Martensitic Transformation Behavior Research Articles

Microstructural evolution and phase transformation in proton-irradiated NiTi films

In the present work, the microstructural characteristics, martensitic transformation behavior, and mechanical performance of NiTi thin films irradiated with different proton fluences were investigated. Multiple-layer structures formed in the NiTi thin films owing to proton irradiation. As the proton fluences were 1.0 × 1015 p/cm2 and 5.0 × 1015 p/cm2, a two-layer structure containing a B2 phase and a B19’ martensite phase was introduced. In addition, thickness of B2 phase layer becomes larger and larger for irradiated NiTi thin films at room temperature, as the proton irradiation fluence increases. With further increasing proton irradiation fluence to 2.0 × 1016 p/cm2, an amorphous layer forms at the outmost layer of NiTi thin film. Moreover, an R phase and GP zones were detected in the interior of the B2 layer. The twin type of B19’ martensite also changed to release the stored elastic energy induced by proton irradiation. The martensite variant related (111‾) type I twins and the substructure of the <011> type II twins gradually evolved into (111) type I twins and (001) compound twins, respectively, with increasing proton irradiation fluence. The formation of a multiple-layer structure resulted in the presence of two endothermic and exothermic peaks corresponding to the B19’ ⇌ B2 martensitic transformation. The introduction of lattice defects was responsible for the reduction in the martensitic transformation temperature. The strength of the irradiated NiTi films was enhanced as a result of dislocation strengthening. However, the introduction of higher density of defects also leads to the deterioration of ductility.

Controllable Martensite Transformation and Strain-Controlled Fatigue Behavior of a Gradient Nanostructured Austenite Stainless Steel.

Gradient nanostructured (GNS) surface layer with a controllable martensite fraction has been synthesized on 316L austenitic stainless steel by means of surface mechanical rolling treatment (SMRT) with temperature being controlled. The mean grain size is in the nanometer scale in the near-surface layer and increases gradually with depth. In addition, the volume fraction of martensite decreases from ~85% to 0 in the near-surface layer while the SMRT temperature increases from room temperature to 175 °C. Fatigue experiments showed that the strain-controlled fatigue properties of the GNS samples are significantly enhanced at total strain amplitudes ≥0.5%, especially in those with a dual-phase surface layer of austenite and pre-formed martensite. Analyses on fatigue mechanisms illustrated that the GNS surface layer enhances the strength-ductility synergy and suppresses the formation of surface fatigue defects during fatigue. In addition, the dual-phase structure promotes the formation of martensite and stacking faults, further enhancing fatigue properties at high strain amplitudes.

Enhanced solute diffusion to promote shear-type reverse transformation recovery in metastable austenitic stainless steel

The martensite reverse transformation behavior and the effect of microstructure differences on mechanical properties were investigated in metastable austenitic stainless steel. During continuous heating, whether annealing or pulsed electric field treatment, the deformation-induced martensite transforms to austenite by diffusionless shear reversion. In subsequent isothermal holding, the samples treated by pulsed electric treated accomplish the reverse transformation from martensite to austenite, and forming the equiaxed, defect-free austenite grains, but the martensitic reversion in annealed samples is incomplete, consisting of the diffusion-type austenite, lath-type austenite and dislocation cells. The difference in microstructure is caused by atomic diffusion in essence. Under a pulsed electric field, the atom receives an electric force that reduces the activation energy required for diffusion. Therefore, the atom diffusion is promoted, which accelerates the dislocations climbing and subgrain boundaries coalescence and then promoting the schedule of recrystallization. As a result, the difference in microstructure has a significant effect on the mechanical properties, i.e., the annealed cold-rolled austenitic stainless steels have higher strength with poor ductility, yet the pulsed steels present an excellent ductility. The reason for the difference in mechanical properties is likely to the phase transformation-induced plasticity responses.

Effects of Cr and Sn additives on the martensitic transformation and deformation behavior of Ti-Cr-Sn biomedical shape memory alloys

The effects of Cr and Sn additives on the phase constitution, lattice deformation strain, transformation temperature, and deformation behavior of Ti-Cr-Sn alloys were systematically investigated to inform the development of a superelastic alloy with high lattice deformation strain. The addition of Cr and Sn changes the reverse martensitic transformation temperature by −190 K/mol% Cr and −141 K/mol% Sn, respectively. The lattice deformation strains (η1, η2, and η3) were evaluated in terms of the lattice parameters of the parent β phase to the martensite α″ phases. In addition, the composition range of the alloys that are expected to exhibit superelasticity and high deformation strain near the human body temperature was established based on the composition dependence of lattice deformation strain and transformation temperature. The deformation behavior of Ti-Cr-Sn alloys at room temperature changes from the shape memory effect to superelasticity with increasing Cr and Sn contents. In particular, Ti-2.5Cr-8.5Sn, Ti-3.0Cr-7.5Sn, Ti-3.0Cr-8.0Sn, Ti-3.5Cr-7.0Sn, and Ti-4.0Cr-6.5Sn alloys exhibit superelasticity at room temperature and a high lattice deformation strain (η2) that exceeds 6.32%. It is concluded that Cr and Sn are more effective than other β-stabilizing elements at lowering the martensitic transformation temperature of β-Ti shape memory alloys without reducing the lattice deformation strain.

Phase Stability of Three Fe–Mn–Al–Ni Superelastic Alloys with Different Al:Ni Ratios

Fe–Mn–Al–Ni superelastic alloy is a potential candidate for diverse engineering applications due to its outstanding properties and low material costs. Recent studies suggest that slight changes in the chemical composition severely affect superelastic response, phase stability and grain growth kinetics. In this paper, we found that the Al stabilizes the parent α phase at high temperature and promotes the formation of β precipitation at lower temperature. An alloy with a 3:1 ratio between Al and Ni produces homogeneously distributed β precipitates with high phase fraction in the alpha matrix after quenching from 1200 °C. The presence of these precipitates stabilizes the α phase, lowers the martensitic transformation temperature and gives the alloy a fully-reversible stress-induced martensitic transformation behaviour without the need to apply an aging step. In alloys with lower Al content the β precipitation produced during quenching is severely restricted and pseudoelasticity is impaired.

Microstructure and Magnetic Properties of Selected Laser Melted Ni-Mn-Ga and Ni-Mn-Ga-Fe Powders Derived from as Melt-Spun Ribbons Precursors

Selective Laser Melting was successfully used as a fabrication method to produce Ni-Mn-Ga and Ni-Mn-Ga-Fe ferromagnetic shape memory alloys. The starting material in a powder form with an average particle size of about 17.6 µm was produced by milling of as melt-spun ribbons. The microstructure, phase composition, and martensitic transformation behavior of both powder precursors and laser melted alloys were investigated by several methods, including high energy X-ray diffraction, electron microscopy, and vibrating sample magnetometry. The as laser melted materials are chemically homogenous and show a typical layered microstructure. Both alloy compositions have a duplex structure consisting either of austenite and 10M martensite (Ni-Mn-Ga) or a mixture of 14M and NM martensitic phases (Ni-Mn-Ga-Fe), contrary to the as milled powder precursors showing fcc structure in both cases. The forward martensitic transformation takes place at 336 and 325 K for Ni-Mn-Ga and Ni-Mn-Ga-Fe, respectively, while the magnetic response is much stronger for Ni-Mn-Ga than for the quaternary alloy. The results show that Selective Laser Melting allows for producing of good quality, homogenous materials. However, their microstructural features and consequently shape memory behavior should be tailored by additional heat treatment.

Transformational, microstructural and superelasticity characteristics of Ti–V–Al high temperature shape memory alloys with Zr addition

This study analyzes the influences of Zr addition on the martensitic transformation behavior, microstructural evolution, mechanical properties and superelasticity effect of arc-melted Ti-12V-4Al-xZr (x = 0, 0.5, 1, 1.5 and 2 wt.%) high temperature shape memory alloys (HTSMA). The results revealed that the austenite transformation temperatures and activation energy values decreased linearly when the Zr addition was greater than 0.5 wt.%. The Ti-V-4Al (wt.%) and Ti12V-4Al-0.5Zr (wt.%) alloys were composed of α″ martensitic phase, while the others consisted of predominant α″ martensitic phase and a small amount of the β austenite phase. The thickness of martensitic plates in the alloys reduced with increased Zr addition. Hardness and reduced elastic modulus values calculated from load-depth curves of the alloys also decreased with increasing Zr addition. Along with the increase in the Zr addition, the alloys’ superelasticity behavior decreased at room temperature (24 °C), while this behavior increased at the high temperatures (450 °C).

Grain Size Effect of the γ Phase Precipitation on Martensitic Transformation and Mechanical Properties of Ni-Mn-Sn-Fe Heusler Alloys.

Isothermal annealing of a eutectic dual phase Ni–Mn–Sn–Fe alloy was carried out to encourage grain growth and investigate the effects of grain size of the γ phase on the martensitic transformation behaviour and mechanical properties of the alloy. It is found that with the increase of the annealing time, the grain size and volume fraction of the γ phase both increased with the annealing time predominantly by the inter-diffusion of Fe and Sn elements between the γ phase and the Heusler matrix. The isothermal anneals resulted in the decrease of the e/a ratio and suppression of the martensitic transformation of the matrix phase. The fine γ phase microstructure with an average grain size of 0.31 μm showed higher fracture strength and ductility values by 28% and 77% compared to the coarse-grained counterpart with an average grain size of 3.31 μm. The fine dual phase microstructure shows a quasi-linear superelasticity of 4.2% and very small stress hysteresis during cyclic loading, while the coarse dual phase counterpart presents degraded superelasticity of 2.6% and large stress hysteresis. These findings indicate that grain size refinement of the γ phase is an effective approach in improving the mechanical and transformation properties of dual phase Heusler alloys.

Martensitic transformation and magnetocaloric effect in Mn&amp;lt;sub&amp;gt;50&amp;lt;/sub&amp;gt;Ni&amp;lt;sub&amp;gt;32&amp;lt;/sub&amp;gt;&amp;lt;sub&amp;gt;&amp;ndash;&amp;lt;/sub&amp;gt;&amp;lt;sub&amp;gt;&amp;lt;italic&amp;gt;x&amp;lt;/italic&amp;gt;&amp;lt;/sub&amp;gt;Cu&amp;lt;sub&amp;gt;&amp;lt;italic&amp;gt;x&amp;lt;/italic&amp;gt;&amp;lt;/sub&amp;gt;Co&amp;lt;sub&amp;gt;8&amp;lt;/sub&amp;gt;Ti&amp;lt;sub&amp;gt;10&amp;lt;/sub&amp;gt; Heusler alloy ribbons

<p indent="0mm">The effects of Cu substitution and annealing on the martensitic transformation behavior and magnetocaloric properties in all-<italic>d</italic>-metal Mn₅₀Ni_32–_{<italic>x</italic>}Cu_{<italic>x</italic>}Co₈Ti₁₀ (<italic>x</italic>=1, 2) alloy ribbons have been mainly investigated. After the substitution, it is observed that the martensitic transformation temperature of the alloys, which was extremely sensitive to the Cu content, was reduced. Moreover, the ferromagnetism of austenite increased, increasing the magnetization difference across the martensitic transformation. The magnetic-field-induced metamagnetic martensitic transformation also occurred. The magnetic entropy change gradually increased, and the effective refrigeration capacity increased by more than twice than that of the Cu-free substitution. Moreover, the sample with <italic>x</italic>=1 was utilized as the research object to study the effect of annealing on the martensitic transformation. After annealing, the martensitic transformation became slow and the austenite ferromagnetism decreased. The change in the magnetization across the phase transition was also reduced. The magnetic-field-induced metamagnetic martensitic transformation was also weakened. Consequently, the magnetic entropy change was greatly reduced. However, the effective refrigeration capacity did not reduce significantly because the refrigeration temperature interval increased largely. In this paper, the physical mechanisms of the effects of Cu substitution and annealing on the martensitic transformation were discussed from the change in the <italic>d</italic>-<italic>d</italic> interaction and the precipitation in the <italic>γ</italic>-phase, respectively.

Martensitic transformation, microstructure and functional behavior of thin-walled Nitinol produced by micro laser metal wire deposition

Additive manufacturing (AM) of Nitinol could enable realizing smart 3D metallic structures that combine the functional properties of shape memory alloys with higher geometrical flexibility. However, powder feedstocks suffer from issues related to availability, chemical composition stability, and long-term stocking. Therefore, wire-based AM processes are considered promising for reducing the time required for the process development stage and for maintaining quality over time. In this study, micro laser metal wire deposition (μLMWD) was used for manufacturing thin walls with heights of up to 40 mm by using commercial superelastic Nitinol wires of 0.4 mm diameter. The microstructure, martensitic transformation (MT), and mechanical behavior were analyzed. The results showed that μLMWD could be used to produce Nitinol walls with high aspect ratio and sub-millimeter thickness, that were free of cracks and produced with relative density as high as 99%. The as-deposited wall section was characterized by an almost homogenous composition, which was associated with a solubilized condition, owing to the multiple heating cycles during the deposition of advancing layers. Limited amount of Ni loss induced an increase in the characteristic temperatures of the MT, and shape memory behavior was detected with a recoverable strain of 4.4% upon heating up to 120 °C.

Effect of Ti/Ni and Hf/Zr ratio on the martensitic transformation behavior and shape memory effect of TiNiHfZr alloys

Abstract The functional properties of shape memory alloys are strongly sensitive to the composition of alloys. In this study, the effect of Ti/Ni ratio and Hf/Zr ratio on the martensitic transformation behavior, shape memory effect and microscopic structures of two different quaternary TiNiHfZr alloys (TixHf15Zr5Ni80-x and Ti31.5HfyZr20-yNi48.5) have been investigated systematically. It is found that increasing Ti/Ni ratio of TixHf15Zr5Ni80-x alloys dramatically increases the martensitic transformation temperature and a maximum value (5%) of the recovered strain during shape recovery on heating is obtained for the alloy with x = 30. On the contrary, changing Hf/Zr ratio of Ti31.5HfyZr20-yNi48.5 alloys does not result in big change of martensitic transformation temperature or recovered strain. The evolution of macroscopic properties of TixHf15Zr5Ni80-x alloys with x can be understood by considering the balanced effect between the Ni content change of the matrix, and the volume fraction of Ti2Ni-like precipitates, which are often brittle and not desired for SMAs. Our results suggest that the TixHf15Zr5Ni80-x (x > 30.5 at. %) alloys are promising shape memory alloys for high temperature applications above 250 °C.

Effect of hydrostatic pressure on martensitic transformation and low-temperature magnetic properties in Ni45Cu5Mn35In15 Heusler alloy

The effect of hydrostatic pressure on martensitic transformation (MT), superparamagnetic (SPM) and exchange bias (EB) behaviors in polycrystalline Ni45Cu5Mn35In15 has been intensively investigated by various experimental methods. The experimental results show that the studied alloy experiences a MT from the austenitic state with a L21-type cubic structure to the martensitic state with a seven-layer (7 M) modulated orthorhombic structure with decreasing temperature, accompanied by a significant change in magnetization. It is found that applying hydrostatic pressure can induce the MT to occur in the higher temperature, while the Curie transition almost remains unchanged due to the suppression of ferromagnetic (FM) coupling. In addition, the applied hydrostatic pressure can suppress the EB field and coercivity of the studied alloy due to the reduction of the competition between FM and AFM interactions. This phenomenon can be attributed to the fact that the application of hydrostatic pressure reduces the size of the SPM cluster and its average magnetic moment.

Martensite Transformation Start Temperature During Quench and Austempering in Fe-8Ni-0.2C Alloys

It is important for the improvement of mechanical properties of quenching and partitioning (QP) steels to control the martensite transformation behavior and the stability of untransformed austenite. In this study, we evaluated the martensite transformation restart temperature (Mr) via a dilatometer during the reheat austempering pattern. This heat pattern corresponding to the typical heat treatment for QP steel involves quenching to TQ, a temperature between the martensite transformation start temperature (Ms) and the finish temperature, and austempering above Ms. In Fe-8Ni-0.2C alloy, Mr equals TQ, regardless of the progress of bainite transformation during austempering at 673 K (400 °C). In Fe-8Ni-1Si-0.2C alloy, Mr decreases parabolically from TQ with the progress of bainite transformation, and this behavior corresponds to the assumed carbon concentration by bainite transformation. These results indicate that the self-stabilization of the entire untransformed austenite during the first quench from Ms to TQ was preserved, regardless the bainite transformation. Moreover, the stabilization by the carbon concentration in untransformed austenite was added to this self-stabilization.

Deformation-induced Martensite Transformation Behavior during Tensile and Compressive Deformation in Low-alloy TRIP Steel Sheets

Deformation-induced Martensite Transformation Behavior during Tensile and Compressive Deformation in Low-alloy TRIP Steel Sheets

Martensitic Transformation and Metamagnetic Transition in Co-V-(Si, Al) Heusler Alloys

This study investigates the crystal structure, martensitic transformation behavior, magnetic properties, and magnetic-field-induced reverse martensitic transformation of Co64V15(Si21–xAlx) alloys. It was found that by increasing the Al composition, the microstructure changes from the martensite phase to the parent phase. The crystal structures of the martensite and parent phases were determined as D022 and L21, respectively. Thermoanalysis and thermomagnetization measurements were used to determine the martensitic transformation and Curie temperatures. Both the ferromagnetic state of the parent phase and that of the martensite phase were observed. With the increasing Al contents, the martensitic transformation temperatures decrease, whereas the Curie temperatures of both the martensite and parent phases increase. The spontaneous magnetization and its composition dependence were also determined. The magnetic-field-induced reverse martensitic transformation of a Co64V15Si7Al14 alloy under pulsed high magnetic fields was observed. Moreover, using the results of the DSC measurements and the pulsed high magnetization measurements, the temperature dependence of the transformation entropy change of the Co-V-Si-Al alloys was estimated.

Tuning microstructure, transformation behavior, mechanical/functional properties of Ti–V–Al shape memory alloy by doping quaternary rare earth Y

The microstructure features, martensitic transformation behavior and mechanical/functional properties of Ti–V–Al alloy were tailored by changing rare element Y content in the present investigation. The results showed that Y doping resulted in the grain refinement and formation of Y-rich phase mainly distributing along grain boundary in Ti–V–Al alloys. The martensitic transformation temperatures of Ti–V–Al alloys slightly increased due to the variation of matrix composition induced by the presence of Y-rich phase. The mechanical and functional properties of Ti–V–Al alloys doped moderate Y addition were significantly improved, which can be ascribed to grain refinement, solution strengthening and precipitation strengthening. The 1.0 ​at.%Y-doped Ti–V–Al alloy exhibited the highest ultimate tensile stress of 912 ​MPa and largest elongation of 17.68%. In addition, it was found that the maximum recoverable strain of 5.42% can be obtained in Ti–V–Al alloy with adding 1.0 ​at.%Y, under the pre-strain of 6% condition, which is enhanced by approximate 0.6% than that of Ti–V–Al alloy without Y addition.

Microstructural and Mechanical Characteristics in Ni-Mn-Ga Alloys Under a Magnetic Field-Assisted Directional Solidification

In the present work, the effect of a transverse magnetic field-assisted directional solidification (MFADS) on the microstructures in Ni-Mn-Ga alloys has been investigated. The results show that the magnetic field is capable of inducing transversal macro segregation perpendicular to magnetic field, causing the emergence of martensite clusters in austenite matrix. Moreover, the magnetic field alleviates the micro segregation on a dendritic scale and promotes the preferred growth of austenite dendrites. On the basis of above investigation, several special samples are designed using the MFADS to study the crystallographic evolution and mechanical behavior during thermal/stress induced martensite transformation. The martensite cluster in austenite matrix is used to investigate the martensite transformation and growth under cooling-heating cycles. The crystallographic relationship and phase boundary microstructure between martensite and austenite have been characterized. In addition, the micro segregation on a dendritic scale can significantly influence the martensite variant distribution, corresponding to the performance during compressive circles based on the analysis about deformation gradient tensor. The stress-induced super elasticity is closely dependent on orientation, well explained from the perspective of different resolved shear stress factors and correspondence variant pair formation transformation strain. The crystallographic evolution has been characterized during in-situ stress-induced transformation. The findings not only deepen the understanding of martensite transformation and mechanical behavior under a thermal/stress field in Ni-Mn-Ga alloys, but also propose a promising strategy to obtain microstructure-controllable functional alloys by MFADS.

Crystallographic texture evolution and martensite transformation in the strain hardening process of a ferrite-based low density steel

The strain-induced martensite transformation is of great importance in the strain hardening process of ferrite based low-density steel. Based on the microstructure analysis, the texture evolution and martensite transformation behavior in the strain hardening process were studied. The results show that martensite transformation accompanied by TWIP effect and high density dislocations maintains the continuous hardening stage. As the strain increases, the texture of retained austenite evolves towards the F orientation {111}<112>, which is not conducive to martensite transformation. After the strain of 5%, the number of austenite grains with high Schmid factor orientations is gradually increased, and then significantly reduced when the strain is over 10 % due to the occurrence of martensitic transformation, which results in a high martensitic transformation rate. However, the unfavorable orientation and the reduced grain size of austenite slow down the martensite transformation at the final hardening stage. Moreover, because of the coordination deformation of austenite grains, strain preferentially spreads between adjacent austenite grains. Consequently, the martensite transformation rate in strain hardening process is dependent on the orientation and grain size evolution of austenite, leading to a differential contribution to each strain hardening stage.

Correlation of orientation relationships and strain-induced martensitic transformation sequences in a gradient austenitic stainless steel

Orientation relationships (ORs) play a crucial role in the phase transformation of face-centered cubic austenite (γ-fcc) to body-centered cubic martensite (α′-bcc) upon plastic deformation. So far, ORs between γ-fcc and α′-bcc are reported frequently; however, the correlation to the transformation sequences is not clear. Here, the gradient structured austenitic stainless 304 steel prepared by ultrasonic nanocrystalline surface modification was examined to clarify the ORs that were involved in the strain-induced martensitic transformation (SIMT) behaviors. The results showed changes in the ORs from Nishiyama–Wassermann (N-W) to Kurdjumov–Sachs (K-S) and Pitsch with an increase in depth from the top-treated surface. The OR of α′-bcc formed directly on γ-fcc was identified as N-W, while those via the hexagonal closed packed e-plate (e-hcp) were the K-S and Pitsch as they were nucleated at the single and two e-plates intersections, respectively. Thus, the SIMT sequence was the direct γ → α′ with the presence of the N-W OR in the high strain depth, while it was the γ → e → α′ as the existence of K-S and Pitsch ORs in the low strain depth. The findings provide essential data for the in-depth and comprehensive study of the polymorphic martensitic transformation mechanisms in various steels and alloys.

Martensitic transformation behaviour and mechanical property of dual-phase Ni-Co-Mn-Sn-Fe ferromagnetic shape memory alloys

Ni-Co-Mn-Sn Heusler alloys possess attractive multifunctional properties, owing to the unique metamagnetic shape memory effect. However, the intrinsic high brittleness limits their applications. In this work, we studied the effects of Fe doping and heat treatment on the microstructures, mechanical properties, martensitic transformation and magnetic behaviours of Ni-Co-Mn-Sn-Fe Heusler alloys. A eutectic dual-phase was successfully introduced in Ni48Co4Mn39-xSn9Fex (x = 3, 4, 5, 6, 7 and 8) alloys by doping Fe for Mn. The engineering strength and ductility increased almost by twofold in a eutectic dual-phase alloy compared to the single-phase alloy. Such microstructure also shows a large quasilinear pseudoelasticity of 4.3% with a very small stress hysteresis and good cyclic stability. After the heat treatment, the γ phase lamellae was much coarsened which resulted in decrease of strength and ductility by ~20%. The magnitude of pseudoelasticity decreased to 2.6% and worse cyclic stability occurred because of the grain coarsening. The metamagnetic transformation was obtained when Fe content is between 2.5 at% and 6.5 at%, with Fe3 alloy showing a A(ferro) ⇔ M(para) type and Fe4-Fe6 alloys showing a A(ferro) ⇔ M(ferro) one.