Plastic Deformation Capability Research Articles

Microstructure and properties of Ti-17 powder compact prepared through powder sintering and isothermal forging

Powder sintering and dual isothermal forging were utilised to prepare titanium alloy. Owing to the disturbance effect of residual pores during sintering, the microstructures of two sintered Ti-17 powder compacts prepared with about 80–150 and −150 mesh powders were composed of large residual pores and low aspect ratio of α platelets and small residual pores and high aspect ratio of α platelets respectively. Residual pores can be closed during the first isothermal forging above the β transus due to the excellent plastic deformation capability of the β phase. The microstructure can be effectively broken during the second isothermal forging below the β transus. Isothermal forging with 10−2 and 10−4 s−1 strain rates can obtain uniform microstructure, whereas isothermal forging with 1 s−1 strain rate just refined the microstructure non-uniformly. Closure of the residual pore and change of the microstructure’s morphology during dual isothermal forging effectively improved the ductility of the compact even though the oxygen and nitrogen contents exceeded the standard requirements.

Study on Failure of Soft Overburden of Fully Mechanized Top Coal Caving

The similar material model experiment takes the NO.S2S9 coal face in DaPing coal mine as a prototype to research the characteristics of movement and failure of the soft overburden under fully mechanized top coal caving conditions. The abutment pressure distribution and periodic weighting, caving span are given by monitoring the stress and strain in the similar material model. The results show that rocks fracture extended from button with the advance of workface. The first caving step distance of main roof is 34m. The periodic caving step distance of main roof is 10m. The internal stress field is 10m and the peak abutment pressure point is 80m far away from the coal wall, while the disturbance range in anterior coal wall is 200m. Some cracks are closed in the later stage. The advance of around 300m is the distance that the water flowing fractured zone’s height reaches the highest value. The biggest height of water- flowing fractured zone is 140m. The soft overburden has strong plastic deformation capability. The synchronism of failure and movement of overburden is strong. The failured overburden is compacted and the cracks keep close with the advance of workface. The simulation experiment provides a reference bases for the application and promotion of thick seam mining technology.

Study on Failure of Soft Overburden of Fully Mechanized Top Coal Caving

The similar material model experiment takes the NO.S2S9 coal face in DaPing coal mine as a prototype to research the characteristics of movement and failure of the soft overburden under fully mechanized top coal caving conditions. The abutment pressure distribution and periodic weighting, caving span are given by monitoring the stress and strain in the similar material model. The results show that rocks fracture extended from button with the advance of workface. The first caving step distance of main roof is 34m. The periodic caving step distance of main roof is 10m. The internal stress field is 10m and the peak abutment pressure point is 80m far away from the coal wall, while the disturbance range in anterior coal wall is 200m. Some cracks are closed in the later stage. The advance of around 300m is the distance that the water flowing fractured zone’s height reaches the highest value. The biggest height of water- flowing fractured zone is 140m. The soft overburden has strong plastic deformation capability. The synchronism of failure and movement of overburden is strong. The failured overburden is compacted and the cracks keep close with the advance of workface. The simulation experiment provides a reference bases for the application and promotion of thick seam mining technology.

On the capability of severe plastic deformation of twining induced plasticity (TWIP) steel

There are few reports showing that severe plastic deformation of relatively high strength materials such as steels is difficult due to the segmented flow. In the present paper, it is shown that twining induced plasticity (TWIP) steel can be processed successfully by simple shear extrusion without segmentation. Two simple shear extrusion dies with the maximum distortion angle of 30° and 45° are considered. For comparison, TWIP steel is also processed by equal channel angular pressing at two strain rates. Results show that equal channel angular pressing leads to the segmented flow due to flow localization while simple shear extrusion has the capability of processing TWIP steel without cracking. The microstructure after one pass of simple shear extrusion consists of many deformation twins due to imposing large plastic strains into the material.

Improved Mechanical Behavior and Plastic Deformation Capability of Ultrafine Grain Alumina Ceramics

The nanoindentation and nanoscratch behavior under ramp loading conditions of three dense bulk alumina ceramic samples with average grain size regimes of ~290 nm, 750 nm, and 1.3 μm, respectively, has been investigated using a nanoindenterXPSsystem in indentation and scratch modes. The mechanical behavior, including hardness from nanoindentation tests, scratch hardness, scratch resistance, and coefficient of friction from nanoscratch tests, can be improved with grain refinement. Moreover, the surface roughness of the scratch grooves and the distinct morphologies of the nanoscratch debris from each sample reveal that alumina ceramic samples with ultrafine grain size exhibit an increase in plastic deformation capability at the nano‐ and micro‐scale.

Effect of composition on fracture behavior of polypropylene–wollastonite–polyolefin elastomer system

The fracture behavior of polypropylene (PP)–wollastonite–polyolefin elastomer (POE) in the mixed mode region was studied using the essential work of fracture (EWF) method. The relationship between the microstructure and the fracture parameters was analyzed. The effect of wollastonite content on the essential work of fracture and the work of plastic deformation was discussed. The energy dissipation during a double-edge-notched tension (DENT) test was calculated with the EWF method. It was found in the mixed mode region that σn increases with shortening of the ligament length region as plastic constraint effect rises and variation of the specific total work of fracture with ligament length was still reasonably linear within the mixed mode region. With increasing wollastonite content, w e (specific essential work of fracture) increases, while the βw p (specific non-essential work of fracture) decreases. The measurements of energy dissipation show that improvement in the fracture toughness of PP–wollastonite–POE is mainly due to the increase in crack propagation resistance during the necking and tearing processes after yielding, while the plastic deformation capability of the material depends mainly on the properties of fracture behavior before yielding. It is also found that the impact strength of the material decreases with increasing wollastonite content. However, the composition with high impact strength has lower specific essential energy of fracture and lower long-term fracture resistance, indicating that EWF is a better indicator of long-term fracture properties than the impact strength. DSC results show that the presence of wollastonite hinders crystallization of the PP.

Effects of drawing on the tensile fracture strength and its reliability of small-sized metallic glasses

The effects of drawing on the structure and mechanical properties of a Co-based metallic glass under tension were thoroughly investigated. Surface changes induced by drawing, including removal of surface flaws, surface chemical homogenization and generation of compressive residual stress tend to increase the fracture strength, whilst open volumes created during drawing, particularly nano-voids, are likely to soften the wires. Initially, the surface changes are decisive factors, but as drawing proceeds, the open volumes gradually become dominant, yielding a maximum fracture strength in the wires with an area reduction ratio of 22%. Moreover, it was found that the fracture strength reliability was enhanced by the drawing, which is due not only to the surface perfection but also to the increase of plastic deformation capability, manifested by the decrease in the activation energy of individual shear transformation zones. Our results imply that the drawing technique could be a promising approach to continuously producing small-sized glassy wires with improved overall properties.

Dual specimen-size dependences of plastic deformation behavior of a traditional Zr-based bulk metallic glass in compression

The plastic deformation behavior of a traditional BMG with the composition of Zr 65Cu 17.5Ni 10Al 7.5 was investigated in compression using the precursory-specimens with different casting sizes and hence different initial states. With increasing the mechanical-testing size of specimen, it is demonstrated that more pronounced action of elastic recoveries of testing machine and specimen led to a decrease in plasticity because of larger shear-strain was introduced in shear bands. Furthermore, for the specimens with the same mechanical-testing size, initial free-volume reduction with increasing the casting size of precursory-specimen also led to a degradation of plasticity as higher free-volume was required for higher shear band emission rate. Even the traditional BMG can be ductile as the casting size and mechanical-testing size of specimen were concurrently decreased to a certain extent. The understanding of the dual specimen-size dependences of plastic deformation capability of BMGs is helpful for the potential and reasonable engineering applications of BMGs.

Equal Channel Angular Pressing in a Pearlitic Structured Steel

Equal Channel Angular Pressing (ECAP) in a fully pearlitic structured steel 65Mn was successfully carried out at 923 K via route C in this study. The severe shear deformation of ECAP was accommodated by periodical bending, periodical shearing and shearing fracture etc in the pearlitic lamellae. The cementite in the pearlite has higher plastic deformation capability. Excessive imperfections may be introduced into the cementite, which supplies additional energy driving for the following spheroidization of cementite in subsequent processing. After five ECAP passes, the fully pearlitic lamellae evolved into a microstructure of ultrafine-grained ferrite matrix uniformly dispersed with finer cementite particles. The ferrite matrix is homogeneous with an average grain size of ~0.3 micrometers. Two possible mechanism for the spheroidization of cementite were proposed：heterogeneous growth of the fractured cementite fragments, and the precipitation of new fine spherical cementite particles through nucleation and growth.

Bulk metallic glasses: Smaller is softer

The authors report on the dramatic effect of sample size on the plastic deformation capability of a Ti-based bulk metallic glass. Compared with the larger one, the smaller size glassy alloy with the same chemical composition exhibits significantly enhanced plasticity, suggesting a “smaller is softer” trend of metallic glasses. The phenomenon is attributed to the fact that the smaller size alloy, which experienced a faster cooling rate during solidification, contains a larger amount of heats of relaxation and crystallization, favoring the preferential nucleation of shear bands and thus allowing enhanced plasticity upon compressive loading.

Experimental determination of the true epoxy resin strength using micro-scaled specimens

In fibre reinforced composite materials, the matrix is the continuous phase, but the inter-fibre distance is rather small. The strength and the capability of plastic deformation is controlled by the matrix physics properties as well as by the acting stress state and the stressed volume. A new method is explained to produce fibres from epoxy resin. The single fibre strength was measured according to the standard test method for single fibre tests. The measured strength data of these thin epoxy resin fibres is close (60%) to the theoretical strength. The mechanism of fracture was identified by fractographic studies as cohesive failure initiated at pre-existing voids or void nucleation and growth. Before final rupture, the fibres showed necking and plastic deformation, which is a surprising behaviour for a brittle epoxy resin.

Cold drawn steel wires—processing, residual stresses and ductility—part I: metallography and finite element analyses

ABSTRACTCold drawing steel wires lead to an increase of their mechanical strength and to a drop of their ductility. The increase of their mechanical strength has long been related to the reduction of the various material scales by plastic deformation, but the mechanisms controlling their elongation to failure have received relatively little attention. It is usually found that heavily deformed materials show a tendency to plastic strain localization and necking. However, in this paper it is shown that, though the steel wires are plastically deformed up to strain levels as high as 3.5, a significant capability of plastic deformation is preserved in as‐drawn wires. This apparent contradiction is resolved by the existence of residual stresses inside the wire. Finite element analyses have been conducted in order to show that residual stresses, inherited from the drawing process, are sufficient to produce a significant hardening effect during a post‐drawing tensile test, without introducing any hardening in the local material behaviour. The main conclusion of this paper is that once the material has lost its hardening capabilities, residual stresses, inherited from the process, control the elongation of cold drawn wires. The finite element method allowed also the determination of the residual stress field that would lead to the best agreement between the simulated and the experimental stress strain curve of as‐drawn wires.

Toughening mechanisms of a Ti-based nanostructured composite containing ductile dendrites

The compressive deformation behavior of a multi-component Ti-Cu-Ni-Sn-Nb composite containing ductile dendritic precipitates embedded in a nanostructured matrix was investigated. The Ti-based composite not only displays a high compressive plasticity of approximate to 21 %, but also exhibits a high fracture strength of approximate to 1.8 GPa, which is comparable to that of monolithic bulk metallic glasses. Pronounced work hardening was observed after yielding. The surface deformation morphology reveals that the work-hardening behavior of the composite is related to the plastic deformation of the dendritic phase and the interaction of shear bands in the nanostructured matrix with the hardened dendrites. At the final stage of compression, most of the dendrites are work hardened, whereby the propagation of the shear bands in the matrix is retarded. The strong interaction between the dendrites and the matrix contributes to the high strength and plastic deformation capability of the composite. The fracture surface exhibits viscous flow traces, indicating that softening or melting of the composite occurs at the moment of fracture, due to the release of the high elastic energy stored in the specimen.

Effect of omega phase on deformation behavior of Ti–7.5Mo– xFe alloys

In an earlier study, it was found that Ti–7.5Mo–1Fe and Ti–7.5Mo–2Fe alloys exhibited markedly different mechanical properties. While Ti–7.5Mo–2Fe alloy exhibited excellent mechanical properties, Ti–7.5Mo–1Fe alloy showed an extremely brittle behavior. The present study is a continual research on these two particular alloys with a focus on the effect of ω phase on the deformation behavior of the alloys. The results indicate that 2Fe alloy exhibited a ductile type fracture surface covered with normal deformation dimples. The fracture surface of 1Fe alloy exhibited a terrace-like morphology with numerous sub-micron-sized dimples. Throughout the entire plastically deformed region of 1Fe alloy, {1 1 2}〈1 1 1〉 type twins were observed, but few twins formed in 2Fe alloy. It is suggested that the numerous, larger-sized ω particles in 1Fe alloy have effectively restricted the slip of dislocations that largely limited plastic deformation capability of the alloy. The less and smaller ω particles could not do the same to 2Fe that led to the observed ductile behavior.

Residual Stresses in Welded Moment Frames and Implications for Structural Performance

Welding-induced residual stresses are usually regarded as secondary stresses and therefore not accounted for in the structural analysis. This treatment, however, was brought into question with the welding research community by the discovery of a widespread structural damage in the 1994 Northridge earthquake. Brittle fracture was identified in many of the prequalified weld joints in steel moment frames. Welding-induced residual stresses are believed to be one of the factors contributing to the brittle fracture. In this paper, a detailed investigation on residual stresses in welded moment frame connections is presented. Both the actual connection configurations of the moment resistant frames and local weld details are considered by means of advanced residual stress analysis techniques. Then, the effects of triaxial residual stress state on plastic deformation capacity are discussed based on finite-element results on a wide-plate specimen loaded in tension. Finally, a series of fracture mechanics analyses were carried out to assess the effects of residual stresses on the fracture behavior. As a result, a great deal of insight has been obtained on the fracture behavior of the moment frame welds. Among other things, the results indicate that welding-induced residual stresses significantly increase the fracture driving force due to the presence of high tensile residual stresses. The high triaxial residual stress states identified in these joints can greatly reduce the plastic deformation capability, consequently promoting brittle fracture. Therefore, it seems reasonable to postulate that weld residual stresses could have played an important role in the occurrence of brittle fracture in the pre-Northridge weld joints.

The effect of double austenitization on the microstructure and fracture toughness of AISI M2 high speed steel

In this study, the effect of double austenitization on microstructure and toughness of AISI M2 high speed steel was investigated. For double austenitization treatment, the specimens, which are hardened initially at 1220°C and quenched in air, were hardened for a second time in the temperature range 1150 – 1050°C. For comparison purposes, another set of specimens is austenitized singly in the temperature range 1150 – 1050°C. Tempering process was carried out between 500 – 640°C. A double austenitization causes a fine carbide precipitation in the matrix, having sizes in the range of 0.10 ± 0.05 μm and volume fractions of between 1 and 6%. It is shown that a double austenitization treatment causes a decrease in fracture toughness (KIc), when compared with single austenitized ones. The reason for the lower KIc values of double austenitized specimens are attributed to these fine carbide precipitates: It is suggested that they limit the plastic deformation capability of the matrix and yield lower fracture toughness values.

Influence of Stress State on the Failure Behavior of Cracked Components Made of Steel

One of the decisive factors influencing the safety of components is the capacity for plastic deformation of the material employed. This depends not only on the actual material properties, such as reduction of area or notch impact energy, but also on the stress conditions prevailing in the component. With sufficiently sharp transitions of geometrical form, or at cracks, such high multiaxial stress states can arise in components, that in spite of excellent plastic deformation capability of the malterial, practically deformationless fractures are inevitable. If one generates from the principal normal stresses (σ1, σ2, σ3) the multiaxiality quotient q, which represents a characteristic quantity for the degree of multiaxiality of the stress state, the effect of the stress states on the strength and deformation behavior of a component can be estimated. With the aid of the Sandel fracture theory, which includes the von Mises yield theory as a special case, the critical q value qc, which characterises the stress conditions leading to cleavage fracture if q < qc, can be calculated. The fracture mechanics evaluation of the sharply notched specimens of dimensions similar to components shows no dependence of the effective crack initiation value on the specimen size or stress state, since at the load free crack tip, plane stress conditions generally prevail. The further failure process after crack initiation in the form of stable crack extension is very strongly controlled by the stress state. This phase could also be estimated from consideration of the pattern of the q value in the remaining cross section. The investigations have shown that the multiaxiality quotient q, which characterizes the degree of multiaxiality of the stress state, represents a characteristic quantity with which, in combination with fracture mechanics methods, the failure behavior of components may be estimated, even with respect to stable crack extension.

LOW‐CYCLE FATIGUE INVESTIGATIONS AND NUMERICAL SIMULATIONS ON DUAL PHASE STEEL WITH DIFFERENT MICROSTRUCTURES

Abstract— Fully reversed low cycle fatigue tests were carried out on 7 mm diameter cylindrical specimens of a dual phase steel treated to give five different microstructures, namely F—ferrite (+ little carbide particles), CF—continuous ferrite + 8.2% martensite, FM—49.1% ferrite + 50.9% martensite mixed structure, CM—continuous martensite + 22.4% ferrite and M—100% martensite. A finite element program was developed based on Eisenberg's cyclic plasticity theory and the low cycle stress‐strain response of the steels with duplex phase microstructures was calculated from the low cyclic curves of the single ferrite (steel F) and martensite (steel M) phase. The experimental results show that fatigue performance of dual phase steel improves as martensite content is increased up to about 50%, thereafter it obviously deteriorates. During cyclic loading, the calculated plastic strain accumulated in ferrite is higher than that in martensite. Inhomogeneity of the plastic strain accumulation in steel CF is more pronounced than that in steel CM. In steel FM a relatively uniform strain distribution was found. Controlling the size of particle phase can result in an optimum strain distribution. Thus, the plastic deformation capability of the constituent phases can be enhanced leading to fatigue performance improvement. Crack initiation occurs easily at the ferrite/martensite interface with a coarse particle phase size, regardless of phase continuity. In steel CF, a crack initiates at the interface perpendicular to the stress axis and propagates in the ferrite matrix by deflecting around coarse martensite particles or by cutting fine martensite particles. In steel CM, a crack initiates at the interface along the stress axis and propagates in the martensite matrix through ferrite particles.