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1.
Medical-grade Ti6Al4V extra-low interstitial (ELI) alloy has widespread applications in the biomedical industry due to its excellent corrosion and wear resistance. Even though 3D printing offers geometry flexibility and rapid means of fabricating customized parts, 3D-printed parts are often plagued with defects including porosity, high residual stresses, and non-equilibrium structures. Thus, post-processing heat treatments may be required to optimize its properties for engineering applications. In this study, the effect of post-processing heat treatment on the microstructure, hardness, and wear properties of 3D-printed and conventionally produced medical-grade Ti6Al4V ELI alloy samples was investigated. In general, distinct α (alpha) lath and basket-weave lath structures with a high degree of orientation were observed within the microstructure of the as-printed samples. Heat treatment led to the growth of distinct continuous and discontinuous α-lath structures along prior β (beta) grain boundaries as well as basket-weave lath and the coalescence of V-shaped structures within the prior β-grains. The hardness of both the 3D-printed and conventionally produced samples increased after heat treatment (≥ 400 HV), regardless of the cooling rate and aging temperature. After being water quenched/aged, the 3D-printed samples at 500 °C had the highest hardness values owing to the presence of coarse V-shaped structures. Furthermore, the V-shaped structures were always harder than all other structures regardless of the heat treatment and manufacturing process used, indicating that these structures dictate the overall mechanical integrity of the material. X-ray diffraction and electron probe microanalysis indicated that the V-shaped structures are rich in aluminum and titanium content, which can form hcp α′ (AlTi3) intermetallic phases. The 3D-printed samples had higher wear resistance overall than the conventionally produced samples regardless of the heat treatment used. Aging at 500 °C led to a higher coefficient of friction after 3D printing owing to an increase in α-phases. Therefore, during heat treatment, the microstructure and properties of medical-grade Ti6Al4V ELI alloy are significantly affected by the starting microstructure, the rate of cooling below the β-transus, and aging temperature and time, regardless of the manufacturing process used. 相似文献
2.
Precipitation sequences in a Cu-14 pct Al-4 pct Ni (wt pct) shape memory alloy were studied by means of transmission electron
diffraction and microscopy as well as X-ray microanalysis techniques. On aging thin foil specimens up to 550 °C in the electron
microscope, an as-quenched sample having a mixture of 2H-type and D0 3-type metastable structures transformed to the stable simple cubic γ 2 phase at or above 450 °C. The remaining matrix either showed precipitates of the fcc α-phase on prolonged annealing at 500
to 550 °C for a longer period, or transformed to martensite on cooling below the M
s temperature (~150 °C). 相似文献
3.
The formation of the ω -phase during the decomposition of a retained βTi (7.1 wt pct Fe) alloy was studied by means of Mössbauer spectroscopy and X-ray diffraction. The linear dependence of (I.S.) ω on the Fe content of the alloy upon quenching was used to derive C ω-the Fe concentration of the ω-phase in alloys aged at 350°C. C ωwas found to decrease with increasing aging time, from 5.91 wt pct Fe after 45 mins, to 4.5 wt pct Fe after 180 h of aging. The kinetic of the ω-phase formation is discussed qualitatively. The dependence of the (I.S.) β and (Q.S.) β on the iron concentration-C β was also determined. 相似文献
4.
The phase transformations occurring in a quenched and aged β-phase (bcc) Ti-40 at. pct V+1 at. pct Si alloy have been investigated using transmission electron microscopy. Upon aging at 570°C, the most probable precipitation sequence is the following: bcc super-saturated solid solution»bcc zones»(Ti, V) xSi y (hexagonal)»(Ti, V) 3Si (tetragonal). The bcc precipitates may be considered Si-rich GP zones which are homogeneously nucleated and coherent with the matrix. The (Ti, V) xSi y particles are a rod-shaped transition phase aligned along the <100> matrix directions. The precipitation reaction causes two peaks in the dependence of the yield stress or hardness upon aging time. The particle diameter and interparticle spacings of the hexagonal silicides indicate that these precipitates are responsible for the second hardness peak. The bcc zones evidently cause the first hardness peak at short aging times at 570°C. 相似文献
5.
The effects of microstructure, impurity content, and testing temperature on the fracture toughness (as measured by the crack
tip opening displacement (CTOD)) and microcrack extension resistance curves ( R curves) of Ti-6Al-4V alloys were examined. At 0 °C, microstructure is the most influential factor in the toughness-strength
relationship. Acicular microstructure specimens have a higher CTOD than specimens with equiaxed microstructures, regardless
of strength (0.2 pct proof stress) and impurity content. At −196 °C, impurity content becomes a controlling factor in the
toughness-strength relationship. Extra-low impurity (ELI) specimens, which have a lower impurity content, show a higher CTOD,
irrespective of microstructure. Microcracks extended from the notch tip before the maximum load was reached during testing
were investigated, and crack initiation ( δ
i) and extension-resistance properties were evaluated by obtaining exact R curves of the microcracks. At 0 °C, specimens with different microstructures and different impurity contents have almost
the same δ
i. But acicular-microstructure specimens with a higher CTOD at a given strength show a greater crack extension resistance.
At −196 °C, ELI specimens, which have a higher CTOD, show a larger crack extension resistance. It is concluded that the crack
extension-resistance property of the microcracks extended from the notch tip before the maximum load is a controlling factor
for the fracture toughness of Ti-6Al-4V alloys. 相似文献
6.
In the current work, several heat treatments were carried out below and above the beta-transition temperature of the Ti–6Al–4V alloy followed by aging at 550 °C for 6 hours. The resultant microstructures and their effects on the mechanical properties of Ti–6Al–4V alloy were investigated. The results showed that solution treatment of Ti–6Al–4V samples followed by water quenching from β and α/β fields raised the alloy hardness from 380 to 575 and 656? HV, respectively, while no remarkable changes were observed after aging. The hot tensile strength of the as-forged sample increased from 671 to 756?MPa after water quenching from the ß- or α/ß- field, while the air cooling from β-phase field decreased the tensile strength to 644 MPa. The fracture mode of the tensile samples was more ductile in case of the solution-treated samples compared to the as-forged samples. A subsurface layer was formed due to the diffusion of oxygen into the surface at high temperatures. This layer which is known as ‘oxygen diffusion layer’ masked the differences of wear behaviour of the specimens. 相似文献
7.
The constitution of the ternary system Al-Cr-Ti is investigated over the entire composition range using X-ray diffraction
(XRD), energy-dispersive X-ray spectroscopy (EDS), differential thermal analysis (DTA) up to 1500 °C, and metallography. Solid-state
phase equilibria at 900 °C are determined for alloys containing ≤75 at. pct aluminum and at 600 °C for alloys containing >75 at. pct
Al. A reaction scheme linking these solid-state equilibria with the liquidus surface is presented. The liquidus surface for
≤50 at. pct aluminum is dominated by the primary crystallization field of bcc β(Ti,Cr,Al). In the region >50 at. pct Al, the ternary L1 2-type phase τ forms in a peritectic reaction p
max at 1393 °C from L + TiAl. Furthermore, with the addition of chromium, the binary peritectic L + α(Ti,Al) = TiAl changes into an eutectic L = α(Ti,Al) + TiAl. This eutectic trough descends monotonously through a series of transition reactions and ternary peritectics
to end in the binary eutectic L = Cr 7Al 45 + (Al). 相似文献
8.
The hardness response, tensile behavior, and phase transformations occurring in a quenched and aged metastable β phase Ti-30
at. pct V-l at. pct Si alloy have been inves-tigated. Upon aging at 570°C, as-quenched samples show a broad hardness peak
which is associated with the formation of rod-like, hexagonal (Ti,V) xSi y transition phase precipi-tates. The equilibrium silicide is observed upon aging at 570°C in the form of faceted, tetragonal
particles. A loss of tensile ductility and a transition to intergranular fracture occurs after extended aging at 570°C and
is related to Si segregation to the grain bound-aries. Comparing the behavior of Ti-30V to that of Ti-30V-lSi shows that the
presence of Si strongly retards α-phase formation. However, a substantial age hardening re-sponse still occurs upon aging
at 450°C, especially after prior cold work (the yield strength increases from 635 to 982 MPa). This hardening response is
combined with a retention of a ductile, transgranular fracture even after extended aging at 450°C. Aging first at 570°C followed
by aging at 450°C results in an increase in the volume fraction of α-phase formed but a subsequent decrease in ductility and
hardness response upon aging at 450°C. These results are discussed in terms of the structure/property relationships which
result from the influence of Si on the formation of, a) (Ti.V) xSi y precipitates, b) the equilibrium silicide, and c) the α-phase. 相似文献
9.
The compressive yield stress of Fe-26Al with additives Ti (0.5 to 4 at. pct), Cr (0.5 to 8 at. pct), Mo (0.5 to 4 at. pct), and V (0.5 to 8 at. pct) at 1073 K (800 °C) has been determined. The effect of the concentration of diverse solutes on the yield stress at 1073 K (800 °C) was compared, and the additivity of the effects of solutes was tested. The effects in iron aluminides with two solutes (V and Ti, Ti and Cr, V and Cr) are compared with those of a single solute V, Ti, and Cr. It is found that the additivity of yield stress increments is valid only for lower solute concentrations. When the amount of the solute atoms increases, the yield stress increment is substantially higher than the sum of the yield stress increments of single solutes. This behavior is related to the high-temperature order in iron aluminides. 相似文献
10.
This study focuses upon the evolution of microstructures during solidification processing of several intermetallic alloys
around the Ll 2 phase in the Al-rich corner of the Al-Ti-Ni ternary system. The alloys were produced by double induction melting and subsequent
homogenization followed by furnace cooling. The microstructure was characterized by means of optical and scanning electron
microscopy with energy-dispersive spectroscopy (EDS) analysis and X-ray diffraction. The microstructural evolution in homogenized
alloys was dependent on both nickel and titanium content. Very fine precipitates of Al 2Ti were observed within the Ll 2 phase in alloys containing 62 to 65 at. pct Al and at least 25 at. pct Ti. The Al 2Ti precipitates are stable at least up to 1000 °C and undergo complete dissolution at 1200 °C. In alloys containing around
66 at. pct Al and 25 to 31 at. pct Ti, phases such as Al 3Ti, Al 5Ti 2, and Al 11Ti 5 were observed. A modified room temperature isotherm in the Al-Ti-Ni ternary system is proposed, taking into account the existence
of Al 2Ti, Al 11Ti 5, Al 5Ti 2, and Al 3Ti in equilibrium with the Ll 2 phase. It seems that at room temperature, the Ll 2 phase field for homogenized alloys is extremely small. It will be practically impossible to obtain a single-phase microstructure
at room temperature in the Al-Ti-Ni ternary alloys after homogenization at 1000 °C followed by furnace cooling.
S. BISWAS, formerly Graduate Student, Department of Mechanical Engineering, University of Waterloo 相似文献
11.
This article presents a detailed assessment of microwave (MW) heating, isothermal sintering, and the resulting tensile properties of commercially pure Ti (CP-Ti), Ti-6Al-4V, and Ti-10V-2Fe-3Al (wt pct), by comparison with those fabricated by conventional vacuum sintering. The potential of MW sintering for titanium fabrication is evaluated accordingly. Pure MW radiation is capable of heating titanium powder to ≥1573 K (1300 °C), but the heating response is erratic and difficult to reproduce. In contrast, the use of SiC MW susceptors ensures rapid, consistent, and controllable MW heating of titanium powder. MW sintering can consolidate CP-Ti and Ti alloys compacted from ?100 mesh hydride-dehydride (HDH) Ti powder to ~95.0 pct theoretical density (TD) at 1573 K (1300 °C), but no accelerated isothermal sintering has been observed over conventional practice. Significant interstitial contamination occurred from the Al 2O 3-SiC insulation–susceptor package, despite the high vacuum used (≤4.0 × 10 ?3 Pa). This leads to erratic mechanical properties including poor tensile ductility. The use of Ti sponge as impurity (O, N, C, and Si) absorbers can effectively eliminate this problem and ensure good-to-excellent tensile properties for MW-sintered CP-Ti, Ti-10V-2Fe-3Al, and Ti-6Al-4V. The mechanisms behind various observations are discussed. The prime benefit of MW sintering of Ti powder is rapid heating. MW sintering of Ti powder is suitable for the fabrication of small titanium parts or titanium preforms for subsequent thermomechanical processing. 相似文献
12.
Two alloys made by adding 5 or 10 at. pct, respectively, of Ni-18.9 at. pct Al to an Ir-15 at. pct Nb alloy were investigated.
The microstructure and compressive strength at temperatures between room temperature and 1800 °C were investigated to evaluate
the potential of these alloys for ultra-high-temperature use. Their microstructural evolution indicated that the two alloys
formed fcc and L1 2-Ir 3Nb two-phase structures. The fcc and L1 2 two-phase structures were examined by transmission electron microscopy (TEM) and scanning electron microscopy (SEM). The
0.2 pct flow stresses were above 1000 MPa at temperatures up to 1200 °C, about 150 MPa at 1500 °C, and over 100 MPa at 1800
°C. The strength of the quaternary Ir-base alloys at 1200 °C was even higher than that of Ir-base binary and ternary alloys.
And the strength of quaternary Ir-Nb-Ni-Al was equivalent to that of the Ir-15 at. pct Nb binary alloy at 1800 °C. The compressive
ductility of quaternary (around 20 pct) was improved drastically compared with that of the Ir-base binary alloy (lower than
10 pct) and the ternary Ir-base alloys (about 11 pct). An excellent balance of high-temperature strength and ductility was
obtained in the alloy with 10 at. pct Ni-18.9 at. pct Al. The effect of Ni and Al on the strength of the Ir-Nb binary alloy
is discussed. 相似文献
13.
This paper addresses the use of alloying additions to titanium alloys for additive manufacturing (AM) with the specific objective of producing equiaxed microstructures. The additions are among those that increase freezing ranges such that significant solutal undercooling results when combined with the rapid cooling rates associated with AM, and so be effective in inducing a columnar-to-equiaxed transition (CET). Firstly, computational thermodynamics has been used to provide a simple graphical means of predicting these additions; this method has been used to explore additions of Ni and Fe to the alloy Ti–6Al–4V (Ti64). Secondly, an experimental means of determining the minimum concentration of these alloying elements required to effect the CET has been developed involving gradient builds. Thirdly, it has been found that additions of Fe to Ti64 cause the alloy to change from an α/β Ti alloy to being a metastable β-Ti alloy, whereas additions of Ni do not produce the same result. This change in type of Ti alloy results in a marked difference in the development of microstructures of these compositionally modified alloys using heat treatments. Finally, hardness measurements have been used to provide a preliminary assessment of the mechanical response of these modified alloys. 相似文献
14.
The coarsening of γ′ and the elevated-temperature hardness have been studied as a function of molybdenum content, time, and
temperature in experimental wrought nickel-base superalloys. The alloys were selected from a systematic series containing
3, 4 1/2, and 6 wt pct Al and 1 wt pct Al plus 3 1/2 wt pct Ti. Each of the aluminum (plus titanium) series consisted of four
alloys containing 0, 2, 5, and 8 wt pct Mo. The alloys were solution-treated plus aged up to 112 h at 1700°F (925°C) and up
to 1000 h at 1400°F (760°C). Molybdenum retards the coarsening of γ′ on aging; this retarding effect is most pronounced in
alloys containing 6 wt pct Al. The coarsening of γ′ particles follows Ostwald ripening kinetics. Hardness testing in vacuo at temperatures up to 1750°F (955°C) shows that molybdenum also increases the elevated-temperature hardness significantly.
The relation of elevated-temperature hardness to the volume fraction of γ′ is considered, and the influence of aluminum and
titanium contents is discussed. 相似文献
15.
The purpose of this investigation was to establish the reaction path during processing of a 25 vol pct TiO 2 preform and molten Al composite by pressure infiltration. Initial preform temperatures between 550° and 850 °C, melt temperatures
from 715° to 850 °C, and two postinfiltration cooling rates were considered. The reaction path between molten Al and TiO 2 under the conditions examined involved three steps: the final products being α-Al 2O 3, Al 3Ti, and excess Al. It was further observed that the morphology of the reactant TiO 2 whisker was maintained throughout the reaction sequence, the M 2O 3 to Al 2O 3 transformation involving diffusion-controlled replacement of Ti atoms by Al. Finally, reaction completion involved heterogeneous
nucleation of Al 3Ti from the Ti-saturated melt at the Al 2O 3 interface. 相似文献
16.
The hot corrosion behavior of a series of wrought nickel-base superalloys containing approximately 13 wt pct Cr was studied
as a function of molybdenum content from 0 to 8 wt pct Mo in each of four Al + Ti levels (3 pct Al, 41/2 pet Al, 6 pct Al,
and 1 pct Al-31/2 pct Ti). Specimens were tested in a burner rig with a 5 ppm sea salt concentration in a 1 pct sulfur diesel
fuel burned at a 30:1 air-to-fuel ratio and were cycled between 1600°F (870°C) or 1800°F (980°C) and room temperature every
50 h during a 1000-h test. It was found that molybdenum significantly reduced the amount of hot corrosion attack at 1600°F
(870°C) for the three aluminum-containing series of alloys. For the titanium-containing series of alloys tested at 1600°F
(870°C) and all alloys exposed at 1800°F (980°C), molybdenum exerted little influence on hot corrosion behavior. Aluminum
was found to markedly increase sulfidation rates at both 1600°F (870°C) and 1800°F (980°C) for all molybdenum levels. Titanium
appeared to be beneficial to the hot corrosion resistance of these alloys at 1600°F (870°C) and detrimental at 1800°F (980°C).
It was further noted that 1600°F (870°C) represented a more severe sulfidation condition than 1800°F (980°C) under these test
conditions. 相似文献
17.
Aluminum (Al)-V master alloys have attracted attention, because they can potentially be efficient grain refiners for wrought aluminum alloys. In this paper, the microstructure and factors affecting the microstructure of Al-4 wt pct V master alloys were investigated by means of controlled melting and casting processes followed by structure examination. The results showed that the type and morphology of the V-containing phases in Al-V master alloys were strongly affected by the temperature of the melt, concentration of vanadium in solution in the melt and the cooling conditions. Two main V-containing phases, Al 3V and Al 10V, which have different shapes, were found in the alloys prepared by rapid solidification. The Al 3V phase formed when there were both a high temperature (1273 K to 1673 K (1000 °C to 1400 °C)) and a relatively high vanadium content of 3 to 4 wt pct, while the Al 10V phase formed at a low temperature (<1373 K (1100 °C)) or a low vanadium content in the range of 1 to 3 wt pct. The results also showed that the type of V-containing phase that formed in the Al-4 wt pct V master alloy was determined by the instantaneous vanadium content. 相似文献
18.
The present study was carried out on four steels containing 0.1 pct C-1.5 pct Mn-0.003 pct B* in common, with additions of 1 pct Cr, 0.5 pct Mo, 0.25 pct Mo + 1 pct Cr, 0.2 pct Ti + 1 pct Cr. They were designated, accordingly, as Cr, Mo, Mo-Cr, and Cr-Ti steels. All the steels exhibited a complete lath martensite microstructure with thin interlaths of retained austenite (≈0.05 pct) in the quenched condition. The normalized microstructures, granular bainite, contained massive areas of ferrite and granules of bainite laths. Both microconstituents contained a fine dispersion of cementite particles (size ≈50 Å) together with high dislocation densities. A mechanism explaining their for-mation has been given. The Cr steel, due to its low hardenability, showed in addition polygonal ferrite in the neighborhood of the so-called M-A constituent (twinned martensite and/or austenite). The annealed microstructure (using a cooling rate of 0.033 °C s ?1) of the Cr steel consisted of coarse ferrite-pearlite. Addition of 0.2 pct Ti to the Cr steel markedly refined the structure, whereas an addition of 0.25 pct Mo altered the microstructure to ferrite-lower bainite. In the 0.5 pct Mo steel, polygonal ferrite was found to be completely missing. The mechanical properties of the four steels after quenching, normalizing, and annealing were investigated via hardness and tensile test mea-surements. An empirical equation, relating the ultimate tensile strength to the steel composition, for steels that had granular bainite microstructures in the normalized condition, was proposed. The fracture surfaces exhibited cleavage and variable-size dimples depending on the microstructure and steel composition. 相似文献
19.
Fractography and metallographic sectioning were used to investigate the influence of microstructure on the fracture mechanism
and fracture toughness (K Ic) of normal interstitial and extra low interstitial (ELI) Ti-5Al-2.5Sn at 20 K (-423°F) and 77 K (-320°F). Plates of each
grade were mill annealed at 815°C (1500°F) followed by either air or furnace cooling. These variations in composition and
cooling rate resulted in differences in the volume fraction and internal structure of the dispersed β phase and in the ordering
of the α matrix. The ELI alloys were tougher than the normal interstitial plates. K Ic of the furnace-cooled ELI plate was 25 pct lower than that of the air-cooled ELI material. Variations in cooling rate had
no influence of K Ic of the normal interstitial alloys. Fractography showed that a large portion of the fracture surfaces were covered with elongated
dimples commonly called “flutes.” Metallographic sections of specimens deformed at 77 K showed that these features form at
the intersections of slip bands or deformation twins with grain or twin boundaries. Ordering and higher interstitial levels
increase the local strain in slip bands resulting in void nucleation at lower macroscopic strains and lower K Ic values.
Formerly Graduate Student, Department of Metallurgy and Materials Science, Carnegie-Mellon University, Pittsburgh, PA. 相似文献
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