共查询到20条相似文献,搜索用时 15 毫秒
1.
Powder metallurgical techniques have been applied to the problem of preparing monolithic samples of Nb 3Sn of a homogeneity, density and stability suitable for unambiguous plastic deformation studies. Cold pressing and reaction
sintering, infiltration, and hot isostatic pressing (HIP) of Nb and Sn powders have been evaluated, with HIP processing producing
a decidedly superior structure. The most homogeneous structure was produced by HIP processing at 1630°C for one h at a pressure
of 160 MPa. A continuous matrix of Nb 3Sn was produced with a porosity of 2.6 pct and a secondary phase content of 3.3 pct. The principal secondary phase was NbO
and no unreacted Nb remained. The Nb 3Sn matrix was quite homogeneous with microprobe analysis revealing an off-stoichiometric composition of 72.2 to 73.2 pct Nb.
An equiaxed grain size of about 60 μm was developed and X-ray diffraction analysis revealed a high degree of long range order.
HIP processing at 1200°C produced a finer grain size, increased porosity, and an incompletely reacted structure involving
3.5 pct unreacted Nb. The composition of the Nb 3Sn phase was nearly the same, regardless of processing technique. Considerable evidence of dislocations arrayed in low angle,
sub-grain boundaries was observed in the 1630°C HIP processed material. Simple, isolated dislocations were predominant in
the 1200°C HIP processed material. 相似文献
2.
The influence of the extrusion ratio on sintering behavior of cold-extruded powder mixture Ti-48A1 has been investigated.
Both pressureless reaction sintering and hot isostatic pressing (HIP) without encapsulation were carried out. Moreover, two-step
sintering, i.e., combination of pressureless sintering and HIP, was conducted. It was found that both porosity and pore size in reactively
sintered specimens largely decrease with increaseing extrusion ratio. For a given extrusion ratio, the porosity after pressureless
sintering decreases with increasing temperature. Although a reduction of porosity can be reached by directly HIP specimens,
the effect of applied pressure in case of combined treatments is strongly dependent on extrusion ratio. By applying an extremely
high extrusion ratio of 350, material with a porosity of only 0.7 pct has been prepared by pressureless sintering and subsequent
HIP without encapsulation while a reverse treatment route led to a porosity of 5%. On the contrary, lower porosities were obtained for low extrusion ratios of 17 and 25 by HIP and following pressureless sintering.
The effect of extrusion ratio, as well as sintering temperature, was discussed. In addition, pore coalescing, gas penetration,
and swelling were considered in order to understand the effect of applying pressure. 相似文献
3.
Metal powders always contain a surface oxide layer, which is particularly tenacious in aluminum alloys. After hot pressing,
this oxide coats the particle boundaries and reduces the ductility. In this article, a study of the Al-17 wt pct Cu alloy
densified from rapidly solidified powder is presented. Different thermomechanical treatments were investigated to improve
the ductility of this material. Channel-die (CD) forging was performed at two temperatures (430 °C and 500 °C). Eight compression
runs were applied to the samples in each CD treatment. At 430 °C, three strain values per run were investigated (35, 50 and
70 pct). A bar was also extruded with a 40:1 ratio. Because of the small size of the samples, the ductility was assessed by
means of the ring expansion test and analyzed by post mortem (fracture surface and cross section) observations. No ductility was measured after CD compression at 430 °C, although it
appears from the fracture surface observations that increasing the strain per run has a beneficial effect. The CD compression
at 500 °C and extrusion were both successful at promoting ductility, extrusion being more effective. 相似文献
4.
Two Mg-1 wt pct Mn alloys containing 0.5 wt pct and 1 wt pct Nd have been processed by indirect extrusion at temperatures ranging from 548 K (275 °C) to 633 K (360 °C) and speeds between 2.8 and 11 mm/s. The microstructure and the texture of the extruded bars were analyzed in order to understand the effect of the processing parameters and of the rare-earth (RE) alloying additions on the texture development. Increasing the Nd content results in weak textures in which the predominant orientations are a function of the extrusion conditions. This may be explained by the occurrence of particle pinning of grain boundaries and by the nucleation of grains with a wider range of orientations. Mechanical tests were carried out in tension and in compression in all the processed samples at 10 ?3 s ?1 and room temperature. It was found that larger RE amounts give rise to the disappearance of the yield asymmetry and to an anomalously high activity of tensile twinning, especially at the lowest extrusion temperatures. This has been attributed to an increase of the critical resolved shear stress of basal slip due to the presence of Mg 3Nd coherent and semi-coherent intermetallic prismatic plates. 相似文献
5.
The flow and fracture behavior of hard-alpha Ti was studied as a function of nitrogen content, stress state, strain rate,
and temperature. Hard-alpha Ti specimens with nitrogen contents ranging from 2 to 11.6 wt pct were fabricated by powder-metallurgy
techniques. Stress-strain curves were obtained under various states of stress by performing uniaxial compression, indirection
tension, indentation, and plane-strain compression tests at two strain rates. By varying the test technique and the specimen
geometry, deformation and fracture in hard-alpha Ti was characterized for mean pressures as high as 6 times the flow stress.
Most of these tests were conducted at 954 °C, but some were performed at 25 °C, 927 °C, and 982 °C. The experimental results
indicated that, during compression testing at 927 °C to 982 °C, hard-alpha Ti exhibited substantial plastic deformation for
nitrogen contents less than 4 wt pct, but showed brittle fracture with little plastic flow for nitrogen contents of 5.5 to
11.6 wt pct. Both the yield and fracture strengths increase with increasing nitrogen content and pressure, but decrease with
increasing temperature. The yield strength increases with strain rate, while the fracture strength shows little or no rate
sensitivity. The fracture strength in tension is substantially lower than that in compression. These observed deformation
and fracture characteristics are explained on the basis of microcrack formation during inelastic deformation. 相似文献
6.
A 5083 Al alloy was synthesized using spray deposition processing with N 2 as the atomization gas. It was noted that the grains that were present in as-spray-deposited 5083 Al were equiaxed with an
average size of 15.2 μm. The matrix of the material was supersaturated with Mg and Mn. The asspray-deposited microstructure
contained irregular pores with porosity in the range of 0.1 to 5.4 vol pct, depending on spatial location in the preform.
The spray-deposited alloy was thermomechanically processed using extrusion and multiple-pass warm rolling to reduce grain
size and close porosity. It was observed that spray-deposited 5083 Al exhibited superplasticity following thermomechanical
processing by extrusion followed by rolling. Superplasticity was observed in the 500 °C to 550 °C temperature range and 3
× 10 −5 to 3 × 10 −3 s −1 strain rate range. The corresponding strain-rate-sensitivity factors were in the 0.25 to 0.5 range and increased with decreasing
strain rate. A maximum elongation of 465 pct was noted at 550 °C and 3 × 10 −5 s −1. The spray-deposited 5083 Al, thermomechanically processed by direct rolling, exhibited superplasticity in the same temperature
and strain rate ranges as those for the extruded and rolled materials. The superplastic elongation of the spray-formed and
rolled material was relatively low, being in the range of 250 to 300 pct. The deformation behavior is discussed in light of
the presence of porosity in the microstructure. 相似文献
7.
Lotus-type porous aluminums with porosities from 10 to 26 pct were fabricated with the Bridgman-type directional solidification method (Gasar). A vacuum atmosphere is critical to obtain high-porosity lotus-type porous aluminum by the Gasar process. The lotus-type porous aluminum was directionally solidified under a pure hydrogen pressure of 0.2 to 16 kPa. The influence of hydrogen pressure on the porosity and pore size in vacuum was investigated. The porosity and pore size increase with decreasing hydrogen pressure, but there exists a maximum porosity at some critical hydrogen pressure. Since a low hydrogen pressure is adopted, the effect of capillary pressure and hydrostatic pressure on the porosity becomes important. With the decreasing of hydrogen pressure, the influence of hydrostatic pressure and capillary pressure on porosity becomes stronger and stronger. The influence of melt height, which is proportional hydrostatic pressure, on porosity and pore size was investigated. The calculated porosities considering capillary pressure and hydrostatic pressure are in good agreement with experimental results. 相似文献
8.
Reduction of ZnO single particles with CO was investigated at atmospheric pressure from 1000° to 1500°C. Weight loss data
up to about 90 pct reduction were easily reproducible for the dense photoconductive grade ZnO particles but not for the American
grade samples, whose scatter was attributed to the 13 pct residual internal porosity and to impurities. The data agreed closely
with a mixed regime model, which pictures external diffusion acting in series with an irreversible first order kinetic process
at the surface. After the diffusional contribution was subtracted, activation energies of 37,900 (±2040) cal per mole and
20,600 (±10,200) cal per mole were obtained for the photoconductive and American grades, respectively. For the photoconductive
grade the mixed regime model gave a good fit over the entire temperature range. Diffusional limitations were approached at
1500°C. 相似文献
9.
This study evaluated the effects of superimposed hydrostatic pressure (138 to 1104 MPa) on densification and plastic flow behavior of porous iron (0.3 to 11.1 % porosity). Pressurization alone caused densification of the porous iron with the effect being most pronounced when the porosity was greater than 3.7% and the pressure above 276 MPa. For the porosities studied, densification as a result of pressurization increased with hydrostatic pressure and initial porosity. The 0.3% porosity iron was the only one whose density did not increase with pressurization or deformation under pressure. The effect of hydrostatic pressure on the flow stress of porous iron was small when densification resulting from pressurization was not a factor. The ductility was found to increase linearly with pressure and the effect of pressure on fracture strain increased with the initial porosity of the iron. Evaluation of the effect of hydrostatic pressure on development of porosity and growth during tensile deformation was limited to hydrostatic pressures of 138 and 276 MPa and iron compacts with initial porosities of 0.3, 1.5, and 3.7% because of the pressurization effects. It appeared that the porosity at fracture was similar in these compacts at both pressures but it was much larger than that observed at 0.1 MPa. The greater ductility of the iron compacts tested under hydrostatic pressure results from a decrease in the growth of pores with deformation and from a greater damage tolerance prior to fracture. As observed for porosity, the average maximum pore diameters at fracture for the compacts tested under pressure were similar but larger than those observed at 0.1 MPa. It appears that a general model of ductile fracture for porous materials cannot be based solely on a critical degree of dilation or on maximum pore extension as a fracture criterion. 相似文献
10.
Currently, consolidation of high Tc superconductor powders is done by sintering, which is not effective in the reduction of
porosity. This work assesses the feasibility of hot isostatic pressing (HIP) to obtain fully dense bulk superconductor using
HIP modeling and experimental verification. It is concluded that fully dense YBa 2Cu 3O 7 can be obtained in reasonable times at temperatures down to around 650 °C. The trade-offs between temperature, time, and
pressure are examined as well as the effects of powder particle size, powder grain size, and trapped gas pressure. The model
has. been verified by experiment under three conditions: 100 MPa HIP at 900 °C for 2 hours, 100 MPa HIP at 750 °C for 2 hours,
and sintering at 950 °C for 16 hours. The additional advantages of HIPing oxide superconductors are also discussed. 相似文献
11.
The Gd-Yb and Lu-Yb phase systems were established by thermal analysis, X-ray diffrac-tion, metallography, electron microprobe
and chemical analyses. The solubility of Yb in α-Gd ranges from 6.5 at. pct at 500°C to 19.0 at. pct at 1161°C. The addition
of Yb to Gd lowers the β (bec) to α (hcp) transformation temperature to an inverse peritectic reaction at 20.0 at. pct Yb and 1161°C. The addition
of Yb to Gd lowers the melting point of Gd to a monotectic horizontal at 1183°C which extends from 21.0 to 71.0 at. pct Yb.
The monotec-tic composition is 49.0 at. pct Yb. The solid solubility of Gd in Yb ranges from 0.2 at. pct at 500°C to 2.3 at.
pct at 819°C. The melting point of Yb is raised from 816°C to 819°C by the addition of Gd while the γ (bee) to β (fee) transformation temperature of Yb is lowered from 796°C to 780°C by the addition of Gd. The solubility of Yb in solid
Lu ranges from 6.0 at. pct at 800°C to 15 at. pct at 1530°C. The addition of Yb to Lu lowers the melting point of Lu to a
monotectic horizontal at 1530°C which extends from 15 to 90 at. pct Yb. The monotectic composition is approximately 30 at.
pct Yb. The solid solubility of Lu in Yb ranges from less than 0.1 at. pct at 500°C to 0.3 at. pct at 817°C. The addition
of Lu raises the melting point of Yb to 817°C and also raises the β (fee) to y (bec) transformation temperature to 798°C. 相似文献
12.
A commercial Al-6 pct Mg-0.3 pct Sc-0.3 pct Mn alloy subjected to equal-channel angular extrusion (ECAE) at 325 °C to a total
strain of about 16 resulted in an average grain size of about 1 μm. Superplastic properties and microstructural evolution
of the alloy were studied in tension at strain rates ranging from 1.4 × 10 −5 to 1.4 s −1 in the temperature interval 250 °C to 500 °C. It was shown that this alloy exhibited superior superplastic properties in
the wide temperature range 250 °C to 500 °C at strain rates higher than 10 −2 s −1. The highest elongation to failure of 2000 pct was attained at a temperature of 450 °C and an initial strain rate of 5.6
× 10 −2 s −1 with the corresponding strain rate sensitivity coefficient of 0.46. An increase in temperature from 250 °C to 500 °C resulted
in a shift of the optimal strain rate for superplasticity, at which highest ductility appeared, to higher strain rates. Superior
superplastic properties of the commercial Al-Mg-Sc alloy are attributed to high stability of ultrafine grain structure under
static annealing and superplastic deformation at T ≤ 450 °C. Two different fracture mechanisms were revealed. At temperatures higher than 300 °C or strain rates less than 10 −1 s −1, failure took place in a brittle manner almost without necking, and cavitation played a major role in the failure. In contrast,
at low temperatures or high strain rates, fracture occurred in a ductile manner by localized necking. The results suggest
that the development of ultrafine-grained structure in the commercial Al-Mg-Sc alloy enables superplastic deformation at high
strain rates and low temperatures, making the process of superplastic forming commercially attractive for the fabrication
of high-volume components. 相似文献
13.
Net-shaped porous Ti-rich Ti 51Ni 49 alloy with well-controlled porosity, pore size, and pore shape are fabricated by pressing-and-sintering compacts containing fine Ti and Ni powders and coarse NaCl powders. After sintering at 1323 K (1050 °C) for 30 minutes in a high vacuum, the NaCl space holder is removed by evaporation, and the remaining Ti and Ni powders are sintered with about 2.3 vol pct liquid phase. The sintered Ti 51Ni 49 compacts have porosities of 26, 64, 70, 78, and 85 pct, and no distortion is observed. DSC tests show that the M S temperature and Δ H are about 347 K (74 °C) and 28 J/g, respectively, and that they are almost independent of the porosity and close to those of wrought Ti-rich TiNi alloys. These porous Ti 51Ni 49 compacts exhibit a homogeneous microstructure, and the compressive properties and porosity are close to those of human bones. 相似文献
14.
The influence of the stress state on the plastic deformation of CMnSi, CMnSi(Nb), and CMnAlSi transformation induced plasticity
(TRIP)-aided steel has been analyzed. Imposing hydrostatic pressures up to 800 MPa during tensile deformation made it possible
to change the stress state of the tensile testing specimens. It was found that the ratio of normal to shear stresses has a
pronounced effect on the evolution of the microstructure, the austenite volume fraction change during straining, and the fracture
surface morphology. The CMnAlSi TRIP steel, which has the largest uniform elongation and the smallest equivalent strain at
fracture in the absence of the hydrostatic pressure, had a more pronounced improvement of all plastic characteristics at increasing
hydrostatic pressure. An increased austenite stabilization, brought about by the high hydrostatic pressure, was clearly observed.
The austenite stabilization results in a decrease of 20 °C to 25 °C of M
s
for an increase of 100 MPa of the hydrostatic pressure. The implications of the observations could be far-reaching for new
sheet forming technologies, such as hydroforming, as the full transformation potential is available for crashsensitive structural
parts by avoiding the formation of the martensite during forming operations. 相似文献
15.
In this study, gas-atomized amorphous Zr 58.5Nb 2.8Cu 15.6Ni 12.8Al 10.3 (Vitreloy 106a) containing 1280 ppmw oxygen was consolidated by equal-channel angular extrusion (ECAE). The powder was vacuum
encapsulated in copper cans and subjected to one extrusion pass in the temperature region above the glass transition temperature
( T
g) and below the crystallization temperature ( T
x). The effects of extrusion temperature and the extrusion rate on microstructure, thermal stability, hardness, and compressive
strength are investigated. Compression fracture surfaces were examined to determine the deformation mechanisms. The consolidates
in which the time-temperature-transformation (TTT) boundary was not crossed during processing exhibit differential scanning
calorimetry (DSC) patterns similar to the initial powder, with a slight decrease in T
x. Compressive strengths of about 1.6 GPa are recorded in the consolidates processed at 30 °C and 40 °C below T
x, which is close to what is observed in cast counterparts. The fracture surfaces exhibit vein patterns covering up to 90 pct
of the surface area in some samples, which are characteristic of glassy material fracture. The slight decrease in T
x after consolidation is attributed to thermal-history-dependent short-range order and formation of nanocrystalline islands.
The present results show that ECAE is successful in consolidation of metallic glass powder. This processing avenue opens a
new opportunity to fabricate bulk metallic glasses (BMGs) with dimensions that may be impossible to achieve by casting methods. 相似文献
16.
The present work is part of an investigation into the use of rapid solidification and powder metallurgy techniques to obtain
iron-boron alloys with good mechanical properties. Two Fe-B binary alloys and two ultrahigh boron tool steels were gas atomized
and consolidated by hot isostatic pressing (HIP) at temperatures ranging from 700 °C to 1100 °C to have a fine microstructure.
Optimum properties were achieved for the binary alloys at low consolidation temperatures, since the solidification mi-crostructure
from the original powders is eliminated and, at the same time, fine microstructures and low porosity are obtained in the alloys.
At high temperatures and low strain rates, three of the four alloys exhibited low stress exponents, but only the Fe-2.2 pct
B alloy showed tensile elongations higher than 100 pct. At low temperatures, only the Fe-2.2 pct B alloy deformed plastically.
This alloy showed values of tensile elongation and ultimate tensile strength that were strongly dependent on testing and consolidation
temperatures.
J.A. JIMéNEZ, Postdoctoral Fellow, formerly with Centra Nacional de Investigaciones Metalúrgicas, C.S.I.C 相似文献
17.
The effects of consolidation temperature on the development of microstructure and resulting mechanical properties of 2XXX
aluminum composites were studied in an effort to fabricate composites with enhanced properties. Type 2009 and 2124 aluminum
composites reinforced with 15 pct SiC particulates were produced at four different consolidation temperatures, i.e., 560 °C, 580 °C, 600 °C, and 620 °C, followed by extrusion at 450 °C. The 2124 Al-SiC
p
composites consolidated at 560 °C showed the most homogeneous and the finest microstructures with the best mechanical properties,
which were even better than the whisker-reinforced counterparts. All the results of the tensile tests, hardness tests, in situ scanning electron microscope (SEM) observations of the fracture process, and the apparent fracture toughness indicated that
the prominent mechanical property improvement observed in the 2124 Al-SiC
p
was associated largely with the reduction of volume fraction of the detrimental coarse and brittle manganese-containing particles,
as well as grain refinement. The detrimental manganese-containing particles that were routinely observed in the 2124 Al-SiC
composites were very effectively refined by the reduction of consolidation temperature, and they rather contributed to the
overall mechanical properties of the composites through Orowan-type strengthening and grain growth inhibition. 相似文献
18.
This work was carried out to investigate the effect of different Ti concentrations as a modifying agent on the microstructure and tensile properties of an in-situ Al-15 pctMg 2Si composite. Cast, modified, and homogenized small ingots were extruded at 753 K (480 °C) at the extrusion ratio of 18:1 and ram speed of 1 mm/s. Various techniques including metallography, tensile testing, and scanning electron microscopy were used to characterize the mechanical behavior, microstructural observations, and fracture mechanisms of this composite. The results showed that 0.5 pctTi addition and homogenizing treatment were highly effective in modifying Mg 2Si particles. The results also exhibited that the addition of Ti up to 0.5 pct increases both ultimate tensile strength (UTS) and tensile elongation values. The highest UTS and elongation values were found to be 245 MPa and 9.5 pct for homogenized and extruded Al-15 pctMg 2Si-0.5 pctTi composite, respectively. Fracture surface examinations revealed a transition from brittle fracture mode in the as-cast composite to ductile fracture in homogenized and extruded specimens. This can be attributed to the changes in size and morphology of Mg 2Si intermetallic and porosity content. 相似文献
19.
The system Cr?Pt has been investigated over the entire composition range by metallography, X-ray diffraction, and electron microprobe studies. There is only one intermediate phase and it has a Cr 3Si(A15)-type crystal structure. The fcc platinum terminal solid solution extends to 71 at. pct Cr at 1530°C and forms a congruent melting maximum at about 1790°C. Atomic ordering within this solid solution range begins at about 17 at. pct Cr and there is a continuous change from the Cu 3Au-type structure to the CuAu-type structure with increasing chromium content. Two eutectic reactions at 1530°C±10°C and at 1500°C ±10°C were indicated and there is evidence of a syntectic reaction at 1580°C±10°C. Platinum is soluble in the bcc chromium terminal solid solution up to about 10 at pct Pt at 1500°C but the solubility decreases rapidly at lower temperatures. 相似文献
20.
The hot compression behavior and microstructure evolution of ingot metallurgy (I/M) and powder metallurgy (P/M) processed
samples of the near- γ Ti-aluminide alloy, Ti-48Al-2Nb-2Cr (at. pct), were determined. Three I/M conditions and two P/M conditions were examined
in this study. Hot compression tests were performed in the temperature range of 1100 °C to 1300 °C at strain rates ranging
from 1.67×10 −1/s to 1.67×10 −4/s. The P/M materials consolidated by either hot isostatic pressing (“hipping”) or extrusion exhibited the best hot workability
in most cases. The P/M materials possessed finer, more homogeneous microstructures than the I/M materials. It was also noted
that improved workability was observed in materials with equiaxed microstructures without any lamellar constituents. 相似文献
|