn–Si-based alloys for moderate prestrains occurs in essentially thermoelastic-like way, in contrast to non-thermoelastic MT of temperature-induced martensite (TIM). We compare, using a number of complementary experimental techniques, the temperatures and the kinetics of the reverse ε→γ martensitic transformation of SIM and TIM for commercial and model Fe–Mn–Si alloys after various thermomechanical treatments. We claim that two studied phenomena are generic in Fe–Mn–Si-based alloys of different compositions and microstructure: first, opposite to the conventional effect of stabilization of martensite by prestrain, the reverse MT during heating of alloys prestrained in bending and tension starts well below (up to 100 K) the start temperature of the reverse MT for the TIM of the same alloy. The difference between engineering temperatures of the start of the reverse transformation, As, for SIM and TIM reaches ca. 50 K, pointing to a need to consider different As for TIM and SIM, AsTIM and AsSIM. Second, the reverse transformations of TIM and SIM show very different kinetics. The reverse MT of SIM is diffuse over the temperature range exceeding 100 K. The reverse MT of TIM, on the contrary, is well localized within the temperature range of ca. 20 K. The experimental observations can be interpreted assuming that, due to the differences in the formation mechanisms of SIM and TIM, the latter shows non-thermoelastic behaviour, whereas the SIM shows similarities with the thermoelastic martensites stabilized by prestrain.

Larger recovery strains in a CoNiSi alloy due to enhanced reversibility of FCC⇌HCP martensitic transformation

It is not clear how coherent ordered nanoparticles affect characteristics of FCC⇌HCP martensitic transformation in disordered alloys. In this paper, the precipitations of coherent ordered nanoparticles and their effects on the FCC⇌HCP martensitic transformation were systematically investigated in a solution-treated Co24Ni6Si shape memory alloy. The results showed that both the coherent ordered L12 structured and L11 structured nanoparticles could precipitate even in the solution-treated Co24Ni6Si alloy followed by water quenching. The L12 structured nanoparticles strongly hindered the occurrence of stress-induced HCP martensitic transformation and its reverse transformation, but their hindrance dramatically reduced with the decrease of size when below 1.5 nm. The coherent ordered nanoparticles did not convert the non-thermoelastic FCC⇌HCP martensitic transformation to the thermoelastic ones owing to the absence of group-subgroup relation between the parent phase and martensite. A maximal recovery strain of 6.7 % was obtained after only aging at 1073 K for 1 min. This value was 1∼2 % higher than the reported maximal ones in conventional FeMnSi-based alloys subjected to complicated thermomechanical treatments. The enhanced reversibility of HCP martensitic transformation could account for this larger recovery strain by the ultrafine coherent ordered L12 structured nanoparticles.

Thermodynamic modeling and computational predictions of NbC precipitation in Fe–Mn–Si-based shape memory alloys by the classical nucleation and growth theories

NbC precipitation in Fe–Mn–Si-based alloys is an effective method to improve the shape memory effect. In this study, the precipitation behavior was investigated using a thermodynamical model to understand the mechanism and optimize the precipitates for a better performance of Fe–Mn–Si-based shape memory alloys. The influence of alloying elements can be considered in the model by introducing interaction parameters. The precipitate size distribution, mean size, precipitate volume faction, and number density of three typical Fe–Mn–Si-based alloys with different NbC addition amounts were calculated. The results indicated that the mean size could be decreased significantly as the NbC addition increased from 0.5% to 1.0%, while the precipitate volume fraction and number density showed obvious increments. The Fe–28Mn–6Si–5Cr alloys exhibited smaller mean sizes and higher number densities than the Fe–14Mn–6Si–9Cr–5Ni and Fe–21Mn–6Si–9Cr–5Ni alloys. It was also found that the precipitate size distribution showed no evident change as the aging time increased from 0.5 h to 2 h except for the Fe–28Mn–6Si–5Cr–0.5NbC alloy in which the precipitates began to coarsen after about 1.25 h.

Evidence supporting reversible martensitic transformation under cyclic loading on Fe–Mn–Si–Al alloys using in situ neutron diffraction

Fe–Mn–Si-based alloys, such as Fe–30Mn–4Si–2Al and Fe–15Mn–10Cr–8Ni–4Si (in mass%), show superior resistance to plastic fatigue compared to the conventional steels, which is ascribed to the reversible back-and-forth movement of {111}〈112¯〉γ Shockley partial dislocations associated with a reversible martensitic transformation between the face-centered cubic γ-austenite and hexagonal close-packed ε-martensite. The purpose of this study was to gather evidence of the reversible martensitic transformation using in situ neutron diffraction under cyclic loading. Three Fe–30Mn–Si–Al alloys with different Gibbs free energy differences at 298 K: Fe–30Mn–6Si (ΔGγ→ε = −250 J/mol), Fe–30Mn–5Si–1Al (ΔGγ→ε = −128 J/mol), and Fe–30Mn–4Si–2Al (ΔGγ→ε = −8.5 J/mol), were studied to unravel the effect of phase stability on the degree of reversibility. The reversible martensitic transformation between γ-austenite and ε-martensite during tension–compression loading is demonstrated as bulk-averaged insights in the Fe–30Mn–4Si–2Al alloy. The forward γ → ε transformation was induced by tensile loading, and the formed ε plates were reversed to γ during unloading and subsequent compressive loading. Furthermore, successive compressive loading induced a different variant of the ε plates from the variant formed under tensile loading, which also reverted to γ by subsequent tensile loading. Such repetition of the γ ↔ ε transformation in the Fe–30Mn-4Si–2Al alloy is thought to increase the plastic fatigue life compared to the Fe–30Mn–6Si and Fe–30Mn–Si–1Al alloys, in which the γ ↔ ε transformation is rarely reversible. Herein, we discuss the mechanism of the deformation-induced reverse transformation and propose an optimum thermodynamic condition: a negative close-to-zero ΔGγ→ε.

Correlation between shape memory effect and density of annealing twin boundary in an FeMnSiCrNi alloy during special heat treatments with different parameters

It remains unclear whether a negative dependence of shape memory effect (SME) still exists on the density of annealing twin boundary (ATB) in FeMnSi-based alloys during special heat treatments without optimal parameters. To address this issue, the evolutions of grain size and martensite amount in an Fe-14.5Mn-5.5Si-8.2Cr-5.2Ni-0.016C alloy were investigated in addition to the evolution of ATB density during the special heat treatments, i.e. training and thermomechanical treatment with different annealing temperatures. The results showed that the SME still showed a strong negative dependence on the ATB density independent of the special heat treatments. An obvious positive dependence of SME existed on the density of low angle grain boundary concomitant with the reduced ATB density. No obvious correlation between the SME and the grain size existed when the pre-deformation was rather lower (<4%). Larger grain size and less pre-existing martensite were conducive to realizing larger recovery strains.

Re-examination of the effect of reducing annealing twin boundary density on the shape memory effect in Fe-Mn-Si-based alloys

Reducing annealing twin boundary (ATB) density was proposed to substantially improve the shape memory effect (SME) of Fe–Mn–Si-based alloys, irrespective of their composition, deformation temperature and thermomechanical treatments. Here, we re-examined the effect of reducing ATB density on the SME of a Fe20Mn5.5Si9Cr5Ni (wt%) alloy. We set off with three starting states, i.e., the as-cast state, the hot forged and solution-treated (ST) state as well as the subsequently δ-annealed (DA) state. Our results reveal that the as-cast sample with the lowest ATB density exhibits the lowest shape recovery ratios, while the shape recovery ratios of the DA sample are close to those of the ST sample although the ATB density in the former is only ~1/3 of that in the latter. In other words, reducing ATB density does not necessarily lead to the improvement of SME. Compared to the ST sample, the lower ATB densities in the as-cast and DA samples are actually concomitant with higher densities of low angle grain boundaries (LAGBs), larger γ grain sizes and a limited amount of χ precipitates (volume fraction < 1.6%). The increase of LAGB density and the precipitation of elongated χ precipitates are deleterious to the SME, while the increase of γ grain size improves the SME. Thus, reducing ATB density cannot result in a monotonic change to the SME, given that the decrease of ATB density is concomitant with other microstructural changes.

Designing Fe–Mn–Si alloys with improved low-cycle fatigue lives

A new design concept for improving the low-cycle fatigue lives of Fe–Mn–Si-based alloys is proposed. The degree of reversibility of the dislocation motion can be increased by setting the stacking fault energy to approximately 20 mJ m−2, enhancing the Ni-equivalent amounts with respect to the Cr-equivalent amounts and setting the Si concentration to 4 wt.%. Under these conditions, an Fe–15Mn–10Cr–8Ni–4Si alloy with a low-cycle fatigue life of approximately 8000 cycles for a total strain range of 2.0% could be developed.

Heat Treatments of Fe-Mn-Si Based Alloys: Mechanical Properties and Related Shape Memory Phenomena

Abstract Heat treatments are usual means for modifying alloy microstructures and, consequently, to control mechanical properties. The aim of this work was to find suitable processes for improving the shape memory effect (SME) of Fe–Mn–Si-based alloys. In particular, we studied mechanisms that affect the plastic deformation of the austenite phase. A Fe–15Mn–5Si–9Cr–5Ni alloy was deformed by rolling at different temperatures and subsequently annealed at recovery- and recrystallization-temperature. The mechanical properties of the material after processing were evaluated by performing tension and flexure tests. The SME of room temperature deformed specimens was measured after heating them to 550°C for shape recovery. We found that the material rolled at 800°C followed by an annealing treatment at 650°C recovers nearly 95 % of a 3 % deformation. In this thermo-mechanical condition, the material has a yield stress of 450 MPa and an ultimate tensile strength of 880 MPa, corresponding to a total elongation of about 16 %. Optical and electron microscopy observations show that the matrix annealed at high temperature contains a low density of defects. As a consequence, there are fewer nucleation sites for martensite and the associated SME is low. On the other hand, annealing at intermediate temperatures (around 650°C) produces a favorable structure containing a large density of stacking faults.

Re-examination of the effect of NbC precipitation on shape memory in Fe–Mn–Si-based alloys

Current literature pertaining to the shape memory effect in the Fe–Mn–Si-based system is critically discussed. It is argued that the enhanced shape memory previously attributed to NbC precipitation is mainly due to the associated thermo-mechanical treatments. It is concluded that the thermo-mechanical processing of the alloy is the dominant factor that determines the shape memory effect in this alloy system.

Stresses related to the shape memory effect in Fe–Mn–Si-based shape memory alloys

The shape memory behaviour of two Fe–Mn–Si-based alloys has been investigated. Two compositions were studied, i. e. Fe29Mn7Si and Fe29Mn7Si5Cr (numbers indicate mass%). Characterizations of the martensitic transformation and the different structural constituents were performed using optical microscopy and X-ray diffraction methods. Transformation temperatures were determined by thermodilatometry on undeformed and deformed samples. The shape recovery was quantified by means of bending tests and dilatometry experiments on compressed samples. A procedure was designed to measure the recovery stresses caused by the dimensional changes of the sample due to the shape memory effect. The recovery stress is defined as the stress that is generated when the recovery of deformation is impeded under constraint. The mechanical results are discussed on the basis of the underlying transformation and deformation processes.

The effect of pre-rolling Fe–Mn–Si-based shape memory alloys: Mechanical properties and transmission electron microcopy examination

Fe–Mn–Si-based alloys have been promising systems for shape memory applications for a long time. However, the need of a complicated training process in order to get the desired properties avoided their practical use. Recently, several new Fe–Mn–Si-based alloys, with interesting properties, containing different types of precipitates have been developed. In these new systems, the shape memory improvement is obtained either by the sole introduction of precipitates, or by the combination of a simple thermomechanical treatment followed by a subsequent precipitation. In this work, we investigate the effect of a thermomechanical treatment performed at a temperature of 870 K on the shape memory properties of an Fe–28Mn–6Si–5Cr (wt.%). We have found that a simple treatment, without the necessity of introducing precipitates, is enough to obtain good shape memory properties. Transmission electron microscopy shows that a large density of stacking faults is produced during the treatment. From these observations we deduce that this large stacking fault density is clearly related to the observed properties.

Effect of second-phase particles on shape memory in Fe–Mn–Si-based alloys

The shape memory behaviour of two Fe–Mn–Si-based alloys has been investigated. One alloy was a reference alloy, and the other alloy was similar in composition except that it contained 0.55 wt% Ti. Following solution treatment and quenching, strip samples were subjected to three types of treatments; isothermal holding, cold rolling followed by isothermal holding, and hot rolling followed by isothermal holding. These treatments resulted in the formation of intermetallic precipitates in the Ti-containing alloy, while the reference alloy remained precipitate-free. In comparing the shape memory of the reference and the particle-containing alloy after identical heat treatments, it was found that the formation of precipitates had a beneficial effect on the shape memory in all cases. In general, the larger precipitates caused a larger increase in the shape memory. The effect of particle size on shape memory has been analysed using the current data and published results for a range of precipitate types.

Optimization of Alloy Design and Hot Rolling Conditions for Shape Memory in Fe–Mn–Si-based Alloys

The effect of composition and hot rolling conditions on the shape memory effect (SME) in the Fe–Mn–Si-based system has been studied to obtain improved shape memory without the need to rely on “training”. It has been found that the texture is not markedly affected by rolling conditions, and texture is therefore not a major factor in explaining variations in SME with processing conditions. Decreasing the pre-deformation temperature to below the Ms was found to have a beneficial effect on shape memory. It was found that the best SME was achieved in an alloy that had Ms just above room temperature, and had been processed by hot rolling followed by recovery annealing. Alloys of different compositions exhibited different optimum rolling temperatures for maximum shape memory performance.

TEM observation of the internal structures in NbC containing Fe–Mn–Si-based shape memory alloys subjected to pre-deformation above room temperature

Abstract Shape recoveries comparable to those of the trained conventional alloys are obtained in Fe–Mn–Si-based alloys containing Nb and C by pre-rolling at 870 K and subsequent short ageing at 1070 K to produce NbC precipitates. In addition, when the amount of thickness reduction by pre-rolling is between 14 and 30%, these newly developed alloys attain recovery stresses high enough to make them useful for practical applications such as pipe joints. In this work, the microstructures resulting from the pre-rolling treatment at 870 K and subsequent ageing are investigated by means of transmission electron microscopy (TEM) in Fe–28Mn–6Si–5Cr–0.53Nb–0.06C and Fe–15Mn–5Si–9Cr–5Ni–0.53Nb–0.06C (mass%) alloys. Observations of the as-rolled microstructure indicate that the pre-rolling treatment produces a high density of stacking faults in the fcc matrix. A fine distribution of semi-coherent, nanometric scale NbC precipitates is obtained after ageing the pre-rolled samples. The amount of pre-rolling affects the size and distribution of precipitates and most of the precipitates are seen to be in close relation with stacking faults in the aged condition. The stress-induced hcp martensite is produced as lamella structure, probably due to the abundance of sites for its nucleation i. e., stacking faults and precipitates, and the width of martensite plates is extremely small. These thin martensite plates are highly recoverable, which is the indispensable condition for obtaining a large shape memory effect.

Correlation of martensitic transformation and surface mechanical behavior with cavitation erosion resistance for some iron-based alloys

Cavitation erosion resistance of four Fe–Mn–Si-based alloys and four Fe–Cr–Ni–Mn stainless steels has been investigated at a speed of 45 m/s using a rotating disc rig. Based on the research of surface mechanical effects and energy absorption of phase transformation, the role of martensitic transformation in cavitation erosion resistance of the tested alloys has been discussed. The results show that the effect of martensitic transformation on the resistance of the tested austenitic alloys is greatly influenced by cavitation-induced martensite structure. The resistance of the Fe–Mn–Si-based alloys is much better than that of the Fe–Cr–Ni–Mn stainless steels. The resistance of the tested alloys correlates with their localized surface elasticity H e and work-hardening capacity. The role of phase transformation in the resistance of the tested austenitic alloys is associated with the localized surface elasticity H e, work-hardening capacity and energy absorption of phase transformation, and it mainly depends on the elasticity H e and work-hardening capacity. As the values of H e of the tested martensitic and austenitic alloys are close, the resistance of the austenitic alloys can be obviously improved due to energy absorption of phase transformation. The excellent resistance of the Fe–Mn–Si-based alloys are mainly attributed to their excellent H e and excellent work-hardening capacity.

Perspectives on the Exploitation of CuZnAl Alloys, FeMnSi-Based Alloys and ZrO&lt;sub&gt;2&lt;/sub&gt;-Containing Shape-Memory Ceramics

Perspectives on the Exploitation of CuZnAl Alloys, FeMnSi-Based Alloys and ZrO&lt;sub&gt;2&lt;/sub&gt;-Containing Shape-Memory Ceramics