Papers by Keyword: Phase Transformation

Abstract: This study numerically investigates functionally graded (FG) interpenetrating phase composites (IPCs) comprising nitinol (NiTi) shape memory alloy (SMA) microstructure as smart architected reinforcement phase. This architected SMA phase is modeled with FG Schwarz-Primitive (P) triply periodic minimal surface (TPMS that is embedded with Pure magnesium (Mg) as a second-phase elastoplastic material. This unique material combination can provide the superelastic and phase transformation capabilities of NiTi alongside the lightweight and damping properties of Mg material. The functional response and phase transformation characteristics of NiTi SMA are embedded by using an in-house developed material subroutine constitutive model in finite element software Abaqus. The effective properties of the Mg-NiTi FG IPCs are evaluated using a three-unit-cell-based Representative Volume Element (RVE) approach subjected to periodic boundary conditions. The effective functional response includes the elastic stiffness and yield strength, as well as the phase transformation characteristics and martensitic phase evolution of the FG P-TPMS lattices within the IPCs. Additionally, the influence of the concentration of NiTi SMA and functional grading of TPMS structures on stress distribution and phase transformation is thoroughly analyzed. These results are evaluated based on the concentration and grading of NiTi TPMS phase on the FG TPMS IPCs. Results show that increasing NiTi content enhances both the elastic stiffness and strength of the Mg-NiTi composite, with phase transformation initiating at stress-concentrated neck regions of the P TPMS lattice. Whereas the functional grading causes localized stress near regions with minimal cross-sectional area, particularly at the necks between adjacent unit cells, making these zones identified as critical to early transformation and potential failure. This novel FG Mg-NiTi TPMS IPC offers a promising pathway toward lightweight, high-performance multifunctional materials.

Abstract: Nano silica was synthesized using the Stober process with ammonia, ethanol, and tetraethyl orthosilicate (TEOS) solution. Equiatomic titanium-nickel pre-alloyed particles were reinforced with silica nanoparticles of constant volume percent with sizes varying as proceeding 50, 100, 250, and 500 nm. The weighed compositions were mixed in a planetary ball mill, followed by compaction via uniaxial compression of 50 MPa. The resultant green pellets were sintered in an argon atmosphere at 1223K for a period of 4 hrs. Following that, by using EDM, the composite pellets were sectioned, soldered, and cold-mounted. Microstructure was analyzed by optical microscopy, mechanical properties by micro-Vickers hardness testing, and electrochemical analysis by Tafel curves, whereas the effect of particle size at constant volume on the densification was determined via Archimedes' Principle. The reinforcement showed increasing hardness up to 120HV and an increase in phase distribution, in addition to the effect complemented by the transformation of silica, whereas the electrochemical evaluation was affected by both reinforcement and phase distribution. Electrochemical corrosion resistance was measured at 6.88mpy in pure TiNi and 10.93mpy in TiNi nano-silica composite.

Abstract: This paper investigates tribocorrosion properties of binder jet additive-manufactured Co-Cr-Mo (F75) parts. Various parameters, including sintering atmosphere and post heat treatment processing, were examined to understand their effect on open circuit potential, friction, wear coefficient, and hardness. Results demonstrated that samples sintered in N₂-5%H₂ atmosphere have more noble potential up to-0.13V and lower wear coefficient down to 4.85e-6 mm³/N.m in comparison with samples sintered in vacuum. Solutionizing and aging (SHT-A) significantly increases hardness up to 626HV and lowers wear coefficient which means that the sample is more resistant to wear compared to as-sintered (AS) samples. However, heat-treated samples present slightly lower initial potential which means that these samples are more chemically active. This is because of the phase transformation of the matrix from FCC Co (γ phase) in AS condition to HCP Co (ε phase) + Co-Cr intermetallic (σ phase) in SHT-A condition, and different precipitate (carbides and nitrides) formation between these samples.

Abstract: Fire-clay blocks were prepared using clay-size sediments from Phong River sieved through mesh number 200 (74 μm) mixed with Rice Husk Ash (RHA) at various ratios of 1:1, 1:2, and 1:3. The clay blocks were cast in a cylindrical mold and fired at 900, 1,000, 1,100, and 1,200°C. A uniaxial compressive test was carried out for all types of specimens acquired from different firing temperatures. In addition, imaging characteristics of the samples were also analyzed using various spectroscopy techniques. Results showed that the compressive strength, and elastic modulus of fired-clay blocks present a linear relationship when compared between two firing temperatures of 900 and 1,200°C but dramatically fluctuates at firing temperatures between 1,000 and 1,100°C. The primary conclusion is that the strength of the fired-clay blocks is governed by the quartz-tridymite-cristobalite phase transformation. It was found that cristobalite transforms from tridymite at temperatures as low as 1,100°C which strongly disagrees with the theoretical temperature of 1,470°C. This can be explained from the presence of RHA in the composite. When sintered, organic carbon in RHA changes into carbon dioxide gas and volatile matters leaving interstitial voids that allow the vibrating atoms of the silica to realign into more open cubic crystal lattices of the cristobalite form. Only when the firing temperature reaches 1,200°C where cristobalite transformation is complete does the strength relationship become more linear with firing temperature.

Abstract: The quenched-in microstructures of Ti-15at%Nb-1at%O alloy after solid solution treatment (SST) in β phase for different SST times were investigated. In the as-rolled sample before SST, the α phase formed at the grain boundaries, and coarse martensite laths of α" phase formed in the grains. In the sample after SST for the time from 0.3 to 2.4 ks, a bundle-microstructure containing α" phase laths nucleating in the same crystallographical direction was formed. In the sample with SST for 4.8ks, the α" phase laths did not form in the area at a certain distance away from the grain boundaries, and the β+ω phase formed in that area. The rest of the areas were covered by the acicular laths of α" phase. The sample after SST for 10.8 ks exhibited the acicular laths of α" phase formed uniformly in the grains. The inhomogeneous oxygen distribution would significantly affect the microstructure formation of an oxygen-containing Ti-Nb alloy.

Abstract: A novel two step austenitization heat treatment process was conceived to develop ADI with optimum combination of strength, ductility and fracture toughness. This novel heat treatment process involved austenitizing the ductile iron in the lower intercritical temperature range and then raising the temperature to the fully austenitic temperature range followed by austempering in the bainitic temperature range. The resulting microstructure consisted of very fine scale bainitic ferrite, high-carbon austenite and pro-eutectoid ferrite. The proeutectoid ferrite is nucleated because of prior austenitization in the intercritical range which resulted in ADI with high fracture toughness without significantly compromising the strength and ductility. An analytical theory has been developed based upon the nucleation of proeutectoid ferrite and graphite nodules during intercritical austenitization, to explain this physical outcome.

Abstract: This study examines the corrosion behavior of 2507 super duplex stainless steel (SDSS) subjected to artificial aging at various temperatures. Using optical microscopy, SEM, EDS, and XRD, we characterized the microstructural and phase changes. Electrochemical tests, including Tafel polarization and EIS, assessed corrosion resistance in 3.5 wt. % NaCl solution. Results show that aging at 1200°C enhances corrosion resistance, attributed to the dissolution of secondary phases and an increase in ferrite content. The optimized microstructure achieved at this temperature displayed the lowest corrosion current density and highest polarization resistance, offering insights for improving SDSS durability in corrosive environments.

Abstract: Jominy end quench test is a standardized metallurgical experiment for obtaining data on steel hardenability. Construction of numerical simulation of the test provides a way for parameterizing and validation of numerical models using the experimental data. In the current work we present the coupled heat transfer, conduction and phase transformation model, which allows for calculation of phase fractions at different positions at the Jominy test piece, and includes the latent heat released by the phase transformations. Also, the temperature and phase fraction dependence of the thermal conductivity is included in the calculation.

Abstract: A systematic study has been made on a Cu-40%Zn alloy treated by an electric current pulse (ECP) and by the examination of the microstructure and the crystallographic features of both the parent and the product phases. The β precipitates under ECP show a Kurdjumov Sachs Orientation Relation (K-S OR) in the vicinity of the grain boundaries (GBs), but a Nishiyama Wasserman (N-W) OR within the grains. Along the GBs the {111}_α /<110>_α dislocation arrays were spotted, whereas the {111}_α /<112>_α stacking faults were observed in the grain interiors. A closer examination of the lattice strain required for the phase transformation revealed that the maximum lattice deformation under the K-S OR is a shear on the {111}_α plane in the <110>_α direction. The dislocations arrays existing along the GBs offer the pre-strain that favors the precipitation of β particles obeying the K-S OR. Oppositely, the stacking faults within the grains provide pre-stains for the formation of the β precipitates respecting the N-W OR. This study sheds some light on the mechanisms by which crystal defects initiate phase transformation in a Cu-40%Zn alloy.

Abstract: The Fe-0.44%C-1.8%Si-1.3%Mn-0.82%Cr-0.28%Mo steel treated by the quenching-partitioning process showed a product of strength and elongation of 30 GPa×% with the yield stress of 1150 MPa. The influence of the partitioning time on the structure and mechanical properties is discussed. It is shown that the volume fraction of retained austenite depends on carbon content in the solid solution of primary martensite. The formation of bainite leads to no deterioration of mechanical properties.