Dislocation Dynamics and Yield Stress Anomalies in Intermetallic Alloys

1994 ◽  
Vol 364 ◽  
Author(s):  
Daniel Caillard
Keyword(s):  
Yield Stress ◽  
Rate Sensitivity ◽  
Short Review ◽  
Dislocation Dynamics ◽  
Positive Temperature ◽  
Intermetallic Alloys ◽  
Transient Effects ◽  
Slip Mechanism ◽  
Double Cross

AbstractMany intermetallic alloys with L12, L10 and BCC related structures exhibit yield-stress anomalies, which have strong influence on their mechanical properties at intermediate temperatures. A short review is made of the anomalous glide systems in various intermetallics. They are shown to exhibit several common characteristics, which allow it to classify them into three groups with possibly similar dislocation mechanisms. In the second half, the well-known case of octahedral glide in Ni3Al is considered again, and a new model is proposed, based on the formation and the unlocking process of incomplete Kear-Wilsdorf locks by a double cross-slip mechanism. This model is supported by several microscopic and macroscopic observations including “in situ” straining experiments in a TEM. It accounts for the positive temperature dependence of the yield-stress with fairly good orders-of-magnitude estimates of the stress. It also explains satisfactorily the small stress-strain rate sensitivity and the transient effects.

Dislocation Dynamics During the Deformation of Intermetallic Alloys and the Flow Stress Anomaly

1998 ◽  
Vol 552 ◽  
Author(s):  
U. Messerschmidt ◽  
M. Bartsch ◽  
S. Guder ◽  
D. Häussler
Keyword(s):  
Flow Stress ◽  
Strain Rate ◽  
Rate Sensitivity ◽  
Dislocation Motion ◽  
Dislocation Dynamics ◽  
Intermetallic Alloys ◽  
Intrinsic Nature ◽  
Voltage Electron ◽  
Macroscopic Deformation

ABSTRACTIn situ straining experiments on NiAl, NiAl-0.2at% Ta, γ-TiAl, and MoSi2 in a high-voltage electron microscope showed a transition from the obstacle controlled dislocation motion or the Peierls mechanism at low temperatures to either an unstable or viscous motion at high temperatures. It is suggested that the viscous motion is due to the formation of point defect atmospheres around the dislocations, which cause additional drag and may be responsible for the flow stress anomaly in some of these materials. The atmospheres may be of an extrinsic or an intrinsic nature. A new model is proposed for the origin of intrinsic atmospheres assuming that the energy of a dislocation in an intermetallic alloy may be lowest if the dislocations contain a number of point defects in their core. The dragging of atmospheres may lead to an “inverse” dependence of the strain rate sensitivity on the strain rate, as observed experimentally. The macroscopic deformation data of the studied materials are discussed in terms of the model.

Dislocation Dynamics in Icosahedral Al-Pd-Mn Single Quasicrystals

2000 ◽  
Vol 643 ◽  
Author(s):  
Ulrich Messerschmidt ◽  
Martin Bartsch ◽  
Bert Geyer ◽  
Lars Ledig ◽  
Michael Feuerbacher ◽  
...  
Keyword(s):  
Electron Microscope ◽  
Stress Exponent ◽  
High Voltage ◽  
Dislocation Dynamics ◽  
Slip Mechanism ◽  
Voltage Electron ◽  
High Voltage Electron Microscope ◽  
Slip Planes ◽  
Slip Traces

AbstractThe paper reviews results from in situ straining experiments on Al-Pd-Mn single quasicrystals in a high-voltage electron microscope. Slip planes were determined from the orientation and width of slip traces. Dislocations are generated by a specific cross slip mechanism. On some slip traces, dislocations move at two distinctly different velocities. A stress exponent was determined on a single dislocation by observing its displacement under decreasing load. The in situexperiments reveal the behaviour of individual dislocations in a temperature range where the deformation of bulk specimens is strongly affected by recovery.

scholarly journals Yield Stress Anomaly in B2 FeAl

1996 ◽  
Vol 460 ◽  
Author(s):  
K. Yoshimi ◽  
S. Hanada ◽  
M. H. Yoo
Keyword(s):  
Single Crystals ◽  
Yield Stress ◽  
Rate Sensitivity ◽  
Recovery Process ◽  
Peak Temperature ◽  
Positive Temperature ◽  
Slip Vector ◽  
Temperature Range ◽  
Tension And Compression ◽  
Increasing Temperature

ABSTRACTOur studies on yield stress anomaly of B2 FeAI single crystals are reviewed in this paper. A positive temperature dependence of yield stress, so-called “yield stress anomaly”, is observed in B2 FeAI in which excess vacancies are fully annealed out. Associated with the anomaly, characteristic asymmetry is found between tension and compression. While the strain-rate sensitivity is almost zero in the temperature range of the yield stress anomaly, the stress relaxation becomes significant with increasing temperature, indicating that a recovery process is thermally activated. It is ascertained by the two-surface trace analysis that slip transition from <111> direction at intermediate temperature to <100> at high temperature occurs around the peak temperature. Even at the peak temperature, in addition, operative slip vector for yielding is confirmed to be predominantly <111> by TEM. Also, it is observed that <111>-type superdislocations are frequently climb-dissociated in the temperature range of the anomaly. APB formation on {111} plane is energetically favorable, which is in agreement with the Flinn's calculation for the B2 superlattice that APB energy on {111} plane is lower than that on {110} plane. Such an anisotropy of APB energy would offer specific driving force for the climb dissociation on <111> superdislocations. On the basis of the observed results, the anomalous strengthening behavior of B2 FeAI single crystals is discussed.

Dislocation Dynamics in Intermetallic and Oxide Dispersion Strengthened (ODS) Alloys

2000 ◽  
Vol 652 ◽  
Author(s):  
Ulrich Messerschmidta ◽  
Susanne Gudera ◽  
Dietrich Häusslera ◽  
Martin Bartscha
Keyword(s):  
Flow Stress ◽  
Dynamic Behaviour ◽  
Diffusion Processes ◽  
Rate Sensitivity ◽  
Oxide Dispersion Strengthened ◽  
Dislocation Dynamics ◽  
Oxide Dispersion ◽  
Intermetallic Alloys ◽  
Wide Range ◽  
Ods Alloys

ABSTRACTIn situ straining experiments in a high-voltage electron microscope allow the observation of the dynamic behaviour of individual dislocations. Such experiments have been performed on the intermetallic alloys NiAl, NiAl containing 0.2 at% Ta, α-TiAl, and MoSi2, and the oxide dispersion strengthened (ODS) alloys INCOLOY MA956 and INCONEL MA754 in a wide range of temperatures. There are many similarities in the dynamic behaviour of dislocations in the different materials. In the intermetallic alloys, a transition occurs between low temperature mechanisms and a viscous motion in the temperature range of the flow stress anomaly. The viscous motion at high temperatures can be explained by diffusion processes in the dislocation cores, whichcan be described by the theory of the Cottrell effect. The diffusing species can be quite different, alloying components or intrinsic point defects like vacancies and antisite defects existing in the lattice or only in the dislocation cores. If the dislocations are straight and crystallographically oriented during their motion, they may be dissociated and move by a succession of glide and conservative climb between the partial dislocations. In the ODS alloys, the dislocations move between the oxide particles again in a viscous way. The relation between the dislocation dynamics and the strain rate sensitivity of the flow stress is discussed for thedifferent materials.

In Situ Straining Experiments On Fe70AL30 Single Crystals

1994 ◽  
Vol 364 ◽  
Author(s):  
G. Molenat ◽  
H. Rösner ◽  
E. Nembach
Keyword(s):  
Mechanical Properties ◽  
Electron Microscope ◽  
Single Crystals ◽  
Yield Stress ◽  
Transmission Electron Microscope ◽  
Intermetallic Alloys ◽  
Temperature Range ◽  
Transmission Electron

AbstractWith an aim to contributing to the understanding of the mechanical properties of Fe3Al-based intermetallic alloys, Fe-30 at. % Al single crystals have been strained in a transmission electron microscope below and in the temperature range of the yield stress anomaly (at 300K and 573K respectively).

The Yield Anomaly in CoTi

2000 ◽  
Vol 646 ◽  
Author(s):  
M. Wittmann ◽  
I. Baker ◽  
N.D. Evans
Keyword(s):  
Yield Stress ◽  
Strain Aging ◽  
Rate Sensitivity ◽  
Dynamic Strain ◽  
Positive Temperature ◽  
Compression Tests ◽  
Strain Rate Change ◽  
Classical Dynamic ◽  
Transmission Electron ◽  
Increasing Temperature

ABSTRACTCompression tests performed on both stoichiometric and cobalt-rich CoTi over a range of temperatures show a positive temperature dependence of the yield stress with increasing temperature, before a decline occurs at high temperatures. In the region of the peak yield stress, serrated yielding and a negative rate sensitivity of the yield stress were observed. Static strain-aging also occurs. These observations are consistent with strong solute-dislocation interactions. Results from quenching experiments and strain rate change tests are presented, together with transmission electron microscope observations of the dislocation structures below, at, and slightly above the peak temperature. The results suggest that the yield anomaly in CoTi can be accounted for by a classical dynamic strain aging mechanism.

scholarly journals Additive Manufacturing of Ti-Based Intermetallic Alloys: A Review and Conceptualization of a Next-Generation Machine

Materials ◽  
2021 ◽  
Vol 14 (15) ◽  
pp. 4317
Author(s):  
Thywill Cephas Dzogbewu ◽  
Willie Bouwer du Preez
Keyword(s):  
Heat Treatment ◽  
High Temperature ◽  
Additive Manufacturing ◽  
Complex Geometries ◽  
Intermetallic Alloys ◽  
Tial Alloys ◽  
Conventional Methods ◽  
Manufacturing Technologies ◽  
In Situ Heat Treatment

TiAl-based intermetallic alloys have come to the fore as the preferred alloys for high-temperature applications. Conventional methods (casting, forging, sheet forming, extrusion, etc.) have been applied to produce TiAl intermetallic alloys. However, the inherent limitations of conventional methods do not permit the production of the TiAl alloys with intricate geometries. Additive manufacturing technologies such as electron beam melting (EBM) and laser powder bed fusion (LPBF), were used to produce TiAl alloys with complex geometries. EBM technology can produce crack-free TiAl components but lacks geometrical accuracy. LPBF technology has great geometrical precision that could be used to produce TiAl alloys with tailored complex geometries, but cannot produce crack-free TiAl components. To satisfy the current industrial requirement of producing crack-free TiAl alloys with tailored geometries, the paper proposes a new heating model for the LPBF manufacturing process. The model could maintain even temperature between the solidified and subsequent layers, reducing temperature gradients (residual stress), which could eliminate crack formation. The new conceptualized model also opens a window for in situ heat treatment of the built samples to obtain the desired TiAl (γ-phase) and Ti3Al (α2-phase) intermetallic phases for high-temperature operations. In situ heat treatment would also improve the homogeneity of the microstructure of LPBF manufactured samples.

scholarly journals Effect of Pre-Shear on Agglomeration and Rheological Parameters of Cement Paste

Materials ◽  
2020 ◽  
Vol 13 (9) ◽  
pp. 2173
Author(s):  
Mareike Thiedeitz ◽  
Inka Dressler ◽  
Thomas Kränkel ◽  
Christoph Gehlen ◽  
Dirk Lowke
Keyword(s):  
Yield Stress ◽  
Mixing Time ◽  
Length Distribution ◽  
Chord Length ◽  
Rheological Parameters ◽  
Reflectance Measurement ◽  
Time Dynamic ◽  
Chord Length Distribution ◽  
In Situ Investigation

Cementitious pastes are multiphase suspensions that are rheologically characterized by viscosity and yield stress. They tend to flocculate during rest due to attractive interparticle forces, and desagglomerate when shear is induced. The shear history, e.g., mixing energy and time, determines the apparent state of flocculation and accordingly the particle size distribution of the cement in the suspension, which itself affects suspension’s plastic viscosity and yield stress. Thus, it is crucial to understand the effect of the mixing procedure of cementitious suspensions before starting rheological measurements. However, the measurement of the in-situ particle agglomeration status is difficult, due to rapidly changing particle network structuration. The focused beam reflectance measurement (FBRM) technique offers an opportunity for the in-situ investigation of the chord length distribution. This enables to detect the state of flocculation of the particles during shear. Cementitious pastes differing in their solid fraction and superplasticizer content were analyzed after various pre-shear histories, i.e., mixing times. Yield stress and viscosity were measured in a parallel-plate-rheometer and related to in-situ measurements of the chord length distribution with the FBRM-probe to characterize the agglomeration status. With increasing mixing time agglomerates were increasingly broken up in dependence of pre-shear: After 300 s of pre-shear the agglomerate sizes decreased by 10 µm to 15 µm compared to a 30 s pre-shear. At the same time dynamic yield stress and viscosity decreased up to 30% until a state of equilibrium was almost reached. The investigations show a correlation between mean chord length and the corresponding rheological parameters affected by the duration of pre-shear.

On a Simplified Method for Solving Impulsively Loaded Structures of Rate-Sensitive Materials

1965 ◽  
Vol 32 (3) ◽  
pp. 489-492 ◽  
Author(s):  
Nicholas Perrone
Keyword(s):  
Yield Stress ◽  
Rate Sensitivity ◽  
Approximate Solutions ◽  
Exponential Rate ◽  
Structural Elements ◽  
Sensitive Material ◽  
Perfectly Plastic ◽  
Simplified Method ◽  
Dynamic Yield ◽  
Constant Yield

In an attempt to assess more completely rate-sensitive material effects, two fundamental structural elements are analyzed: A wire with an impulsively loaded end mass, and an impulsively loaded ring. The ring and wire are made of perfectly plastic, rate-sensitive materials. In each case, exact and approximate solutions are obtained for an exponential rate-sensitivity law. The results suggest that very good approximations to the exact solutions may be found by utilizing a rate-insensitive material with constant yield stress equal to the initial dynamic yield stress.

Injectable in situ gelling delivery system for the treatment of jawbone infections

2021 ◽  
Author(s):  
Shreshtha Dash ◽  
Somnath Singh ◽  
Alekha K Dash
Keyword(s):  
Lactic Acid ◽  
Sustained Release ◽  
Cell Viability ◽  
Yield Stress ◽  
Delivery System ◽  
Pluronic F127 ◽  
Sustained Delivery ◽  
In Situ Gelling ◽  
Sustained Rate

Aim: A polymeric in situ gelling delivery system for localized and sustained delivery to jawbone infections was developed. Materials & methods: In situ gelling delivery systems were prepared using either poly-dl-lactic acid or chitosan and Pluronic F127/Pluronic F68. Metronidazole nanoparticles were prepared using poly (dl-lactide-co-glycolide) or chitosan. Poly (dl-lactide-co-glycolide) was used for microparticles. Particles were characterized for size, charge and morphology. Results: Viscosity and yield stress of the gels were 0.4 Pa.s and 2 Pa, respectively, with 70% cell viability over 72 h. Around 90% of loaded metronidazole was released at a sustained rate over 1 week. Conclusion: Use of appropriate amount of nano/microparticles in the gel resulted in a sustained release over a period of 1 week – needed for jawbone infection.

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