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At least 1,531 records · Page 85
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DOE OSTI2015

Microstructural characterization, formation mechanism and fracture behavior of the needle δ phase in Fe–Ni–Cr type superalloys with high Nb content

Microstructural characterization, formation mechanism and fracture behavior of the needle δ phase in Fe–Ni–Cr type superalloys with high Nb content (GH4169, equivalent to Inconel 718) have been quantitatively investigated in this research. The typical microstructures of δ phases with the stick, mixed and needle shapes obviously present in Inconel 718 after the isothermal upsetting at the temperature of 980–1060 °C with the initial strain rate of 10{sup −3}–10{sup −1} s{sup −1}. It is found that the shape of the δ phase has a great effect on the mechanical properties of the alloy, viz., the stick δ phase behaves good plasticity and the needle δ phase has good strength. In addition, the needle δ phase can be used to control the grain size as it can prevent grain growth. The combined effect of the localized necking and microvoid coalescence leads to the final ductile fracture of the GH4169 components with the needle δ phase. Both dislocation motion and atom diffusion are the root-cause for the needle δ phase to be firstly separated at grain boundary and then at sub-boundary. The formation mechanism of the needle δ phase is the new finding in this research. Furthermore, it is the primary mechanism for controlling the needle δ phase in Fe–Ni–Cr type superalloys with high Nb content. - Highlights: • Shape of the δ phase takes great effect on mechanical property. • Needle δ phase plays a great role to prevent grain growth. • Needle δ phase can enhance the fracture strength. • Microstructure mechanism of the needle δ phase has been investigated. • Fracture behavior of the needle δ phase has been studied.

36 MATERIALS SCIENCE↗
DOE OSTI2016

Mechanical behavior and microstructure of Ti{sub 20}Hf{sub 20}Zr{sub 20}Ta{sub 20}Nb{sub 20} high-entropy alloy loaded under quasi-static and dynamic compression conditions

The microstructure and the mechanical behavior of equimolar Ti{sub 20}Hf{sub 20}Zr{sub 20}Ta{sub 20}Nb{sub 20} high-entropy alloy in a wide range of initial strain rates between ~ 10{sup −3} s{sup −1} and ~ 3.4 × 10{sup 3} s{sup −1} were studied. A significant increment in the yield strength with increasing strain rate was observed. The yield strength at ~ 3.4 × 10{sup 3} s{sup −1} was about 40% higher than that measured at ~ 10{sup −3} s{sup −1}. Analysis by electron backscatter diffraction shows that in the low strain rate regime (up to ~ 10 s{sup −1}) the deformation occurs mainly in evenly distributed bands, while in the dynamic regime the deformation is strongly localized in macroscopic shear bands accompanied by softening even after the onset of yielding. The Kernel Average Misorientation technique reveals a high level of lattice rotation within these bands that also carries intense shear. In addition, X-ray diffraction line profile analysis indicates that the sharp increase in the flow stress is mostly related to an increase of the dislocation density. - Highlights: • Strain rate effect on the plastic behavior of Ti{sub 20}Hf{sub 20}Zr{sub 20}Ta{sub 20}Nb{sub 20} HEA was studied. • Low strain rate regime was characterized by a continuous hardening. • At high strain rates softening occurred shortly after the onset of yielding. • Intense strain localization in shear bands occurred in the high strain rate regime. • High dislocation density explained the upturn of flow stress at high strain rates.

36 MATERIALS SCIENCE↗
DOE OSTI2016

Room temperature creep behavior of Ti–Nb–Ta–Zr–O alloy

The room temperature creep behavior and deformation mechanisms of a Ti–Nb–Ta–Zr–O alloy, which is also called “gum metal”, were investigated with the nanoindentation creep and conventional creep tests. The microstructure was observed with electron backscattered diffraction analysis (EBSD) and transmission electron microscopy (TEM). The results show that the creep stress exponent of the alloy is sensitive to cold deformation history of the alloy. The alloy which was cold swaged by 85% shows high creep resistance and the stress exponent is approximately equal to 1. Microstructural observation shows that creep process of the alloy without cold deformation is controlled by dislocation mechanism. The stress-induced α' martensitic phase transformation also occurs. The EBSD results show that the grain orientation changes after the creep tests, and thus, the creep of the cold-worked alloy is dominated by the shear deformation of giant faults without direct assistance from dislocations. - Highlights: •Nanoindentation was used to investigate room temperature creep behavior of gum metal. •The creep stress exponent of gum metal is sensitive to the cold deformation history. •The creep stress exponent of cold worked gum metal is approximately equal to 1. •The creep of the cold-worked gum metal is governed by the shear deformation of giant faults.

36 MATERIALS SCIENCE↗
DOE OSTI2016

High temperature oxidation behavior of Ni-based superalloy GH202

The isothermal oxidation kinetic and oxidation behavior of Ni-based superalloy GH202 were investigated at temperatures from 800 °C to 1100 °C. The microstructure of GH202 was characterized by scanning electron microscope (SEM) and electron back-scattered diffraction (EBSD). The composition and morphologies of oxide scales of the surface and cross-section samples were examined by X-ray diffraction (XRD), electron probe microanalysis (EPMA), and SEM combined with energy-dispersive spectroscopy (EDS). The results show that the growth of the oxide scales on GH202 obeyed a parabolic law with an activation energy of 247.8 KJ·mol{sup −1} from 800 °C to 1100 °C. The continuous and dense oxide scales that formed at 800 °C and 900 °C were composed of Cr{sub 2}O{sub 3}, NiCr{sub 2}O{sub 4} and a small amount of TiO{sub 2}. Beneath the oxide scales, aluminum-rich oxides formed along the grain boundaries due to internal oxidation. After treatment at 1000 °C for 150 h, the oxide scale was separated into two layers, the outer layer with sparse holes that was mainly composed of Cr{sub 2}O{sub 3} and TiO{sub 2} and an inner layer of dense Cr{sub 2}O{sub 3}. At 1100 °C for 100 h the outer layer was spalled, due to the looser oxide scale with large blocks of TiO{sub 2} and NiCr{sub 2}O{sub 4} which provided channels for oxygen diffusion. - Highlights: •Isothermal oxidation kinetic of GH202 obeys a parabolic law from 800 °C to 1100 °C. •The activation energy of GH202 is closed to the diffusion activation of Cr{sup 3+}. •The evolution process of oxide scales from 800 °C to 1100 °C is displayed by EPMA. •The oxidation behavior of GH202 is mainly controlled by Cr{sup 3+} and Ti{sup 4+}.

36 MATERIALS SCIENCE↗
DOE OSTI2016

Improving precipitation hardening behavior of Mg−Zn based alloys with Ce−Ca microalloying additions

The precipitation hardening behavior of newly developed Mg−Zn−Ca−Ce alloys, with modified texture and improved ductility, is studied to delineate the microstructural characteristics that lead to effective hardening upon ageing treatments. Advanced electron microscopy and atom probe techniques are used to analyze the structural characteristics in relevance to the hardening potential. It has been found that the formation of a new basal precipitate phase, which evolves from a single atomic layer GP zone, and is finely distributed in both under-aged and peak-aged microstructures, has a significant impact in the improvement of the hardening response compared with the base Mg−Zn alloys. It has also been found that the β′{sub 1} rod precipitates, commonly formed during ageing treatments of Mg−Zn alloys, have their size and distribution significantly refined in the Ca−Ce containing alloys. The role of alloy chemistry in the formation of the fine basal plate GP zones and the refinement in β′{sub 1} precipitation and their relationships to the hardening behavior are discussed. It is proposed that Ca microalloying governs the formation of the GP zones and the enhancement of hardening, particularly in the under-aged conditions, but that this is aided by a beneficial effect from Ce. - Highlights: • Ce−Ca microalloying additions improve hardening in Mg−Zn, over Ce or Ca alone. • Improved hardening is due to refined β′{sub 1} rods, and fine basal plate precipitates. • Atom probe tomography identifies Ca in both β′{sub 1} and the fine basal plates. • The fine basal plates originate as ordered monolayer GP zones with 1:1 Zn:Ca (at.%). • With ageing GP zones become more Zn-rich and transform to the fine basal plates.

36 MATERIALS SCIENCE↗
DOE OSTI2017

Microstructure evolution and dynamic recrystallization behavior of a powder metallurgy Ti-22Al-25Nb alloy during hot compression

The flow behavior of a powder metallurgy (P/M) Ti-22Al-25Nb alloy during hot compression tests has been investigated at a strain rate of 0.01 s{sup −1} and a temperature range of 980–1100 °C up to various true strains from 0.1 to 0.9. The effects of deformation temperature and strain on microstructure characterization and nucleation mechanisms of dynamic recrystallization (DRX) were assessed by means of Optical microscope (OM), electron backscatter diffraction (EBSD) and transmission electron microscope (TEM) techniques, respectively. The results indicated that the process of DRX was promoted by increasing deformation temperature and strain. By regression analysis, a power exponent relationship between peak stresses and sizes of stable DRX grains was developed. In addition, it is suggested that the discontinuous dynamic recrystallization (DDRX) and continuous dynamic recrystallization (CDRX) controlled nucleation mechanisms for DRX grains operated simultaneously during the whole hot process, and which played the leading role varied with hot process parameters of temperature and strain. It was further demonstrated that the CDRX featured by progressive subgrain rotation was weakened by elevating deformation temperatures. - Highlights: •Flow behavior of a P/M Ti-22Al-25Nb is studied by hot compression tests. •Microstructure evolution of alloy is affected by deformation temperature and strain. •The relationship between peak stress and stable DRX grain size was developed. •The process of DRX was promoted by increasing deformation temperature and strain. •Nucleation mechanisms of DRX were identified by EBSD analysis and TEM observation.

36 MATERIALS SCIENCE↗
DOE OSTI2017

Aging behavior of an in-situ TiB{sub 2}/Al-Cu-Li-x matrix composite

Transmission electron microscopy, differential scanning calorimetry and hardness tests have been performed on an in-situ TiB{sub 2}/Al-3.3Cu-1.0Li-0.60Mg-0.40Ag-0.14Zr-0.13Si composite to study its aging behavior at 175 °C. A cubic phase suspected to be the σ (Al{sub 5}Cu{sub 6}Mg{sub 2}) phase or its variant is precipitated at all aging stages studied, and this phase is not typically observed in the Al-Cu-Li alloys. The primary hardening (aging for 3 h) phases consist of δ′ (Al{sub 3}Li), β′ (Al{sub 3}Zr) and the cubic phase. After aging for 18 h, all precipitates including T{sub 1} (Al{sub 2}CuLi), S (Al{sub 2}CuMg), θ′ (Al{sub 2}Cu), δ′, β′ and the cubic phase have appeared, and the formation of T{sub 1} and S results in a rapid increase in hardness. With prolonging of aging time, the apparent coarsening of T{sub 1} and S results in a decline in hardness. - Highlights: •The aging behavior of an in-situ TiB{sub 2}/Al-Cu-Li-x composite was studied. •A cubic phase suspected to be σ (Al{sub 5}Cu{sub 6}Mg{sub 2}) or its variant was precipitated. •The hardness change was dominated by the evolution of T{sub 1} (Al{sub 2}CuLi) and S (Al{sub 2}CuMg).

36 MATERIALS SCIENCE↗
DOE OSTI2017

Hot deformation behaviors and processing maps of B{sub 4}C/Al6061 neutron absorber composites

In this study, the hot deformation behaviors of 30 wt.% B{sub 4}C/Al6061 neutron absorber composites (NACs) have been investigated by conducting isothermal compression tests at temperatures ranging from 653 K to 803 K and strain rates from 0.01 to 10 s{sup −1}. It was found that, during hot compression, the B{sub 4}C/Al6061 NACs exhibited a steady flow characteristic which can be expressed by the Zener-Hollomon parameter as a hyperbolic-sine function of flow stress. High average activation energy (185.62 kJ/mol) of B{sub 4}C/Al6061 NACs is noted in current study owing to the high content of B{sub 4}C particle. The optimum hot working conditions for B{sub 4}C/Al6061 NACs are found to be 760–803 K/0.01–0.05 s{sup −1} based on processing map and microstructure evolution. Typical material instabilities are thought to be attributed to void formation, adiabatic shear bands (ASB), particle debonding, and matrix cracking. Finally, the effect of the plastic deformation zones (PDZs) on the microstructure evolution in this 30 wt.% B{sub 4}C/Al6061 composite is found to be very important. - Highlights: •The hot deformation behavior of the 30 wt.% B{sub 4}C/Al6061 NACs was first analyzed. •The 3D efficiency map and the instability map are developed. •The optimum hot working conditions were identified and validated by SEM and TEM. •The hot deformation schematic diagram of 30 wt.% B{sub 4}C/Al6061 NACs is developed.

36 MATERIALS SCIENCE↗
DOE OSTI2017

Effect of cooling rates on the structure, density and micro-indentation behavior of the Fe, Co-based bulk metallic glass

The experiments demonstrate that ductility of the samples of bulk metallic glass (BMG) with the same chemical composition increased with decreasing sample size. It is shown that microhardness and density increases with decreasing the cooling rate. The fracture morphology of rods after compressive fracture were different on the cross section. Two characteristic features of the compressive fracture morphologies of metallic glasses (MGs) were observed in samples: smooth region and the vein pattern. Many parallel shear bands were observed on the deformed specimen with ϕ = 2 mm in diameter. The results provide more understanding on the relationship among the cooling rate, structure and micro-indentation behavior of the Fe-Co-based BMGs. - Highlights: •Fracture morphology and micro-indentation behavior is studied. •The smaller BMG sample exhibits the larger plasticity. •Microhardness and density increase with decreasing the cooling rate. •Formation of shear bands has been reported in deformed specimens. •Structure and mechanical properties of BMGs can be controlled by the cooling rate.

36 MATERIALS SCIENCE↗
DOE OSTI2017

Through-thickness microstructural evolution during grain boundary engineering type thermomechanical processing and its implication on sensitization behavior in austenitic stainless steel

Highlights: • GBE-treated 304L steel exhibits thickness-dependent microstructural evolution. • Degree of sensitization increases from surface to interior in GBE specimen. • Low fractal dimension and large TRD size lead to lesser sensitization at surface. • As-received specimen shows high albeit thickness-independent sensitization behavior. • Higher residual strain leads to higher sensitization at surface of deformed sample. - Abstract: Through-thickness microstructural variations and their synergistic influence on degree of sensitization in grain boundary engineering (GBE) treated 304L stainless steel was compared with solution-annealed and marginally cold-worked (~ 3% thickness reduction) specimens. The solution-annealed specimen has shown higher degree of sensitization due to the presence of lower fraction of Σ3 (as well as other low Σ CSL) boundaries and dominant random high-angle boundaries (RHAGBs) connectivity. However, it has shown thickness-independent sensitization behavior as grain boundary character distribution, residual strain and grain boundary network topology towards thickness direction are similar. Owing to higher residual strain, as revealed by local misorientation analysis, the cold-worked specimen showed maximum degree of sensitization (higher than solution-annealed condition) at surface. The GBE specimen has shown much lower degree of sensitization at surface due to higher-fraction of Σ3 boundaries and discontinuous RHAGBs connectivity. However, increasing fraction of RHAGBs and their connectivity, as confirmed by twinning-related domain and fractal analysis, cause more sensitization at interior of this specimen. In spite of this, the degree of sensitization at the interior of GBE specimen is lower than the solution-annealed or cold-worked specimen. Even though the residual strain in GBE specimen is higher than the solution-annealed condition, the lower degree of sensitization indicates that the grain boundary character distribution and grain boundary network topology have higher implication than residual strain in controlling sensitization.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Mechanical behavior and texture development of over-aged and solution treated Al-Cu-Mg alloy during multi-directional forging

The effects of initial heat treatments, i.e. solution treating and over ageing, on deformation behavior of an age hardenable Al-Cu-Mg alloy, including mechanical properties and texture evolution, during multi-directional forging (MDF) are investigated. Hardness measurements, electron back scatter diffraction (EBSD) maps, and differential scanning calorimetry (DSC) are carried out. Mechanical behaviors of the alloy in terms of compressive stress and hardness test after MDF are investigated. In both solution treated and over-aged alloys, the compressive stress during MDF shows an increase up to the second pass, and by further straining the flow stress is decreased. However, shear and deformation bands are formed in both solution treated and over-aged alloys, the density of these instabilities in the solution treated alloy is higher than that in over-aged alloy. In the solution treated alloy, not only large particles induce deformation bands, but also the dynamically formed GPB zones/Cu-Mg co-clusters. Texture analysis shows that the shear component of C {001}<110> (Φ{sub 1} = 90°, Φ = 0°, Φ{sub 2} = 45°) is developed with 18 mrd during MDF of the over-aged alloy. In the solution treated alloy, due to the high density of deformation bands and different microchemistry of the matrix in comparison with that of the over-aged alloy, the Copper type texture component {112}<111> (Φ{sub 1} = 90°, Φ = 35°, Φ{sub 2} = 45°) is developed with 11 mrd.

36 MATERIALS SCIENCE↗
DOE OSTI2018

The effects of Cr particles addition on the aging behavior and mechanical properties of SiCp/7075Al composites

Highlights: • The novel investigated the effects of Cr particles on the aging behavior and mechanical properties of SiC{sub p}/7075Al composites. • The addition of Cr particles didn't change precipitation sequence, but significantly degraded precipitation kinetics of composites. • Cr particles had a significant effect on the tensile properties of the peak-aged composite because of Mg aggregation. - Abstract: The effects of Cr particles addition on the aging behavior and mechanical properties of SiC{sub p}/7075Al composites were systematically investigated. The results show that the precipitation sequence of (5 vol% Cr + 50 vol% SiC)/7075Al composites (C-7075Al) is still: GP zone → η′ → η (MgZn{sub 2}), but Cr particles degrade the precipitation kinetics of the C-7075Al composite in comparison to (5 vol% SiC{sub p} + 50 vol% SiC{sub p})/7075Al (B-7075Al) because of reduced dislocation density present in the matrix and higher thermal activation energy of the η′ phase. Besides, the addition of Cr particles has a significant effect on the mechanical properties of the C-7075Al composite during the aging process. In contrast to the as-cast state, the ultimate tensile strength of the C-7075Al composite obtains an increase of 120 MPa at the peak-aged period, significantly lower than those of 7075Al alloy (207 MPa) and the B-7075Al composite (151 MPa), which is attributable to the severe aggregation of elemental Mg at the interface between Cr particles and the Al matrix.

36 MATERIALS SCIENCE↗
DOE OSTI2018

High-temperature deformation behavior of the extruded Ti-22Al-25Nb alloy fabricated by powder metallurgy

Highlights: • The high-temperature deformation behavior of the extruded Ti-22Al-25Nb alloy was analyzed. • A true strain-compensated Arrhenius-type constitutive model was established. • The processing maps of true strains 0.2, 0.4, and 0.6 were constructed to optimize the processing parameters. • Microstructural evolution and deformation mechanism in different conditions were explained. - Abstract: The hot-deformation behavior of the extruded Ti-22Al-25Nb alloy was investigated by compression testing in the 1213–1333 K temperature range, under a 0.001–1.0 s{sup −1} strain-rate range. The activation energy, Q, at a deformation strain of 0.3, was calculated to be 574.80 kJ mol{sup −1}. A strain-compensated Arrhenius-type constitutive model was established for this alloy, with an average absolute relative error (AARE) and correlation coefficient (R) of 7.64% and 0.994, respectively. Based on the dynamic materials model (DMM), processing maps were developed by combining the power dissipation and instability maps. An instability region was identified in the 1213–1278 K temperature range and 0.022–1.0 s{sup −1} strain-rate range. The microstructure of specimens deformed under different conditions were analyzed to characterize the corresponding deformation mechanisms. The main softening mechanisms in the stability regions were dynamic recrystallization (DRX) and dynamic globularization. The grain size of the specimen deformed at 1313 K/0.001 s{sup −1} was approximately 15.2 μm.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Pre-processing related recrystallization behavior in β annealing of a near-β Ti-5Al-5Mo-5V-3Cr-1Zr titanium alloy

Highlights: • Recrystallization behavior of Ti-55531 alloy in β annealing under four typical pre-processing conditions is studied. • Recrystallization nucleation mode is governed by the size and distribution of substructures. • The sequence of phase transformation and β recrystallization significantly affects recrystallized grain size. • The uniform substructures produce a higher fraction of low angle grain boundaries after full recrystallization. - Abstract: Recrystallization behavior of Ti-55531 alloy in β annealing is explored for grain refinement during primary hot working. Four typical pre-processing conditions were adopted to characterize the diverse processing routes of primary hot working: deformation and annealing in the two-phase region, deformation of globularized structure in two-phase region, deformation of lamellar structure in two-phase region and deformation in single β region. The microstructure evolution in β annealing was quantified by Optical Microscopy (OM), Electron Backscattered Diffraction (EBSD), X-ray diffraction (XRD) and quantitative metallography. The results indicate that the nucleation mode of static recrystallization is determined by the size and distribution of substructures prior to annealing which is related to pre-processing route. The uniform substructures in as-deformed globularized structure produce a higher fraction of low angle grain boundaries after full recrystallization. Fine β recrystallized structure can be obtained when β recrystallization is accompanied by α-to-β phase transformation.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Effects of W on creep behaviors of novel Nb-bearing high nitrogen austenitic heat-resistant cast steels at 1000 °C

Highlights: • The synergistic CALPHAD and experimental approaches are adopted. • Creep behaviors of high nitrogen austenitic alloys are investigated at 1000 °C. • Creep properties are improved through the W solid solution strengthening. • W additions promote the formation of (Cr,Fe,W){sub 23}C{sub 6} instead of (Cr,Fe,W){sub 7}C{sub 3}. • (Cr, W)-rich precipitates severely degrade the creep properties. - Abstract: Recently, there is an urgent demand from automotive industries to develop new austenitic heat-resistant cast steels that withstand exhaust gas temperatures as high as 1000 °C. In this study, the effect of W additions on the creep behavior of a series of Nb-bearing high nitrogen austenitic heat-resistant cast steels was investigated at 1000 °C and 50 MPa. Microstructures before and after creep rupture tests were carefully characterized to illustrate the microstructural evolution during creep deformation. Microstructural analyses revealed that the W addition did not affect the formation of primary Nb(C,N), whereas significantly promoted the precipitation of primary (Cr, W)-rich phases, particularly those cellular structures along grain boundaries. The W solid solution strengthening and the secondary precipitation strengthening of submicron-scale (Cr, W)-rich phase improved the creep resistance of the alloys through increasing the hardness of γ-austenite and pinning the dislocations. However, the primary cellular (Cr, W)-rich structures significantly accelerated the nucleation and propagation of creep cracks along grain boundaries, thereby increasing the creep rate and decreasing the creep life.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Effect of equal channel angular pressing on the mechanical and tribological behavior of Al-Zn-Mg-Cu alloy

Highlights: • The correlation of microstructure, mechanical and wear properties has been studied. • The correlation between tensile properties and shear properties has been also studied. • Mechanical and tribological properties have been improved through the ECAP process of the Al-7075 alloy. • The dominant wear mechanisms in the initial material were adhesive and oxidative wear as well as delamination. • Abrasive and oxidative wear were the dominant wear mechanisms in ECAP processed specimens. - Abstract: Microstructure, mechanical properties and wear behavior of severely deformed Al-7075 alloy by equal channel angular pressing (ECAP) were studied. Microhardness, tensile and shear strength tests were performed to evaluate the mechanical properties of the material. The relationship between tensile and shear strength was also extracted. In addition, with the aim of study the wear behavior, the wear tests were performed by using a pin-on-disk machine under different normal loads. The microscopic observations i.e. transmission electron microscopy (TEM) and scanning electron microscope (SEM), were used to analyze the microstructure of specimens as well as the surface, subsurface and debris of worn specimens. Mechanical properties results showed that the hardness and strength (tensile and shear) of the ECAP-processed alloy were higher than that of the annealed alloy and increased with the increment of pass number. The wear test data revealed that the wear rate and friction coefficient decreased with increasing ECAP passes. The SEM analysis of worn specimens demonstrated that the dominant wear mechanisms of the annealed alloy were adhesion wear and delamination and the dominant wear mechanisms of ECAP processed alloy were abrasive wear and plowing bands. Oxidative wear was also observed at both annealed and ECAPed alloy. Totally, these results demonstrated that ultrafine-grained structure fabricated by ECAP, improves the wear resistance of the Al-7075 alloy.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Precipitation behavior of α phase during aging treatment in a β-quenched Ti-7333

Highlights: • The precipitation behaviors of α precipitate are investigated in the hot-rolled Ti-7333. • The formation of the V-shaped α configuration is closely related to the variant selection of α phase. • The angle between the closely packed direction of α{sub GB} and GB plane plays a key role on the precipitation of α{sub GB}. - Abstract: In this work, the precipitation behavior of α phase during the isothermal aging treatment was investigated in the β-quenched Ti-7333. The results indicate that dislocations contribute to the precipitation of α phase with a dot-like morphology at the beginning of aging treatment, and these dot-like α precipitates holding the same Burgers orientation relationship (BOR) with respect to the β matrix coalesce into the platelet morphology. The platelet-like α precipitates that orientate to different growth directions combine with each other to form the V-shaped configuration. And the formation of this configuration is closely related with the variant selection (VS) of α phase, which is resulted from the perfect accommodation of transformation strain within the β matrix. Meanwhile, it is found that low-angle grain boundaries (LAGBs) and high-angle grain boundaries (HAGBs) can be decorated by α phase during the aging treatment, and its precipitation degree significantly depends on the angle between the closely packed direction of αGB and the GB plane. The volume fraction of α phase increases with increasing aging time, and this increasing volume fraction eventually results in the increase in micro-hardness of the alloy.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Effects of carbon content and size on Ti-C reaction behavior and resultant properties of Cu-Ti-C alloy system

Highlights: • TiC-strengthened Cu alloys were synthesized by an in situ powder metallurgy route. • Effect of C/Ti ratio on electrical and mechanical properties was studied. • Ti-C reaction behavior during powder metallurgy process was discussed. • Microstructure evolution during present process was analyzed. - Abstract: TiC-strengthened Cu alloys with various nominal compositions were prepared by a high-energy ball-milling process, followed by consolidation using spark plasma sintering (SPS), and a subsequent heat treatment. The results indicated that the lower atomic ratio of C/Ti, the more residual Ti in copper matrix, resulting in a lower electrical conductivity but higher microhardness of alloy. The optimal atomic ratio was found to be close to1.0. The reaction behavior of Ti-C in the Cu-Ti-C system during this process was investigated. Microstructural results suggested that TiC formation is through a diffusion-controlled mechanism. A thin TiC layer was formed at the surface of C particles, and the growth of TiC layer was controlled by interdiffusion of C and Ti atoms across the TiC layer. An incomplete chemical reaction between Ti and C, characterized by a core-shell structure, was observed when the initial C particles were large, and the TiC particle size was found to strongly depending on the C particle size.

36 MATERIALS SCIENCE↗