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1.
The influence of microstructure and strength on the fracture mode and toughness of 7XXX series aluminum alloys 总被引:5,自引:0,他引:5
The effects of microstructure and strength on the fracture toughness of ultra high strength aluminum alloys have been investigated.
For this study three ultra high purity compositions were chosen and fabricated into 1.60 mm (0.063 inches) sheet in a T6 temper
providing a range of yield strengths from 496 MPa (72 ksi) to 614 MPa (89 ksi). These alloys differ only in the volume fraction
of the fine matrix strengthening precipitates (G. P. ordered + η′ ). Fracture toughness data were generated using Kahn-type
tear tests, as well asR-curve andJ
c
analyses performed on data from 102 mm wide center cracked tension panel tests. Consistent with previous studies, it has
been demonstrated that the toughness decreases as the yield strength is increased by increasing the solute content. Concomitant
with this decrease in toughness, a transition in fracture mode was observed from predominantly transgranular dimpled rupture
to predominantly intergranular dimpled rupture. Both quantitative fractography and X-ray microanalysis clearly demonstrate
that fracture initiation for the two fracture modes occurred by void formation at the Cr-dispersoids (E-phase). In the case of intergranular fracture, void coalescence was facilitated by the grain boundary η precipitates. The
difference in fracture toughness behavior of these alloys has been shown to be dependent on the coarseness of matrix slip
and the strength differential between the matrix and precipitate free zone (σM-σPFZ). A new fracture mechanism has been proposed to explain the development of the large amounts of intergranular fracture
observed in the low toughness alloys.
Formerly a Research Assistant at Carnegie-Mellon University 相似文献
2.
Slip behavior, fracture toughness, and fatigue thresholds of a high purity Al-Li-Cu alloy with Zr as a dispersoid forming
element have been studied as a function of aging time. The fracture toughness variation with aging time has been related to
the changes in slip planarity,i.e., slip band spacing and width. Although the current alloy exhibits planar slip for all aging conditions examined, the crack
initiation toughness,Klc, compares favorably with those of 2XXX and 7XXX aluminum alloys. Near threshold fatigue crack growth results in air and vacuum
suggest that irregularities in the crack profile and the fracture surfaces and slip reversibility are some of the major contributing
factors to the crack growth resistance of this alloy. 相似文献
3.
The influence of Mn content on the ductile-brittle transition in 16 to 36 wt pct Mn steels was investigated and interpreted
in light of the evolving microstructure. It was found that when hcp ε martensite is present in the as-quenched condition or
forms during deformation, it lowers the toughness. In 25Mn steel, the stress concentrations at e plate intersections result
in the formation of planar void sheets along the {111}γ planes. The deformation-induced α’ martensite in 16 to 20 pct Mn alloys enhances the toughness, but leads to a ductile-to-brittle
transition at low temperatures that is due to the intrusion of an intergranular fracture mode. Binary alloys with greater
than 31 pct Mn also fracture in an intergranular mode at 77 K although the impact energy remains quite high. Auger spectroscopy
of the fracture surfaces shows no evidence of significant impurity segregation, which suggests the importance of slip heterogeneity
in controlling intergranular fracture in these alloys. 相似文献
4.
Crack extension in commercial aluminum alloys proceeds by the “ductile” or fibrous mode. The process involves the large, ~1 μm to ~10μm, Fe-, Si-, and Cu-bearing inclusions which break easily, and the growth of voids at the cracked particles. The linking-up of the voids is accomplished by the rupture of the intervening ligaments, and this is affected by the fine, ~0.01μm precipitate particles that strengthen the matrix. The ~0.1μm Cr-, Mn-, and Zr-rich intermediate particles are more resistant to cracking and may enter the process in the linking-up stage. The fracture toughness of aluminum alloys therefore depends on a) the extent of the heavily strained region ahead of the crack tip, which is a function of the yield strength arad modulus, b) the size of the ligaments which is related tof c, the volume fraction of cracked particles, and c) the work of rupturing the ligaments. An approximate analysis predicts KIc varies asf c-1/6, and this is in agreement with measurements on alloys with comparable yield strength levels. Studies in which the aging conditions are altered for the samef cshow that the toughness decreases with increasing yield strength level. This degradation in toughness is related to the localization of plastic deformation. The tendency for localization is illustrated with the help of “plane strain” tension and bend specimens whose behavior is related to the toughness. Measurements of the strain distribution on the microscale show that slip is relatively uniformly distributed in a 7000-type alloy with low inclusion and particle content when the material is in the as-quenched and overaged conditions. In contrast the distribution is highly nonuniform in the peak aged condition where slip is concentrated in widely spaced superbands involving coarse slip bands with large offsets that crack prematurely. The connection between the tendency for slip localization and the fine precipitate particles which strengthen the matrix remains to be established. In overaged alloys grain boundary ruptures occur within the superbands. The amount of intergranular failure increases with grain size and is accompanied by a loss of fracture toughness. 相似文献
5.
The influence of Mn content on the ductile-brittle transition in 16 to 36 wt pct Mn steels was investigated and interpreted in light of the evolving microstructure. It was found that when hcp ε martensite is present in the as-quenched condition or forms during deformation, it lowers the toughness. In 25Mn steel, the stress concentrations at e plate intersections result in the formation of planar void sheets along the {111}γ planes. The deformation-induced α’ martensite in 16 to 20 pct Mn alloys enhances the toughness, but leads to a ductile-to-brittle transition at low temperatures that is due to the intrusion of an intergranular fracture mode. Binary alloys with greater than 31 pct Mn also fracture in an intergranular mode at 77 K although the impact energy remains quite high. Auger spectroscopy of the fracture surfaces shows no evidence of significant impurity segregation, which suggests the importance of slip heterogeneity in controlling intergranular fracture in these alloys. 相似文献
6.
Joseph S. Santner 《Metallurgical and Materials Transactions A》1978,9(6):769-779
Ten different alloys based on the 7075 composition were used to study the effect of purity level, dispersoid type, and heat
treatment on fracture toughness. Five purity levels ranging from 0.03 to 0.30 wt pct Fe + Si and two dispersoid types were
investigated. Each alloy was given two heat treatments: the standard T651 heat treatment or a special thermomechanical treatment
(TMT). Fracture toughness was measured using notched round tensile specimens taken from both the longitudinal and long-transverse
directions. The notched round tensile test was modified to give the “plastic energy per unit area”. This fracture toughness
parameter gave the same ranking for corresponding alloy/heat treatment combinations as the total energy per unit area measured
on precracked Charpy specimens. The fracture toughness ranking for the ten alloys was the same in the longitudinal and long-transverse
directions. This suggests the elongated distribution of constituent particles in the rolling direction does not change the
failure mechanism. Fractographic evidence showed a bimodal distribution of ductile dimple size in all ten alloys. The number
of large ductile dimples decreased with increasing purity level while the number of small ductile dimples increased. This
is interpreted to mean that the smaller dispersoid and hardening particles become increasingly important in controlling the
fracture toughness as the large intermetallic particles are eliminated by increasing the purity of these aluminum alloys.
Since thermomechanical processing controls the amount and type of these smaller particles, it is a useful means for increasing
fracture toughness in high purity aluminum alloys. 相似文献
7.
Metallurgical factors affecting fracture toughness of aluminum alloys 总被引:13,自引:0,他引:13
Crack extension in commercial aluminum alloys proceeds by the “ductile” or fibrous mode. The process involves the large, ~1
μm to ~10μm, Fe-, Si-, and Cu-bearing inclusions which break easily, and the growth of voids at the cracked particles. The
linking-up of the voids is accomplished by the rupture of the intervening ligaments, and this is affected by the fine, ~0.01μm
precipitate particles that strengthen the matrix. The ~0.1μm Cr-, Mn-, and Zr-rich intermediate particles are more resistant
to cracking and may enter the process in the linking-up stage. The fracture toughness of aluminum alloys therefore depends
on a) the extent of the heavily strained region ahead of the crack tip, which is a function of the yield strength arad modulus,
b) the size of the ligaments which is related tof
c, the volume fraction of cracked particles, and c) the work of rupturing the ligaments. An approximate analysis predicts KIc varies asf
c-1/6, and this is in agreement with measurements on alloys with comparable yield strength levels. Studies in which the aging
conditions are altered for the samef
cshow that the toughness decreases with increasing yield strength level. This degradation in toughness is related to the localization
of plastic deformation. The tendency for localization is illustrated with the help of “plane strain” tension and bend specimens
whose behavior is related to the toughness. Measurements of the strain distribution on the microscale show that slip is relatively
uniformly distributed in a 7000-type alloy with low inclusion and particle content when the material is in the as-quenched
and overaged conditions. In contrast the distribution is highly nonuniform in the peak aged condition where slip is concentrated
in widely spaced superbands involving coarse slip bands with large offsets that crack prematurely. The connection between
the tendency for slip localization and the fine precipitate particles which strengthen the matrix remains to be established.
In overaged alloys grain boundary ruptures occur within the superbands. The amount of intergranular failure increases with
grain size and is accompanied by a loss of fracture toughness.
This paper is based on an invited presentation made at a symposium on “Advances in the Physical Metallurgy of Aluminum Alloys”
held at the Spring Meetings of TMS-IMD in Philadelphia, Pennsylvania, on May 29 to June 1, 1973. The symposium was co-sponsored
by the Physical Metallurgy Committee and the Non-Ferrous Metals Committee of TMS-IMD 相似文献
8.
Crystallographic and fractographic studies have been carried out on hydrogen charged purified iron and on iron-silicon alloys
with silicon contents up to 3 pct. The specimens could be cracked by cathodically charging with hydrogen even without the
application of an external stress. An experimental technique was developed which enabled the exposure of the fracture surface
formed purely by hydrogen charging, and to contrast this with an adjacent mechanically induced fracture surface. In the case
of purified iron, hydrogen induced cracks are found to occur on potential slip planes whereas in the case of iron-3 pct silicon,
the crack follows the observed cleavage plane. Intermediate silicon content alloys showed transitional behavior. In agreement
with the variation of crack plane in the alloys, the fracture surface appearances was also drastically different, reflecting
the change in intrinsic toughness with alloy content. The observed transition from slip plane cracking to cleavage plane cracking
was found to occur near a silicon content of 0.7 pct. The observed behavior is discussed in terms of how the intrinsic toughness
of the ironbased lattice affects how hydrogen-induced cracks are formed.
Formerly at Carnegie-Mellon University. 相似文献
9.
10.
N. R. Moody R. E. Stoltz M. W. Perra 《Metallurgical and Materials Transactions A》1987,18(8):1469-1482
Fracture toughness values and tensile properties were determined in the Fe-Ni-Co superalloy IN903* as a function of hydrogen
concentration, loading rate, and grain size to define the effects of hydrogen on fracture toughness and failure modes. Tests
using precracked, precharged three point bend samples showed that fracture toughness decreased from 90 to 50 MPa-m1/2 as hydrogen increased from zero to 5000 appm. The decrease in fracture toughness was accompanied by a fracture mode change
from microvoid coalescence in the uncharged samples to principally slip band fracture with some twin band and ductile intergranular
fracture in the hydrogen charged samples. Fractographic observations and application of ductile fracture toughness models
showed that fracture initiated at matrix carbides in all samples. These carbides established the critical fracture distance
for all fracture processes observed in the fracture toughness samples. Hydrogen promoted the secondary fracture processes
of slip band, twin band, and ductile intergranular fractures which lowered both the critical fracture strain and the fracture
toughness of IN903. 相似文献
11.
Effect of Y, Sr, and Nd additions on the microstructure and microfracture mechanism of squeeze-cast AZ91-X magnesium alloys 总被引:7,自引:0,他引:7
Sunghak Lee Seung Hyuk Lee Do Hyang Kim 《Metallurgical and Materials Transactions A》1998,29(4):1221-1235
This study aims to investigate the effects of Y, Sr, and Nd additions on the microstructure and microfracture mechanism of
the four squeeze-cast magnesium alloys based on the commercial AZ91 alloy. Microstructural observation, in situ fracture tests, and fractographic observation were conducted on the alloys to clarify the microfracture process. Microstructural
analyses indicated that grain refinement could be achieved by small additions of alloying elements, although the discontinuously
precipitated Mg17Al12 phases still existed on grain boundaries. From in situ fracture observation of an AZ91-Sr alloy, it was seen that coarse needle-shaped compound particles and Mg17Al12 phases located on the grain boundary provided easy intergranular fracture sites under low stress intensity factor levels,
resulting in the drop in toughness. On the other hand, the AZ91-Y and AZ91-Nd alloys showed improved fracture toughness, since
deformation and fracture paths proceeded into grains rather than to grain boundaries, as the planar slip bands and twinnings
actively developed inside the grains. These findings suggested, on the basis of the well-developed planar slip bands and twinnings,
that the small addition of Y or Nd was very effective in improving fracture toughness. 相似文献
12.
《Acta Metallurgica Materialia》1993,41(9):2755-2764
A study of the mixed mode I and III fracture resistance of four 2034 aluminum alloys with varying manganese content is presented in this paper. The mixed mode fracture tests are carried out using modified compact tension specimens. As there is no standard test method for mixed mode fracture, special formulations of the J integral are used to characterize the mixed mode fracture resistance. The results indicate that the overall effect of mode III shear component is to lower the critical J-integral energy by enhancing the tendency for shear instability and early void formation. Manganese present in small amounts forms intermediate size dispersoids which increase the strength and work hardening ability without the loss of fracture toughness. In larger amounts manganese forms large particles which lower the fracture toughness significantly. These micromechanisms and those of mixed mode fracture are discussed. 相似文献
13.
Mechanical behavior of double-aged AA8090 总被引:3,自引:0,他引:3
The short-transverse fracture toughness of AA8090 is dramatically improved by double aging treatments, which produce a transition from coarse planar slip to homogeneous deformation. Although the fracture mode remains intergranular, stress concentrations across the weak, highangle boundaries are reduced by homogeneous deformation, ultimately increasing fracture toughness. This behavior is attributed to dissolution of the shearable phase(δ’) and growth of the strong precipitate (S’). Predictions of slip distribution agree fairly well with observed deformation behavior. A number of tempers with improved strength-toughness relationships were developed, and fatigue crack growth behavior in laboratory air was not affected by double aging. 相似文献
14.
Melts of commercially pure liquid aluminum, and an Al-7Si-0.3Mg alloy, were cast into molds designed to produce entrainment
of oxide film defects. The melts were held for periods of up to 20 minutes to investigate whether changes in the oxide film
defects in the melt could occur, once sufficient time had elapsed for consumption of their internal atmosphere. The alloys
were characterized by the determination of their Weibull modulus, examination of fracture surfaces under scanning electron
microscopy (SEM), and determination of their porosity characteristics. The Weibull moduli of the ultimate tensile strength
(UTS) values of the Al-7Si-0.3Mg alloy were reduced initially by holding in the liquid state for 10 minutes, but then the
values increased after holding for 20 minutes. This high Weibull modulus was found despite oxide films being observed on the
fracture surfaces. In the case of the commercial purity Al, the UTS Weibull moduli increased only slightly with holding for
20 minutes. The results suggested that holding of Al alloys in the liquid state influenced the scatter of mechanical properties
by influencing the porosity content of the castings, which was related to their oxide film content. Some evidence for healing
of a double oxide film defect with time was also found in the commercial purity Al alloy. 相似文献
15.
Effect of size and shape of tungsten particles on dynamic torsional properties in tungsten heavy alloys 总被引:3,自引:0,他引:3
Dong-Kuk Kim Sunghak Lee Heung-Sub Song 《Metallurgical and Materials Transactions A》1999,30(5):1261-1273
The effect of the size and shape of tungsten particles on dynamic torsional properties in tungsten heavy alloys was investigated.
Dynamic torsional tests were conducted on seven tungsten alloy specimens, four of which were fabricated by repeated sintering,
using a torsional Kolsky bar, and then the test results were compared via microstructure, mechanical properties, adiabatic shear banding, and deformation and fracture mode. The size of tungsten particles
and their hardness were increased as sintering temperature and time were increased, thereby deteriorating fracture toughness.
The dynamic torsional test results indicated that in the specimens whose tungsten particles were coarse and irregularly shaped,
cleavage fracture occurred predominantly with little shear deformation, whereas shear deformation was concentrated into the
center of the gage section in the conventionally fabricated specimens. The deformation and fracture behavior of the specimens
having coarse tungsten particles correlated well with the observation of the in situ fracture test results, i.e., cleavage crack initiation and propagation. These findings suggested that there would be an appropriate tungsten particle
size because the cleavage fracture mode would be beneficial for the “self-sharpening” of the tungsten heavy alloys. 相似文献
16.
The fracture toughness of Nb-Cr-Ti solid-solution alloys has been shown to be greatly improved by Ti addition, but the mechanism
of toughness enhancement has not been established. In this study, critical experiments were performed on the tough Nb-Cr-Ti
alloy to characterize the crack-tip fracture process and to investigate the origin of fracture toughness. In addition, theoretical
calculations of the unstable stacking energy (USE) and the Peierls-Nabarro (P-N) energy and stress were performed as a function
of Ti content in the Nb-Cr-Ti alloys. The experimental results indicate that the fracture toughness in the tough Nb-Cr-Ti
alloy originates from extensive dislocation emission that suppresses cleavage crack propagation from the crack tip. The theoretical
calculation indicates that Ti addition lowers the P-N energy and stress, but has little effect on the USE. These results are
used to elucidate the effects of Ti addition on cleavage fracture in Nb-Cr-Ti alloys by considering the influence of the P-N
energy and stress values on (1) dislocation mobility, (2) crack-tip dislocation emission, (3) fracture toughness, and (4)
brittle-to-ductile fracture transition. It is concluded that dislocation emission in the Nb-Cr-Ti alloys appears to be controlled
by the P-N energy, which influences dislocation mobility, rather than by the USE, which influences dislocation nucleation.
Ti increases the fracture toughness of Nb-Cr-Ti alloys by increasing dislocation mobility and dislocation emission from the
crack tip through a reduction of the P-N energy and stress.
An erratum to this article is available at . 相似文献
17.
《Acta Metallurgica》1985,33(3):527-537
Investigation of the deformation behaviour of AlLi based alloys containing zirconium as a grain-refining addition shows that the poor toughness properties are attributed to the intense coplanar slip associated with δ' (Al3Li) precipitation being unimpeded by the grain structure as a result of the pronounced deformation texture present in the sheet product; fracture proceeds via a transgranular shear failure mode which limits toughness. Changes in composition and thermomechanical treatment have been utilised in order to encourage the formation of additional precipitate phases, and, whilst δ' confers the major increment of strength to all AlLi based alloys, widespread precipitation of S phase (Al2CuMg) within AlLiMgCuZr alloys is shown to influence strongly the deformation behaviour; in particular the propensity towards slip coplanarity is reduced, and significant improvements in toughness are obtained. Additionally, by promoting homogeneous deformation within the grain structure, the presence of S phase causes the material to display isotropic properties even though a strong texture remains in the zirconium-refined sheet product. 相似文献
18.
The effect of hydrogen on the tensile behavior of austenitic stainless steel welds was studied in two AISI 300 series alloys
and two nitrogen strengthened alloys. The microstructure of these welds typically contained several percent ferrite in an
austenite matrix. Hydrogen was found to reduce the ductility of all welds; however, the severity of ductility loss increased
with increasing tendency to deform via a planar slip mode. In materials exhibiting large degrees of slip planarity, 304L and
308L, hydrogen changed the fracture mode from dimple rupture to a mixed mode of ductile and brittle fracture associated with
the austenite-ferrite interface. The two alloys, 22-13-5 and 309S, which tend to deform by cross slip mechanisms, showed smaller
losses in ductility even though hydrogen assisted the ductile rupture process by aiding void growth and coalescence, without
changing the fracture mode. Varying the amount of ferrite from approximately one to 10 pct had no significant effect on performance
in hydrogen. 相似文献
19.
L. A. Simpson 《Metallurgical and Materials Transactions A》1981,12(12):2113-2124
In many ductile commercial alloys, fracture is initiated at second phase particles. In this work, the initiation process in
Zr-2.5 pct Nb pressure tube alloy is examined. In particular, the conditions for fracture of a hydride platelet in a zirconium
matrix are sought, by testing tensile specimens containing hydrides oriented with the normals of the platelet parallel to
the tensile axis. Acoustic emission is monitored to signal the fracture event. It is concluded that some plastic deformation
must precede hydride fracture and that fracture is encouraged by a triaxial stress state. The fracture mechanism appears to
be one of slip-induced crack nucleation in the hydrides with the critical event being the growth of this crack to a critical
size.
Formerly on leave with the Materials Research Laboratory, Brown University, Providence, RI 02912 相似文献
20.
K. T. Venkateswara Rao Weikang Yu R. O. Ritchie 《Metallurgical and Materials Transactions A》1989,20(3):485-497
Mechanisms influencing the plane-strain fracture toughness behavior of commercial aluminum-lithium alloys at cryogenic temperatures
are investigated as a function of microstructure and plate orientation. It is confirmed that certain alloys show a markedincrease in tensile ductility and toughness withdecrease in temperature, although such behavior is not found in the short-transverse orientations, or for all alloys and aging conditions.
Specifically at lower temperatures, the majority of Al-Li alloys, namely 2090-T8E41, 8091-T8X, 8090-T8X, and 2091-T351, show
a significantincrease in fracture toughness in the in-plane orientations (L-T, T-L), without any apparent change in fracture mode. Such behavior
is attributed primarily to loss of through-thickness constraint resulting from enhanced short-transverse delamination (termed
crack-divider delamination toughening), consistent with observed reductions in plane-strain ductility and short-transverse
(S-L, S-T) toughness. Conversely, in underaged microstructures of 8091, 8090, and peak-aged 2091, a decrease in toughness
with decreasing temperature is found for both L-T and S-L orientations, behavior, which is associated conversely with a fracture-mode
change from ductile void coalescence to brittle transgranular shear and integranular delamination at lower temperatures.
W{upeikang} Y{upu}, formerly with the Department of Materials Science and Mineral Engineering, University of California, Berkeley 相似文献