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1.
The influence of strain rate on the stress-strain curve of materials showing yield point phenomenon are determined in the range of small strains and high strain rates by means of the indirect impact tension test. With increasing deformation velocity, the Lüders strain is found to increase considerably. The stress reduction after reaching the upper yield point, is not abrupt and takes place gradually with increasing strain. These phenomena are investigated on Armco iron, on the tempered steel 42 CrV 6, on pure tantalum and tungsten alloys. A simple mechanical model is introduced which shows that the Lüders front velocity increases with strain rate approaching asymptotically the value of the plastic wave velocity as an upper limit. With this restriction, the local strain within the Lüders band must increase by increasing the rate of elongation of the tensile specimen. The Lüders strain could be estimated quantitatively as a function of strain rate. The model describes also the stress-strain function after reaching the upper yield point considering the limited stiffness of the test arrangement. 相似文献
2.
J. K. Chen 《国际钢铁研究》2008,79(9):708-712
Lüders strain and aging index were employed to characterize the aging properties of hot rolled low carbon steels during production. The Lüders strain was found to increase with aging time while the aging index decreased. It is interpreted that the aging index represents the susceptibility of steels to aging, while the Lüders strain measures the effects of carbon and nitrogen atmosphere pinning on dislocations. By comparing accelerated aging with room temperature aging behaviours of low carbon steels for up to two years, an algorithm deriving the activation energy of solute carbon and nitrogen migration from aging property measurements was proposed. The equation derived for accelerated aging of hot‐rolled low carbon steels investigated was in reasonable agreement with that reported in the literature. 相似文献
3.
4.
This study investigates the effect of austenite reverted transformation (ART) annealing temperature and temper-rolling on the microstructure, mechanical properties, and deformation behaviors of cold-rolled Fe–0.25C–5.9Mn–1.0Al–1.57Si transformation-induced plasticity (TRIP) steel. The cold-rolled steel annealed at 700 °C demonstrates excellent mechanical properties. The ultimate tensile strength, total elongation, and product of strength and elongation are observed as 1212 MPa, 31.8%, and 38.6 GPa%, respectively. The excellent combination of strength and ductility is related to the discontinuous TRIP effect; still, an inhomogeneous deformation is observed during tensile deformation, known as the Lüders strain. Temper-rolling is used for the ART-annealed specimens at 700 and 720 °C, and yield point elongation decreases when temper-rolling reduction increases. When the temper-rolling reduction increases by 8%, the yield point elongation of the specimen annealed at 700 °C is noted at 1%, while the specimen annealed at 720 °C exhibits continuous yielding. The strain-induced martensite transformation and increased dislocation density in the ferritic matrix improve the early-stage strain hardening rate, thus suppressing the Lüders band's formation. 相似文献
5.
R. Nogueira de Codes O. S. Hopperstad O. Engler O.-G. Lademo J. D. Embury A. Benallal 《Metallurgical and Materials Transactions A》2011,42(11):3358-3369
The spatial and temporal characteristics of propagating deformation bands in the Al-Mg alloy AA5182 in O temper were studied
experimentally at room temperature. Tensile tests were carried out on flat specimens at strain rates in the range from 10−5 to 10−1 s−1. Digital image correlation (DIC) and digital infrared thermography (DIT) were applied to monitor the propagating bands. It
was found that the material exhibits a sharp yield point, and Lüders bands were seen at all the strain rates. Jerky flow took
place all along the Lüders plateau. It thus seems that the Portevin–Le Chatelier (PLC) effect starts at incipient yielding
and that there is no critical strain. At the end of the Lüders plateau, PLC bands immediately started to propagate back and
forth along the gage section of the specimen. The work hardening of the material decreased consistently with increasing strain
rate, while the flow stress on the Lüders plateau was rather unaffected by the strain rate. This indicates that the dynamic
strain aging (DSA) mainly affects the strength of the interaction between mobile and forest dislocations. The strain to necking
was found to decrease gradually with strain rate for this alloy, which is consistent with the lower work-hardening rate at
the higher strain rates. 相似文献
6.
Hsun Hu 《Metallurgical and Materials Transactions A》1983,14(1):85-91
The effects of phosphorus, tin, and silicon on the magnitude of Lüders strain in low-carbon sheet steels have been studied.
At equivalent grain sizes the Lüders strain was effectively reduced by the P or Sn additions. However, when various amounts
of Sn were added to a steel containing 0.07 pct P, incremental effects of increasing Sn content could not be recognized; the
combined effect of Sn and P on Lüders strain was nearly the same as the effect produced by a slightly higher P alone. Additions
of Si to a 0.05 pct P-treated steel, even up to fairly high concentrations, produced very little effect on the Lüders strain.
The magnitude of Lüders strain, as influenced by the grain size, is a linear function of d-1/2, this relation being similar in form to the Hall-Petch relation for yield or flow stress and grain size. The effect of P
or Sn on this relation is to decrease the slope of the straight-line plot. These observations have been analyzed on the basis
of Lüders strain in relation to the band-front velocity and the contribution of the mobile dislocation density and velocity
product (ρv) to the strain rate. Solutes that effectively increase the density of mobile dislocations generated from the grain-boundary
sources should decrease the magnitude of Lüders strain. 相似文献
7.
Development of radiation tolerant materials for use in fusion reactors requires testing of small volumes of candidate materials
in high energy neutron irradiation facilities. As part of an effort to develop methods for extracting mechanical property
information from small specimens, this study investigated techniques to characterize flow properties of steels exhibiting
Lüders strains from ball indentation hardness/microhardness tests. A quantitative relationship was found between the amount
of pile-up formed during indentation and the magnitude of the Lüders strain exhibited during tensile testing in SAE 1015 steel.
Moreover, by nonlinear regression analysis, a single set of equations was found which correlated hardness data to the homogeneous
true-stress-true-plastic strain relationship of steels in a variety of metallurgical conditions in the strain range 1 to 10
pct at room temperature. 相似文献
8.
C. X. Huang G. Yang C. Wang Z. F. Zhang S. D. Wu 《Metallurgical and Materials Transactions A》2011,42(7):2061-2071
The technique of equal-channel angular pressing (ECAP) was used to refine the microstructure of an AISI 301 austenitic stainless
steel (SS). An ultrafine-grained (UFG) microstructure consisting mainly of austenite and a few martensite was achieved in
301 steel after ECAP processing for four passes at 523 K (250 °C). By submitting the as-ECAP rods to annealing treatment in
the temperature range from 853 K to 893 K (580 °C to 620 °C) for 60 minutes, fully austenitic microstructures with grain sizes
of 210 to 310 nm were obtained. The uniaxial tensile tests indicated that UFG 301 austenitic SS had an excellent combination
of high yield strength (>1.0 GPa) and high elongation-to-fracture (>30 pct). The tensile stress–strain curves exhibited distinct
yielding peak followed by obvious Lüders deformation. Measurements showed that Lüders elongation increased with an increase
in strength as well as a decrease in grain size. The microstructural changes in ultrafine austenite grains during tensile
deformation were tracked by X-ray diffraction and transmission electron microscope. It was found that the strain-induced phase
transformation from austenite to martensite took place soon after plastic deformation. The transformation rate with strain
and the maximum strain-induced martensite were promoted significantly by ultrafine austenite grains. The enhanced martensitic
transformation provided extra strain-hardening ability to sustain the propagation of Lüders bands and large uniform plastic
deformation. During tensile deformation, the Lüders bands and martensitic transformation interacted with each other and made
great contribution to the excellent mechanical properties in UFG austenitic SS. 相似文献
9.
The mechanics involved in the development of type-A Portevin-Le Chatelier plastic flow in Al 6061 alloy tensile specimens were investigated using strain differential measurements. Specimens with tapered as well as uniform diameter gage sections were used. In the former the Lüders bands tend to move into a negative strain gradient, while in the latter they move into a positive gradient. In both cases plastic instability leads to strain rate fluctuations that grow catastropically into the Lüders phenomena. Once formed the bands tend to move under a stress increment that is nearly independent of the strain and only weakly dependent on strain rate. The near constancy of the stress required to drive the bands can account for the increase in Lüders strain, Δ∈L, with increasing number of band passages, and the fact that Δ∈L increases as a band moves into a positive strain gradient, while it decreases when it moves into a negative gradient. 相似文献
10.
《Metallurgical and Materials Transactions A》1975,6(2):461-466
The mechanics involved in the development of type-A Portevin-Le Chatelier plastic flow in Al 6061 alloy tensile specimens
were investigated using strain differential measurements. Specimens with tapered as well as uniform diameter gage sections
were used. In the former the Lüders bands tend to move into a negative strain gradient, while in the latter they move into
a positive gradient. In both cases plastic instability leads to strain rate fluctuations that grow catastropically into the
Lüders phenomena. Once formed the bands tend to move under a stress increment that is nearly independent of the strain and
only weakly dependent on strain rate. The near constancy of the stress required to drive the bands can account for the increase
in Lüders strain, Δ∈L, with increasing number of band passages, and the fact that Δ∈L increases as a band moves into a positive strain gradient, while it decreases when it moves into a negative gradient. 相似文献
11.
The ultrafine grained duplex steels were fabricated by austenite reverted transformation annealing of the medium manganese steels after quenching or cold rolling. The microstructures were examined by scanning electron microscopy (SEM) and transmission electron microscopy (TEM) and X‐ray diffraction (XRD). The mechanical properties were determined by uniaxial tensile test. It was demonstrated that both the quenched and cold rolled structures were transformed into ultrafine grained duplex structures with large fractioned austenite by ART‐annealing. Long time annealing is essential to obtain the large fractioned austenite in quenched steel, but only short time annealing is needed to get large fractioned austenite in the cold rolled sheet. The mechanical examinations indicated that ART‐annealing results in the superhigh tensile elongation (>40%) and superhigh strength (1000 MPa) in quenched steels after long time annealing but in cold rolled steels after short time annealing. Based on the analysis on the work hardening behaviors of these ART‐annealed steels, the abnormal work hardening rate was presented and analyzed. The substantially enhanced ductility was attributed to the Lüders band propagation of the ferrite matrix and/or the TRIP effects of the large fractioned austenite. At last the dynamic phase natures of both fraction and stress was proposed to interpret the abnormal hardening behaviors and the “S” shaped stress–strain curves. 相似文献
12.
13.
《Acta Metallurgica Materialia》1995,43(12):4281-4287
The phenomenology of Lüders bands formation in a rapidly solidified Ni-20Al-12Cr-1.8Mo intermetallic alloy ribbon in the temperature range of 300-770 K is discussed. It was observed that strength and Lüders bands aspect on the specimen were irrespective of temperature. The flow characteristics in the Lüders region of the load-elongation curve were, however, very temperature sensitive. At low temperatures (<470 K), a flat plastic region with few instabilities was seen; but at higher temperatures (>470 K), a clear serrated behavior was manifested and the amplitude of serration increased with temperature. It is suggested that yielding occurs by dislocation generation at grain boundaries and that the stress required for dislocation generation (σeff) is athermal. A temperature dependent stress originated by the dynamic pile-up of dislocations at grain boundaries (dynamic stress) is, however, introduced as rate controlling for Lüders front motion and responsible for serration appearance. 相似文献
14.
The development of macroscopic plastic flow heterogeneities in metals in the form of Chernov–Lüders bands and the Portevin–Le Chatelier effect is studied. The main laws of deformation development in these two cases are found, and the kinetics of the motion of Chernov–Lüders band fronts and the serrated deformation during the Portevin–Le Chatelier effect is investigated. It is shown that both versions of heterogeneities can be considered as macroscopic autowave processes of switching and excitation in deformable media. 相似文献
15.
Juan J. Aróztegui José J. Urcola Manuel Fuentes 《Metallurgical and Materials Transactions A》1989,20(9):1657-1668
Commercial electric arc melted low-carbon steels, provided as I beams, were characterized both microstructurally and mechanically
in the as-rolled, copper precipitation, and plastically pre-deformed conditions. Inclusion size distribution, ferrite grain
size, pearlite volume fraction, precipitated volume fraction of copper, and size distribution of these precipitates were deter-mined
by conventional quantitative optical and electron metallographic techniques. From the tensile tests conducted at a strain
rate of 10-3 s-1 and impact Charpy V-notched tests carried out, stress/strain curves, yield stress, and impact-transition temperature were
obtained. The spe-cific fractographic features of the fracture surfaces also were quantitatively characterized. The increases
in yield stress and transition temperature experienced upon either aging or work hard-ening were related through empirical
relationships. These dependences were analyzed semi-quantitatively by combining microscopic and macroscopic fracture criteria
based on measured fundamental properties (fracture stress and yield stress) and observed fractographic parameters (crack nucleation
distance and nuclei size). The rationale developed from these fracture criteria allows the semiquantitative prediction of
the temperature transition shifts produced upon aging and work hardening. The values obtained are of the right order of magnitude. 相似文献
16.
研究了平整对罩式退火生产的BH钢板力学性能和自然时效性能的影响。结果表明,平整对BH钢的BH性能有显著影响,平整延伸率在1.0%~2.0%之间时BH值有最大值;平整使得BH钢的屈服强度先下降而后上升,屈服强度最小时的平整延伸率消除了钢板单向拉伸时的屈服点延伸现象;足够的平整延伸率是BH钢抗自然时效性能的有效保证。实验结果在工业生产中得到了应用。 相似文献
17.
Lüders band usually apprears in medium Mn steel, and they are difficult to be eliminated without sacrificing the mechanical properties. In this study, a tailored two-step intercritical annealing approach is proposed to eliminate Lüders band and maintain excellent mechanical behavior. A strong ferritic matrix and austenite with appropriate stability are obtained, achieving excellent mechanical behavior. Lüders band is fully removed by the early stress-induced martensitic transformation. 相似文献
18.
The tensile deformation behavior of mechanically-stabilized austenite is investigated in Fe-Mn binary alloys. A 30 pct thickness
reduction by rolling at 673 K (above the Af temperature) largely suppresses the austenite (γ) to hcp epsilon martensite (ε) transformation in 17Mn and 25Mn steels. However,
the deformation behavior of the mechanically stabilized austenite in the two alloys differs significantly. In 25Mn steel,
the onset of plastic deformation is due to the stress-induced γ→ ε transformation and results in a positive temperature dependence of the yield strength. The uniform elongation is enhanced
by the γ → ε transformation during deformation. In 17Mn steel, bccα′ martensite is deformation-induced along with e and a plateau region similar to Lüders band deformation appears at the beginning
of the stress-strain curve. The mechanical stabilization of austenite also suppresses the intergranular fracture of 17Mn steel
at low temperatures.
M. STRUM, formerly Candidate for Ph.D. at the University of California at Berkeley 相似文献
19.
采用SEM、XRD、力学性能测试等实验分析方法对冷轧中锰钢(0.1C-7Mn)在奥氏体逆相变不同温度退火过程中碳化物演变对Lüders应变的影响进行了分析。结果表明:随着退火温度的升高,碳化物先析出后溶解,在640℃退火时碳化物全部溶解,逆转变奥氏体的稳定性适中,强塑积最高为25 GPa·%。退火温度偏低导致奥氏体稳定性过高,同时碳化物会抑制位错运动,使得屈服点延伸较为明显;退火温度适中则高密度位错开始回复,变形时能持续地产生TRIP效应硬化基体,提高材料的综合性能;退火温度偏高时,碳化物的溶解导致位错对消重排,Lüders应变消失,奥氏体稳定性降低,应变诱导马氏体快速形成,导致中锰钢抗拉强度较高但均匀伸长率降低。 相似文献
20.
B. O. Han E. J. Lavernia Z. Lee S. Nutt D. Witkin 《Metallurgical and Materials Transactions A》2005,36(4):957-965
Cryomilled 5083 Al alloys blended with volume fractions of 15, 30, and 50 pct unmilled 5083 Al were produced by consolidation
of a mixture of cryomilled 5083 Al and unmilled 5083 Al powders. A bimodal grain size was achieved in the as-extruded alloys
in which nanostructured regions had a grain size of 200 nm and coarse-grained regions had a grain size of 1 μm. Compression
loading in the longitudinal direction resulted in elastic-perfectly plastic deformation behavior. An enhanced tensile elongation
associated with the occurrence of a Lüders band was observed in the bimodal alloys. As the volume fraction of coarse grains
was increased, tensile ductility increased and strength decreased. Enhanced tensile ductility was attributed to the occurrence
of crack bridging as well as delamination between nanostructured and coarse-grained regions during plastic deformation. 相似文献