Low-temperature Transformation Research Articles

An investigation on stabilisation and phase transformation characteristics of Cu-Al-Be shape-memory alloys quenched in different media

ABSTRACT The aim of the present investigation is to fabricate the ternary Cu87.59, Al11.99 and Be0.42 (wt.%) shape memory alloy (SMA), and to study the effect of quenching medium on the phases, phase transformation, stabilisation, and shape recovery ratio. SMA was fabricated, betatized at high temperature (β) and then quenched into different media and quench types. The quenched SMAs are examined for phases, microstructure, phase transformation temperatures, martensite stabilisation and shape recovery ratio. XRD results reveal that all the quenched alloys are of complete martensite of 18 R and 2 H variants. Meta quench oil quenched SMAs possess pure martensite, smaller grain size, lower transformation temperatures, and rapid recovery, whereas silicone oil exhibits larger grain size and delay in recovery and the mechanisms are discussed in detail in the discussion part. MQ oil quenched SMAs are suitable and recommended for rapid response/recovery actuator applications with enhanced properties.

Correction to “Use of a low transformation temperature effect for the targeted reduction of welding distortion in stainless chromium‐nickel steel for an application in rail vehicle construction”

Correction to “Use of a low transformation temperature effect for the targeted reduction of welding distortion in stainless chromium‐nickel steel for an application in rail vehicle construction”

Mechanical properties of various thermomechanical processed Ti-Mo-Al-Zr and pseudo-binary (Ti-Zr)-Mo shape memory alloys for biomedical applications

Shape memory alloys (SMAs) have attracted much attention due to the great demand for biomedical materials. Among the SMAs, the Ti-based SMAs are promising materials owing to their high biocompatibility, non-toxicity, excellent functionalities, and proper mechanical properties. Concerning the Ti-based SMAs, adding other elements is an often-used strategy, and the combinations of the addition materials are unlimited. Among the various Ti-based alloy systems, the Ti-Mo-Al-Zr alloy is one fundamental alloy system, since Mo and Al are served as the specific metals for the calculations of the Mo equivalent and Al equivalent for the evaluation of the alloy stability. While Zr is a neutral element for fine-tuning the properties of the alloys. This fundamental quaternary alloy was thus chosen in this study. In this research series, it has been found that the Ti-5.5Mo-8Al-6Zr alloy performed proper mechanical properties. Therefore, this specific alloy was further subjected to various thermomechanical treatments to further improve its properties. In addition to the Ti-5.5Mo-8Al-6Zr alloy, pseudo-binary alloys of (Ti-Zr)-Mo were also investigated due to their intrinsic low phase transformation temperature compared to the pure Ti and Zr elements. Strategies for the development of these SMAs by conducting thermomechanical treatments have been proposed in this study.

Interpass temperature strategies for compressive residual stresses in cladding low-transformation-temperature material 16Cr8Ni via wire arc additive manufacturing

Low-transformation-temperature (LTT) materials induce compressive residual stresses within an effective depth in wire arc additive manufacturing (WAAM), which is well suited for cladding applications. However, this role was found to be sensitive to the phase transformation sequence. We developed an LTT material—16Cr8Ni (16Cr8Ni-LTT)—and to explore its potential, it was clad onto a KA36 substrate via WAAM. Three different interpass temperature strategies were designed as follows: (1) no interpass temperature control; (2) interpass temperature controlled at approximately 270 °C above the martensite transformation start temperature (Ms = 200 °C) for simultaneous phase transformation; and (3) interpass temperature controlled at approximately 50 °C below the martensite transformation finish temperature (Mf = 60 °C) for sequential phase transformation. A thermal elastic-plastic numerical model considering the phase transformation-induced plasticity (TRIP) was developed using in-house software, JWRAIN-Hybrid, to reproduce the temperature field and residual stresses of 16Cr8Ni-LTT cladding. X-ray diffraction (XRD) and contour methods were employed for residual stress measurements. The high accuracy of the model was validated in terms of the temperature, deformation, and residual stress. The reproduced maximum historical temperature distributions were combined with hardness tests to identify the depths and temperatures of the heat-affected zone (HAZ). The measured maximum compressive residual stresses induced by the three interpass temperature strategies for cladding 16Cr8Ni-LTT were −503, −420, and −720 MPa. Moreover, irrespective of the adopted interpass temperature strategy, both longitudinal and transverse residual stresses were compressive in the 16Cr8Ni-LTT cladding. This study provides scientific insights into LLT-induced compressive residual stresses and highlights the superiority and flexibility of cladding LTT materials via WAAM for improving the resistance of large metal components to fatigue and corrosion.

Use of a low transformation temperature effect for the targeted reduction of welding distortion in stainless chromium‐nickel steel for an application in rail vehicle construction

Use of a low transformation temperature effect for the targeted reduction of welding distortion in stainless chromium‐nickel steel for an application in rail vehicle construction

Assessment of fatigue crack growth resistance of newly developed LTT alloy composition for the repair of high strength steel structures

Tensile residual stress (TRS) is a well-known factor that deteriorate the integrity of welded joints. Fatigue failure is accelerated by the existence of TRS introduced during the welding process. There have been efforts in the last two decades to develop filler alloys that can reduce TRS by introducing compressive residual stress (CRS) to oppose the TRS in high strength steel welded joints. These works are based on the theory of austenite (γ) to martensite (α’) transformation and the filler is often called a low transformation-temperature (LTT) alloy. Many studies have reported that the fatigue strength (FS) of weld joint made with LTT alloy is many times better than that of the conventional fillers. It is reported to be particularly useful in the repair of high strength steel structures. However, studies on the fatigue crack growth (FCG) behaviour of these LTT alloys is scarce. In this work, we developed Fe-CrNiMo based LTT weld metal composition, assessed its FCG behaviour and compared the results with that of a conventional welding wire (ER70S-6). It is found that ER70S-6 weld metal obtained under relatively fast cooling is extremely tough, but the associated heat affected zone (HAZ) has poor resistance to FCG which obscured the benefit of the tough weld metal. High heat input or condition that results to slow cooling of the ER70S-6 weldment deteriorates its resistance to FCG. Unfortunately, despite its low martensite start temperature of 231±7 and the anticipated beneficial effect of induced CRS, the LTT alloy studied had the lowest FCG resistance. The LTT alloy appears to have an intrinsic microstructural feature or a ‘fault line’ that reduced its resistance to FCG. While the LTT alloy weld metal has poor resistance to FCG, the associated HAZ resisted FCG more than the HAZ associated with ER70S-6 weld metal. It is observed that aligning the ER70S-6 weld metal perpendicular to the crack front produced the highest resistance to fatigue crack initiation and propagation. In the case of ER70S-6, it is believed that the weld metal induced a CRS at the notch tip which resulted to the high fatigue resistance. In the case of the LTT alloy, perpendicular alignment of the weld metal produced slight improvement.

Effects of microalloying element addition on mechanical properties of SA508 Gr.1A low-alloy steels

SA508 Gr.1A low-alloy steel is being considered as a candidate material for main steam line piping in nuclear power plants. Therefore, improving its strength and toughness is essential for enhancing the leak-before-break (LBB) margin. In this study, six types of model alloys were fabricated by varying the contents of microalloying elements (C, Cu, B, Ti, and Nb) to enhance the mechanical properties of the specimens. The addition of a few ppm of B led to the formation of a fine-grained low-temperature transformation microstructure, resulting in the highest strength among the model alloys. However, the addition of Nb and Ti increased the formation of coarse ferrite, significantly decreasing the strength of the alloys. Reducing the C content while adding a small amount of B simultaneously maintained strength and enhanced toughness. Furthermore, the LBB margins of model alloys and commercial steel were evaluated to validate the influence of varying microalloying content. The model alloys exhibited a substantial increase in yield strength and fracture resistance, resulting in a more than 10% increase in the LBB margin. Notably, the LBB margin of the alloy with 15 ppm B was 1.39, approximately 25% higher than that of commercial steels.

Polymorphic transformation from metastable β to stable α phase in MnTe flake

Manganese telluride (MnTe) is an intriguing polymorphic semiconductor. Among them, the metastable β-MnTe phase, characterized by a wurtzite-type structure, has been successfully fabricated as a sputtered film on substrates. However, bulk or free-standing β-MnTe without substrates has never been realized. In this study, we fabricated a β-MnTe flake using a lift-off process. Notably, the X-ray diffraction patterns of the flake sample approached the predicted peak positions of bulk β-MnTe, indicating strain relaxation. This study represents a pioneering experimental success in demonstrating a bulk-like β-MnTe. Furthermore, we conducted thermal analyses associated with the β→α transformation through differential scanning calorimetry. The β-MnTe flake exhibited a lower transformation temperature than films, suggesting diminished thermal stability of the β phase. The enthalpy change was quantitatively determined as −45.1 kJ mol−1, which arises from the substantial volume change. These experimental demonstrations are expected to contribute to the application of free-standing β-MnTe in optoelectronic and piezoelectric fields.

열가공제어한 저탄소강의 용접부 기계적성질에 미치는 용접기법의 영향

This study investigated the mechanical properties of thermal mechanical control process steel and obtained the following results. In a welded metal (WM) subjected to both single submerged arc welding (SSAW) and tandem SAW (TSAW), grain boundary ferrite (GBF) was generated along with a prior austenite grain boundary and ferrite side plate (FSP). In particular, TSAW with low basicity and a high heat input yielded coarser results than those of SSAW. In the heat affected zone (HAZ), as the heat input increased, the fraction of low-temperature transformation products such as bainite and acicular ferrite decreased, whereas the fraction of primary ferrites such as GBF, FSP, and polygonal ferrite (PF) tended to increase, and the grain size tended to coarsen. The hardness value of the WM increased in the order of electro gas welding, SSAW, and TSAW regardless of the heat input, and the hardness value of the HAZ tended to increase the softening width and decrease the minimum value with an increase in the heat input. Finally, the joint strength of the weld zone tended to decrease as the heat input increased. However, the joint strength of the weld zone under all the applied welding techniques met the value required by the base metal.

해양구조물용 YS 420MPa 강재 Flux Cored Arc 용접금속의 기계적 성질에 미치는 용접 자세의 영향

This study investigated the mechanical properties of flux-cored arc weld metal of 420-MPa yield-strength (YS) steels for offshore structures based on welding position. Results showed that the recovery rate of alloying elements in the horizontal-position weld metal was higher than that in the vertical-up-position weld metal, and the fraction of low-temperature transformation phases with high dislocation density in the horizontal-position weld metal was high. Consequently, the YS, tensile strength, and impact toughness of the horizontal-position weld metal were higher than those of the vertical-up-position weld metal. In addition, the impact toughness of the weld metal was affected to a greater extent by the microstructure and fraction of the weld metal itself than by the fraction of the martensite-austenite constituents.

Development of an in situ alloying method for high-performance welding processes to achieve an LTT effect by local modification of the alloy content

One possible option for increasing the fatigue strength of welded joints is the use of so-called low transformation temperature (LTT) alloys. The aim is to introduce residual compressive stresses into the weld to counteract crack initiation and propagation. Until now, there has been no application of an LTT effect to high-performance welding processes such as the laser beam submerged arc hybrid welding process (LUPuS hybrid). First, the LUPuS hybrid single-wire process was further developed into the LUPuS tandem hybrid process. This makes it possible to equip the two submerged arc welding torches with different commercially available filler wires. The aim of the work is to further develop the LUPuS tandem hybrid welding process to enable the use of the LTT effect. The in situ alloying process for obtaining the LTT effect from commercially available material combinations was extended to the two-wire process. The alloy obtained was investigated by means of energy dispersive x-ray spectroscopy and hardness measurements and the influence on residual stresses was determined by the borehole method supported by electronic speckle pattern interferometry.

Effect of laser beam wobbling and welding speed in X80 steel wire-fed laser root welds

A wobble laser was combined with a wire feed during laser root welding of X80 pipeline steel. The enlarged laser irradiation area and molten pool resulting from the laser beam wobbling provided a better gap-bridging ability, which is critical during laser welding of thick pipes. Applying laser beam wobbling enabled better control of filler metal dilution, which determined the weld metal microstructure and mechanical properties. In the present work, more bainite along with reduced acicular ferrite was formed in the wobble laser weld, due to the effect of dilution resulting from increased base material melting during wobbling beam, along with an inhomogeneous fusion zone microstructure. The instrumented indentation testing was utilized for the first time to distinguish the local yield strength variation of different regions of the fine weld zones, allowing one to verify if the weld metal strength meet the target value in industrial standards. As the productivity being improved when the welding speed increased from 1.0 to 1.5 m/min, the presence of more low temperature transformation products generated with a faster cooling led to a 26 HV higher hardness and a 70 MPa higher yield strength in the upper region of the fusion zone.

Modelling the Evolution of Phases during Laser Beam Welding of Stainless Steel with Low Transformation Temperature Combining Dilatometry Study and FEM

In this study, the evolution of volume fractions during laser beam welding (LBW) of stainless steel, with a specific focus on incorporating the low transformation temperature (LTT) effect using the dilatometer, has been proposed. The LTT effect refers to the phase transformations that occur at lower temperatures and lead to the formation of a martensitic microstructure, which will significantly influence the residual stresses and distortion of the welded joints. In this research, the LTT conditions are achieved by varying the Cr and Ni content in the weld seam by varying the weld parameter, including laser power, welding speed and filler wire speed. The dilatometer analysis technique is employed to simulate the thermal conditions encountered during LBW. By subjecting the stainless steel samples to controlled heating and cooling cycles, the kinetics of the volume fractions can be measured using the lever rule and empirical method (KOP and Lee). The phase transformation simulation model is computed by integrating the thermal and metallurgical effects to predict the volume fractions in LBW joints and has been validated using dilatometer results. This provides valuable insight into the relationship between welding parameters and phase transformations in stainless steel with the LTT effect during laser beam welding. Using this relationship, the weld quality can be improved by reducing the residual stresses and distortion.

Phase transitions, baro- and piezocaloric effects in single crystal and ceramics of ferroelectric NH4HSeO4

A study of heat capacity, thermal dilatation and sensitivity to hydrostatic and uniaxial pressure was carried out on single-crystal and ceramic samples of NH4HSeO4. The main parameters of low-temperature successive phase transitions B2 (T1) ↔ incommensurate IC (T2) ↔ ferroelectric P1 (T3) ↔ non-ferroelectric did not depend on the type of samples. The behavior of the volumetric strain and the results of direct measurements of T3(p) contributed to the resolution of the longstanding problem associated with the ambiguity of the sign of the corresponding volumetric baric coefficient. The role of thermal expansion anisotropy in the formation of the piezocaloric effect (PCE) near the ferroelectric phase transition at T3 has been studied. Due to the strong difference in the linear baric coefficients, the main contribution to the barocaloric effect (BCE) comes from the inverse intensive and extensive PCE associated with the a-axis. Compared to a single crystal, ceramics demonstrate lower BCE values, which, however, exist in a wider temperature range, which leads to close values of integral caloric parameters. The strong decrease in both BCE and PCE at low-temperature transformations in NH4HSeO4 compared to the ferroelectrics NH4HSO4 and NH4NH4SO4 is associated with a small change in entropy during three low-temperature phase transitions, ΣΔSi = 2.52 J/mol∙K, which is a consequence of a high degree of structural ordering in selenate as a result of a high-temperature transformation at T0 between the superionic and B2 phases, accompanied by a giant change in entropy, ΔS0≈Rln21.

Reduction of distortion during laser beam welding by applying an in situ alloyed LTT effect and considering influencing factors

In rail vehicle construction, austenitic stainless steels are used to achieve lightweight design concepts, as the higher specific strength of the material allows the thickness of parts to be reduced. However, when these are welded to create complex structures, increased welding distortion occurs even with low-heat joining processes such as laser beam welding.In order to reduce distortion, an in situ low transformation temperature (LTT) effect has been achieved using commercially available materials rather than specially manufactured LTT alloys. The LTT effect introduces compressive stresses into the weld seam, which counteract the formation of welding distortion due to tensile stresses. However, in the case of complex structures, several other factors influence the formation of distortion. The influence of the LTT effect and other factors such as tack welding, clamping and cooling conditions were analysed by distortion measurements and a maximum distortion reduction was determined.

Description of an original molecular ordering process into a disordered crystalline form: the atypical low-temperature transformation of the disordered form III of linezolid.

Form III of linezolid was prepared by heating the commercial form above 150 °C and subsequently analyzed upon cooling down to -160 °C, by low- and high-frequency Raman spectroscopy, differential scanning calorimetry and powder X-ray diffraction (PXRD). It was observed that form III was preserved down to 0 °C. At lower temperatures a soft mode was clearly detected by low-frequency Raman spectroscopy associated with the detection of additional Raman bands distinctive of additional intermolecular H-bond interactions. Raman spectroscopy investigations performed in a wide frequency range revealed a continuous transformation characterized by both displacive and order-disorder signatures. By contrast, PXRD highlighted the absence of symmetry breaking, Bragg peaks being still indexed in the same unit cell from room temperature down to -160 °C. Additionally, a significant broadening of Bragg peaks was observed with decreasing temperature interpreted as being a consequence of a distribution of frozen molecular conformations.

Effect of Mo and Mo+Nb Additions on the Phase Transformation and Microstructure of a Developed Low-Carbon CrNiMnB Ultrahigh-Strength Steels with a Preceding Hot Deformation

The influence of molybdenum, and molybdenum with niobium addition on the phase transformation behaviour of a developed low-carbon CrNiMnB ultrahigh-strength steels, was investigated. Gleeble 3800 thermomechanical simulator was employed to simulate the hot-rolling process and to get the dilatation curves. After austenitization at 1250 °C for the complete dissolution of carbides, specimens received 0.6 total strain (i.e., 0.2 at 1100 °C and 2 x 0.2 at 900 °C) followed by cooling at various cooling rates (CRs) in the range of 2-60 °C/s. The final microstructures were investigated using laser scanning confocal microscopy, field emission scanning electron microscopy, and hardness measurements. Then the continuous cooling transformation diagrams were constructed based on the dilatation curves, microstructure, and hardness values. The electrolytic extraction method was used to assess the elements' distribution and the composition of the forming precipitates. The addition of Mo increased the hardenability, decreased the transformation temperatures, and promoted the formation of low-temperature transformation products i.e., martensite and bainite ferrite, at different CRs and inhibit the formation of polygonal ferrite. The formation of coarse precipitates neglected the effect of Mo+Nb addition, decreased the hardenability and expanded the region of BF formation to high CRs. The variation in the hardness with microstructural changes was discussed.

EVIDENCE OF ULTRAHIGH-PRESSURE EVOLUTION OF GARNET PERIDOTITES IN THE POLAR URALS

In garnet peridotites of the Marunkeu eclogite-gneiss complex (Polar Urals), segregations of carbonates in association with chlorapatite were first established. They contain relics of dolomite, which is “replaced” by calcite with symplectite (graphic) intergrowths of brucite and hydrocarbonate. The formation of peridotites with unusual carbonate structures is the result of the evolution of this complex at different depths. During deep subduction and progressive with the formation of garnet peridotites, segregated dolomite experienced a solid-phase breakdown into aragonite and magnesite at the pressure peak (≥5 GPa). During exhumation and retrograde metamorphism, garnet peridotites, together with segregations, underwent low-temperature transformations; aragonite was replaced by calcite, and magnesite by brucite and hydrocarbonate. The presence of carbonates with deep-seated structures confirms that the garnet peridotites and eclogites of the Polar Urals belong to ultrahigh-pressure complexes.

Characteristics of residual stress distribution in wire-arc additive manufactured layers of low transformation temperature material

High heat input in wire and arc additive manufacturing (WAAM) tends to result in high tensile residual stress, which adversely affects performance and service reliability. However, low-transformation-temperature (LTT) materials are susceptible to compressive residual stresses due to their low phase-transformation point. Here, the LTT material 10Cr10Ni was used as the deposited material to fabricate a pipe structure, and the contour method was applied to measure the residual stress distribution in the cross-section. A thermomechanical coupling analysis model was developed to investigate the residual stress generation mechanism in 10Cr-10Ni(LTT) during the WAAM process. The simulated deformation and residual stress distributions agree well with the measurements. Through simulation analysis of the deposition of different layers, we found the key to the generation of compressive residual stresses in the LTT layers lies in whether there is a transformation from austenite to martensite during the final thermal cycle. When the deposition height exceeded 5.405 mm (approximately four layers), compressive residual stresses were only generated in the last four LTT layers and transformed into tensile residual stresses in the other layers. This results in a concept called the effective region, which can be used as a guide for reducing stress in WAAM using LTT materials.

Operando neutron diffraction reveals mechanisms for controlled strain evolution in 3D printing

Residual stresses affect the performance and reliability of most manufactured goods and are prevalent in casting, welding, and additive manufacturing (AM, 3D printing). Residual stresses are associated with plastic strain gradients accrued due to transient thermal stress. Complex thermal conditions in AM produce similarly complex residual stress patterns. However, measuring real-time effects of processing on stress evolution is not possible with conventional techniques. Here we use operando neutron diffraction to characterize transient phase transformations and lattice strain evolution during AM of a low-temperature transformation steel. Combining diffraction, infrared and simulation data reveals that elastic and plastic strain distributions are controlled by motion of the face-centered cubic and body-centered cubic phase boundary. Our results provide a new pathway to design residual stress states and property distributions within additively manufactured components. These findings will enable control of residual stress distributions for advantages such as improved fatigue life or resistance to stress-corrosion cracking.