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1.
D. Eylon J. A. Hall C. M. Pierce D. L. Ruckle 《Metallurgical and Materials Transactions A》1976,7(11):1817-1826
To establish correlations between microstructure and mechanical properties for the Ti-ll alloy, twelve different combinations
of hot die forging and heat treatment, in the a + 8 and Β phase regions, were investigated. The resulting heat treated forgings
were classified into four distinct categories based on their microstructural appearance. The room temperature tensile, post-creep
tensile, fracture toughness and fatigue crack propagation properties were measured along with creep and low cycle fatigue
at 566‡C. The creep, tensile, fatigue crack propagation and fracture toughness properties, grouped in a manner similar to
the microstructural categories. The fracture appearance and behavior of the cracks during propagation in fatigue and in fracture
toughness tests were examined, and correlations with the microstructure discussed. In the case of the fully transformed acicular
microstructure, it was found that the size and the orientation of colonies of similarly aligned α needles are dominant factors
in the crack behavior.
Formerly a National Research Council Associate, Air Force Materials Laboratory
Formerly with AFML 相似文献
2.
3.
P. Schmidt A. El-Chaikh H.-J. Christ 《Metallurgical and Materials Transactions A》2011,42(9):2652-2667
Aging of highly β-stabilized titanium alloys commonly leads to the formation of precipitate-free zones being susceptible to fatigue crack initiation.
Duplex aging improves the fatigue properties of metastable β titanium alloys by enhancing a homogeneous α phase formation. In this study a duplex-aging cycle was designed for Ti 38-644 (β-C). Depending on the prior processing history heat treatment parameters were adapted on the basis of microstructure studies,
hardness measurements and comparative tensile tests. The fatigue limit and fatigue crack growth threshold were determined
for duplex-aged β-C. The results indicate that duplex aging promotes a homogeneously precipitated α phase providing excellent values of the fatigue limit. Surface-related fatigue crack initiation was observed. Comparing the
fracture surfaces of direct- and duplex-aged β-C a transition of the tensile fracture mode from intergranular to predominantly transgranular was observed accompanied by
a gain in ductility at comparable yield strengths. This was assumed to be the reason for the slightly improved fatigue crack
growth behavior of duplex-aged as compared to direct-aged β-C. Along the entire heat treatment cycle the microstructure response was evaluated with regard to the particular effects
on the fatigue properties. The results indicate clearly that key to success is a completely recrystallized β microstructure and the reasonably controlled aging response. 相似文献
4.
The grindability of Ti-based alloys was analyzed by considering the fracture behavior of individual alloys in response to
the stress field of a grinding wheel. First, the stress field under a grinding wheel was computed by treating the grinding
wheel as a cylindrical disk with a flat region acting on a flat substrate. The initiation and propagation of microcracks in
the substrate was then examined on the basis of the contact stress field and one of two fracture criteria: (1) a critical
stress criterion for the onset of cleavage crack initiation, and (2) a critical stress intensity factor criterion for the
initiation and propagation of shear cracks. Grindability was computed as a function of grinding speed and microstructure for
several Ti-based cast alloys containing α, α + β, or β microstructure with or without the intermetallic precipitates. Model
predictions indicated that the grindability of Ti alloys increases with decreasing fracture toughness or tensile ductility.
The theoretical results are compared against experimental data in the literature to elucidate the roles of microstructure
in grindability. The comparison revealed that alloying addition that leads to the formation of brittle intermetallics enhances
grindability by reducing fracture toughness, tensile ductility, and the resistance to crack initiation and propagation. 相似文献
5.
In order to better understand the large scatter in the fatigue results associated with β-annealed microstructures of α+ β titanium alloys, the fatigue crack initiation and propagation behavior of thin center notched Ti-11 specimens with a large
colony α platelet microstructure was investigated. Colonies with a mean intercept diam of greater than 1 mm were grown in
2 mm thick specimens by means of a vacuum β-annealing process. This enabled crack path morphologies and crack propagation
rates to be determined within single colonies by means of optical microscopy on the polished and etched surfaces. The results
showed that the fracture is related to a shear mechanism across the colonies from the initiation stage through the overload
fracture. Intense shear bands were observed ahead of and in the same direction as the propagating cracks. The density of the
shear bands increased with increasing stress intensity. Since the colonies are randomly oriented, the fatigue cracks propagated
at various angles with respect to the tensile axis. The crack propagation rate across a single colony is no faster than the
propagation rate in the equiaxed α+ β microstructure of the same material. However, cracks were halted at the colony boundaries and forced to reinitiate through
a cycle consuming process into the next colony. It is mainly this reinitiation process and microstructurally dependent growth
which are responsible for the slower crack growth rates and large scatter band obtained for the β-annealed microstructures
when compared to the α+ β microstructures. It is suggested that by reducing the colony size the crack growth rates will be reduced and the fatigue
scatter band will be narrowed.
Formerly with the Metals and Ceramics Division, Air Force Materials Laboratory, Wright-Patterson Air Force Base. 相似文献
6.
7.
G. Terlinde H. -J. Rathjen K. -H. Schwalbe 《Metallurgical and Materials Transactions A》1988,19(4):1037-1049
Fracture mechanics and tensile tests have been performed on the metastable β-Ti alloy Ti-IOV-2Fe-3AI. A variety of microstructures
was established by several combinations of forging and heat treatment resulting in different types, morphologies, and volume
fractions of the a-phase which precipitates from the matrix-β phase. Both fracture toughness and ductility are strongly reduced
by increasing hardening by the secondary a-phase. An elongated primary a-phase (α
p
) shows higher toughness compared to a globular α
p
-phase. A thick, continuous subgrain boundary a-film lowers the toughness significantly. For microstructures without primary
a a grain boundary α-film does not affect the toughness, while the ductility is drastically reduced. Very attractive combinations
of fracture toughness and ductility were found for a microstructure without primary a and without grain boundary α. The results
are discussed based on the fractographic observations, and a model is proposed which includes the effect of microstructure
and slip distribution on the crack nucleation, the crack growth path, and the crack deviation. 相似文献
8.
The strength,fracture toughness,and low cycle fatigue behavior of 17-4 PH stainless steel 总被引:1,自引:0,他引:1
The influence of microstructure on the strength, fracture toughness and low cycle fatigue behavior of 17-4 PH stainless steel
has been examined. Aging hardening involves initial formation of coherent copper-rich clusters which transform to incoherent
fee ∈-copper precipitates upon further aging. The changes in strength level and strain hardening rates observed during aging
are consistent with previously suggested models for precipitation hardening based on differing elastic moduli. The fracture
toughness and fatigue crack growth rates were shown to be a function of microstructure and environment. At equivalent strength
levels overaging resulted in a higher fracture toughness than did underaging. The fatigue crack growth rates increased with
increasing strength level and humidity but were not a function of toughness level. Attempts to correlate the fatigue crack
growth rates with monotonie tensile properties were unsuccessful. However when final failure obeyed a critical strain criteria,
the fracture toughness behavior could be reasonably described and related to preferential void nucleation and growth at δ-ferrite-matrix
interfaces. 相似文献
9.
The fatigue and fracture resistance of a TiAl alloy, Ti-47Al-2Nb-2Cr, with 0.2 at. pct boron addition was studied by performing
tensile, fracture toughness, and fatigue crack growth tests. The material was heat treated to exhibit a fine-grained, fully
lamellar microstructure with approximately 150-μm grain size and 1-μm lamellae spacing. Conventional tensile tests were conducted as a function of temperature to define the brittle-to-ductile
transition temperature (BDTT), while fracture and fatigue tests were performed at 25 °C and 815 °C. Fracture toughness tests
were performed inside a scanning electron microscope (SEM) equipped with a high-temperature loading stage, as well as using
ASTM standard techniques. Fatigue crack growth of large and small cracks was studied in air using conventional methods and
by testing inside the SEM. Fatigue and fracture mechanisms in the fine-grained, fully lamellar microstructure were identified
and correlated with the corresponding properties. The results showed that the lamellar TiAl alloy exhibited moderate fracture
toughness and fatigue crack growth resistance, despite low tensile ductility. The sources of ductility, fracture toughness,
and fatigue resistance were identified and related to pertinent microstructural variables. 相似文献
10.
Friction-stir (FS) processing was used to modify the coarse, fully lamellar microstructure of investment cast and hot isostatically
pressed (HIP’ed) Ti-6Al-4V. The effect of FS processing on mechanical properties was investigated using microtensile and four-point
bend fatigue testing. The tensile results showed a typical microstructure dependence where yield strength and ultimate tensile
strength both increased with decreasing slip length. Depending on the processing parameters, fatigue strength at 107 cycles was increased by 20 pct or 60 pct over that of the investment cast and HIP’ed base material. These improvements have
been verified with a statistically significant number of tests. The results have been discussed in terms of the resistance
of each microstructure fatigue crack initiation and small crack propagation. For comparison, a limited number of fatigue tests
was performed on α + β forged Ti-6Al-4V with varying primary α volume fraction and also on investment cast material heat treated to produce a bi-lamellar condition. 相似文献
11.
The present study compares the fatigue and fracture properties of the high-strength β titanium alloy β-Cez with the conventional α+β titanium alloy Ti-6Al-4V, because of increasing interest in replacing α+β titanium alloys with β titanium alloys for highly stressed airframe and jet engine components. This comparison study includes the Ti-6Al-4V alloy
in an α+ β-processed condition (for a typical turbine blade application) and the β-Cez alloy in two distinctly different α+β-processed and β-processed conditions (optimized for a combination of superior strength, ductility, and fracture toughness). The comparison
principally showed a much lower yield stress for Ti-6Al-4V (915 MPa) than for both β-Cez conditions (1200 MPa). The Ti-6Al-4V material also showed the significantly lower high-cycle fatigue strength (resistance
against crack initiation) of 375 MPa (R=−1) as compared to the β-Cez alloy (∼600 MPa, R=−1). Particularly in the presence of large cracks (>5 mm), the fatigue crack growth resistance and fracture toughness of
the Ti-6Al-4V material is superior when compared to both β-Cez conditions. However, for small crack sizes, the conditions of both the alloys under study show equivalent resistance
against fatigue crack growth. For the β-Cez material, where microstructures were optimized for high fracture toughness (conventional large crack sizes) by thermomechanical
processing, maximum K
Ic-values of 68 MPa√m of the β-processed β-Cez condition (tested in the longitudinal direction) decreased by ∼50 pct in the presence of small cracks (1 mm). A similar
decrease in fracture toughness was obtained by loading the β-processed β-Cez condition perpendicular to the flat surfaces of the pancake-shaped β grain structure (tested in the short transverse direction). These results were discussed in terms of the effectiveness of
the crack front geometry in hindering crack propagation. Further, the results of this study were considered for alloy selection
and optimized microstructures for fatigue and fracture critical applications. Finally, the advantage of the α+β-processed β-Cez condition in highly stressed engineering components is pointed out because of its overall superior combination of fatigue
crack initiation and propagation resistance (especially against small fatigue cracks). 相似文献
12.
R. M. Horn 《Metallurgical and Materials Transactions A》1975,6(9):1525-1533
The relationships between microstructure and fatigue crack propagation behavior were studied in a 5Mo-0.3C steel. Microstructural
differences were achieved by varying the tempering treatment. The amounts, distribution, and types of carbides present were
influenced by the tempering temperature. Optical metallography and transmission electron microscopy were used to characterize
the microstructures. Fatigue fracture surfaces were studied by scanning electron microscopy. For each heat treatment the fatigue
crack growth properties were measured under plane strain conditions using a compact tension fracture toughness specimen. The
properties were reported using the empirical relation of Paris [da/dN = CoΔKm]. It was found that secondary hardening did influence the fatigue crack growth rates. In particular, intergranular modes
of fracture during fatigue led to exaggerated fatigue crack growth rates for the tempering treatment producing peak hardness.
Limited testing in a dry argon atmosphere showed that the sensitivity of fatigue crack growth rates to environment changed
with heat treatment. 相似文献
13.
14.
The mechanisms of low-cycle, fatigue crack nucleation and early growth were investigated at room temperature for Ti-17 (an
α+ β titanium alloy). The investigation includedβ processed, high temperature α+ β processed, and low temperaturea + β processed material. The mechanisms of crack nucleation and the path of early crack growth were found to vary with microstructure.
Low temperature α+ β processed material demonstrated later crack nucleation but more rapid early crack growth than the other two thermomechanically
processed forms. High temperature α+ β processed material containing microstructural flaws, commonly referred to as β-flecks, showed a degradation of fatigue life
at high cyclic strain ranges. Results are discussed in terms of microstructural sites of plastic strain concentration.
Formerly a member of the Research Technology Program, General Electric Research and Development Center 相似文献
15.
The low cycle fatigue behavior of IMI-685 alloy withβ-processed andβ-annealed microstructures was investigated. Material with large colony structure ofα-platelets oriented in the same direction, resulting from insufficientβ-work and slow cooling rate from theβ-phase region, exhibited lower fatigue strength than material with basketweave arrangement of theα-platelets. Most of the fatigue crack initiation and propagation processes were dominated by cracking related to intense shear
across a colony. The size of the shear related initial cracks could be limited by reducing the colony size, which resulted
in an increased fatigue strength. In the large colony microstructure, it was possible to cause a substantial fatigue life
debit by introducing a small planar defect on the surface or by applying 5 min dwell time at peak load. The combination of
planar defect and dwell time caused the highest life debit. Residual porosity of negligible size caused, in the large colony
microstructure, a fairly large, subsurface, cleavage-like planar defect that participated in the initiation of fatigue cracks.
Due to its appearance on the fracture surface, the defect which is characterized in detail in the paper, was named cleavage
rosette.
J. A. HALL, formerly with the Air Force Materials Laboratory 相似文献
16.
Experimental data concerning the influence of compositional, microstructural and textural variations on the fracture toughness
and deformation kinetics of α and α-β titanium alloys are presented. In particular, the influence of these parameters on the
thermal and athermal components of the flow stress is compared to their influence on fracture toughness. The similarities
in deformation behavior among alloys are noted and contrasted to the dissimilarities in fracture toughness behavior. 相似文献
17.
18.
L. W. Tsay Y. S. Ding W. C. Chung C. Chen 《Metallurgical and Materials Transactions A》2010,41(6):1502-1510
Fatigue crack growth tests of Ti-4.5Al-3V-2Fe-2Mo (SP-700) laser welds after various postweld heat treatments (PWHTs) were
investigated. The welds and the mill-annealed base metal had similar fatigue crack growth rates (FCGRs) at a stress ratio
(R) of 0.1. After increasing the stress ratio to 0.5, the peak-aged (482 °C) weld exhibited higher FCGRs due to increased notch
brittleness of the material. The tough microstructure as well as tortuous crack path of the overaged (704 °C) weld could account
for the reduced FCGRs, particularly at a higher R. The fatigue fracture appearance of the welds varied from transgranular to intergranular failures, depending on the stress
intensity factor ranges and PWHTs. Experimental results also demonstrated that the 704 °C-aged weld with coarsened α + β structures had better impact toughness than the base metal with banded structures. 相似文献
19.
J. J. Kruzic J. H. Schneibel R. O. Ritchie 《Metallurgical and Materials Transactions A》2005,36(9):2393-2402
Ambient- to elevated-temperature fracture and fatigue-crack growth results are presented for five Mo-Mo3Si-Mo5SiB2-containing α-Mo matrix (17 to 49 vol pct) alloys, which are compared to results for intermetallic-matrix alloys with similar compositions.
By increasing the α-Mo volume fraction, ductility, or microstructural coarseness, or by using a continuous α-Mo matrix, it was found that improved fracture and fatigue properties are achieved by promoting the active toughening mechanisms,
specifically crack trapping and crack bridging by the α-Mo phase. Crack-initiation fracture toughness values increased from 5 to 12 MPa√m with increasing α-Mo content from 17 to 49 vol pct, and fracture toughness values rose with crack extension, ranging from 8.5 to 21 MPa√m at
ambient temperatures. Fatigue thresholds benefited similarly from more α-Mo phase, and the fracture and fatigue resistance was improved for all alloys tested at 1300 °C, the latter effects being
attributed to improved ductility of the α-Mo phase at elevated temperatures. 相似文献
20.
Peravudh Lowhaphandu John J. Lewandowski 《Metallurgical and Materials Transactions A》1999,30(2):325-334
The effects of Cu infiltration on the monotonic fracture resistance and fatigue crack growth behavior of a powder metallurgy
(P/M) processed, porous plain carbon steel were examined after systematically changing the matrix strength via heat treatment. After austenitization and quenching, three tempering temperatures were chosen (177 °C, 428 °C, and 704 °C)
to vary the strength level and steel microstructure. The reductions in strength which occurred after tempering at the highest
temperature were accompanied by the coarsening of carbides in the tempered martensitic steel matrix, as confirmed by optical
microscopy and by microhardness measurements of the steel. Each steel-Cu composite, containing approximately 10 vol pct infiltrated
Cu, had superior fracture toughness and fatigue properties compared to the porous matrix material given the same heat treatment.
Although the heat treatments given did not significantly change the fatigue behavior of the porous steel specimens, the fatigue
curves (da/dN vs ΔK) and fracture properties were distinctly different for the steel-Cu composites given the same three heat treatments. The
fracture toughness (K
IC
and J
IC
), tearing modulus, and ΔK
TH
values for the composites were highest after tempering at 704 °C and lowest after tempering at 177 °C. In addition, the fracture
morphology of both the fracture and fatigue specimens was affected by changes in strength level, toughness, and ΔK. These fractographic features in fatigue and overload are rationalized by comparing the size of the plastic zone to the microstructural
scale in the composite.
This article is based on a presentation made in the symposium “Fatigue and Creep of Composite Materials” presented at the
TMS Fall Meeting in Indianapolis, Indiana, September 14–18, 1997, under the auspices of the TMS/ASM Composite Materials Committee. 相似文献