The effect of yttrium and erbium dispersoids on the deformation behaviour of titanium

The deformation characteristics at 295 K and 575 K of polycrystalline Ti-Y and Ti-Er alloys containing 20 to 90 nm diameter dispersions were investigated by stress-strain measurements and by transmission electron microscopic observations of deformation substructures. The presence of the dispersoids increases the yield stress at both 295 K and 575 K, with the dispersion strengthening being more pronounced for the larger grain size and at the higher temperature. In dispersoid-free titanium the yield stress varies with grain size at both 295 K and 575 K in accordance with the Hall-Petch relation, but the yield stress of the Ti-Er alloys does not show a well-defined linear dependence on the inverse square root of grain size. The work hardening is less sensitive to grain size in the Ti-Er alloys than in pure titanium. The extent of twinning is significantly higher in the Ti-Er and Ti-Y alloys than in titanium at both temperatures. The influence of the dispersoids on deformation substructure and grain size related deformation behaviour is discussed.     

Size effect on deformation behavior and ductile fracture in microforming of pure copper sheets considering free surface roughening

In meso/micro-scaled plastic deformation, material deformation and ductile fracture are quite different from those in macro-scale. The roughness of the free surfaces of workpiece increases with deformation and the decrease of grain number in the sample thickness direction, leading to the nonuniformity of specimen thickness. The so-called size effect and free surface roughening may in turn affect the deformation behavior, ductility and fracture morphology of the samples. To explore the coupled effect of workpiece geometry and grain size on material flow behavior in meso/micro-scaled plastic deformation, uniaxial tensile test of pure copper sheets with different thicknesses and comparable microstructure was performed. The experimental results reveal that the material flow stress, fracture stress and strain, and the number of microvoids on fracture surface are getting smaller with the decreasing ratio of specimen thickness to grain size. In addition, the modified Swift’s equation and the corrected uniform strain are closer to the experimental ones considering the thickness nonuniform coefficient induced by the free surface roughening. Furthermore, the observation of fracture morphologies confirms that the local deformation caused by the free surface roughening leads to strain localization and a decreased fracture strain when there are only a few grains involved in plastic deformation.     

The size effect on micro deformation behaviour in micro-scale plastic deformation

When the geometry of metal deformed part is scaled down to micro-scale, the understanding and prediction of micro deformation behaviour becomes difficult. This is because the conventional material deformation models are no longer valid in micro-scale due to the size effect, which affects the deformation behaviour in micro plastic deformation, and thus leveraging the traditional knowledge of plastic deformation from macro-scale to micro-scale is not meaningful. In this paper, the size effect on micro-scale plastic deformation and frictional phenomenon are investigated via micro-cylindrical compression test, micro-ring compression test and Finite Element (FE) simulation. The experimental results show the occurrence of various size-effect related deformation phenomena, including the decrease of flow stress and the increases of: (a) irrational local deformation, (b) the amount of springback, and (c) the interfacial friction stress with the decreasing specimen size. The research further verifies that the established surface layer models, with the identified surface grain, the internal grain properties and the measured friction coefficients, are able to predict micro deformation behaviour. The research thus provides an in-depth understanding of size effect on deformation and frictional behaviours in micro-scale plastic deformation.     

Effects of temperature and grain size on deformation and fracture in recrystallized Ni3Al doped with boron

The effects of temperature and grain size on the deformation and fracture behaviour of recrystallized Ni₃Al doped with boron were investigated by tensile tests at temperatures up to 973 K as a function of grain sizes from 1.6 to 105m. The yield stress showed a positive temperature dependence to a peak temperature in somewhat different manners depending on the grain size. For coarse-grained specimens, a rapid drop in elongation was observed with increasing temperature. The predominant fracture mode changed with temperature from the transgranular fracture of {1 1 1} cracking to brittle intergranular fracture. This embrittlement at elevated temperatures was considered to occur by a high stress concentration at grain boundaries arising from increased flow stress level and the occurrence of grain boundary sliding (GBS). In contrast, the elongation was not so markedly decreased with temperature for intermediate- and fine-grained specimens which exhibited ductile intergranular fracture and cavitation fracture, respectively, at elevated temperatures, and a slant-type fracture and cup-cone fracture, respectively, at low temperatures. The suppression of serious high-temperature embrittlement for intermediate-grained specimens was explained in terms of the slow propagation of a crack formed by GBS, owing to stress relaxation by dynamic recrystallization (DR) and plastic deformation. In the case of ultra-fine-grained specimens a large elongation was developed at elevated temperatures, which was interpreted as that the further occurrence of DR with increasing volume fraction of grain boundaries reduces the cavitation promoted by GBS, and that the limited sliding length due to extremely small grain diameter raises the stress for cavity formation.     

Mechanisms governing cyclic fracture in an Al–Cu–Li alloy

Abstract

The fracture behaviour of a peak-aged, partially recrystallized Al–4·5Cu–1·21Li–0·51Mn–0·20Cd alloy has been investigated as a function of strain amplitude, stress intensity, and environment. It was found that the failure was predominantly intergranular separation, regardless of the environment, stress intensity, or strain amplitude, and that the fracture behaviour was influenced mostly by intrinsic microstructural features, rather than the nature of the environment. The shearable nature of matrix strengthening precipitates, large recrystallized grains, and precipitate-free zones along the high-angle grain boundaries aid in localizing the deformation, resulting in low-energy intergranular fracture. The iron- and silicon-rich intermetallic precipitates in the alloy promote void nucleation following fracture of the particle. A model is proposed which suggests the need for high stresses and strains for the initiation and spontaneous growth and coalescence of microvoids. The mechanisms of fracture behaviour of the alloy are discussed in terms of several concurrent processes involving strength of the material, intrinsic microstructural effects, deformation behaviour, state of stress, and strain.

MST/497     

Computer modelling of phase diagrams

Abstract

An investigation has been made of the tensile behaviour between 20 and 600°C of two ultrahigh boron steels (Fe–2·2B and Fe–4·9B), consolidated by hot isostatic pressing at temperatures ranging from 700 to 1100°C. Tensile tests showed plastic deformation only in the Fe–2·2B alloy. A decrease in yield and ultimate tensile stresses occurred when the consolidation temperature was increased. This was accompanied by an increase in the elongation to failure. This alloy satisfies the Hall–Petch relation for all testing temperatures. The slope of the yield stress versus d^?1/2 curve (d is grain size) decreases as the temperature increases, indicating that the mechanism controlling plastic deformation becomes independent of grain size at high testing temperatures. The fracture mode observed was brittle at room temperature and ductile, shown by the presence of dimples, at temperatures above 400°C.

MST/2050     

Geometry and grain size effects on the fracture behavior of sheet metal in micro-scale plastic deformation

The demand on micro-parts is significantly increasing in the last decade due to the trend of product miniaturization. When the part size is scaled down to micro-scale, the billet material consists of only a few grains and the material properties and deformation behaviors are quite different from the conventional ones in macro-scale. The size effect phenomena occur in micro-scale plastic deformation or micro-forming and there are still many unknown phenomena related to size effect, including geometry and grain size effects. It is thus critical to investigate the size effect on deformation behavior, especially for the fracture behavior in micro-scale plastic deformation. In this research, tensile test was conducted with annealed pure copper foils with different thicknesses and grain sizes to study the size effects on fracture behavior. It is found that flow stress, fracture stress and strain, and the number of micro-voids on the fracture surface decrease with the decreasing ratio of specimen size to grain size. Based on the experimental results, dislocation density based models which consider the interactive effect of specimen and grain sizes on fracture stress and strain are developed and their accuracies are further verified and validated with the experimental results obtained from this research and prior arts.     

Effects of ferrite grain size and martensite volume fraction on dynamic deformation behaviour of 0·15C–2·0Mn–0·2Si dual phase steels

Abstract

Effects of ferrite grain size and martensite volume fraction on quasistatic and dynamic deformation behaviour of 0·15C–2·0Mn–0·2Si dual phase steels were investigated in this study. Dynamic torsional tests were conducted on six steel specimens that had different ferrite grain sizes and martensite volume fractions, using a torsional Kolsky bar, and then the test data were compared in terms of microstructures, tensile properties, fracture mode, and adiabatic shear band formation. Under dynamic torsional loading, maximum shear stress and fracture shear strain increased with decreasing ferrite grain size and increasing martensite volume fraction. Observation of the deformed area beneath the fracture surface after the dynamic torsional test indicated that adiabatic shear bands of 5 to 15 μm in width were formed along the shear stress direction, and that voids or microcracks initiated at ferrites or martensite/ferrite interfaces below the shear band. The width of the shear band decreased as the ferrite grain size increased or the martensite volume fraction decreased. These phenomena were then analysed by introducing concepts of theoretical critical shear strain.     

不同热输入下F460钢焊接粗晶热影响区韧脆转变的内在机理

使用Gleeble 3800热模拟试验机模拟F460钢单道次焊接条件下焊接粗晶热影响区的热循环过程,通过光镜(OM)、扫描电镜(SEM)分析热影响区的显微组织、确定临界事件,通过ABAQUS软件计算临界解理断裂应力σf,进而系统分析不同焊接热输入E下韧脆转变温度变化的内在机理。结果表明:随着E的提高,焊接粗晶热影响区显微组织依次为少量板条马氏体和大量细密的板条贝氏体,板条贝氏体较多的板条/粒状贝氏体,粒状贝氏体较多的板条/粒状贝氏体,粗大的粒状贝氏体。原始奥氏体晶粒、贝氏体团的最大尺寸随着E的提高而变大。在完全解理断裂的冲击断口上,寻找停留在缺口尖端附近的残留裂纹,通过对比残留裂纹长度、原始奥氏体晶粒大小、贝氏体团尺寸,发现不同E下解理断裂的临界事件尺寸都是贝氏体团大小,而临界事件尺寸越小,韧脆转变温度越低。此外,通过有限元模拟缺口尖端的应力分布得到σf,σf越大冲击韧度越好,随着E的提高σf降低,故进一步说明随着E的提高韧脆转变温度Tk上升的内在机理。     

The role of grain boundary sliding and reinforcement morphology on the creep deformation behaviour of discontinuously reinforced composites

In this study, the role of grain boundary sliding behaviour on the creep deformation characteristics of discontinuously reinforced composites is investigated numerically together with the other influencing parameters: reinforcement aspect ratio, grain size and interfacial behaviour between the reinforcement and the matrix. The results obtained for the composites are compared with results obtained for a polycrystalline matrix material having identical grain size and morphology. The results indicate that, with sliding grain boundaries, the stress enhancement factor for the composites is much higher than the one observed for the matrix material and its value increases with increasing reinforcement aspect ratio, reduction in the matrix grain size and sliding interfacial behaviour between the reinforcement and the matrix. In the composites, the contribution of the grain boundary sliding to overall steady state creep rates occurs in a larger stress range in comparison to the matrix material. Experimentally observed higher creep exponent values or stress dependent creep exponent values for the composites could not be explained solely by the mechanism of grain boundary sliding. However, experimentally observed large scale triple point grain boundary cavitation in the composites could occur due to large grain rotations resulting from grain boundary sliding.     

In situ observations on shear and creep-fatigue fracture behaviors of SnBi/Cu solder joints

Shear and creep-fatigue fracture behaviors of the SnBi/Cu solder joints were investigated in this study. The deformation and fracture morphologies were in situ observed by scanning electron microscope, and the fracture mechanisms were discussed based on the observation results. It is found that the SnBi solder in the solder joint shows good ductility under shear stress, there is serious deformation mismatch between the Sn and Bi phases in micro-scale but no macro-scale cracking occurs inside the solder, and the shear fracture occurs along the Cu/solder interface. Under creep-fatigue loadings, the strain of the solder joints increases rapidly during the initial few cycles, but the increase rate decreases due to strain hardening. After the strain hardening becomes saturated, the strain increases exponentially with increasing cycles and the damage inside the solder keeps developing, final fracture occurs inside the solder near the joint interface. As the plastic deformation of the SnBi solder concentrates at the grain boundary, it is predicated that grain-boundary sliding is the major creep deformation mechanism. The influencing factors on creep-fatigue resistance include the stress range, holding time and grain size of the solder. Based on the understandings, techniques to enhance the creep-fatigue resistance were proposed.     

Ductility and fracture of copper bicrystals with boundary SiO2 particles

Abstract

High temperature deformation andfracture ofCu-SiO₂ bicrystals with [001] twist boundaries of various misorientation angles were investigated under the condition of non-activation of grain boundary sliding. As the misorientation angle increases, the bicrystals became more susceptible to intergranular brittle fracture. Clear intermediate temperature embrittlement was observed in bicrystals with a random high angle boundary. The boundary segregation of O atoms was found to enhance intergranular fracture. Although the boundary SiO₂ particles provide stress concentration sites which cause early formation of boundary cavities, the boundary dependent deformation and fracture behaviour is essentially determined by inherent boundary strength, which is afunction of the misorientation angle.

MST/1969     

Deformation-mechanism map for nanocrystalline metals by molecular-dynamics simulation

Molecular-dynamics simulations have recently been used to elucidate the transition with decreasing grain size from a dislocation-based to a grain-boundary-based deformation mechanism in nanocrystalline f.c.c. metals. This transition in the deformation mechanism results in a maximum yield strength at a grain size (the 'strongest size') that depends strongly on the stacking-fault energy, the elastic properties of the metal, and the magnitude of the applied stress. Here, by exploring the role of the stacking-fault energy in this crossover, we elucidate how the size of the extended dislocations nucleated from the grain boundaries affects the mechanical behaviour. Building on the fundamental physics of deformation as exposed by these simulations, we propose a two-dimensional stress-grain size deformation-mechanism map for the mechanical behaviour of nanocrystalline f.c.c. metals at low temperature. The map captures this transition in both the deformation mechanism and the related mechanical behaviour with decreasing grain size, as well as its dependence on the stacking-fault energy, the elastic properties of the material, and the applied stress level.     

Deformation and fracture in Al–Li-base alloys

Abstract

Aluminium–lithium-base alloys are of considerable interest because of their low density and high modulus. However, they have been shown to have low ductility and poor fracture toughness. This has been attributed to a variety of factors, including intense shear band formation, segregation to grain boundaries, and weakened grain boundaries due to precipitation and precipitate-free zones. The authors have investigated the deformation structures observed in binary and more complex commercial alloys. As would be expected, considering the microstructure of the alloys, extensive strain localization and shear band formation occurs in these alloys. However, it is shown that the commercial alloys are less sensitive to strain localization than the model binary alloy systems investigated. The stresss–train behaviour has been investigated. The alloys exhibit jerky flow, which is indicative of negative strain rate sensitivity, and strain rate change tests showed this to be the case. This is consistent with the deformation structures observed. The effect of weakened grain boundaries due to precipitation and precipitate-free zones has been studied by comparing the fracture characteristics of aged and unaged material. It is shown that the mode of failure is identical under appropriate conditions. It is concluded that segregation to grain boundaries is the major cause of the lower ductility and toughness of Al–Li alloys. This possibility has been investigated using in situ fracture surface analysis techniques. Results are presented on grain boundary segregation, and methods of reducing its influence on fracture behaviour are indicated.

MST/570     

Influence of polycrystal grain size on fracture toughness of and fatigue threshold in Armco iron

Influence of polycrystal grain size on ductile fracture toughness of and fatigue threshold stress intensity in Armco iron has been studied over a grain size range 40 to 1050 μm. Both ductile fracture toughness and fatigue threshold stress intensity have been found to decrease with increasing grain size and the variation in either case follows a relationship similar to that proposed by Hall-Petch for strength. The variation of toughness with grain size can be understood in terms of plastic zone size whereas the fatigue threshold behaviour in Armco iron appears to be controlled by the critical value of crack tip opening displacement range.     

Sintering,microstructure and mechanical properties of commercial Y-TZPs

The sintering behaviour of Y-TZP ceramics, their resulting microstructures and properties are influenced not only by the characteristics of the raw materials but also were found to be dependent on the thermal history during the fabrication process. It is generally understood that fracture toughness increases as grain size increases up to a certain limit but in the present investigation, the results obtained challenge this view. The work is concerned with grain size dependence on the mechanical properties, in particular on the fracture toughness. Two commercially available powders based on two different processing techniques (i.e. coated and co-precipitated) were studied. It has been found that both materials exhibited different fracture toughness trends. Smaller grains of coated Y-TZP resulted in high fracture toughness >12 MPa m^1/2 while the opposite effect was seen in the co-precipitated material which showed enhanced fracture toughness with increasing grain size above a certain lower limit from a nonconventional heat treatment.     

Superplasticity

Superplasticity is the phenomenon of extraordinary ductility exhibited by some alloys with extremely fine grain size, when deformed at elevated temperatures and in certain ranges of strain rate. To put the phenomenology on a proper basis, careful mechanical tests are necessary. These are divided into (i) primary creep tests, (ii) steady state deformation tests, and (iii) instability and fracture tests, all of which lead to identification of macroscopic parameters. At the same time, microstructural observations establish those characteristics that are pre-requisites for superplastic behaviour. Among the macroscopic characteristics to be explained by any theory is a proper form of the equation for the strain rate as a function of stress, grain size and temperature. It is commonly observed that the relationship between stress and strain rate at any temperature is a continuous one that has three distinct regions. The second region covers superplastic behaviour, and therefore receives maximum attention. Any satisfactory theory must also arrive at the dependence of the superplastic behaviour on the various microstructural characteristics. Theories presented so far for microstructural characteristics may be divided into two classes: (i) those that attempt to describe the macroscopic behaviour, and (ii) those that give atomic mechanisms for the processes leading to observable parameters. The former sometimes incorporate micromechanisms. The latter are broadly divided into those making use of dislocation creep, diffusional flow, grain boundary deformation and multimechanisms. The theories agree on the correct values of several parameters, but in matters that are of vital importance such as interphase grain boundary sliding or dislocation activity, there is violent disagreement. The various models are outlined bringing out their merits and faults. Work that must be done in the future is indicated.     

Experimental investigation on low cycle fatigue and fracture behaviour of a notched Ni‐based superalloy at elevated temperature

In order to investigate the effects of stress concentration on low cycle fatigue properties and fracture behaviour of a nickel‐based powder metallurgy superalloy, FGH97, at elevated temperature, the low cycle fatigue tests have been conducted with semi‐circular and semi‐elliptical single‐edge notched plate specimens at 550 and 700 °C. The results show that the fatigue life of the notched specimen decreases with the increase of stress concentration factor and the fatigue crack initiation life evidently decreases because of the defect located in the stress concentration zone. Moreover, the plastic deformation induced by notch stress concentration affects the initial crack occurrence zone. The angle α of the crack occurrence zone is within ±10° of notch bisector for semi‐circular notched specimens and ±20° for semi‐elliptical notched specimens. The crack propagation rate decreases to a minimum at a certain length, D, and then increases with the growth of the crack. The crack propagation rate of the semi‐elliptical notched specimen decelerates at a faster rate than that of the semi‐circular notched specimen because of the increase of the notch plasticity gradient. The crack length, D, is affected by both the applied load and the notch plasticity gradient. In addition, the fracture mechanism is shown to transition from transgranular to intergranular as temperature increases from 550 to 700 °C, which would accelerate crack propagation and reduce the fatigue life.     

THE BRITTLE FRACTURE OF 475°C EMBRITTLED CAST DUPLEX STAINLESS STEEL

Abstract— The fracture behaviour of cast duplex stainless steels, heat treated to different ferrite contents and hardness was investigated using tensile and notched bend tests. The purpose was to identify the microstructural features which controlled the ductile-to-brittle fracture transition of 475°C embrittled duplex stainless steel. The results indicate that twin nucleated cleavage has a tensile stress fracture criteria and the brittle-to-ductile transition temperature depends on ferrite microhardness, ferrite grain size and constraint.