Dual-phase Region Research Articles

Effect of intercritical annealing and subsequent ageing on the microstructure and mechanical properties of a medium Ni–Mn low carbon steel

The aim of this research was to develop a medium Ni–Mn dual phase steel with a combination of relatively high strength and good formability for industrial applications. Accordingly, this research was conducted to examine the effects of intercritical annealing and subsequent ageing treatments on the microstructure and mechanical properties of an Fe-9.5Ni-6.5Mn (wt.%) low carbon martensitic steel. Electron back scattering diffraction (EBSD) analysis revealed that intercritical annealing treatment at 610 °C in the ferrite + austenite (α+γ) dual phase region for various holding times led to formation of the reversed austenite phase in the martensite matrix, producing a medium Ni–Mn dual phase steel. It was resulted that the volume fraction of austenite and its grain size were significantly increased with increasing the holding time. Also, high resolution transmission electron microscope (HRTEM) images illustrated the formation of nano-sized θ-NiMn precipitates within the martensite phase after subsequent ageing at 450 °C for 7.2 ks. Intercritical annealing followed by ageing yielded a good combination of an ultimate tensile strength (∼1160 MPa) and fracture strain (∼11.5%) in the developed dual phase steel compared to the initial martensitic steel. The high strength and good ductility of the developed dual phase steel were respectively attributed to the presence of θ-NiMn precipitates with a hard character and reversely formed austenite with a ductile nature in the microstructure. Furthermore, the fully brittle fracture mode in the aged initial steel changed to a mixture of brittle and ductile behavior in the dual phase steel produced by intercritical annealing and subsequent ageing.

Temperature influenced microstructure evolution and strengthening of 4.5 vol% TiBw/TA15 composites with columnar-reinforced structure fabricated by pre-sintering and hot extrusion

In this paper, 4.5 vol% TiBw/TAl5 composite with columnar network reinforced structure was prepared by pre-sintering and subsequent hot extrusion process. Pre-sintering was conducted at 1100 °C for 0.5 h, and billets were extruded with an extrusion ratio of 10: 1. The extrusion speed was 10 mm/s, and water cooling was adopted. An extrusion temperature range of 925–1100 °C (which covers the phase transition) was used to study the influence of the extrusion temperature on the microstructure evolution and its correlation with strengthening. The results showed that the extrusion temperature played a decisive role in the evolution of the matrix morphology. When the extrusion temperature increased from the (α + β) dual-phase region to the β single-phase region, the microstructure of the composite matrix was mainly composed of elongated α phase and recrystallized α phase. It gradually transformed into a lamellar structure from a high-temperature β phase. TiB whiskers continued to grow and coarsen. The corresponding texture also changed from 〈10−10〉 α//ED component in the dual-phase region to 〈0001〉α//ED and 〈10−11〉 α//ED mixed components, which were related to the high-temperature 〈101〉β//ED component in the β-phase region. Further performance analysis showed that the matrix strength was positively correlated with the content of the lamellar α phase, which increased with the extrusion temperature; however, the strengthening of TiB whiskers was affected by its growth and fragility and displayed a weakening trend. As a result, although the strength of the composite was always better than that of the matrix, its overall strength decreased upon increasing the extrusion temperature, showing the best performance when extruded at 925 °C. The stress of strengthening effect could be accurately estimated by the shear lag model.

Constitutive and microstructural characteristics of Ti-5Al-2.5Sn alloy during isothermal and non-isothermal multi-stage hot deformation across different phase regions

The flow behaviors and microstructural evolutions during multi-stage hot deformation are essential aspects in the processing of high performance titanium alloys. In the present work, both isothermal multi-stage deformation (IMD) and non-isothermal multi-stage deformation (NMD) over α + β and single β phase regions were employed to study the constitutive behaviors and microstructural evolutions of a near- α Ti-5Al-2.5Sn alloy. The results showed that the flow stress, restoration fraction in α + β phase region of NMD were lower than that of IMD, because that the predominant lamellar microstructures formed during NMD increased α/β boundaries and thereby promoted grain boundaries gliding. In β phase region, the flow behaviors showed slight differences with temperature and deformation passes during both IMD and NMD processes. Clear transitions were observed from the mean flow stress between different phase regions during NMD. In addition, an obvious α + β → β dynamic phase transformation occurred during deformation at 990 °C, which was identified to be a displacive nucleation and diffusion-controlled growth process. In addition, numerous FCC-Ti precipitations were observed during IMD at 960 °C and strain rate of 1 s −1 with the orientation relationship of (0 0 0 1) HCP matrix //{1 1 1} FCC . • Isothermal and non-isothermal multi-stage deformation across different phase regions were performed on a Ti-Al-Sn alloy. • The flow stress during isothermal multi-stage deformation in dual phase region was higher than that of non-isothermal ones. • Clear transitions were observed from the mean flow stress between different phase regions during non-isothermal deformation. • An obvious α→β dynamic phase transformation occurred during isothermal multi-stage deformation at 990 °C. • Numerous FCC-Ti precipitates were observed during isothermal multi-stage deformation at 960 °C.

Microstructure Evolution and Deformation Mechanisms of As-Cast Antibacterial Ti6Al4V-5Cu Alloy for Isothermal Forging Process.

The hot workability behavior of antibacterial Ti6Al4V-5Cu alloy was investigated using a hot compression experiment in the temperature range of 790–1040 °C and strain rate of 10−3–10 s−1 with a strain of 0.4. The deformation behavior of the alloy was characterized by Gleeble 3800 compression experiment, and the relationship among deformed microstructures and deformation parameters was established. The deformations of Ti6Al4V-5Cu alloy were temperature and strain rate-dependent. Higher temperature and lower strain rate made power dissipation efficiency (η) increase and reach 89%. The activation energies (Q) in the dual-phase (α + β) and single β phase regions were calculated as 175.43 and 159.03 kJ mol−1, respectively. In the dual (α + β) phase region, with an increase in strain rate, flow-softening behavior was dominated, however in the single β phase region such as processing at 940 °C. Flow stress increased slightly in which work-hardening behavior was dominated (especially between strain rates of 10−3–1 s−1). The deformation at various conditions exhibited different stress-strain profiles, providing an insight that work hardening and flow softening coexisted in Ti6Al4V-5Cu alloy. The relative intensity of oscillatory change in flow stress profile decreased as the strain rate decreased. The hot workability of Ti6Al4V-5Cu alloy was also accessed from the viewpoint of the sub-grain structure.

Nanoscale origin of the crystalline-to-amorphous phase transformation and damage tolerance of Cantor alloys at cryogenic temperatures

Based on recent experimental studies, crystalline-to-amorphous phase transformation has been proposed as a mechanism to enhance the damage tolerance of Cantor alloys at cryogenic temperatures. In this study, we provide atomistic insights, via molecular dynamics simulations, into the origin of the solid-state amorphization ahead of a crack tip, and report the deformation mechanisms contributing to cryogenic damage-tolerance. We show that the amorphization stems from the formation of multi-dislocation junctions due to the low stacking fault energy. This leads to high lattice resistance to dislocation glide and facilitates nucleation of amorphous nuclei. The deformation mechanisms in the amorphous/crystalline dual phase regions include high-density Shockley partial dislocations and multi-dislocation junctions in the crystalline region, as well as radiation-shaped shear bands and amorphous bridges in the amorphous region, which are rarely found in conventional alloys at low temperature. The amorphous bridges lead to crack shielding. Furthermore, altering the chemical composition changes the work-of-fracture and hence the damage tolerance. The Rice-criterion ductility (ratio between surface energy and unstable stacking fault energy) is an important factor affecting the degree of amorphization, which is useful for the mechanics-based design of Cantor alloys.

中Mn鋼の中断焼入-二相域焼鈍により形成されるコア-シェル型第二相組織の制御

Medium manganese steel (Fe-5.0%Mn-1.2%Si-0.10%C alloy) was subjected to interrupted quenching from the austenite single-phase region to a temperature between Ms and Mf followed by intercritical annealing in the ferrite and austenite dual-phase region at 923 K. As a result, a core-shell type second phase, which consisted of a fresh martensite core surrounded by a film-like retained austenite shell, was formed. The mechanism and kinetics of reversion for the interrupted-quenched specimens were analyzed with DICTRA simulation and TEM observation. With regard to the effect of the core-shell type second phase on mechanical properties, it was inferred that the fresh martensite core functioned as a hard second phase and enhanced work hardening by stress partitioning similar to DP steel, while the film-like retained austenite contributed to improved ductility due to the TRIP effect. As the interrupted quenching temperature decreased, the volume fraction of the fresh martensite core decreased, while the stability of the retained austenite shell increased. This showed potential for controlling the strength and ductility balance of medium manganese steel. A possible beneficial effect of the core-shell type second phase on the ductile fracture behavior was also discussed in terms of stress/strain relaxation at the interfaces between hard martensite and ferrite matrix.

Effect of hydrogen on the mechanical properties and fracture modes of annealed 430 ferritic stainless steel

The change of fracture morphology after traditional electrochemical hydrogen charging and the microstructure of the hot-rolled AISI430 ferritic stainless steels (FSSs) specimens annealed at different temperatures (720°C–1060 °C) was characterized by scanning electron microscopy (SEM) and electron backscatter diffraction (EBSD) technology. We systematically illustrated the effect of annealing temperature on its hydrogen embrittlement sensitivity, hydrogen-induced mechanical property changes, and the transformation of microstructure on the change of hydrogen-induced fracture mode. The experimental results prove that: the hydrogen embrittlement susceptibility of FSSs increases significantly from the α single-phase region to the α+β dual-phase region. Due to the presence of intergranular carbides, the annealed specimens in the α single-phase region still retain high plasticity during the tensile test after hydrogen charging, and the yield strength is significantly improved, which is attributed to the change of the strain field around the intergranular carbide by hydrogen. In the presence of hydrogen, the formation of crack holes of annealed specimens in the α single-phase zone and the formation of quasi-cleavage (QC) + cleavage fracture (C) morphology of annealed specimens in α+β dual-phase zone are the result of the synergistic effect of hydrogen enhanced localized plasticity (HELP) and hydrogen enhanced decohesion (HEDE) mechanisms. When the annealing temperature reaches 940 °C, transgranular secondary cracks were observed in the vicinity of the fracture of the annealed specimen, which mainly propagate towards {001}//ND-oriented ferrite grains, accompanied by the high strain zone of the crack boundary.

Hot deformation behavior and flow stress modeling of a novel CoNi-based wrought superalloy

The thermal deformation behavior and flow stress modeling of a novel CoNi-based wrought superalloy after forging was investigated at γ' sub-solvus temperature (1050 °C and 1075 °C) and γ' super-solvus temperature (1100 °C and 1125 °C) with strain rate range from 0.001 s−1 to 0.1 s−1 under a 50% strain. Scanning electron microscopy (SEM), electron backscatter diffraction (EBSD) and transmission electron microscope (TEM) techniques were used to characterize the microstructures and explain the flow behavior. The results show that the flow stress increases with decreasing temperature and increasing strain rate. Dynamic recrystallization (DRX) occurs under all deformation conditions. When the alloy was deformed sub-solvusly at 1075 °C/0.01 s−1 and 0.001 s−1 conditions, the γ' phase dissolved resulting in substantial grain growth. The constitutive equations were established via the flow stress data with the respect to the γ' phase dissolution. The activation energies for the γ + γ' dual-phase region and the γ single-phase region of the studied alloy are about 650 and 390 kJ/mol, respectively, which is comparable with traditional Ni-based superalloys.

Peculiar microstructural evolution and tensile properties of β-containing γ-TiAl alloys fabricated by electron beam melting

The microstructure and tensile properties of β-containing Ti–44Al–4Cr alloy rods additively manufactured by electron beam melting (EBM) process were examined as a function of input energy density determined by the processing parameters. To the best of our knowledge, this is the first report to demonstrate that two types of fine microstructures have been obtained in the β-containing γ-TiAl alloys by varying the energy density during the EBM process. A uniform α 2 /β/γ mixed structure containing an α 2 /γ lamellar region and a β/γ dual-phase region is formed at high energy density conditions. On the other hand, a lower energy density leads to the formation of a peculiar layered microstructure perpendicular to the building direction, consisting of a ultrafine α 2 /γ lamellar grain layer and a α 2 /β/γ mixed structure layer. The difference in the microstructures originates from the difference in the solidification microstructure and the temperature distribution from the melt pool, which are dependent on the energy density. Furthermore, it was found that the strength of the alloys is closely related to the volume fractions of the β phase and the ultrafine α 2 /γ lamellar grains which originates from the massive α grains formed by rapid cooling under low energy density conditions. The alloys with high amounts of these peculiar microstructures exhibit high strength comparable to and higher than the conventional β-containing γ-TiAl at room temperature and 1023 K, respectively.

The texture evolution related to the interactions between α and β phases in dual-phase region annealing of a near α titanium alloy pipe after Pilger cold rolling

This work focused on the texture evolution related to the interactions between α phases and β phases for a Ti80 Pilger cold rolling pipe during dual-phase region annealing of 1223 K, 1243 K and 1263 K. The key observation was that the annealing temperature significantly affected the components and intensities of the texture. At 1223 K, the main factor affecting the texture evolution was the globularization of the primary α phases (αP), and the globularization of αP was greatly influenced by the crystallographic orientation in reference to the loading direction during cold rolling. Those globularized αP maintaining the Burgers orientation relationship (BOR) with the residual β phases were more prone to growing up. At 1243 K, more αP were dissolved and residual β phases grew up accompanying by the precipitation of more secondary α phases (αS). The texture was closely related to the preferential growth of the β phase due to the same crystallographic orientation of αS with adjacent αP grains. Those β phases suffering little deformation during the cold rolling were more likely to grow up since the initial volume advantage and the maintaining of BOR with αP. The β phases undergoing larger deformation would be pinned by αP during the growing up, which was attributed to the recrystallization of β phases and the broken of BOR with αP. At the annealing temperature of 1263 K, the degree of variants selection (DVS) of αS was significantly reduced due to the dissolution of abundant αP during β→αS transformation, thus the texture intensities were reduced.

The effect of inter critical heat treatment on mechanical and wear properties of AISI 1015 steel

ABSTRACT Dual-phase (DP) steels mainly consist of ferrite and martensite phases. In this paper, DP steel was developed by inter-critical heat treatment followed by water quenching using AISI 1015 steel which is a type of plain low carbon steel. After developing DP steel, the mechanical and wear properties of AISI 1015 have been investigated. Inter-critical heat treatment was done at four different temperatures 740°C, 770°C, 800°C, 830°C in a muffle furnace. After SEM, it was found that the percentage of martensite increases with the heating temperature in the dual-phase region followed by water quenching. The results revealed that inter-critical heat treatment increased the tensile and hardness properties but decreased the ductility as the temperature increased. For wear properties, it was concluded that the specimen heat-treated at higher temperature showed a higher wear rate due to lower ductility. This was due to higher ductility at a lower temperature in comparison to lower ductility at a higher temperature. So, basically, the percentage of martensite affects the properties of dual-phase steel.

Temperature Increase during Isothermal Forging of Ti-5Al-2Sn-2Zr-4Cr-4Mo Alloy Using a 1500-Ton Forging Press

In this study, the temperature increase of the Ti-17 alloy (Ti–5Al–2Sn–2Zr–4Cr–4Mo, wt.%) during isothermal forging in the (α + β) dual-phase region was investigated using large-size workpieces forged between hot dies in a 1500-ton forging press. The temperature increase was predicted using finite element analysis (FEA). The tip of a thermocouple was inserted into the center of the workpiece (diameter: 100 mm; height: 50 mm). The forging temperatures were 1023 K (750 °C) and 1073 K (800 °C) in the (α + β) dual-phase region. The strain rate was 0.05 s−1 and 0.5 s−1 at each temperature. Meanwhile, the compression percentage was 75%. The true stress–true strain curves were inversely obtained by fitting the load–stroke curves using the FEA. The predicted temperature was in a good agreement with that in the experimental results when the value of 1.0 was used as the conversion factor for plastic deformation energy to heat energy in the FEA under the current forging conditions. At the strain rate of 0.5 s−1, the temperature instantly increased to a β-transus temperature in 3 s at 1073 K (800 °C). In contrast, the temperature logarithmically increased at both 1023 K (750 °C) and 1073 K (800 °C) at 0.05 s−1 in 28 s (e.g., 42 K at 1023 K (750 °C)). The obtained true stress–true strain curves indicate that flow softening occurred during the forgings, which is attributed to dynamic recrystallization and/or dynamic recovery. The temperature increase in the Ti-17 alloy was smaller than that in the Ti–6Al–4V alloy under the same forging condition.

Effect of Strain Rate on the Microstructure and β-Texture Evolutions in β-Processed Forging of a Near-β Titanium Alloy

The effect of strain rate on the β texture evolution during two-step hot forging of Ti-6246 alloy was investigated. The two-step forging consisted of 15% or 50% prior-β forging at 980°C and subsequent 60% or 25% forging at 870°C in the (α + β) dual-phase region. The total compression ratio was 75%, and the investigated strain rates were 0.01 and 1.0 s−1. The β forging texture showed typical {001} and {111} body-centered cubic textures. With increasing compression ratio in the (α + β) region and at a strain rate of 0.01 s−1, the amount of precipitated α phase increased. Dynamic recrystallization was rarely observed after forging in the (α + β) region at a strain rate of 0.01 s−1. Large amounts of α precipitates lowered the {001} β texture intensity through slip transmission between the α and β phases under the Burgers orientation relationship. However, in specimens forged at a strain rate of 1.0 s−1, as the compression ratio in the β single-phase region increased, the growth of dynamic-recrystallized β grains was promoted at the prior-β grain boundaries, where α-phase precipitation was not substantial. These effects resulted in a higher {001} texture intensity of the β phase in specimens forged at 1.0 s−1 compared with that of the β phase in specimens forged at 0.01 s−1.

Nanoscale Origin of the Crystalline-to-Amorphous Phase Transformation and Damage Tolerance of Cantor Alloys at Cryogenic Temperatures

Based on recent experimental studies, crystalline-to-amorphous phase transformation has been proposed as a mechanism to enhance the damage tolerance of Cantor alloys at cryogenic temperatures. In this study, we provide atomistic insights, via molecular dynamics simulations, into the origin of the solid-state amorphization ahead of a crack tip, and report the deformation mechanisms contributing to cryogenic damage-tolerance. We show that the amorphization stems from the formation of multi-dislocation junctions due to the low stacking fault energy. This leads to high lattice resistance to dislocation glide and facilitates nucleation of amorphous nuclei. The deformation mechanisms in the amorphous/crystalline dual phase regions include high-density Shockley partial dislocations and multi-dislocation junctions in the crystalline region, as well as radiationshaped shear bands and amorphous bridges in the amorphous region, which are rarely found in conventional alloys at low temperature. The amorphous bridges lead to crack shielding. Furthermore, altering the chemical composition changes the work-of-fracture and hence the damage tolerance. The degree of amorphization can be interpreted in terms of the Rice-criterion ductility parameter (ratio between surface energy and unstable stacking fault energy), which is useful for the mechanics-based design of Cantor alloys.

Phase evolution and mechanical properties of an intercritically-annealed Fe–10Ni–7Mn (wt. %) martensitic steel severely deformed by high-pressure torsion

Experiments were conducted on an Fe–10Ni–7Mn (wt. %) martensitic steel processed by high-pressure torsion (HPT) after intercritical annealing in the dual phase (α+γ) region at 600 °C for 2 h. Microstructural examination and phase analysis using electron backscatter diffraction (EBSD) confirmed that post-HPT processing led to a phase transformation of austenite to α′-martensite (γ→α′ and γ→ε→α′) such that after 20 turns of HPT the microstructure included multi-phases of α′-martensite, austenite and ε-martensite. The HPT processing also produced a significant reduction in the α′-martensite and austenite grain sizes from initial values of ~3.8 and ~3.9 μm to ~225 and ~60 nm after 20 HPT turns, respectively, while the ε-martensite final grain size was ~44 nm. Micro-hardness measurements and tensile testing showed a significant increase in hardness and in the ultimate tensile strength to about 680 Hv and 2235 MPa after 20 turns, respectively. Finally, the fracture surfaces of samples after HPT processing showed a combination of dimples, cracks and shear bands which confirmed a transition in fracture behavior from ductile to a mixture of a ductile and brittle mode.

β-Texture Evolution of a Near-β Titanium Alloy During Cooling After Forging in the β Single-Phase and (α + β) Dual-Phase Regions

β-Texture evolution during cooling after forging in the β single-phase region and (α + β) dual-phase region was investigated in Ti-6Al-2Sn-4Zr-6Mo alloy. Typical {001} and {111} β fiber textures of the β phase developed during forging. Dynamic-recrystallized β grains formed and inherited the orientation of their parent grains during forging in the β single-phase region. The {001} texture was enhanced with decreasing cooling rate after forging in the β region, that is, as cooling time in the β region increased. The results of the post-forging holding in the β region demonstrated that the recrystallized grains that grew in the β region consumed the adjacent {111}-oriented parent grains and initially enhanced the {001} texture. However, the enhancement of the {001} texture was moderated with increasing holding time because of the increased orientation deviation between the recrystallized grains and the parent grains. In addition, the {001} β texture was weakened by the growth of static-recrystallized grains after its incubation period during the holding time. These results indicate that growth of dynamic-recrystallized and static-recrystallized grains moderated the increase of the {001} texture intensity at low cooling rates of specimens forged in the β region. In specimens forged near the β-transus temperature and in the (α + β) dual-phase region, the {001} texture intensity was only slightly affected by the cooling rate. Recrystallization of the β phase was suppressed by the precipitation of the α phase during forging and cooling at low temperatures. A slow cooling rate of − 2 °C/s was determined by measuring the temperature change of a large specimen similar in size to those used in real-world applications.

Amphiphilic micelle-forming PDMS-PEGBEM comb copolymer self-assembly to tailor the interlamellar nanospaces of defective poly(ethylene oxide) membranes

An amphiphilic comb copolymer was synthesized via free-radical polymerization using two macromonomers: monomethacryloxypropyl-terminated poly(dimethyl siloxane) (PDMS-MA, 10,000 g mol−1) and poly(ethylene glycol)–behenyl ether methacrylate (PEGBEM, 1500 g mol−1). The PDMS-PEGBEM comb copolymer underwent self-assembly to form micellar structures in ethanol/tetrahydrofuran solvent. The micelle-forming PDMS-PEGBEM copolymer was incorporated into the low-cost, commercially available polymer, and non-selective polymer poly(ethylene oxide) (PEO, 106 g mol−1), resulting in highly selective all-polymeric membranes. The alignment of the PDMS-PEGBEM micelles in the interlamellar region of PEO and the corresponding increase in the domain spacing were observed by transmission electron microscopy and small-angle X-ray scattering, confirming the successful encapsulation of PDMS-PEGBEM in the interlamellar region. The PDMS functions as a rubbery filler between the crystallites, whereas PEGBEM has good affinity towards PEO because of their chemical similarity. The CO2/N2 and CO2/CH4 selectivities of PEO membranes was significantly enhanced from 0.78 and 0.71 to 41.7 and 13.3, respectively, on the loading of 10 wt.% of the copolymer. The highest CO2 permeability reached 240 barrer at 50 wt.% copolymer loading, with the CO2/N2 and CO2/CH4 selectivities of 34.6 and 12.2, respectively. Herein, the evolution of the interlamellar nanospaces and dual-phase region, as well as the microstructures of the membranes are reported.

Typical Microstructural Characteristics of Ti–5Al–5Mo–5V–3Cr–1Fe Metastable β Ti Alloy Forged in α + β Region

In this study, typical microstructural characteristics of a metastable β Ti alloy (Ti–5Al–5Mo–5V–3Cr–1Fe) forged in a dual-phase region (strain of 54% at 820 °C) were investigated in detail by the combined use of X-ray diffraction, energy dispersive spectroscopy, electron channeling contrast imaging and electron backscatter diffraction techniques. Results show that the microstructure of the forged alloy is composed of bulk α grains, α plates and β matrix. The bulk α grains correspond to retained primary α phase (αp, average grain size ~ 2.4 μm), while the α plates are secondary α phase (αs, width ~ 70 nm) precipitated from the β matrix during air cooling. During forging, the β matrix experiences dynamic recovery with many subgrains and significant orientation gradients formed. Analyses of the orientation relationship between the α and β phases show that the Burgers orientation relationship is not maintained between some αp and β phases, which should be related to thermal deformation-induced changes of their orientations. In contrast, all of the αs plates are found to maintain well the Burgers orientation relationship with the β phase.

New Zr–25Ti–xMo alloys for dental implant application: Properties characterization and surface analysis

A series of Zr–25Ti–xMo (x = 0, 2.5, 5, 7.5, 10 and 12.5 wt %) as-cast alloys were designed to advance a new-brand Zr-based alloy with low Young's elastic, moderate compression strength, superior corrosion resistance and good wear behavior for the application of dental implant materials. In the present study, the microstructures of the alloys were evaluated by X-ray diffraction, scanning electron microscopy and X-ray photoelectron spectroscopy. The microhardness and uniaxial compression measurement were performed to evaluate the mechanical properties of the alloys. The electrochemical behaviors of the alloys was investigated in artificial saliva solution. The friction and wear performances of the Zr–25Ti–xMo alloys were examined by pin-on-disk under the load of 15 N. The results shows that the crystalline structure transforms from hexagonal close-pack (α phase, x = 0 wt %) to body-centered cubic structure (β phase, 5 ≤ x ≤ 12.5 wt %) through a α + β dual-phase region (x = 2.5 wt %). They exhibits excellent mechanical properties with low elastic modulus (from 17.7 to 24.4 GPa) and moderate compression strength (from 1154.4 to 1310.8 MPa). The Zr–25Ti–xMo alloys possess good corrosion resistance due to the formation of a protective passive film consisting mainly of ZrO2, TiO2 and minor MoO3. Especially, the polarization curves demonstrates that the Zr–25Ti–7.5Mo alloy has a wider passive region than the other five alloys and it possesses the lower corrosion current density and corrosion rate. Furthermore, this alloy exhibits good abrasion resistance with the lowest coefficients of friction and volume wear loss. Thus, the Zr–25Ti–7.5Mo alloy has a combination of excellent mechanical properties such as low elastic modulus, moderate yield strength and the compression strength, good electrochemical stability and well wear resistance, it is considered a promising candidate for orthopedic materials as dental implant.

Acquiring a low yield ratio well synchronized with enhanced strength of HSLA pipeline steels through adjusting multiple-phase microstructures

Intercritical treatments within the dual-phase (α+γ) region were applied on HSLA pipeline steels, for acquiring a low yield ratio (YR) well balanced with desirable strength. Intercritical cooling treatment (ICT), step cooling treatment (SCT) as well as direct cooling treatment (DCT) after full austenization were designed to obtain an optima multiphase microstructure. Effects of cooling rate in DCT routine and intercritical temperatures in ICT and SCT routines on microstructural evolution and corresponding mechanical properties were also investigated. Especially, the SCT treatment applied with an intercritical temperature of 750 °C produces a microstructure composited of “soft” coarse polygonal ferrite, and “hard” acicular bainite and lath martensite containing large amounts of dislocation tangles or networks generated by deformation. Such multiple phase constituents guarantee the high strength and remarkable ductility on deformation, meanwhile cleavages propagation is hindered by the high-angle boundaries of bainite and martensite sheaves, which leads to the lowest YR ~0.61 combined with highest tensile strength among all. In addition, by using the Swift equation to elucidate the relationship between the phase component and yield ratio, it is found that simply increasing the fraction of low-temperature transformed phases, like high-strength acicular bainite and lathed martensite, or the percentage of soft polygonal ferrite for good ductility, can hardly solve the problem how to achieve ultralow-YR pipeline steels balanced with enhanced strength. The present result proves that, through utilizing the proposed SCT heat treatment on pipeline steels, an ultralow yield ratio ~0.61 achieved is synchronized with a desirable strength, which efficiently overcomes the trade-off limit between the strength and yield ratio when applying conventional heat-treatment routines. The fact indicates that, rationally adjusting the content of multi-phase microstructure through optimizing the intercritical treatment conditions, can enable us of realizing the synchronous improvement of the YR and strength in HSLA pipeline steels for real engineering.