Solid-state Phase Transformations Research Articles

The simulation of microstructural evolutions in friction stir additive manufacturing

Both recrystallization and solid state phase transformation take key role for the determination of final mechanical properties in friction stir additive manufacturing (FSAM) of titanium alloy. Monte Carlo model is developed to simulate the microstructural changes and a two scale strategy is used to simulate both the recrystallization and the solid state phase transformation in FSAM of duplex titanium alloy. Results indicate that the selection of the building direction can lead to different temperature variations in FSAM due to the different heat accumulations. Lower temperature leads to lower cooling rate in FSAM. This is the reason that the volume fraction of α phase is decreased when the process temperature is decreased. Higher temperature leads to the formation of bigger grains when the rotating speed is increased or the transverse speed is decreased.

Microstructure formation during laser powder bed fusion of Ti-22Al-25Nb with low and high pre-heating temperatures

We compare microstructure formation during laser powder bed fusion (LPBF) of a Ti-22Al-25Nb alloy applying low and high pre-heating build plate temperatures. Fast cooling rates during low-temperature LPBF lead to metastable weakly ordered β phase, i.e., bcc–(Ti,Al,Nb) with pronounced 〈100〉 texture in build direction and nanosized segregations within grains elongated in the build direction. For high-temperature LPBF a Widmanstätten microstructure was observed with lenticular O phase precipitates within the β matrix. Microscopical and in situ high-energy synchrotron diffraction investigations demonstrate that the microstructure formation can be widely explained by the precipitation of O phase from supersaturated β rather than by a sequence of solid-state phase transformations, as it would be expected under thermodynamic equilibrium conditions. A detailed analysis of the microstructural gradient in the subsurface region, however, demonstrates that precipitation from metastable β cannot fully explain the observed microstructures, and that the process energy density, i.e., the intensity of the intrinsic heat treatment of LPBF, plays a relevant role. The investigation of the graded area near the surface of the LPBF materials produced with high pre-heating temperature is particularly interesting as it reveals preferred nucleation of the O phase at subgrain boundaries and dislocations.

Process-structure-property modeling for postbuild heat treatment of powder bed fusion Ti-6Al-4V

Postbuild heat treatment is an important component in optimized manufacturing processing for laser beam powder bed fusion (PBF-LB) Ti-6Al-4V. The development of predictive modeling, based on the understanding of the relationships between process parameters, microstructure evolution, and mechanical properties, is a potentially key ingredient in this optimization process. In this paper, a process-structure-property (PSP) model is developed to predict the effect of postbuild heat treatment on yield strength, which is a key tensile property for PBF-LB Ti-6Al-4V. The process-structure part is developed with a focus on the prediction of solid-state phase transformation, especially dissolution of martensite during the heating phase. Subsequent tensile properties are quantified by a microstructure-sensitive yield strength model based on the predicted microstructure variables. The integrated PSP model is validated via experimentally measured phase fraction, α lath width and monotonic tensile tests on PBF-LB Ti-6Al-4V with different heat treatment temperatures, for identification of optimal process parameters.

New insights of the nucleation and subsequent phase transformation in duplex stainless steel

The solidification behavior, especially involving multi-phase, of duplex stainless steel (DSS) directly determines its microstructures. However, thermodynamic calculations and other reports, which indicates that δ-ferrite is always thermodynamically stable as the primary phase, cannot precisely describe the nucleation and subsequent phase transformations in DSS. Both high temperature confocal microscopy (HTCM) and differential thermal analysis (DTA) are well-established tools for studying solidification behaviors and phase transformation. Here, an in-house build apparatus was employed to investigate the solidification process of DSS. It enables contemporaneous in-situ observation of the surface and thermal analysis of the bulk thermal events. Meanwhile, N2 and Ar are used as protective gas. Dual-peak DTA signals of solidification were recorded under an N2 atmosphere. Furthermore, through the real-time comparison between the DTA signals and the in-situ observations, a solid-state phase transition was accurately located. We propose the following new insight into the solidification mechanism for DSS, by combining experimental results, thermodynamic arguments (Gibbs free energy), and microstructural characterization using electron back scattered diffraction (EBSD): The γ nucleates and grows via a competitive nucleation process, and completely transforms into δ-ferrite in a solid-state phase transformation. After that, part of the δ-ferrite transforms into γ, and the duplex structure remains (L → γ → δ → δ + γ). Heterogeneous nucleation of γ on the liquid surface was also observed, which is quite different from the nucleation and growth pattern of δ-ferrite. These new insights contribute to the improvement of solidification mechanisms of high nitrogen stainless steels and, in turn, microstructures and physical properties of steels in the course of materials production processes.

Microstructure evolution and tribological properties of (TiB+TiC)/Ti–6Al–4V composites fabricated via in situ laser-directed energy deposition of wire and powders in an underwater environment

Titanium-matrix composite-reinforced restored layers are used to repair cracks and dents on titanium alloy shells to impart the deposition layer with the same properties as the base metal. However, there are few studies on their underwater applications. Underwater laser-directed energy deposition is a rapidly-developing technique that shows great application potential for underwater restoration projects. This paper reports the effect of the Ti/(B+C) ratio on the microstructure and mechanical properties of a layer restored underwater using CWPLDED. The results showed that the ceramic-reinforced phase transitioned from a quasi-continuous mesh distribution to a multi-point distribution as the Ti/(B+C) ratio changed. The matrix grain was refined because the reinforcing phase promoted the non-spontaneous nucleation of β-Ti and α-Ti and limited grain growth during both solidification and solid-state phase transformation. The composite layer restored underwater had a maximum ultimate tensile strength of 1231.18 ± 6.37 MPa and a maximum yield strength of 1158.11 ± 8.71 MPa. When Ti: B: C = 3: 4: 4, the improved wear resistance of the restored layer was attributed to the higher microhardness caused by an increased precipitated phase content and Ti matrix grain refinement. When Ti: B: C = 3: 4: 1, the restored layer exhibited better wear resistance due to the excellent spatial configuration of the reinforced phase. The intrinsic relationship between the (B+C) content, reinforced phase space, and dry sliding wear performance was revealed. This provides a design window for preparing (TiB+TiC)/Ti64 composite restored layers with excellent comprehensive performance by CWPLDED and offers new insights for repairing titanium alloys underwater.

Thermo-Mechanical Study of TIG Welding of Ti-6Al-4V for Residual Stresses Considering Solid State Phase Transformation

To overcome the detrimental effect of residual stress in welded joints, which affects the overall performance of the welded structure, this paper studies the magnitude and distribution of residual stress after welding and local post-weld heat treatment (PWHT). The coupled thermo-metallurgical-mechanical model for welding 6 mm thick Ti-6Al-4V (TC4) titanium alloy plates was established, the evolution of the SSPT and its effect on the residual stress were quantitatively analyzed, and a parametric analysis of local PWHT was performed. The results demonstrated that there was good agreement between the numerical results and the experimental data. Due to the cooling rate reaching 327 °C/s, the volume fraction of α、 in the fusion zone (FZ) reached 0.218 after welding and decreased by 90.83% after PWHT when the heating temperature was 700 °C. The peak value of the longitudinal residual stress can reach 686.4 MPa after welding with SSPT, which was 11.38% lower than that without SSPT, and it decreased by 65.6% after PWHT when the heating temperature was 900 °C. The research results demonstrate that SSPT has a significant effect on residual stress, and PWHT can obviously reduce the residual stress, which provides a certain reference for welding TC4 titanium alloy plates.

Morphologies and growth mechanism of (TiNb)C in TiNb-based surface reinforced layer

The TiNb-based surface-reinforced layer, which has a layered structure with various (TiNb)C particles, fabricated by in-situ solid-state diffusion carbonization reaction, (TiNb)C particles were investigated in three-dimensional space. The shape evolution and growth mechanism during fabrication were also discussed. To clarify the relationship between growth mechanism and the morphologies of the particles, the methods of low-temperature brittle fracture and stress corrosion cracking were adopted to investigate the morphological characteristics of particles in each layer. In the surface-reinforced layer, there are three kinds of particles with different sizes and diversified morphologies (quasi nano-sized near-spheroidal, micro-sized fusiform-like, and submicro-sized near-cubic), and the fusiform-like particles in the middle layer have lamellar structure. The results of EBSD indicate that the orientation relationship of each layer exhibits a random-directed-random relationship from outside to inside. The variation of size, morphologies, and orientation relationship is caused by the directed diffusion of carbon atoms and changes in the nucleation-growth mechanism. The analyses reveal that the growth mechanism of the (TiNb)C particles changes from spherical to lamellar and then back to spherical. The solid-state phase transformation condition induces the formation of the special middle part in the surface-reinforced layer, which has a lamellar structure and special < 110 > orientation relationship.

Additive manufacturing of fine-grain fully lamellar titanium aluminide alloys

Additive manufacturing (AM), or 3D printing, has attracted increased attention in producing metallic parts with complex geometries, but it has proved difficult to prepare equiaxed fine-grain parts because the high thermal gradient in solidification commonly conduces the formation of coarse columnar grains. This work shows a solution to fine-grain titanium aluminide (TiAl) alloys by designing a high-frequency thermal cycling to control the solid-state phase transformations in layer-by-layer AM. After solidification, the specially-designed high-frequency thermal cycling can significantly refine the microstructure by repeatedly inducing the nucleation of new grains and suppressing the growth of these newborn fine grains. Therefore, even if solidification leads to coarse columnar grains, equiaxed fine-grain microstructure can still be obtained by solid-state phase transformations. The resulting TiAl alloys have fine heteromorphic grains (∼50 μm) and a fully lamellar microstructure. These fine grains contribute to good strength-ductility balance at room temperature, and their irregular shape and fully lamellar microstructure restrict the flow and distortion of grains at high temperatures, which stands a chance to significantly increase the operating temperature of TiAl parts.

On the underlying material response of pseudoelastic NiTi

In some temperature regimes NiTi can be deformed to strain of 5–7% that recovers fully on unloading. Under tensile loading it traces a closed hysteresis with two stress plateaus associated with the reversible solid-state phase transformation between the austenitic (A) and the martensitic (M) phases, during which the deformation localizes and propagates through the specimen. Hallai and Kyriakides (2013) extracted the underlying softening response associated with A-M transformation from a tensile test on a NiTi strip laminated between two hardening steel face-strips that ensured that it deformed uniformly. The test finishes with the laminate permanently deformed to a strain of 6%. Extraction of the softening response of the M-A transformation requires compressing the thin laminate back to zero strain. The present paper presents a new experimental set-up that enables completion of the load/reverse load test on such a laminate by laterally supporting the specimen during compression to prevent buckling. The extracted response of NiTi exhibits softening branches with up-down-up trajectories for both the A-M and M-A transformations, and Maxwell stresses that match those of the stress plateaus of the tensile hysteresis of a NiTi strip. The extracted response is used to calibrate a custom SMA constitutive model, which when incorporated in a finite element analysis reproduces the measured hysteresis of NiTi nearly perfectly for the first time. Extraction of the complete underlying partially unstable response of SMAs enables modeling of the unstable behavior of SMA structures with the confidence provided by measured data.

Investigation on residual stress of EQ56 high strength steel butt weld

In the welding process of high-strength steel (HSS), the intense temperature change in joints induces the formation of welding residual stress (WRS), which increases the fatigue crack growth rate. Developing an effective finite element (FE) model for accurately predicting the WRS distribution is critical to improve the quality of welded structures. Experimental tests and numerical simulations were carried out to investigate the WRS distribution of EQ56 HSS. The thermal expansion curves at various heating and cooling rates were measured through thermal simulation experiments for achieving the continuous cooling transformation (CCT) diagram of EQ56 HSS. Based on the CCT diagram, the thermo-metallurgical-mechanical (TMM) FE model was developed to consider the influence of solid-state phase transformation (SSPT) on distribution and magnitude of WRS for welded EQ56 HSS joints, as well as material hardening law. Comparison of predicted WRS distribution and experimental data shows that plastic hardening laws of materials significantly affect the precision of WRS distribution, and combined hardening law gives most reliable prediction. The FE model considering SSPT behavior provides more accurate estimation of WRS distribution than those without considering SSPT behavior.

Effect of Pore Formation on Redox-Driven Phase Transformation.

When solid-state redox-driven phase transformations are associated with mass loss, vacancies are produced that develop into pores. These pores can influence the kinetics of certain redox and phase transformation steps. We investigated the structural and chemical mechanisms in and at pores in a combined experimental-theoretical study, using the reduction of iron oxide by hydrogen as a model system. The redox product (water) accumulates inside the pores and shifts the local equilibrium at the already reduced material back toward reoxidation into cubic Fe_{1-x}O (where x refers to Fe deficiency, space group Fm3[over ¯]m). This effect helps us to understand the sluggish reduction of cubic Fe_{1-x}O by hydrogen, a key process for future sustainable steelmaking.

Microstructure, mechanical properties and corrosion resistance of an as-cast fine-structure Cr-Fe-Ni-Al-Si high entropy alloy with Mo addition

A novel Cr-Fe-Ni-Al-Si-Mo high-entropy alloy (HEA) is designed and prepared by arc-melting. This as-cast HEA consists of fine FCC sideplates, as well as BCC phase in the inter-sideplate region containing well-dispersed nanoprecipitates, which is possibly derived from solid-state phase transformation. It exhibits excellent mechanical properties including ultimate tensile strength of > 1 GPa and plasticity of ∼15.5%. It also shows superb corrosion resistance higher than typical stainless steels in 3.5 wt% NaCl solution. Our work indicates that this low-cost HEA has outstanding comprehensive properties in as-cast state, and Mo plays an important role in microstructure, mechanical properties and corrosion resistance of this series of HEAs, which is beneficial for their further applications under complex and harsh environments.

Decomposition of γ-Fe in 0.4C–1.8Si-2.8Mn-0.5Al steel during a continuous cooling process: A comparative study using in-situ HT-LSCM, DSC and dilatometry

Continuous cooling transformation (CCT) diagrams represent roadmaps for producing all heat-treatable steels. CCT curves provide valuable information on the solid-state phase transformation sequence, depending on the defined cooling strategies, the alloying concept of the steel and previous processing steps. The experimental characterization of CCT diagrams is usually done on a laboratory scale applying thermal analysis of dilatometry. In current research studies, however, also other in-situ methods such as high-temperature laser scanning confocal microscopy (HT-LSCM) or differential scanning calorimetry (DSC) are frequently used to investigate phase transformations during thermal cycling. In the present study, HT-LSCM observations and DSC analysis are critically compared with dilatometry results by investigating the CCT diagram of a 0.4%C-1.8%Si-2.8%Mn-0.5%Al (in mass pct.) advanced steel grade. Furthermore, classical examinations by optical microscopy and hardness measurements were performed to support the analysis. In general, very good consistencies between all experimental techniques were identified in determining the transformation start temperature for pearlite, bainite and martensite. The optical microscopy confirmed the observed phase transformations and the results correlated with the measured hardness response. Based on the results, coupling of HT-LSCM and DSC is considered as a valuable novel approach to plot CCT diagrams in future research.

Influence of External Conditions on the Black-to-Yellow Phase Transition of CsPbI3 Based on First-Principles Calculations: Pressure and Moisture

All-inorganic CsPbI3 perovskite has emerged as a promising candidate for next-generation solar cells. However, owing to the poor structural stability of black phase CsPbI3 perovskite at room temperature, it spontaneously transforms to the yellow, photoinactive, nonperovskite phase at room temperature, limiting further development of CsPbI3 perovskite solar cells. To better understand the mechanism driving such undesirable phase transformation, we examine the thermodynamics and kinetics associated with γ-to-δ phase transformation using density functional theory calculation. A solid-state nudged elastic band method was employed to find minimum energy pathways assuming a concerted solid-state phase transformation between these two phases. The gas molecules representing atmospheric (H2O, O2, and N2) and inert (Ar) ambient environments, as well as applied external pressure, were considered to examine the influence of external conditions on the black-to-yellow phase transition. Our calculation reveals that, with increasing pressure, the thermodynamic driving force for converting the γ-phase to the δ-phase increases, whereas a larger activation energy must be overcome for the phase transition to occur. We also investigate the moisture-induced phase transformation with an H2O molecule and its dissociated species (H+/OH–) and demonstrate that the reaction energy barriers can be significantly lowered in the presence of H2O or OH–. On the other hand, other nonpolar species (O2, N2, and Ar) have a negligible effect on the phase transformation kinetics, while they might reduce the thermodynamic driving force for the phase transformation and suppress the undesirable phase transformation. Our theoretical prediction could support recent observations that the γ-to-δ phase transformation occurs rapidly and catalytically in the presence of moisture, whereas in dry argon and dry oxygen atmospheres, the γ-CsPbI3 remains stable.

Phase-field Modeling and Simulation of Solid-state Phase Transformations in Steels

The phase-field method is used as a powerful and versatile computational method to simulate the microstructural evolution taking place during solid-state phase transformations in iron and steel. This review presents the basic theory of the phase-field method and reviews recent advances in the phase-field modeling and simulation of solid-state phase transformations in iron and steel, with particular attention being paid to the modeling of the austenite-to-ferrite, pearlitic, bainitic, and martensitic transformations. This review elucidates that the phase-field method is a promising computational approach to investigate the microstructural evolutions (e.g., interface migration, solute diffusion, and stress/strain evolutions) that take place during the phase transformations. It also indicates that further improvements are required to enhance the predictive accuracy of the phase-field models developed to date. Finally, this review discusses the critical challenges and perspectives for the further improvement of the phase-field modeling of solid-state phase transformations in steel, i.e., the modeling of heterogeneous nucleation, the abnormal effect of the diffusion interface, and material parameter identification.

Modeling of welding residual stress in a dissimilar metal butt-welded joint between P92 ferritic steel and SUS304 austenitic stainless steel

In this study, a butt-welded joint between P92 ferritic steel and SUS304 austenitic stainless steel was fabricated with a nickel-based filler material. Based on Sysweld software, three-dimensional thermal-metallurgical-mechanical coupled finite element analyses were performed to investigate the hardness and welding residual stress in the welded joint. In the material behavior model, various factors influencing the formation and distribution of hardness and welding residual stress were carefully considered, including the austenitic transformation, martensitic transformation, and the tempering of martensite in P92 steel; the isotropic strain hardening, annealing effect, and melting effect of all materials; and the differential thermal expansion between dissimilar metals. The neutron diffraction method and hole-drilling strain gauge method were applied to measure the profiles of residual stress of the welded joint, respectively. Hardness mapping was performed on a weld cross-section specimen. The predicted results of welding residual stress and hardness are in good agreement with the measurements, respectively. The peak residual stresses in longitudinal and transverse directions are above 600 MPa in P92 steel. There are high tensile residual stress and significant cold work near the weld root in SUS304 stainless steel. The solid-state phase transformation induces compressive longitudinal residual stress in the heat-affected zone, high tensile transverse residual stress near the weld toe in P92 steel, and a significant stress gradient. The tempering effect can reduce the hardness and tensile residual stress in the heat-affected zone of P92 steel. The formation mechanism of welding-induced residual stress was illustrated based on the Satoh test. The effects of peak temperature, internal restraint, and differential thermal expansion are found to be decisive for the formation of residual stress. Large tensile residual stress can be generated with internal restraints, which promote the stress increase and lead to material yielding. A restraint factor was proposed for the quantitative assessment of stress evolution.

Microstructure estimation and validation of ER110S-G steel structures produced by wire and arc additive manufacturing

Wire and Arc Additive Manufacturing (WAAM) emerged as a manufacturing process for large scale structures with extensive form and design freedom. WAAM can be fully exploited once the relation between the transient thermal history and its relation to microstructure development and resultant mechanical properties is established. This relation can be further used for computational design tools such as Topology Optimization. This paper presents a model to predict the relation between the thermal history and solid-state phase transformations in a widely applicable High Strength Low Alloy steel ER110S-G. The transient thermal history of parts manufactured by WAAM is modelled using finite element analysis. The modelled thermal history is validated with thermocouple measurements. Our results show that a critical cooling cycle is responsible for the solid-state phase transformation in an AM part. The cooling rate of this particular cooling cycle is superimposed onto an experimentally constructed Continuous Cooling Transformation (CCT) diagram to determine the local solid-state phase fractions. The predicted phase fractions in three wall samples with different design and processing conditions of AM parts are used to predict the hardness. The predicted hardness is 10% higher than the measured hardness of AM samples. The effect of tempering is also considered in the model through JMAK equation. The results show that the tempering is caused in regions with high martensite content and it lowers the hardness by 8 − 10%. The micrographs of the AM parts show that the microstructural features are same for the AM parts with similar critical cooling rates.

Microstructure, mechanical properties and corrosion resistance of an as-cast fine-structured Cr-Fe-Ni-Al-Si high entropy alloy

In this paper, a CrFeNiAl0.28Si0.12 high entropy alloy (HEA) was designed and fabricated by arc-melting. It is found that, this Co-free as-cast HEA is mainly composed of fine FCC and BCC phases that have close correlation with solid-state phase transformation during cooling after solidification. It shows high mechanical properties combining ultimate tensile strength of ∼1130 MPa and plasticity of ∼8%. Moreover, it also has good corrosion resistance comparable to stainless steels. This work provides a novel route for designing high-performance low-cost HEAs with outstanding mechanical properties and corrosion resistance produced simply via casting under relatively low cooling rates, which is beneficial for wide applications of this series of materials under harsh environment.

Evolution of iron-rich intermetallics and its effect on the mechanical properties of Al–Cu–Mn–Fe–Si alloys after thermal exposure and high-temperature tensile testing

Si addition is commonly used to modify the iron-rich intermetallics in Al–Cu–Mn–Fe alloys, which is beneficial to increasing the use of recycled aluminum. Most of the available research has focused on the effect of Si content on the room-temperature mechanical properties of Al–Cu–Mn–Fe alloys. To expand the application of Al–Cu–Mn–Fe–Si alloys as light heat-resistant structural components in the automotive and aerospace industries, it is of great importance to investigate the evolution of iron-rich intermetallics and its effect on the fracture behavior of Al–Cu–Mn–Fe–Si alloys after thermal exposure and high-temperature tensile testing. In this work, the evolution of iron-rich intermetallics and the high-temperature mechanical properties of heat-treated Al-6.5Cu-0.6Mn-0.5Fe alloys with different Si contents after thermal exposure and high-temperature tensile testing were assessed by tensile tests, image analysis, scanning electron microscopy, X-Ray diffraction, transmission electron microscopy, and atomic probe tomography. The results indicate that the Al-6.5Cu-0.6Mn-0.5Fe alloys with 0.1Si and 0.5Si additions have excellent and stable high-temperature mechanical properties after long thermal exposure, which are better than those of most heat-resistant Al alloys. The high performance of the high-temperature mechanical properties is attributed to the high heat resistance of secondary intermetallics and precipitated particles. The addition of Si is detrimental to the strength of Al-6.5Cu-0.6Mn-0.5Fe alloys after long thermal exposure. This can be attributed to the solid-state phase transformation of iron-rich intermetallics from α-Fe to β-Fe, which results in the increase of needle-like Fe-rich phases and Si particles, the agglomeration of secondary intermetallics, and the consumption of Al2Cu phases.

Crystallization and phase-transformation diagrams of Nb-doped CuZrAl metallic glass obtained by fast-scanning calorimetry and in-situ synchrotron XRD upon flash-annealing

The crystallization mechanism and solid-state phase transformations in Cu46.5Zr48Al4Nb1.5 metallic glass upon isokinetic and isothermal annealing are studied by using conventional and fast-scanning differential calorimetry, electric current flash-annealing, electromagnetic levitation, and in-situ high-energy synchrotron X-ray diffraction. Continuous-heating-transformation and time-temperature-transformation diagrams mapping the formation and evolution of the Cu10Zr7, B2 CuZr, τ3 Zr51Cu28Al21 and τ4 Cu2ZrAl phases in dependence on heating rate and temperature are constructed. Cu46.5Zr48Al4Nb1.5 metallic glass shows an increased propensity for the Cu10Zr7 phase formation compared to the parent Cu47.5Zr47.5Al5 glass.