Incomplete Martensitic Transformation Research Articles

Frictional shear stress-induced incomplete martensitic transformation in mono-crystalline B2-CuZr spherulites

An austenitic B2-CuZr phase, when deformed, can undergo a martensitic transformation (MT) that produces a martensitic B19′-CuZr phase, resulting in remarkable transformation-induced plasticity (TRIP) and shape memory effects in B2-structured alloys. However, the shear mechanism as well as the crystallographic characteristics, which underlie the MT and, in turn, the TRIP and shape memory effects, remain unclear and somewhat controversial. Here we report that a single shear on {110}B2<001>B2, along with {1 1 5.5}B2 being the habit plane, are responsible for the MT in B2-CuZr, where {110}B2 and <001>B2 are the shear plane and the shear direction, respectively. These were revealed by determining unambiguously the orientation relationship between B2 and B19′, co-existing in individual spherulites contained in a B2-CuZr-containing bulk metal glass-matrix composite (BMGC). This exotic microstructure, featuring a core-shell like distribution in each single spherulite in which all the 12 possible martensite (B19′) variants constitute the core and the parent B2 forms the shell, was made from an incomplete MT in originally mono-crystalline B2 spherulites of the as-cast BMGC when subjected to moderately applied frictional shear stresses upon mechanical grinding. Finite element analysis suggests that a core-shell like shear stress distribution inside a B2 spherulite, caused by the confinement of the stiffer glass matrix, may account for it. These results provide direct experimental evidence to elucidate the B2→B19′ MT and its variant selections under various stress states, and pave the way for developing ductile B2-structured alloys, including but not limited to, shape memory intermetallics, high entropy alloys and B2-containing composite materials.

Uncovering the hierarchical clusters in the heat-affected zone of an electron beam welded α/β titanium alloy joint

Using high-resolution transmission Kikuchi diffraction (TKD) and transmission electron microscopy (TEM), we examined the hierarchical clusters that form in situ in the heat-affected zone (HAZ), which are commonly referred to as “ghost” structures, of bimodal titanium alloy Ti-5Al-2Sn-2Zr-4Mo-4Cr (wt%, TC17). The ghost structures are enriched with Al elements but poor in Mo and Cr compared to the surrounding β matrix. TKD results show that the ghost structure in middle-HAZ mainly consists of αL laths with a high-angle grain boundary, which exhibits the classic Burgers orientation relationship (BOR) with the host matrix, while it encircles the αP grains in far-HAZ. And the ghost structure is evidenced to form via incomplete martensitic transformation. TEM results further confirm that the ghost structure is composed of αL and tiny βL laths with BOR, with the former being enriched with Al and poor with Cr and Mo, while the latter is the opposite. Interestingly, two αL variant clusters with a check-mark morphology are frequently observed viewed along [0001]αL//[110]βL directions, which are dominated by the crystallographic and geometrical relationships between α and β phases. Based on the microstructural characterization, it is hypothesized that the ghost structure is transformed from the initial αP phase, due to the coupling effect of high thermal stress (which induces the formation of a large number of dislocations) and element diffusion caused by sudden temperature increase and plunge cooling in the HAZ during the welding process.

Characterization of iron meteorites by scanning electron microscopy, X‐ray diffraction, magnetization measurements, and Mössbauer spectroscopy: Gibeon IVA

AbstractGibeon IVA iron meteorite fragment was characterized using optical microscopy, scanning electron microscopy (SEM) with energy dispersive spectroscopy (EDS), X‐ray diffraction (XRD), magnetization measurements, and Mössbauer spectroscopy. Optical microscopy and SEM made on the polished section of the meteorite, show the presence of α‐Fe(Ni, Co) and γ‐Fe(Ni, Co) phases and plessite structures. There are no troilite inclusions observed in the studied section. EDS studies indicate some variations in the Ni concentrations: (i) within the α‐Fe(Ni, Co) phase in the range ~5.0 ± 0.1 – ~7.5 ± 0.1 at% and (ii) within the γ‐Fe(Ni, Co) phase in the range ~26.0 ± 0.2 – ~36.1 ± 0.2 at%. The latter Ni concentration range indicates the presence of small amount of the paramagnetic γ‐phase in addition to the ferromagnetic γ‐phase. EDS also shows that Ni content in two plessite structures is varying in the range ~16–37 at%, which can indicate the presence of only the α2‐Fe(Ni, Co) and γ‐Fe(Ni, Co) phases in the duplex plessite structure. This may be a result of the γ‐phase decomposition with the incomplete martensitic transformation: γ → α2 + γ due to a faster cooling rate. XRD indicates the presence of ~1.3 wt% of the γ‐Fe(Ni, Co) phase in Gibeon VIA. The saturation magnetization moment of 185(2) emu g−1 obtained also confirms the presence of phases with low and high Ni concentrations. The most appropriate fit of the Gibeon IVA Mössbauer spectrum demonstrates the presence of five magnetic sextets and one paramagnetic singlet which are assigned to the ferromagnetic α2‐Fe(Ni, Co), α‐Fe(Ni, Co), γ‐Fe(Ni, Co), and paramagnetic γ‐Fe(Ni, Co) phases. The relative average Fe contents in these phases are: 13.4% in the α2‐Fe(Ni, Co) phase, 78.3% in the α‐Fe(Ni, Co) phase, and 8.3% in the ferromagnetic and paramagnetic γ‐Fe(Ni, Co) phases.

Variant selection and morphology by the different cooling rates in Zr-xNb-0.4Mo alloys

Although many efforts have been made on understanding the variant selection rules, the interplay of these rules and their combined effects on variant selection have not been well addressed. In this work, variant selection and morphology by the different cooling rates in Zr-xNb-0.4Mo (x=0.2, 0.6 and 1.0 wt.%) alloys are characterized using scanning electron microscopy (SEM) and electron backscatter diffraction (EBSD) technique. The results show that the variation of variant selection and morphology is attributed to the Nb content, the parent β grain size, the cooling rate, the β grain boundary (GB) characteristics and the interplay among them. The Nb addition (Nb≤0.6 wt.%) decreases the average grain size of parent β grains, and the smaller parent β grain size generally results in the larger degree of variant selection (DVS) values in the individual parent β grain due to their resistance to the uniform nucleation and growth of α-variants. The slow cooling rate promotes the diffusional transformation and favors the formation of special β GBs and the β GBs with high dislocation density, which usually lead to a stronger DVS in the individual parent β grains. The very large weighted area percentage of α-variant in the individual parent β grains, which accounts for the stronger DVS in the individual parent β grains, is due to the special β GBs, the β GBs with high dislocation density and the incomplete martensitic transformation (martensitic-diffusional mixed transformation). The β GB with high dislocation density has a larger contribution to the strong DVS of individual parent β grains than the special β GB. In addition, the result shows that the low density of statistically stored dislocations (SSDs) can inhibit the nucleation and growth of the α-variant produced by the special β GB.

Compressive stress-induced martensitic transformation and elastocaloric effect in Cu-Al-Mn single-crystal alloy

In this study, the strain distribution and elastocaloric effect during compression of a superelastic [116]-oriented single-crystal Cu–Al–Mn shape memory alloy were investigated. Its superelastic behavior exhibited low critical stress (213 MPa), narrow stress hysteresis (111 MPa), and excellent superelasticity (near 9%). The detailed evolution of strain field during the superelastic cycle was analyzed by digital image correlation method. Multiple variants were triggered during its superelastic deformation, and uneven strain distribution was observed in the strain field. The temperature and strain fields of the four side surfaces showed inhomogeneous transformation in the single-crystal alloy. The elastocaloric temperature drops ranged from 3.1 K to 12.3 K when the sample was unloaded from prestrains of 2 %–9 %. The calculated theoretical elastocaloric effect was compared with the measured temperature drop, and the differences were confirmed to be related to non-uniform and incomplete martensitic transformation. This study presents a comprehensive analysis of the inhomogeneous transformation behavior and elastocaloric effect in [116]-oriented Cu–Al–Mn single-crystal alloy.

Deep learning based phase transformation model for the prediction of microstructure and mechanical properties of hot-stamped parts

Hot stamping of boron steel is an effective way for weight reduction of automobiles parts, but suffers from the loss of mechanical properties due to incomplete martensitic transformation. Different from general heat treatment, phase transformation in hot stamping relates to both thermal loading path and deformation history. Unfortunately, the existing models have difficulty in capturing the history-dependent phase transformation behavior of boron steel under hot stamping conditions. In the present work, the gated recurrent unit (GRU) are combined with fully connected neural network (FCNN) to capture the coupling effect of both nonlinear strain history and thermal loading path on phase transformation. A unified deep-learning based phase transformation (DLPT) model is developed to integrate both diffusive and diffusionless transformation to cover all feasible thermo-mechanical loading path in hot stamping process. A hybrid driven thermo-mechanical-metallurgical (TMM) framework is developed by integrating the DLPT model into general TMM framework, which providing a novel approach for two-scale simulation of hot stamping process. A comprehensive database of phase transformation is constructed for model training based on well-designed thermo-mechanical loading experiments to cover hot stamping conditions, and the optimal topology of neural network in DLPT model is obtained by training on this database. The accuracy and reliability of DLPT model is systematically demonstrated by dieless hot V-bending and hot stamping of T-shaped parts besides CCT and DCCT tests. The net effect of local plastic strain on subsequent phase transformation is clarified with accuracy by dieless V-bending with the removal of disturbing factors. Non-uniform distribution in hardness of the final hot stamped parts can be predicted with the DLPT model successfully, which is better in accuracy than in case of working with the conventional model.

Thermokinetic EMF during a reverse phase transition in titanium nickelide as a way of information recording

The external factors that influence on the thermokinetic EMF value in the Ti – 50 at.% Ni samples were determined. A method for setting thermokinetic EMF in certain sections of the TiNi wire was developed. The thermokinetic EMF value was measured directly using a digital millivoltmeter MNIPI V7-72. The sections of the Ti – 50 at.% Ni wire samples were subjected to tensile tests on a tensile machine IP 5158-5. On the basis of calorimetric studies, the kinetics of martensitic transformations was investigated. It was found that the direct phase transition affects the thermokinetic EMF value of the Ti – 50 at.% Ni during thermal cycling. Thermal cycling in the temperature range of the complete martensitic transformation causes the thermokinetic EMF value reduction by 0.16 mV by the 15th temperature cycle. The degradation of the thermokinetic EMF value by 0.04 mV took place during thermal cycling in the temperature range of the incomplete martensitic transformation by the 70th thermal cycle. The thermokinetic EMF value was restored to 0.22 mV with increasing temperature to 240 °С, as in the case of annealing at temperatures of 400÷800 °С. The thermokinetic EMF value is associated with a change in physical and mechanical properties of the alloy during thermal cycling. It is characterized by a change in stages of the phase transition and a shift of the characteristic temperatures. On the basis of the obtained experimental data, a method was proposed for a purposeful setting of extended TiNi wire sections with the thermokinetic EMF value from 0 to 0.6 mV, using different methods of influence on its value (thermal cycling, deformation, temperature change in heating zone). The proposed technical solution can be used as a method for information recording.

Stasis mechanism of γ → ε martensitic transformation in Fe-17Mn alloy

In the present study, we observed incomplete martensitic transformation in Fe-17Mn alloy that γ austenite → ε martensitic transformation stops at a certain temperature during continuous cooling and the ε phase fraction does not increase with decreasing temperature further. The mechanism of this transformation stasis (TS) was investigated through microstructural observation, thermodynamic calculation and tensile testing using Fe-17Mn specimens with various γ grain sizes. The TS phenomenon was observed in all specimens regardless of prior γ grain size, and the TS temperature was lowered with decreasing prior γ grain size. TS occurred due to the dynamic grain refinement that ε platelets subdivide a γ grain to make several subgrains. The γ/ε interphase boundaries surrounding subgrains revealed the higher suppression effect of γ → ε martensitic transformation than prior γ grain boundaries. Therefore, at the same level of subgrain size, the fine γ-grained specimen revealed more active γ → ε martensitic transformation due to the higher fraction of prior γ grain boundaries than the coarse γ-grained specimen. Considering the difference in boundary characteristic between prior γ grain boundaries and γ/ε interphase boundaries, the driving force for γ → ε martensitic transformation was newly calculated and used to explain the TS phenomenon.

Numerical analysis of NiTi actuators with stress risers: The role of bias load and actuation temperature

NiTi wires and thin ribbons are used in shape memory actuators as active elements undergoing repeated thermal cycles. The NiTi actuators operate either under constant force (CF loading constraint) or they act against a variable force (VF loading constraint) from an elastic bias spring. Any stress riser represents a danger for potential actuator failure upon cycling. Although NiTi actuators always include stress risers either inherently in their design or as material flaws, their effect on actuation performance has, however, not been studied thoroughly yet. This issue is addressed in the present paper through numerical analysis of the effects of actuation load type (CF or VF loading constraint), maximum temperature, and stiffness of the bias springs (in VF loading constraint) on the local evolution of martensitic transformation (MT) and mechanical fields around the notch upon the cyclic thermomechanical loading of a thin NiTi shape memory notched ribbon. The analyses clearly show a strong amplification effect of the transformation strain on the stress concentration factor. The results reveal that the evolution of the forward MT upon cooling causes a sharp stress peak around the notch-tip the magnitude of which depends on the bias spring stiffness while upon heating, the stress at the notch-tip area relaxes due to the reverse MT of the surrounding bulk. Moreover, the simulations indicate that any overheating during the actuation is harmful to the notch-tip as the complete reverse MT at the notch-tip has stress rising effect while incomplete MT at the notch-tip has stress relaxing effect.

Electrostatic levitation processing and microscopic hardness property of hyperperitectic Ti60Ni40 alloy

The solidification mechanism of hyperperitectic Ti60Ni40 alloy was systematically investigated by using the electrostatic levitation (ESL) technique, and the maximum undercooling achieved was up to 373 K (0.25 TL). When the undercooling is below a threshold of 281 K, typical peritectic solidification prevailed within the levitated liquid alloy, and the double recalescence processes were observed by a high-speed camera. In this case, the primary TiNi compound preferentially grew as well-defined dendrites, and the interdendritic Ti2Ni phase was subsequently produced by the peritectic reaction and transformation. TEM analyses showed that an incomplete martensitic transformation occurs in the primary TiNi phase, which results in the complicated structure of B19’ martensite and R-phase. Once the undercooling exceeded 281 K, the coupled-growth mode between TiNi and Ti2Ni compounds was induced, leading to the lamellar and anomalous eutectic-like microstructures. The enhancement of undercooling promoted the migration speed of liquid-solid interface in both typical peritectic solidification and metastable coupled-growth modes, but it exhibited an abrupt reduction in critical undercooling. The microscopic hardness of the primary TiNi phase processed by ESL was 48.1% higher than that of the Ti60Ni40 master alloy, while it increased by 29.0% for the peritectic Ti2Ni phase. These indicated that the peritectic-type superalloys with advanced performances could be designed by modulating the solidification process and solid solubility through different undercoolings.

Deformation-force effects of shape memory of TiNi alloys under conditions of incomplete martensitic transformation

The article is devoted to studying the influence of preliminary and plastic processing, applied stress, degree of phase transformation on the deformation effects manifested in conditions of incomplete martensitic transformation. The laws of generation and relaxation of reactive stresses were studied depending on the stiffness of the reaction, the magnitude of the preliminary deformation and the selected interval of heat changes. It is shown that the accumulation and return of deformation, the effect of generation and relaxation of reactive stresses in a narrow temperature range occurs only in structurally inhomogeneous objects, determined with the statistical spread of characteristic temperatures.

In situ high-energy X-ray diffraction study of thermally-activated martensitic transformation far below room temperature in CuZr-based bulk metallic glass composites

Abstract The martensitic transformation within B2 phase plays a critical role for the ductility of CuZr-based bulk metallic glass (BMG) composites, yet the underlying mechanism for the martensitic transformation process needs further investigations. Here, we found that the onset temperature of the thermally-activated martensitic transformation for CuZr-based BMG composites was reduced to 162.7 ± 2.5 K, which is far lower than those of polycrystalline samples, due to the stabilization of the austenitic phase induced by crystalline-amorphous interfaces. In situ high-energy X-ray diffraction measurements (HEXRD) further demonstrate that the thermally-activated martensitic transformation is governed by an extremely incomplete martensitic transformation from B2 to B19’ and B33 phases without the decomposition of B2 phase which generally appears in polycrystalline samples. Due to unconfined stress states during the thermally-activated martensitic transformation, B33 phase forms more early during thermal process than deformation process. The present study provides a further understanding on the martensitic transformation behaviors of shape memory type CuZr-based BMG composites.

An observation of the binder microstructure in WC-(Co+Ru) cemented carbides using transmission Kikuchi diffraction

The microstructure of WC-(Co+Ru) cemented carbides was studied using transmission Kikuchi diffraction (TKD). We report that the CoFCC grains contain Σ3-annealing twins which undergo an incomplete martensitic transformation to form CoHCP laths on cooling below TMs. The parent CoFCC and product CoHCP phases are related via the Shoji-Nishiyama orientation relationship. Four variants of the CoHCP plates exhibit a special misorientation of 70.5 ± 0.2°〈112¯0〉. A higher transformed fraction was observed in the sample possessing the lower binder content and finer WC grains. We suggest that this is due to the availability of more WC-binder interfaces, which facilitate CoHCP plate nucleation.

Thermally-Activated Martensitic Transformation Behaviour Below Room Temperature in CuZr-Based Bulk Metallic Glass Composites

During the thermally-activated martensitic transformation (MT) of CuZr-based bulk metallic glass (BMG) composites containing B2 CuZr crystals, the MT temperatures were reduced to 162.7 ± 2.5 K due to the constraint effect from glassy matrix, which is far lower than those of crystalline counterparts. In situ high-energy X-ray diffraction measurements demonstrate that the thermally-activated MT is governed by an extremely incomplete MT from B2 to B19' and B33 phases without the decomposition of B2 phase which appears in crystalline counterparts. Due to free stress states during the thermally-activated MT, B33 phase forms more easily during thermal process than deformation process.

Detwinning of preloaded martensite in shape memory alloys and its effect on the cyclic behavior of NiTi cylindrical actuators

A multi-mechanism material model is used to investigate the effect of the degree of martensitic detwinning/reorientation occurring at the end of mechanical preloading, on the performance of shape memory tubular and solid cylindrical actuators. A high-temperature ternary Ni50.3Ti29.7Hf20 alloy, showing almost complete transformation behavior, and an ordinary binary Ni49.9Ti50.1 material, depicting incomplete martensitic transformation, are used. The results indicated a clear correlation between the shape memory cyclic actuation behavior and the martensitic deformation character of the selected alloy. More specifically, while the actuation strokes produced by the Ni50.3Ti29.7Hf20 systems consistently followed a direct pattern for the different geometries and torque magnitudes, the results for the Ni49.9Ti50.1 cases indicated a behavior that is counterintuitive to what may be expected under pure mechanical loading conditions. In particular, when subjected to the same, high preload torques, Ni49.9Ti50.1 solid actuator under lesser stresses generated higher twist strokes than a tubular counterpart experiencing higher stresses.

Effects of dilution and baseplate strength on stress distributions in multipass welds deposited using low transformation temperature filler alloys

Transformation plasticity can be utilised to control residual stresses in steel welds. This requires special filler alloys that transform at a sufficiently low temperature to compensate for accumulated thermal contraction strains. However, the welding parameters needed to optimise the effect in multipass joints have yet to be established. This topic has been investigated by characterising the residual stress distribution in multipass welds fabricated with different welding alloys and baseplates using neutron diffraction to assess the effects of dilution and baseplate strength. While the use of richly alloyed weld metal does enhance fatigue performance in single pass joints, the extent of stress relief that can be derived from transformation plasticity is reduced due to incomplete martensitic transformation when further layers are deposited. For all cases studied, compressive stresses were measured in the weld metal with balancing tensile stress in the heat affected zone of the plate. The magnitude of the tension was observed to be a function of the strength of the baseplate. Recommendations are also presented for the combination of welding and material parameters that lead to the optimum exploitation of transformation plasticity as a method for boosting the fatigue performance of multipass welded joints.

Improvement of NiTi Shape Memory Actuator Performance Through Ultra‐Fine Grained and Nanocrystalline Microstructures

AbstractUltra‐fine grain sizes have been shown to enhance some key mechanical and functional properties of engineering materials, including shape memory alloys. While the effect of ultra‐fine and nanocrystalline grain sizes on pseudoelastic shape memory materials is well‐appreciated in medical device engineering, the effect of such microstructures on actuators has not been sufficiently characterized. In the present work, it is demonstrated that NiTi spring actuators with ultra‐fine grained microstructures can be obtained by conventional wire drawing in combination with heat treatments and that the final grain size can be controlled by varying the final annealing temperature. Annealing at 400 °C for 600 s allows for the evolution of microstructures with median grain sizes of about 34 nm, while annealing at 600 °C for the same length of time results in median grain sizes of about 5 µm. It is observed that the grain size strongly affects the elementary processes of the martensitic phase transformation. Small austenite grain sizes inhibit twinning accommodation of transformation strains, such that a higher driving force is required to nucleate martensite. This increase in the martensite nucleation barrier decreases the martensite transformation temperatures such that only partial transformation to martensite is possible upon cooling to room temperature. The incomplete martensitic transformation reduces the exploitable actuator stroke; however, a reduction in grain size is shown to improve the functional stability of the material during thermal and thermomechanical cycling by reducing the irreversible effects of dislocation plasticity.

Wide Range Temperature Memory Effect of an In Situ NiTi Alloys Composite

Incomplete martensitic transformation characteristics of NiTi shape memory alloys after cold deformation were studied. By designing the shape of the original surface curve, macroscopic domains with different dislocation density were introduced into the NiTi alloys and materials possessing the characteristics of composites were obtained. Due to the interactions between the dislocation texture and martensite variants, the reverse transformation temperatures were found to expand over a large temperature window, which gives temperature memory effect a higher potential for application.

Crystal structure of intermediate structures in shape-memory alloys as realization of patterns of algebraic geometry

Three-dimensional models of the structure of 7R (14M) seven-layer martensite in NiAl-base alloys and of R-phase in NiTi-base alloys are suggested. Both models treat the seven-layer martensite and the R-phase as products of incomplete martensitic transformation, the structure of which is represented by combinations of coordination polyhedrons constituting the crystal lattices of the initial B2-phase and ω-phase.

Evidence for incomplete martensitic transformation in Ni0.62Al0.38 by neutron diffraction

Abstract High-resolution neutron powder diffraction was used to investigate the evolution of the crystal structure of Ni 0.62 Al 0.38 with a martensitic transformation temperature, M S , of 65 K as a function of temperature. Although the β 2 parent phase has the propensity to nucleation instability, no precursor effects are observed until ∼40° above M S . Below M S , the transition does not occur homogeneously and it is shown that the martensite coexists with the austenite over a wide temperature range. The low temperature phase seems to be largely described by the 7R structure that accomodates the transition from the parent to the martensite and minimizes the lattice mismatch between the two.