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At least 1,621 records · Page 90
All sourcesNASA NTRSDOE OSTI
DOE OSTI2018

Investigation of the Tribological Properties of AlxSi-1.2Fe(Mn) (x = 5-13 wt.%) Alloys

The effect of Fe-impurity and Mn modification on the sliding wear behavior of Al-xSi-1.2Fe(Mn) (x = 5-13 wt.%) alloys was investigated in dry sliding against AISI 52100 steel. According to the results, in the Si range of 5-9 wt.%, Fe-impurity promotes the formation of finely distributed β-Al{sub 5}FeSi intermetallics in the matrix increasing its hardness and potential to support the tribolayer leading to the improved wear characteristics. The presence of Fe in the composition of Al-(9-13)Si alloys, however, leads to the formation of primary β-platelets that their sizes and volume fractions increase with Si concentration. The formation of primary compounds increases the hardness, but makes the alloys less ductile and more vulnerable to microcracking and wear. Manganese modification converts the β-Fe platelets to the α-Al{sub 15}(Fe,Mn){sub 3}Si{sub 2} compounds with different morphologies. The substitution of fine β-platelets by α-Chinese scripts in Al-(5-9)Si-1.2FeMn alloys reduces the wear resistance, but the formation of primary α-compounds with star-like or polyhedral morphologies in Al-(9-13)Si-1.2FeMn alloys improves their wear characteristics. Moreover, it was found that the formation of Fe(Mn)-rich compounds affects the friction coefficient which is likely due to the fracture of particles in the subsurface region, their interaction with contacting surfaces, and their effect on the stability of the tribolayer.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Uniaxial Ratcheting Behavior of Dual-Phase Steel Sheet

Uniaxial ratcheting behavior of an automotive-grade dual-phase steel sheet of 1.6 mm thickness has been investigated through engineering stress-controlled asymmetric cyclic loading experiments in the elastoplastic regime at ambient temperature (~ 28°C). Experiments have been performed keeping mean stress, stress amplitude or maximum stress constant. The effect of stress combinations on ratcheting deformation has been rationalized in terms of plastic strain amplitudes, both in loading and in unloading directions. It is found that for all stress combinations the steady-state ratcheting rate follows a perfect power relationship. Scanning electron fractography reveals that failure under asymmetric cyclic loading conditions occurs in a ductile manner through the formation of voids as it happens under tensile loading.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Notch Sensitivity and Failure Behavior of TiAl and K418 Alloys

The notch sensitivity of tensile specimens of TiAl and K418 alloys has been investigated, and the notch strength has been quantitatively analyzed. The fracture surface of the specimens has also been observed and analyzed by scanning electron microscopy (JEOL-6700F). By comparing notch rod tensile specimens and special notch rod tensile specimens, it was found that the basic nature, fracture driving force, and fracture criteria of the brittle TiAl alloy and the ductile K418 alloy are different. The final fracture of the K418 alloy is controlled by strain and not by stress, and the specimens do not exhibit notch sensitivity. However, the final fracture of the TiAl alloy is controlled by stress, and the specimens exhibit small notch sensitivity. For the special notch rod tensile specimens, the K418 specimens do not exhibit notch sensitivity. However, for the TiAl alloy, when the notch depth reaches 10% of the specimen diameter, the tensile strength decreases, and when the notch depth reaches 20% of the specimen diameters, the tensile strength sharply decreases.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Cryorolling-Induced Texture, Mechanical Properties and Fracture Behavior of Al-Mg-Si Alloys During Cold Deformation

This study investigated the texture, mechanical properties, and fracture behavior during cold working of cryorolled (CR) Al-Mg-Si alloys. An x-ray texture goniometer was used to examine the crystallographic texture, and tensile tests were performed to understand the deformation behavior in more detail. SEM/EBSD/TEM was used to observe the microstructures and fracture morphology of the alloy during cryorolling. The microstructures indicated that a large number of ultrafine grains and dislocations formed after cryorolled. The recrystallization was prohibited due to low temperature during deformation. The ultrafine grains and dislocation strength improved the tensile strength from 279.38 MPa for the room-rolled alloy to 313.98 MPa for the cryorolled alloy; better plasticity (4.6%) was observed in the CR sheet. Ductile and intergranular fractures were observed in the CR sheet deformation from 20 to 90%.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Microstructure and Mechanical Properties of Rapidly Cooled Friction Stir Welded Ti-6Al-4V Alloys

Two mm-thick Ti-6Al-4V alloy plates were friction stir butt welded under natural- and rapid-cooling conditions. The microstructures and mechanical properties of joints were investigated and compared. The rapid cooling via spraying cooling liquid nitrogen on the plate top surface was beneficial to obtaining a narrower cross section and finer and more uniform grains in the stir zone than those obtained by natural cooling. Joint tensile strength of 1021 MPa and elongation of 1.88% under the rapid-cooling condition respectively reached 102.7 and 150.4% of those under the natural-cooling condition, respectively. The uniform-and-compact equiaxed grains led to equiaxed dimples on the fracture surface, which demonstrated a ductile fracture of the welding joints.

36 MATERIALS SCIENCE↗
DOE OSTI2018

The Effect of Heat Treatment Process Parameters on Mechanical Properties, Precipitation, Fatigue Life, and Fracture Mode of an Austenitic Mn Hadfield Steel

In this study, the effect of cooling rate after heat treatment on mechanical properties, fatigue life, precipitation and fracture mode of an austenitic Mn Hadfield steel was investigated. Cast samples of the Hadfield steel were heat treated at 1100 C for 2 h. The samples were subsequently quenched in pure water and also in 3 wt.% NaCl salt bath. Optical microscope (OM) and scanning electron microscope (SEM) were used to analyze microstructure and fracture surfaces. Transmission electron microscope (TEM) was used to assess the precipitates. X-ray diffraction (XRD) was used to determine the phases formed. Mechanical properties and fatigue life were determined by uniaxial tensile test, bending fatigue and hardness measurements. Results showed that the sample quenched in salt bath had lower Mn{sub 3}C precipitates, hardness, yield and tensile strengths. Instead, this situation resulted in a more ductile fracture mode and higher formability. Finally, the fatigue life of the sample quenched in salt bath was longer than the one quenched in pure water.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Enhanced Strength of 304 SS-Ti6Al4V Laser-Welded Joints Containing Composite Interlayers

This paper investigates the microstructural characterization of the fracture propagation edge during laser beam welding of Ti6Al4V and 304 stainless steel using Cu, V, and Ni interlayers as a single Cu interlayer and composite interlayer structures with different Cu interlayer thicknesses. This study attempts to increase the Ti-SS joint strength through the use of the composite interlayers. X-ray diffraction characterization based on a 0.5-mm-thick Cu interlayer showed that the SS/Cu/Ti joint contained considerable brittle IMCs; however, the fracture surface of the SS/Cu-V/Ti composite interlayer sample did not contain any Fe-Ti IMCs or Cr{sub 2}Ti. EBSD observation indicated that the crack propagated transgranularly from solidified CuTi in the interdendritic regions of the FeTi compounds in the composite interlayer sample; however, the crack propagated intergranularly along Fe{sub 2}Ti grain boundaries in the single interlayer sample. The investigation based on a 1-mm-thick Cu interlayer showed that the SS/Ni-Cu1/Ti sample was stronger than the SS/Cu1/Ti sample due to a lack of CuTi at 3.5 kW. The tensile strength of the SS/Cu1-V/Ti joint was greater than that of the SS/Ni-Cu1/Ti joint due to the formation of ductile NiTi instead of brittle NiTi{sub 2}. The microstructures of the fracture edges after tensile testing illustrated that the eutectic CuTi + CuTi{sub 2} fractured at the SS/Cu1/Ti joint after tensile testing, while CuTi{sub 2} and eutectoid αTi + CuTi{sub 2} compounds fractured during the tensile experiment at the SS/Ni-Cu1/Ti and SS/Cu1-V/Ti joints, respectively. Investigations based on both the 0.5- and 1-mm-thick Cu interlayers showed that the tensile strength and elongation of the composite interlayer joints at a laser power of 3.5 kW were greater than those of the single interlayer joints.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Fracture Analysis of ARB-Processed CP-Ti Sheets Under Unidirectional Tensile Loading

The fracture behavior of commercially pure titanium (CP-Ti) processed by accumulative roll bonding (ARB) was investigated in this study. Monolithic and Ti-SiC composite samples were first produced by ARB process and then subjected to uniaxial tensile testing at room temperature. Two different ductile fracture mechanisms including shear dimple rupture and equiaxed dimple rupture were observed in the initial and final ARB cycles, respectively. The difference in the rupture mechanism was attributed to different stress states at the crack tip and different densities of metallurgical defects. A non-uniform distribution of dimple size was obtained for fracture surfaces of the samples processed by a low number of ARB cycles. This was attributed to the heterogeneity of microstructure in the primary ARB cycles. The fracture surface of the samples processed by high ARB cycles represented more uniform dimples. This was attributed to more homogenous microstructure in the final cycles. Moreover, SiC particles showed a major role in fracture of samples, so that they affected the size and depth of the dimples, as well as the number of ARB cycles in which the transmission of the fracture mechanism occurred.

36 MATERIALS SCIENCE↗
DOE OSTI2019

Laser Additive Manufacturing of (Ti-6Al-4V)/10 wt.% Ag Composite Using Spherical Powders to Reduce Wear and Friction

In order to extend the service lifetime of Ti-6Al-4V-based components, friction and wear behaviors of Ti-6Al-4V alloys needed to be further improved. Laser additive manufacturing with a moving laser beam was used to prepare (Ti-6Al-4V)/10 wt.% Ag samples, on a three-dimensional RC-SLM400 printer with laser powers of 2-10 kW. (Ti-6Al-4V)/10 wt.% Ag samples were successfully printed using spherical powders prepared by gas atomization process (SPGAP) and combustion reaction (SP-CR). The tribological properties of the as-prepared samples sliding against Si{sub 3}N{sub 4} balls were examined using a ball-on-disk tribometer. The results showed that the tribological behavior of (Ti-6Al-4V)/10 wt.% Ag printed by SP-CR was better than that of the sample printed by SPGAP. The low friction and reduced wear were mainly attributed to the formation of the lubrication layer containing large amounts of silver, which can exhibit an excellent deformation and plastic ductility. It effectively reduced the friction resistance and material loss and provided the good lubrication for 0-80 min, resulting in low friction and reduced wear of the samples prepared by SP-CR.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Effect of Annealing on the Microstructure and Properties of Electron Beam Welded 4J34 Kovar Alloy

Electron beam welding of 4J34 Kovar alloy was conducted, and the microstructure and mechanical properties of the joints in as-welded and postweld annealing treatment (PWAT) states were investigated. The martensitic transformation occurred and led to the generation of α lath martensite due to the rapid cooling in the as-welded condition. After annealing treatment, part of the α martensite transformed to γ austenite and subsequently developed the bulk γ phase and α/γ lamellar microstructures in the fusion zone (FZ) of the postwelded joint. Grains in the heat-affected zone (HAZ) clearly developed and penetrated into the FZ, causing the disappearance of the fusion line between the FZ and the HAZ. All the joints in both the as-welded and PWAT conditions exhibited ductile fracture. The maximum tensile strength of 505 MPa was obtained in the as-welded joint with the welding speed of 150 mm/min. The linear expansion coefficients of the joint and base metal decreased with the increase in temperature. However, the decreasing rate of PWAT joints was lower than that of the as-welded joints.

36 MATERIALS SCIENCE↗
DOE OSTI2019

Microstructural Properties of Heat-Treated LENS In Situ Additively Manufactured Titanium Aluminide

This study reports on the microstructure, phase identification and hardness property of the LENS-manufactured binary Ti-Al alloy which was achieved via laser in situ alloying. The in situ alloying method, using laser beam as an energy source, indicated that it is possible to achieve a binary gamma phase microstructure from elemental powders of Ti and Al. The as-produced sample, however, was characterized of having different microstructure at the top, middle and bottom regions. The middle and top layers had similar hardness which was lower than that of the bottom region. The as heat-treated sample was characterized of lamellar grain microstructure at the top and just grains at the bottom and the middle. The observed grains were different in size, phase and hardness. This study indicated that in addition to the pores and the precipitation of α{sub 2} phase or aluminum segregation on the grain boundary veins intermetallics lead to severe cracking. Overall, the hardness values of the as-produced and heat-treated samples were similar, and due to extensive cracking, it was inferred that both samples will lack ductility.

36 MATERIALS SCIENCE↗
DOE OSTI2019

Role of Si in the Surface Damage Mechanism of RB-SiC/Si Under Mechanical Loading

Indentation test (Nanoindentation and Vickers indentation), diamond scratching and high spindle speed grinding are conducted to investigate the role of silicon (Si) in the surface damage behavior of reaction-bonded SiC/Si composites (RB-SiC/Si). Even though the addition of Si contributes to densifying the bulk materials and improving the toughness, the indentation and diamond scratching results firstly indicate that the cracks initiate at the SiC/Si interfaces due to the non-uniform deformation caused by the existence of Si, and the phase transformation of Si also leads to the pop-out effect during the nanoindentation and the diamond scratching test. The ground surface of RB-SiC/Si is characterized by scratching grooves and brittle fracture, indicating the ductile material removal mode and brittle material removal mode for RB-SiC/Si, respectively, and the surface reliefs form on the ground surface due to the different hardness between Si and SiC phases. Moreover, the phase transformation of Si contributes to the easy fracture of phase boundaries under the mechanical loading, and the accompanied volume change also results in the dislodgement of hard particles and the generation of surface burs on the ground surface.

36 MATERIALS SCIENCE↗
DOE OSTI2019

Nonuniformity in Mechanical Properties of Linear Friction Welded Mild Steel Joint

Mild steel was successfully joined by linear friction welding. Grain size at peripheral region was coarser than that at the other zones of the weld interface, which had no significant effect on microhardness. The local tensile properties were considerably decreased when oxides or lack of bonding appeared at the interface. Ductile fracture dominated the fracture mode for all joints. Variation in local tensile properties could be relieved under a medium friction pressure, a relatively high amplitude with a sufficient friction time. It was attributed to the facts that plastic metal layer of joint became uniform to some extent and interfacial defects were eliminated. The standard deviation of plastic metal layer thickness (χ{sub p}) could be used to describe the combined effects of welding parameters on joint integral performance. It was found that the standard deviation of plastic metal layer thickness showed a strong inverse correlation with the joint integral performance. Well-formed and defect-free joints exhibiting 90% of the base metal tensile strength could be produced when processing at a frequency of 20 Hz with a friction pressure of 103 MPa or 155 MPa, an amplitude of 2.5 mm and a friction time of at least 4 s.

36 MATERIALS SCIENCE↗
DOE OSTI2019

Sulfide Stress Corrosion Cracking Behavior of G105 and S135 High-Strength Drill Pipe Steels in H{sub 2}S Environment

Slow strain rate testing was employed to investigate the sulfide stress corrosion cracking (SSCC) behavior of low-carbon high-strength drill pipe steels (G105 and S135) in the simulated drilling environment containing H{sub 2}S. Results reveal that both G105 and S135 steels have a high SSCC susceptibility in the simulated drilling environment containing H{sub 2}S. In the range of experimental slow strain rate, the SSCC sensitivity of drill pipe steel increases with the decrease in slow strain rate, and the ductility and fracture properties of drill pipe steels also decrease obviously. With the increase in the steel strength, the SSCC sensitivity of the drill pipe steel increases, and the S135 drill pipe steel has higher SSCC sensitivity than G105. There is no obvious plastic deformation on the fracture surface of G105 and S135 steels in H{sub 2}S solution. The cracks of fracture sprout on the surface of the specimen, the instantaneous fracture zone is in the middle, and the fracture can be divided into the cracking and the final fracture zone. The SSCC mechanisms of two drill pipe steels are hydrogen embrittlement. The hydrogen enters the steel and reduces the binding force of the iron atom, resulting in cleavage fracture.

36 MATERIALS SCIENCE↗
DOE OSTI2019

Thermally Assisted Adhesion Enhancement in High-Phosphorous Electroless Nickel Plating

High-phosphorus electroless nickel is a widely recognized protective plating for oilfield metallic parts subject to corrosion and wear. In the present investigation, thermally activated diffusion treatment was applied to enhance electroless nickel (10-13 wt.% P) plating adhesion on low-alloy steel (UNS G41400) and reduce the risks of spalling. High-phosphorus electroless nickel (10-13 wt.% P) was applied to a variety of UNS G41400 low-alloy steel test samples as per ASTM B733 that were subsequently heat-treated in air for four distinct temperatures, i.e., 510, 540, 570, and 600 °C for 2-10 h. Following post-plating diffusion heat treatments, the initial amorphous electroless nickel was replaced by Ni{sub 3}P precipitates dispersed in a crystalline nickel-rich phase, with an average hardness improvement from 500 to 700 HK. An increased atomic interdiffusion (from 400 to 3.5 µm) was observed at plating–substrate interfaces, resulting in enhanced adhesion and reduced susceptibility toward spalling. This improvement in adhesion was measured repeatedly through post-bending crack analyses and subsequently validated by interface microindentation. Along with diffusion across plating–substrate interface, nickel oxide formation on surface by oxidation was observed to grow at quasi-parabolic rate with no detrimental impact on spalling. In all cases, plating adhesion was improved by heat treatments due to the diffusion-controlled formation of a seemingly ductile layer.

36 MATERIALS SCIENCE↗
DOE OSTI2019

Microstructure and Mechanical Properties of a Multiphase FeCrCuMnNi High-Entropy Alloy

A FeCrCuMnNi high-entropy alloy was produced using vacuum induction melting, starting from high-purity raw materials. The microstructure and mechanical properties of the as-cast FeCrCuMnNi alloy were studied, considering x-ray diffraction (XRD), scanning electron microscopy, and hardness and tensile tests. XRD results revealed the existence of two FCC phases and one BCC phase. Microstructural evaluation illustrated that the as-cast alloy has a typical cast dendritic structure, where dendrite regions (BCC) were enriched in Cr and Fe. Interdendritic regions were saturated with Cu and Ni and revealed G/B(T) {110} 〈111〉 and Brass {110} 〈112〉 as the major texture components. The produced alloy revealed an excellent compromise in mechanical properties due to the mixture of solid solution phases with different structures: 300 HV hardness, 950 MPa ultimate tensile strength and 14% elongation. Microhardness test results also revealed that the BCC phase was the hardest phase. The fracture surface evidenced a typical ductile failure. Furthermore, heat treatment results revealed that phase composition remained stable after annealing up to 650 °C. Phase transformation occurred at higher temperatures in order to form more stable phases; therefore, FCC2 phase grew at the expense of the BCC phase.

36 MATERIALS SCIENCE↗
DOE OSTI2019

The Effects of Sc Addition on the Microstructure and Mechanical Properties of Be-Al Alloy Fabricated by Induction Melting

This study investigated the alloying effects of 0.4 wt.% Sc on the microstructure, mechanical properties, and fracture behavior of a cast Be-Al alloy prepared by vacuum induction melting. The micromechanics of Sc-containing secondary-phase particles (SPs) were characterized by nanoindentation, and the fracture behavior of the alloy was investigated via tensile fractography and optical metallography. The addition of 0.4 wt.% Sc significantly modified the alloy microstructure by decreasing the secondary dendritic arm spacing (SDAS), weakening the dendritic structure of Be, and producing an equiaxed/cellular alloy microstructure with refined Be grains. Sc alloying increased the Vickers hardness by 36%, elastic modulus by 28%, yield strength by 35%, ultimate strength by 52%, but decreased the elongation from 3.6 to 2.9%. Sc-containing SPs had a higher elastic modulus, yield strength, microhardness, and a lower plasticity index than the Be and Al matrices, resulting in interface separation during the fracture process. SPs located at the Be/Al interfaces did not act as microcrack sources for the matrices, whereas highly faceted SPs within the Be grains induced microcrack formation and decreased the elongation. The failure mode of the cast Be-Al-0.4Sc alloy still involved the brittle cleavage failure of Be regions and ductile dimple failure of Al regions, which was not the case with the cast Be-Al alloy.

36 MATERIALS SCIENCE↗
DOE OSTI2019

Experimental Research on Tensile Behavior of Advanced High-Strength Steel DP600 at High Strain Rate

We conducted a series of experimental investigations on the mechanical properties of DP600 dual-phase steel under strain rates from 0.001 to 3000 s{sup −1} at room temperature. A split Hopkinson tensile bar was used to capture a high strain rate response. A comparison of the quasi-static and dynamic experiment results showed that the yield strength and ultimate tensile strength increased significantly under dynamic conditions. Fractographs were analyzed by scanning electron microscopy, which indicated a better ductility at a high strain rate. Based on the experimental results, the Johnson–Cook constitutive model was applied to describe the tensile deformation behavior under various strain rates. A reasonable agreement was observed between the experimental data and the Johnson–Cook model. A metallographic analysis of tensile specimens showed that the average ferritic grain size of the fractured specimens at a high strain rate was smaller than that of the fractured specimens under quasi-static conditions. Ferritic grain refinement proved that high-speed forming could enhance the forming ability of the DP600 steel.

36 MATERIALS SCIENCE↗