Martensitic Films Research Articles

The abnormal carbon redistribution in lath martensite during tempering in Mn-patterned steels

Martensite tempering is a critical step to achieve high strength with decent ductility. It is well-known that, during tempering, the carbon is sequentially arranged as segregation, transition carbide and cementite. Abnormally, this study shows the absence of carbide precipitation in Mn-depleted martensite during tempering when Mn distribution is heterogeneous between austenite and martensite. After fast heating and short austenitization from Mn-depleted ferrite and Mn-enriched cementite, the high-temperature austenite with Mn heterogeneous distribution has formed, resulting in the formation of alternating Mn-depleted lath martensite and Mn-enriched film retained austenite (RA) after quenching. In comparison with the carbide precipitation in conventional lath martensite with Mn homogeneous distribution during tempering at 150–500°C, carbide formation is completely inhibited in Mn-depleted lath martensite. Instead, carbon atoms diffuse from Mn-depleted martensite to its neighboring Mn-enriched film RA, leading to an increased carbon content in RA. This is mainly attributed to the strong interaction between Mn and carbon, further assisted by high dislocation density and small width of lath martensite. Additionally, this RA can promote carbon diffusion in the interior due to its gradient Mn distribution.

Pitting Performance of Cold- and Hot-Rolled Nickel-Saving High-Strength Metastable Austenitic Stainless Steel

Nowadays, nickel-saving metastable austenitic stainless steel (MASS) has become the right solution to meeting the growing requirement of higher strength, better corrosion resistance and more cost saving for the automobile industry. Better understanding of the pitting mechanism of the MASS after either cold- or hot-rolled can offer guidance for the producing of high-performance automobile steel. In the current work, for uncovering the pitting mechanism of the cold- and hot-rolled MASS, the microstructural evolution and pitting performance of nickel-saving metastable austenitic stainless (MASS) steel after cold- (CR) and hot-rolling (HR) were researched via electron microscopy technique and electrochemical methods. Austenite composites the main phase of the MASS. Small amounts of martensite film were proven to form in the MASS. The precipitation of Cr-rich M23C6 carbides was observed in the CR-MASS, while no carbides existed in the HR-MASS. The pitting resistance of the HR-MASS was better than the CR-MASS, which could be attributed to the fact that the stable pits in CR-MASS were initiated near the carbides, whereas the MnS inclusion would serve as the initiation sites for stable pits in HR-MASS. Findings in this work will provide a guidance for developing new generation MASS for automobile industry.

Corrosion Behavior of Passivated Martensitic and Semi-Austenitic Precipitation Hardening Stainless Steel

This research aimed to conduct a passive layer state study on martensitic and semi-austenitic precipitation hardening stainless steels (PHSS) passivated in citric acid and nitric acid baths at 49 and 70 °C for 50 and 75 min and subsequently exposed in 5 wt.% NaCl and 1 wt.% H2SO4 solutions. Corrosion behavior of the passivated material was observed by using potentiodynamic polarization (PP) according to the ASTM G5-11 standard. The microstructural analysis was performed by optical microscopy and scanning electron microscopy (SEM), while the passivated layer was characterized by X-ray photoelectron spectroscopy (XPS). The results indicated that the semi-austenitic-NA-50 min-70 °C sample showed the best corrosion resistance behavior in both solutions. The XPS characterization confirmed that the martensitic and semi-austenitic surface film presented a mixture of chemical compounds, such as Cr2O3 and Fe(OH)O, respectively.

Nano Scaled Checkerboards: A Long Range Ordering in NiCoMnAl Magnetic Shape Memory Alloy Thin Films with Martensitic Intercalations

Magnetic shape memory Heusler alloys, such as NiCoMnAl, are considered as promising candidates for magnetocaloric cooling applications. Grown in thin film systems of adjacent layers with austenite and martensite crystal structures of almost equal thicknesses, a long-range ordering phenomenon in the shape of a 3D checkerboard pattern occurs in NiCoMnAl samples. The crystallographic origin of the pattern is proven by transmission electron microscopy (TEM) techniques. The darker fields of the arrangement consist of martensite nuclei superposed with austenite, while the purely austenite regions appear bright in TEM cross sections. The nucleation process is presumably triggered by inhomogeneous local elastic stray fields of primary martensitic nuclei in the austenite matrix and limited by the thicknesses of the martensite and austenite thin films.

Ferromagnetic resonance in nanotwinned Ni–Mn–Ga film undergoing martensitic transformation

The ferromagnetic resonance (FMR) in the shape memory alloys (SMAs) exhibiting structural cubic-tetragonal phase transition has been studied. The influence of instability and spatial inhomogeneity of crystal lattice on the FMR spectra of twinned ferromagnetic SMAs films has been analyzed using statistical model of ferromagnetic martensite. It has been shown that the abnormally strong temperature dependence of lattice parameters of martensitic film gives a noticeable contribution to the temperature dependence of the resonance value of external magnetic field. The narrowing of the resonance peak, which had been experimentally observed on cooling of the film slightly below the Curie temperature, has been explained. It is argued that the martensitic films are good candidates for the experimental study of local elastic strains influence on the shape of the FMR peak.

Pseudoelasticity and Shape Memory Effects on Deformation Recovery of Indented Titanium-Nickel Alloy Films

The deformation behavior of shape-memory titanium-nickel (TiNi) alloy films indented by diamond tips of various nominal radius of curvature under different contact loads was investigated by nanoindentation. Pseudoelasticity and shape-memory effect (SME) strongly affected the deformation recovery of the indented films. The effects of tip radius and maximum indentation load on the deformation recovery of austenitic and martensitic TiNi films are interpreted in terms of associated deformation mechanisms controlled by pseudoelasticity, SME, and dislocations. The results reveal the important roles of pseudoelasticity and SME on the capacity of TiNi films to either fully annihilate or partially recover from deformation induced by nanoindentation, illustrating a high potential for applications requiring large reversible deformations.

Oxidation and concomitant effects on the microstructure and high temperature tensile properties of a DS Ni-base superalloy applied with different thicknesses of Pt-aluminide (PtAl) bond coat

The oxidation resistant diffusion PtAl coatings applied on the hot-section Ni-superalloy components of aero-engines exhibit phase transitions during thermal exposure in air: B2-NiAl➔B2-NiAl/L10-NiAl martensite+γ´-Ni3Al➔γ-Ni + γ´-Ni3Al. The present study correlates the oxidation-microstructure-tensile properties for DS CM-247LC superalloy coated with 60 and 90 μm thick inward grown PtAl bond coat. Cyclic oxidation of the superalloy, in the uncoated condition and after application of 60 and 90 μm thick PtAl coating, has been carried out at 1100 °C for various durations of 10, 100, 750 and 1000 h. The microstructure changes occurring in the coating and superalloy substrate during oxidation for various durations are examined. The concomitant tensile properties for the various oxidized samples are evaluated at 870 °C and their fracture behavior ascertained. Both coatings prevented surface degradation and imparted superior tensile properties, despite 750–1000 h of cyclic oxidation at 1100 °C. The morphology of the phases formed in the coatings affected the tensile properties of the coated superalloy. Cracks formed along the inter/intra-phase boundaries in the coatings. The predominant formation of martensitic L10 laths and films of single-phase γ´ at the prior B2/L10 phase boundaries in the thicker coating during intermediate-term oxidation, such as after 100 h, promoted the initiation of sharp cracks, which penetrated into the substrate and deteriorated the strength and ductility appreciably. On the other hand, if the B2/L10 martensitic phase was enveloped by the γ´ phase or the coating was comprised of γ´ surrounded by the γ phase, then the coating cracks had blunt tips and the tensile properties were not affected adversely. Albeit early phase transformations, the thin coating imparted superior tensile properties.

Structural, magneto-static and magneto-dynamic properties of Fe substituted Ni-Mn-In films

Off-stoichiometric Ni-Mn-In films of 500 nm thickness were deposited on Si (100) substrate by dc magnetron sputtering at ambient temperature. As-deposited films were annealed ex situ at 550 °C and 700 °C for 1 h under high vacuum. As-deposited Ni-Mn-In films which were amorphous, exhibited orthorhombic martensite structure upon annealing. The amorphous film was found to be paramagnetic whereas the annealed (martensite) film exhibited weak ferromagnetic behavior. Fe substituted Ni-Mn-In films of the same thickness annealed at 550 °C exhibited orthorhombic martensite structure, whereas films annealed at 700 °C exhibited dual phase (cubic L21 austenite and orthorhombic martensite) structure. Martensite Ni-Mn-Fe-In films showed large magnetic moment as compared to the ternary alloy film. Interestingly, films with dual phase structure yielded much higher magnetic moment. Ni-Mn-Fe-In films with higher thicknesses annealed at 700 °C showed enhanced magnetic moment due to higher L21 phase content in them. Analysis of polar angle variation of resonance field and linewidth of the dual phase Ni-Mn-Fe-In films revealed very low effective perpendicular magnetic anisotropy (3.0 × 103 J/m3) and low Gilbert damping constant (0.009 ± 0.001) for the first time. The new results obtained are interpreted in terms of the crystalline nature of the films.

Microstructural evolution and mechanical behaviour of high strength steel treated by quenching and partitioning process

The present steel has been processed through thermo-mechanical controlled processing (TMCP) with the finish rolling temperature (FRT) of 850°C followed by air cooling (AC). Subsequently, the steel was subjected to partial (800°C) and full austenitisation (930°C) followed by quenching and partitioning (Q&P) process at 345°C below MS (365°C) temperature. The steel in as-rolled condition reveals mixed microstructures comprising ferrite and bainite with yield strength of 560 MPa and tensile strength of 1063 MPa along with an average total elongation of 15%. The attractive combination of tensile strength (933-1000 MPa) and ductility (26-29% total elongation) are attributed to the multiphase microstructure comprising ferrite, lath martensite and thin film like carbon enriched retained austenite after one step Q&P process.

The relationships between sputter deposition conditions, grain size, and phase transformation temperatures in NiTi thin films

Sputter-deposited NiTi films on full 100mm wafers exhibit different phases, depending on the wafer region. Comprehensive characterization and analysis show that, unlike the common assumption, this effect is not related to the Ti/Ni stoichiometry. Instead, we show that the different phases are related to differences in grain size. Our results indicate the existence of a critical grain size of approximately 50–100nm, below which a significant inhibition of the martensitic transformation occurs. Further, we study the effect of deposition conditions and show that a desired uniform martensitic film can be achieved by decreasing the impact energy of the deposited atoms.

Refinement of Retained Austenite in Super-bainitic Steel by a Deep Cryogenic Treatment

The effect of a deep cryogenic treatment on the microstructure of a super-bainitic steel was investigated. It was shown that quenching the super-bainitc steel in –196°C liquid nitrogen resulted in the transformation of retained austenite to two phases: ~20 nm thick martensite films and some nano carbides with a ~25 nm diameter. Some refinement of the retained austenite occurred, due to formation of fine martensite laths within the retained austenite. The evolution of these new phases resulted in an increase in the average hardness of the super-bainitic steel from 641 to ~670 HV1.

Local elasticity and mobility of twin boundaries in martensitic films studied by atomic force acoustic microscopy

Nanoscale elastic properties of twinned martensite NiMnGa films were characterized by means of atomic force acoustic microscopy using cantilever contact-resonance spectra to measure the local contact stiffness k* and the local damping Q−1, which contains information on the crystallographic anisotropy of martensitic twin variants and the dissipative motion of twin boundaries (TBs). Images of k* and indentation modulus maps were obtained. Similar to topography images measured by conventional atomic force microscopy in contact mode, they show the nature of the twin structure and thus a regular variation in local elastic modulus. A correlation between k* and Q−1 was observed and mirrors the motion of the TB accompanied by a viscoelastic procedure. The k*-image and the topography image measured are opposite in contrast, which likely arises from mobile and immobile TBs depending on the geometry of twinning. Multi-resonance spectra were measured, which can be related to martensitic multivariants and are explainable as different types of nanotwins. A critical stress, defined as the starting point of softening due to TB movement was determined to be about 0.5 GPa for a thick film (1 μm) and 0.75 GPa for a thin film (0.15 μm), respectively. The values are much larger than that measured for bulk materials, but reasonable due to a large internal stress in the films.

Nanoindentation response of substrate-attached and freestanding single-crystalline Fe7Pd3 ferromagnetic shape memory thin films around the martensite transition: The impact of constraints and beyond

Ferromagnetic shape memory alloys offer great potential in the fields of engineering and medical sciences as integrated actuators or sensors. However, their physical properties, when miniaturized and connected to a substrate or mounted as active elements, are still insufficiently understood. The present work explores the impact of miniaturization and external boundaries on one of the most central features, namely twin boundary mobility. By measuring the nanoindentation response of substrate-attached films and freestanding foils around the austenite↔martensite transformation temperature in classical indentation, as well as dynamical quasi-continuous stiffness-measurement mode, dramatic softening and increasing recovery after film lift-off are discovered. The atomistics of these findings are explored with the help of classical multimillion-atom molecular dynamics simulations on indentation into martensite and austenite films on rigid or flexible substrates, as well as freestanding or mounted thin foils. They clearly demonstrate how substrate or lateral constrains hinder twin boundary motion, while complete untwinning only prevails in the presence of a flexible substrate or completely free foils. Experimentally observed pop-in events can be rationalized as local austenite→martensite transitions. Surface softness, which is observed by low indentation moduli, when compared to predictions from the bulk elastic constants, might indicate a more fundamental scenario close to the martensite transformation.

X-ray diffraction study of the reverse martensitic transformation in NiTi shape memory thin films

The development of stresses, phase fractions and the microstructure of thin equiatomic NiTi substrate-bound films was investigated during the reverse transformation from martensite to austenite. Synchrotron X-ray diffraction (XRD) experiments were performed during the heating portion of thermal cycling applied to the thin films to capture, in particular, the reverse martensitic phase transformation (monoclinic martensite→cubic austenite). The phase fractions and microstructure, as a function of temperature and thermal cycling, were analyzed through the application of Rietveld refinement to the diffraction data. Further, using the XRD data, the overall macroscopic stress in the film (derived from the curvature of the film/substrate system determined by XRD rocking curve measurements) and the stress in the austenite phase (derived from the lattice strain) during the transformation were tracked as a function of the degree of the transformation. The state of the stress in the austenite was found to remain biaxially, rotationally symmetric, even in the two-phase (martensite and austenite) film. The developments of the total stress in the film and the stresses in each of the two phases are discussed in terms of the transformation-induced volume misfit and its accommodation by elastic deformation.

Nanoscale magneto-structural coupling in as-deposited and freestanding single-crystalline Fe7Pd3 ferromagnetic shape memory alloy thin films

Ferromagnetic shape memory alloys are characterized by strong magneto-mechanical coupling occurring at the atomic scale causing large magnetically inducible strains at the macroscopic level. Employing combined atomic and magnetic force microscopy studies at variable temperature, we systematically explore the relation between the magnetic domain pattern and the underlying structure for as-deposited and freestanding single-crystalline Fe7Pd3 thin films across the martensite–austenite transition. We find experimental evidence that magnetic domain appearance is strongly affected by the presence and absence of nanotwinning. While the martensite–austenite transition upon temperature variation of as-deposited films is clearly reflected in topography by the presence and absence of a characteristic surface corrugation pattern, the magnetic domain pattern is hardly affected. These findings are discussed considering the impact of significant thermal stresses arising in the austenite phase. Freestanding martensitic films reveal a hierarchical structure of micro- and nanotwinning. The associated domain organization appears more complex, since the dominance of magnetic energy contributors alters within this length scale regime.

Fabrication conditions and transformation behavior of epitaxial Ni–Mn–Ga thin films

Epitaxial Ni–Mn–Ga films have been grown onto heated substrates by sputtering. Their chemical composition depends on the sputtering argon pressure. Representative epitaxial films of Ni52.3Mn26.8Ga20.9, 0.5 μm-thick, transform martensitically at about 120 °C, accompanied by sharp changes in the lattice parameter and resistivity, and orders ferromagnetically below 98°. The observed high transformation temperature, orthorhombic martensitic structure, twinning mode and film morphology, indicate a potential multifunctional behavior of the film, such as high-temperature shape-memory effect and magnetic field actuation.

Epitaxial high saturation magnetization FeN thin films on Fe(001) seeded GaAs(001) single crystal wafer using facing target sputterings

It was demonstrated that Fe–N martensite (α′) films were grown epitaxially on Fe(001) seeded GaAs(001) single crystal wafer by using a facing target sputtering method. X-ray diffraction pattern implies an increasing c lattice constant as the N concentration increases in the films. Partially ordered Fe16N2 films were synthesized after in situ post-annealing the as-sputtered samples with pure Fe8N phase. Multiple characterization techniques including XRD, XRR, TEM, and AES were used to determine the sample structure. The saturation magnetization of films with pure Fe8N phase measured by VSM was evaluated in the range of 2.0–2.2 T. The post annealed films show systematic and dramatic increase on the saturation magnetization, which possess an average value of 2.6 T. These observations support the existence of giant saturation magnetization in α″-Fe16N2 phase that is consistent with a recent proposed cluster-atom model and the first principles calculation [N. Ji, X. Q. Liu, and J. P. Wang, New J. Phys. 12 063032 (2010)].

Microstructure and atomic configuration of the (001)-oriented surface of epitaxial Ni–Mn–Ga thin films

The (001)-oriented surface of epitaxial off-stoichiometric Ni–Mn–Ga ferromagnetic shape memory alloys was studied in both austenitic and martensitic phases. Scanning tunneling microscopy (STM) imaging of the austenitic surface reveals a well-ordered and reconstruction-free surface exhibiting predominantly Mn–Ga termination. We found that only one of the two atomic species (Ga or Mn) is visible in STM, which is attributed to a pronounced geometric corrugation of the surface layer. After a transformation of the sample from the initial austenitic phase to the martensitic phase upon a high-temperature annealing step, a thorough investigation of the martensitic surface was conducted. On a larger scale, pronounced corrugation lines arise from the macroscopically twinned surface. A second corrugation feature is found on a distinctly smaller scale and is shown to originate from the modulated nature of the martensitic film structure. The irregularly spaced corrugation lines support the model of adaptive martensites.

Nanoindentation of NiTi shape memory thin films at elevated temperatures

The shape memory effect and nanoindentation response of various phases of sputtered NiTi shape memory thin films were investigated as a function of temperature. The phase transformation temperatures of NiTi films were observed to be sensitive to a compositional shift. The mechanical properties of NiTi thin films also presented a significant response to phase transformations. At the same load, the maximum indentation depth for austenite is smaller than for martensite, indicating that martensite is softer than austenite. A martensite thin film was converted to austenite via in situ heating nanoindentation and displayed the mechanical properties similar to the austenite film at room temperature. These results underscore the validity of elevated temperature nanoindentation methods as a means of interrogating the mechanical properties of materials that undergo thermally-induced phase transformations. The details of the load–displacement curves are also described.

Microstructure and twinning in epitaxial NiMnGa films

Although magnetic shape memory alloys have attracted large scientific interest, miniaturization as single-crystalline thin films is still a greatly unresolved issue. In the present work we investigate the microstructure of epitaxial NiMnGa thin films which are fabricated by sputter deposition on magnesium oxide substrates at elevated temperatures. Transmission and scanning electron microscopy as well as atomic force microscopy studies are employed to relate surface topography to twin formation in 7 M martensitic NiMnGa films. Additional findings include pore formation in substrate proximity as well as minor precipitation with reduced nickel and gallium contents.