Transition In Deformation Mechanism Research Articles

Microstructure and mechanical behavior of arc-evaporated Cr–N coatings

Abstract Cr–N coatings were grown by arc evaporation onto high speed steel substrates. The coatings were each grown using a different negative substrate bias voltage, VS, between 20 and 400 V. X-ray diffraction showed a microstructure containing primarily the NaCl structured CrN phase along with the BCC-Cr and HCP-Cr2N. Auger electron spectroscopy and transmission electron microscopy indicate a substoichiometric (N/Cr=0.85±0.08) composition and dense columnar microstructure, respectively. At VS=400 V, equiaxed grains and microcracks parallel the substrate–coating interface were observed. The fiber textured coatings are in a compressive residual stress state that increases from 2.9 (VS=20 V) to 8.8 GPa (VS=100 V). At higher bias voltages, a decrease of the compressive residual stress is seen, which is discussed in terms of lattice defect annihilation in the collision cascade and lattice defect diffusion during deposition. Nanoindentation showed a maximum hardness at VS=100 V of 29 GPa. The critical loads for cohesive failure in a scratch test decreased monotonically with increasing negative substrate bias. The scratch results suggest a transition in deformation mechanism from plastic deformation to cracking, which occurs at lower applied loads when VS is increased. Similar behavior was also seen in a crater grinding wear test where a shift in wear mechanism from plastic deformation to chipping occurred at VS=200 V. The influence of the microstructure on the deformation transition is discussed in terms of lattice defect density and the presence of equiaxed grains.

Superplastic Deformation of Aluminum Systems at 500-650 °C with or without Presence of Liquid Phase

Three aluminum base materials were studied over the high temperature range of 500-650 °C, in an attempt to examine the role of liquid phase on controlling deformation mechanisms based the observed m- and Q-values. For the two near-pure At systems, the 1050 At and AO, no solute-induced incipient partial melting was expected, and the solidus and liquidus temperatures should merge to near 655±5 °C. The m-values of these materials were consistently scattered with 0.1-0.25 up to the extrahigh temperature of 650 °C. The high activation energy in the A0 material at T>600 °C is postulated to be related to the load transfer effect or bubble-induced stress reduction, but not the liquid phase role in explaining the deformation mechanism. In the 6061A1, an appreciable amount of liquid should have been present at 590 and 610 °C. The low activation energy of 20 kJ/mole over the temperature range of 570-610 °C might reflect the transition of deformation mechanisms, though no pronounced increase in m-values was observed even at 610 °C. This result was controversial and needs future studies.

High Temperature Deformation Behavior and Microstructural Evolution of Ti-47Al-2Cr-4Nb Intermetallic Alloys

The deformation behavior in the isothermal compression of Ti-46.5Al–2Nb–2Cr (at. %) including flow stress and microstructure evolution at the deformation temperatures ranging from 1373 to 1533 K and strain rates from 0.001 to 1 s−1 were investigated. The phenomenological-based model was established by describing the variation of the strain hardening exponent with the increasing of strain. The strain rate sensitivity exponent ranging from 0.065 to 0.449 indicated the transition of deformation mechanism from dislocation creep to grain boundary sliding, and the model by considering the effect of the processing parameters on the strain rate sensitivity was presented. The effect of the processing parameters on the apparent activation energy for deformation was analyzed. Furthermore, the novel constitutive model coupling with the strain hardening exponent and strain rate sensitivity exponent in the deformation was put forward and applied to predict the deformation behavior of Ti-46.5Al–2Nb–2Cr.

Fracture toughness of polypropylene copolymers: influence of interparticle distance and temperature

A heterophasic reactor grade polypropylene-ethylene copolymer (RAHECO®) was diluted with a propylene-ethylene random copolymer to get materials with constant EPR/PE-particle diameter but various interparticle distances. According to the results of instrumented impact tests, brittle-to-tough transitions were found at −20°C, −10°C, 0°C, +10°C and room temperature. The critical interparticle distance shifts linearly over the range of temperatures from about 0.4 μm at −20°C to 1.3 μm at room temperature. The results were compared with measurements of conventional notched Charpy impact strength at room temperature. The transition from brittle to tough impact behaviour was correlated to a transition of micromechanical deformation mechanisms from cavitation bands to croids. Cavitation of the rubber particles was always the first step of deformation. The in situ HVEM investigations additionally show that the particle distribution should be as homogeneous as possible.

Abyssal peridotite mylonites: implications for grain-size sensitive flow and strain localization in the oceanic lithosphere

Microstructures preserved in abyssal peridotites dredged from the oceans record several different physical regimes of deformation. Fabrics associated with deformation processes at slow-spreading mid-ocean ridges form two major classes of abyssal peridotites based on detailed microstructural observations. The most abundant class are medium- to coarse-grained tectonites with microstructures that reflect deformation processes during mantle upwelling and emplacement to the base of the lithosphere. These tectonites give geothermometric temperatures of ∼755°C or higher, interpreted to represent lower temperature limits for diffusive exchange in coarse-grained abyssal peridotites during cooling. This conclusion is consistent with flow laws for olivine at these temperatures. The second class of abyssal peridotites, previously largely undescribed for the mid-ocean ridge environment, include fine-grained mylonites associated with faulting and shear zones that develop during extension and cooling of the oceanic lithosphere when the brittle-plastic transition extends into mantle rocks. These mylonites give temperatures of ∼600°C, which we suggest represent a lower temperature limit for plastic deformation. Reduced grain size in mylonites allows for diffusive exchange to continue to these low temperatures. Relict augen in the mylonitic samples preserve equilibration temperatures similar to those exhibited by the coarse-grained tectonites.Based on flow laws for olivine, we suggest that deformation in some fine-grained mylonites occurred by diffusion creep down to ∼600°C. Rheological data for olivine indicate that dislocation creep is not likely to occur at this temperature. We conclude that a reduction in grain size by cataclasis, or dynamic recrystallization, resulted in a transition in deformation mechanisms from dislocation- to diffusion-creep during uplift (and/or cooling). The observation of these fine-grained mylonites indicates that shear zones that extend into the upper mantle will be weaker than expected if deformation was accommodated by brittle processes or dislocation creep. Weak faults may promote the development of long-lived detachments in the upper mantle. This inference supports mid-ocean-ridge tectonic models that suggest that ultramafic rocks exposed at the inside corners of ridge-axis discontinuities are exhumed along long-lived detachment faults.

Superplasticity and hot rolling of two-phase intermetallic alloy based on TiAl

The deformation behavior in the isothermal compression of Ti-46.5Al–2Nb–2Cr (at. %) including flow stress and microstructure evolution at the deformation temperatures ranging from 1373 to 1533 K and strain rates from 0.001 to 1 s−1 were investigated. The phenomenological-based model was established by describing the variation of the strain hardening exponent with the increasing of strain. The strain rate sensitivity exponent ranging from 0.065 to 0.449 indicated the transition of deformation mechanism from dislocation creep to grain boundary sliding, and the model by considering the effect of the processing parameters on the strain rate sensitivity was presented. The effect of the processing parameters on the apparent activation energy for deformation was analyzed. Furthermore, the novel constitutive model coupling with the strain hardening exponent and strain rate sensitivity exponent in the deformation was put forward and applied to predict the deformation behavior of Ti-46.5Al–2Nb–2Cr.

Compositional Limitations on Rock Types Contributing to Zones of Sealing Phyllosilicate-Rich Fault Gouge: ABSTRACT

A common approach to fault-zone seal analysis in siliciclastic strata involves analyzing the lithologic composition of the displaced stratigraphic interval along a fault as a means of predicting the spatial extent of sealing phylloilicate-rich gouge zones. It is often assumed that only the mudstones within the stratigraphic section contribute to the formation of this phyllosilicate-rich gouge. This study documents composition-dependent contrasts in natural deformational style of sandstones and mudstones that challenge this assumption. Faulting of phyllosilicate-poor (<20%), porous sandstones results in the formation of deformation bands. In contrast, the deformation of mineralogically less-mature sandstones generates gouge with a fine-grained, phyllosilicate-dominated matrix. Capillary-pressure tests indicate that this phyllosilicate-rich gouge is capable of sealing large hydrocarbon columns, whereas the deformation bands generally do not have small enough pore throats to serve as good seals. Most mudstones; deform by intergranular sliding and other grain-scale mechanisms to form [open quotes]scaly[close quotes] gouge typical of sealing [open quotes]shale smear[close quotes] fault zones. Other mudstones, however, deform by transgranular fracturing and brecciation, potentially leading to the formation of fluid conduits rather than seals. A transition in deformation mechanism from fracturing to intergranular sliding typically occurs when phyllosilicate content of the original mudstone exceeds 20-30%. At naturalmore » conditions suitable for petroleum preservation, faulting of sandstones and mudstones containing in excess of 20-30% phyllosilicates generally produces phyllosilicate-rich fault gouge. Thus, during fault-seal analysis in siliciclastic sequences, this compositional criterion can be used to distinguish between lithologies that do and do not contribute to the formation of high-quality sealing fault gouge.« less

Transitions in creep mechanisms and creep anisotropy in Zr1Nb1Sn0.2Fe sheet

The creep characteristics of a Zr1Nb1Sn0.2Fe alloy sheet were investigated at temperatures from 773 to 923 K and at stresses ranging from 9 to 150 MPa along both the rolling and transverse directions. Transitions in creep mechanisms are noted, with diffusional viscous creep at low stresses, viscous-glide-controlled microcreep in the intermediate stress regime and the climb of edge dislocations at high stresses. The creep anisotropy decreases with a decrease in the stress exponent and the creep rates differ by only 30% in the viscous creep regime, while an order-of-magnitude difference is noted at high stresses. The solute-strengthening effect of Nb addition is evident in the stress regime where appropriate data are available. These transitions in creep mechansims clearly reveal the dangers in blind extrapolation of short-term high stress data to low stresses and long times relevant to in-reactor conditions. The creep behavior of these materials is similar to that noted in Class I alloys, while the transitions in deformation mechanisms in Zircaloy-4 resemble those found in pure metals or Class II alloys with no viscous glide mechanism.

The effect of particles on failure modes of filled polymers

AbstractThe effect of rigid particles on the fracture mode of polymers that yield with necking was analyzed theoretically with a model of regularly arrayed spherical particles. The adhesion between a polymer and particles was assumed to be weak, and particles were assumed to debond from the polymer before necking. A linear decrease in engineering draw stress with an increase in the filler content was derived. An increase in filler content leads to a transition in deformation mechanism. The transition depends on the ability of the polymer to strain‐harden. If the ability to strain‐harden is insignificant and the engineering fracture stress (strenght) of the polymer is lower than its yield stress, the transition is from ductile to brittle fracture. If the ability to strain‐harden is essential and the strength of the unfilled polymer is higher than its yield stress, the transition (ductile‐to‐ductile) is from neck propagation to uniform ductile yield. The critical filler contents were determined for both transitions from the properties of an unfilled polymer. The ductile‐to‐ductile transition without embrittlement is possible if the strength of the unfilled polymer is higher than its yield stress. Results for polymers filled by weakly bonded particles were compared with polymers filled by particles that debond after the yield stress.

Constitutive relations between stress, strain-rate and temperature of copper.

Strain-rate and temperature dependence on the flow stress of OFHC copper is discussed. Tensile tests were made ranging in strain-rate from 2.5 × 10-4 s-1 to 1 s-1 and temperature from 25°C to 600°C with a cam-plastometer type testing machine built as a trial. Experimental results were compared with a theory proposed previously by the first author for constitutive relationships of elastic/visco-plastic materials. On the temperature and strain-rate range discussed in this paper, it was recognized that three fields of deformation mechanism exist. The proposed equation represented well constitutive relation in individual field and correlation of neighboring fields. Significant variations of final reduction in area and total elongation also were discussed in connection with transition of deformation mechanism.

Solvent‐induced embrittlement in a crystallizable polyarylate

AbstractThe effects of solvent‐induced crystallization on the micromechanical properties of thin films of polyarylate (PAr) were studied. Under uniaxial extension amorphous polyarylate was observed to deform exclusively by shear deformation with no evidence of crazing. Upon exposure to methylethyl ketone, vapor, or liquid, PAr crystallizes and is subsequently embrittled. Our transmission electron microscopy results clearly show that this embrittlement results from a transition in plastic deformation mechanism from shear yielding to crazing. A detailed examination of the samples revealed that the crazes formed preferentially within the noncrystalline regions and that the craze tips followed a complex trajectory around the crystallites. In some cases the craze tip advance deviated by as much as ±30 from a direction normal to the tensile axis. Because crazes are inherently more susceptible to forming cracks than shear deformation zones, crystallization reduces the fracture toughness of the polymer. This type of embrittlement, via a transition in plastic deformation mechanism, is believed to be a general behavior for solvent‐crystallizable thermoplastics.

The brittle-ductile transition in a deformation-mechanism map for halite

The brittle-ductile transition in rock deformation has been studied extensively, but is still poorly quantified. Progress in our understanding of this and other transitions in mechanical behavior and deformation mechanisms is facilitated by use of deformation-mechanism maps. A preliminary deformation-mechanism map for halite is presented, based on shear experiments at room temperature (22°C) over the range of normal stress up to 350 MPa and average shear-strain rates between 10° to 10 −6 s −1. The map illustrates that transitions in mode-of-failure, mechanical behavior and dominant deformation mechanism, which often are referred to collectively as the brittle-ductile transition, do not necessarily coincide. The analysis also suggests that the transition from dominantly cataclastic to crystal-plastic deformation-mechanism fields is adequately described by the distinct and independent mechanism field of semi-brittle flow.