Small Prestrain Research Articles

Damage of the martensite interfaces as the mechanism of the martensite stabilization effect in the NiTi shape memory alloys

The aim of the paper was a comprehensive experimental study of the martensite stabilization effect (MSE) in NiTi alloys under various conditions to find the mechanism behind this phenomenon. The MSE was studied in the quenched Ni50Ti50 and Ni51Ti49 alloys subjected to pre-deformation to various strains in three ways: deformation in the martensite state, cooling under stress through a temperature range of the forward transformation and deformation in the austenite state due to the stress-induced martensite formation. It was found that after pre-deformation by the martensite reorientation, the MSE was observed even after a small pre-strain. After pre-deformation by cooling under stress or by stress-induced transformation, the MSE was small if the pre-strain was less than 5%; otherwise, the value of this effect was significant. A new mechanism of the martensite stabilization effect was considered. It was assumed that the damage to the martensite boundaries during the pre-deformation (without plastic strain) decreased their mobility and a larger thermodynamic force needed to be provided to move the interfaces during the reverse transformation.

Influence of Initial Textures on Microstructure and Mechanical Properties of Commercially Pure TA2 Titanium Sheet Pre-Strained by Cryorolling

Titanium alloys are one of the most important structural materials for the aerospace applications, which bring active study on microstructural and mechanical optimization of the alloys. In the present study, commercial TA2 titanium sheets with different initial textures were rolled to a strain of 4% at liquid nitrogen temperature. Microstructures of the rolled sheets were characterized using a scanning electron microscope equipped with an EBSD detector and their mechanical properties were tested through quasi-static uniaxial tension tests at room temperature. The influence of the initial texture on the microstructure and mechanical performance of the cryorolled sheet were discussed. Twinning is found to occur in the TA2 sheets after rolling even at a small strain. Initial texture is an important factor affecting both numbers and types of the twins. {11–22} contraction twinning is always the dominating type of twins for all three sheets having different initial textures. A lot of {11–22} and {11–24} twins can be found in the sheets with the initial texture of C-axes//ND, while the sheet with an initial texture of C-axes away from the ND contains few twins, with a high density of dislocations developed instead. Along with contraction twinning, many {10–12} and {11–21} twins are detected in the sheet with the initial texture of C-axes⊥ND. The cryorolled sheet having the densest twins shows the best mechanical properties, indicating a beneficial effect of a small pre-strain.

Twinning, grain orientation, and texture variations in Mg alloy processed by pre-rolling

A pre-rolling (PR) process with a small pre-strain was applied along the transverse direction (TD) and the extrusion direction (ED) of non-basal and basal textured AZ61 Mg alloys at room temperature to investigate the induced twinning behavior. The microstructural evolution of the alloys was used to evaluate the grain-boundary effect in terms of the texture variation. The results demonstrated that {101¯2} extension twinning was introduced by the PR process and that the volume fraction of twins increased with increasing thickness reduction. The subdivision of grains via twinning induced grain reorientation that generated a prominent basal texture with the c-axis parallel to the normal direction (ND) after PR. The textured Mg alloy with this c-axis//TD feature can promote twinning activity. The twinning performance was critical to initiate plastic deformation, therefore to determine deformation behavior at room temperature. The deformation mechanism was also addressed related to the extension twin lamellae.

Bifurcations of vortex-induced vibrations of a fixed membrane wing at Re $$\le $$≤ 1000

Vortex-induced vibrations (VIVs) of a fixed two-dimensional perimeter-reinforced (PR) membrane wing at $$0\le \alpha $$ Re (Reynolds number) $$\le $$ 1000 and $$0^\circ \le \alpha $$ (angle of attack) $$\le $$ 30 $$^{\circ }$$ are investigated using fluid–structure interaction simulations. By employing very fine increments for Re and $$\alpha $$ , bifurcation boundaries of the dynamic response of the membrane wing in the Re– $$\alpha $$ plane are captured. With increase in Re and/or $$\alpha $$ , it is found that the VIV state of a fixed PR membrane wing will change progressively from static state to period 1 via a Hopf bifurcation and then from period 1 to multiple period and chaos via a succession of period-doubling bifurcations. The Hopf bifurcation is triggered by the shedding of the leading- and/or trailing-edge vortices, while the period-doubling bifurcations are induced by the appearance and evolution of the secondary vortices on the upper surface of the membrane wing at higher Re and $$\alpha $$ . With an increase in the structure rigidity or pre-strain, the overall responses of the membrane wing are not changed much in the Re– $$\alpha $$ plane except that the period 1 response near $$700\le Re\le 1000$$ and $$14^{\circ }\le \alpha \le 16^{\circ }$$ is destroyed, due to the significant change of the shedding process of the leading-edge vortices. Moreover, it is also found that unsteady responses of the PR membrane wing at $$\alpha =0^{\circ }$$ can be suppressed by small pre-strain.

Impurity-Induced Grain Boundary Strengthening in Polycrystalline Graphene

First-principles density functional theory (DFT) calculations were performed to investigate the effects of boron (B) and nitrogen (N) doping on the fracture behaviors of a series of symmetric graphene grain boundaries (STGBs) under biaxial straining. Doping was found to generally enhance the fracture strength of STGBs, which was shown to be attributed to dopants mechanically lowering the local stretching energy or chemically strengthening the critical bond. We also showed that doping may also induce crack deflection to further improve the fracture resistance of graphene grain boundaries (GBs). Furthermore, we showed that the presence of multiple B and N dopants in the pentagon-heptagon ring(s) can contribute to promoting intergranular fracture in the presence of a small prestrain along the GBs. Our findings clarify the role of B and N dopants in GBs strengthening of polycrystalline graphene.

Reverse transformation behavior of TiNi shape memory alloys prestrained in the parent phase

The reverse martensitic transformation of TiNi alloy wires prestrained in the parent phase was studied. Experimental results show that the reverse transformation of the TiNi alloys prestrained in the parent phase is significantly different from that of the TiNi alloys prestrained in the martensite phase. Three continual peaks appear on the DSC curves of wires with a small prestrain and one high temperature peak appears on the DSC curves of wires with a large prestrain.

A theoretical comparison of two possible shape memory processes in shape memory alloy reinforced metal matrix composite

Two possible shape memory processes, austenite to detwinned martensite transformation and twinned martensite to detwinned martensite transformation of a shape memory alloy have been modeled and examined. Eshelby’s equivalent inclusion method with Mori-Tanaka’s mean field theory is used for modeling of the shape memory processes of TiNi shape memory alloy reinforced aluminum matrix composite. The shape memory amount of shape memory alloy, plastic strain and residual stress in the matrix are computed and compared for the two processes. It is shown that the shape memory amount shows differences in a small prestrain region, but the plastic strain and the residual stress in the matrix show differences in the whole prestrain region. The shape memory process with initially martensitic state of the shape memory alloy would be favorable to the increase in the yield stress of the composite owing to the targe compressive residual stress and plastic strain in the matrix.

Thermomechanical properties of TiNiCu 12 wire reinforced Kevlar/epoxy composites

TiNi12 wt%Cu shape memory alloy wire reinforced Kevlar/epoxy composites were produced and the thermomechanical behaviors of the composites were studied. Results showed that the prestrain level does not affect significantly the stress–strain and stress–temperature behaviors of the TiNiCu composites. Only the strain–temperature behavior of the composites is sensitive to the prestrain level. Composites with a small prestrain level show a larger hysteresis and a larger strain rate upon heating than those with a large prestrain level.

Grain Boundary Engineering for Intergranular Corrosion Resistant Austenitic Stainless Steel

Optimum parameters in the thermomechanical treatment during grain boundary engineering (GBE) were investigated for improvement of intergranular corrosion resistance of type 304 austenitic stainless steel. The grain boundary character distribution (GBCD) was examined by orientation imaging microscopy (OIM). The intergranular corrosion resistance was evaluated by electrochemical potentiokinetic reactivation (EPR) and ferric sulfate-sulfuric acid tests. The sensitivity to intergranular corrosion was reduced by the thermomechanical treatment and indicated a minimum at a small roll-reduction. The frequency of coincidence-site-lattice (CSL) boundaries indicated a maximum at the small pre-strain. The ferric sulfate-sulfuric acid test showed much smaller corrosion rate in the thermomechanical-treated specimen than in the base material for long time sensitization. The optimum thermomechanical treatment introduced a high frequency of CSL boundaries and the clear discontinuity of corrosive random boundary network in the material, and resulted in the high intergranular corrosion resistance arresting the propagation of intergranular corrosion from the surface.

Effects of Heat Treatment, Pre-Strain and Magnetic Field on the Formation of α′ Martensite in Fe–25.5Ni–4Cr and 304L Steels

The effects of solution heat treatment temperature, cooling rate from this temperature, pre-strain and re-heating on α' martensite formation were examined in Fe-25.5Ni-4Cr steel and 304L steels. The distribution of the potential energy of nucleation sites for α' martensite was also estimated using results obtained in magnetic fields of up to 20.70 MA/m. In Fe-25.5Ni-4Cr steel, solution heat treatment temperature and cooling rate from this temperature had almost no effect on the amount of α' martensite formed in magnetic fields over about 8 MA/m. In contrast, the amount of α' martensite formed in magnetic fields lower than about 8 MA/m increased as solution heat treatment temperature increased and as cooling rate decreased. This cooling rate effect in Fe-25.5Ni-4Cr steel is opposite to that in 304L steel, and the reason for this remains to be clarified. In Fe-25.5Ni-4Cr steel, re-heating almost did not affect the amount of α' martensite. From this result, it was expected that internal strain or lattice defects generated by water-quenching almost do not act as nucleation sites in this steel. In Fe-25.5Ni-4Cr steel, pre-strain of up to about 10% suppressed the formation of α' martensite and pre-strain over 10% enhanced the formation of α' martensite. Such a change of the amount of α' martensite with pre-strain was reduced as the magnetic field increased. In Fe-25.5Ni-4Cr steel, it was predicted that specimens that were solution heat treated at higher temperatures would have two peaks on the curve of the potential energy distribution of nucleation sites and that the small peak with a high potential energy would he easily annihilated by a small pre-strain. However this peak was not thought to be annihilated by re-heating. On the other hand, the large peak was expected to increase at larger pre-strain. This enhancing effect of pre-strain was not observed in 304L steel even in high magnetic fields. This supported the view that pre-strain suppresses the formation of α' martensite not through work hardening but through the annihilation of nucleation sites. In contrast, in Fe-25.5Ni-4Cr steel, some strain concentration induced by a large pre-strain was expected to generate nucleation sites. As mentioned above, clear differences were revealed in the effects of heat treatments and pre-strain on the formation of α' martensite between Fe-25.5Ni-4Cr steel and 304L steel. This shows that the nature of the nucleation sites and the procedure for the formation of α' martensite differ between these two kinds of steels.

New thermomechanical treatment of high temperature aging nickel alloys with high dispersed γ — Phase

A new thermomechanical treatment that permits a significant increase in the time to failure, as well as the resistance in stress rupture tests of the hard aged nickel-based alloys has been proposed. It is shown that new treatment consisting of a small prestrain in creep regime and subsequent one- or two-step aging brought about the formation of a high-dispersed microstructure of gamma-prime phase particles from several nm to 50 nm. This microstructure is more stable towards coarsening upon subsequent annealing. Improvement of the mechanical and creep properties is explained by a stable microstructure of the alloy, produced by the thermomechanical treatment, a higher degree of order inside particles, and coexistence of the mechanisms of cutting and by-passing of particles by single and pair dislocations.

On the Decrease of Fatigue Limit Due to Small Prestrain

Fatigue behavior of plain specimens of low carbon steel subjected to small tensile prestrain is investigated through rotating bending tests and the mechanism of the decrease of fatigue limit due to the prestrain is discussed. It is found that 3 percent prestraining causes the acceleration of both slip and crack initiations, and increases the growth rate of a small surface crack of less than 0.3 mm. It also decreases the fatigue limit. If prestrained material is aged, the fatigue limit increases. These effects of the small prestrain are explained based on the unpinning of locked dislocations due to the prestrain.

EFFECTS OF SMALL PRESTRAIN ON THE FATIGUE BEHAVIOUR OF SPHEROIDAL PEARLITE STEEL

AbstractIn order to clarify the effect of a small prestrain on the fatigue strength of a carbon steel, the fatigue behaviour of plain specimens of a eutectoid steel with spheroidal pearlite, previously subjected to a tensile prestrain, is investigated and compared with that of annealed and strain‐aged specimens. It is found that a small prestrain of a few percent does not decrease the fatigue limit nor the crack initiation or propagation lives. Fatigue slip initiation, however, is accelerated by the small prestrain. It is also observed that cyclic strain ageing does not occur. From these results, it is inferred that dislocations are unpinned by the small prestrain and this causes fatigue slip to occur more readily, but the fatigue behaviour following slip initiation is not affected because the spheroidal cementite particles act as obstacles to further slip deformation and Stage I crack propagation.

Effects of small prestrain on fatigue strength of eutectoid steel with spheroidal pearlite structure.

In order to clarify the effect of small prestrain on the fatigue strength of carbon steel, fatigue behavior of plain specimens of eutectoid steel subjected to tensile prestrains is investigated, and compared with that of annealed or strain-aged specimens. It is found that small prestrain of a few percent does not decrease either the fatigue limit or the crack initiation and propagation lives. Fatigue slip initiation, however, is accelerated by the small prestrain. It is also found that no cyclic strain aging occurs. From these results, it is concluded that dislocations are unpinned by the small prestrain and this causes fatigue slip to occur easily, but the fatigue behavior after the slip initiation is not affected because spheroidal cementite acts as obstacles to further slip deformation and the stage I crack propagation.

Effect of small prestrain on the fatigue strength of low carbon steel.

Effect of small prestrain on the fatigue strength of low carbon steel.

Channel die tests on Al and Cu polycrystals: Study of the prestrain history effects on further large strain texture

Al and Cu polycrystals have been prestained in a channel die to a compressive strain of 0.04-0.08 (small prestrain) and 0.40-0.55 (large prestrain) and then secondarily strained in different orientations—still in the channel die—up to a second strain of 0.75 in compression. The polycrystal hardening anisotropy is measured at small and large strain and reported under a form similar to the latent hardening ratio curves for single crystals, say here the ratio of the initial (back extrapolated) yield stress for the secondary straining to the maximum stress at the end of the prestrain. Texture measurements have been performed at the end of the secondary straining for the copper samples and are reported under the form of {111}, {200} and {110} pole figures. The observed differences on the measured textures are discussed with regard to the prestrain orientation and amplitude with special attention to the associated polycrystal hardening anisotropy and its more probable causes, geometrical or microstructural. The main observations are: (1) for copper, there is a clear hardening anisotropy after small strains, which does not seem due to texture because of too mild associated rotations, but more reasonably to an intracrystalline “latent hardening” effect in polycrystals. This effect appears greater in Cu than in Al in agreement with theoretical expectations (from the stacking fault energy of the two materials) and experiments on single crystals confirming the above interpretation. This hardening anisotropy depends on the way the load path is changed and differs for the various secondary orientations; (2) a weak hardening anisotropy occurs after large strains where one can expect more effect of the associated texture, which here appears to act in the opposite direction of that due to the latent hardening; (3) the correlation between changes on deformation textures at large strain and path changes during the sample loading appears as follows: while a path change after a large prestrain has a strong effect on the texture pattern after a secondary strain of 0.75 both on intensity and location of the maxima, it has only a mild effect after a weak prestrain but still showing noticeable differences for some orientations on the intensity maxima, which are the orientations associated with the highest measured hardening values at the end of the prestrain.

Particle size and orientation effects on softening in a cu matrix containing rod-shaped iron particles

Cu-Fe alloy samples have been thermomechanically processed to produce dispersions of rod-shaped a iron particles in single crystal matrices. These particles are aligned preferentially along one of the matrix (110) directions. Softening, following a small prestrain, has been examined as a function of the particle size and of the particle axis/slip system orientation relationship. Both factors were found to influence significantly softening in a manner consistent with the Orowan-loop pipe diffusion theory of recovery for dispersion hardened alloys.

The effect of small pre-strain on the internal friction and modulus of high-purity molybdenum II. Transient peaks between 400–540 K, the region of the γ peak

The internal friction of high-purity molybdenum has been measured at a frequency of about 500 Hz. After small pre-strains the damping curves showed a number of peaks between 400–540 K which disappe...

The effect of small pre-strain on the internal friction and modulus of high-purity molybdenum I. The Occurrence of a large frequency ‘kink’ associated with β peaks

Abstract The internal friction and frequency of high-purity molybdenum samples mounted as cantilevers vibrating at about 550 Hz have been measured between 80 and 530 K. The effect of small pre-strains was investigated. The main feature observed in one sample was a large frequency ‘kink’ between 350 and 450 K, which was most pronounced for pre-strains between 0·13 and 0·37%. Six damping peaks were identified in the high-strain-amplitude (10−5) curves taken in the same temperature range. We suggest that these peaks are β peaks arising from various interactions between dislocations and point defects. Since the modulus defect associated with a β peak can be ten times larger than that associated with a Debye relaxation and can be direct or inverse, the presence of β peaks can also account for the unusual frequency ‘kink’.

Effects of a small prestrain at high temperatures on the creep behaviour of AISI 304 stainless steel

Dans la phase de deformation initiale, les vitesses de fluage decroissent par precontrainte. Les vitesses de fluage minimales sont aussi influencees par la quantite de precontrainte