An experimental investigation of tensile deformation of round bars

Round specimens of different sizes and made of aluminum and copper were subjected to uniaxial tension; and on the basis of data thus obtained, a semi-empirical approach was developed to analyze tensile deformation and the effects of specimen size there on both before and after the onset of nonuniform deformation. Results reveal that, while a diffused instability sets in as the load approaches its maximum, the initiation of localized neck is delayed well beyond this point. This delay, however, is found to depend on the length as well as the diameter of the specimen.     

Superplastic deformation characteristics of two microduplex titanium alloys

A comparison of the superplastic deformation behaviour of Ti-6Al-4V (wt%) between 760 and 940‡ C and Ti-6Al-2Sn-4Zr-2Mo between 820 and 970‡ C has been carried out on sheet materials possessing similar as-received microstructures. High tensile elongations were obtained with maximum values being recorded at 880‡ C for Ti-6Al-4V (Ti-6/4) and at 940‡ C for Ti-6Al-2Sn-4Zr-2Mo (Ti-6/2/4/2), under which conditions both alloys possessed aΒ phase proportion of approximately 0.40. For a given deformation temperature the Ti-6/4 alloy had a slightly lower flow stress than the Ti-6/2/4/2, and this was attributed to the lowerΒ phase proportion in the latter alloy. However, at the respective optimum deformation temperatures the Ti-6/2/4/2 alloy had the lower flow stress. The results show that suitably processed Ti-6/2/4/2 alloy is capable of withstanding substantial superplastic strains at relatively low flow stresses, although the optimum deformation temperature is higher for this alloy than for Ti-6/4 material possessing a similar microstructure.     

Response of titanium alloys to high strain rate deformation

Abstract

The stress-strain response of samples of Ti64 and Ti550 at strain rates from 10^?1 s^?1 to 10³ s^?1 and samples of Ti811 and Ti153 at a strain rate of 10³ s^?1 have been assessed. It has been found that the influence of the imposed strain rate on the stress-strain response of Ti64 and Ti550 alloys is very similar – in both alloys the yield stress increases with increase of strain rate and the energy absorbed to fracture increases. At high strain rates localised deformation occurs in the form of shear bands in Ti64 and Ti550 but no shear banding was seen in Ti811 and Ti153. The fracture surfaces of Ti64 and of Ti550 show an increased tendency to brittle failure and an increase in necking with increase of strain rate. The influence of alloy microstructure and composition on the response to changes in imposed strain rate are discussed in terms of adiabatic heating and the factors controlling the flow stress in these alloys.     

An investigation of weld fracture surfaces and mechanical properties of two gamma titanium aluminide alloys

Two gamma titanium aluminide sheet materials were investigated to characterize the fusion zone cracking and mechanical properties that occur for specimens subjected to gas tungsten arc welding (GTAW). The materials used in this study consisted of essentially the same composition; however, one was received in the primary annealed (PA) condition and the other in the designed fully lamellar (DFL) condition. For both materials, all of the specimens welded with a stationary torch (spot welds) cracked catastrophically immediately after welding. The PA specimens exhibited a different mechanism of fracture in the fusion zone compared with the DFL specimens. The fractures that occurred in the PA specimens followed an interdendritic path through regions that did not completely solidify prior to the occurrence of cracking, whereas for the DFL specimens the primary mechanism of fracture was cleavage. For the butt welded specimens, no cracking occurred. For both materials, specimens welded parallel to the rolling direction exhibited more distinct preferred growth of columnar grains than those welded perpendicular to the rolling direction. The weld structure/property relationship is also discussed.     

Thermoplastic shear localisation in titanium alloys during dynamic deformation

Abstract

Cylindrical specimens (4 mm diameter and 4 mm height) of titanium alloy bar were given various heat treatments to provide a wide range of microstructures and mechanical parameters. These specimens were then subjected to high plastic strain at a large strain rate (10³ s^-1 ) during dynamic compression by a split Hopkinson bar at ambient temperature. The microstructures of the localised shear bands were examined by optical and transmission electron microscopy. The results show that there are two types of localised shear bands: deformed and white shear bands. A detailed observation reveals that there is no difference in the nature of the deformed and white shear bands, but they occur at different stages of localised deformation. It is found that there is a burst of strain, corresponding to a critical strain rate at which the white shear band occurs and no phase transformation occurs in the shear bands.     

An experimental investigation of the structure of some internally oxidised ferromagnetic alloys

Results are presented of optical, scanning electron and transmission electron microscope observations on some internally oxidised ferromagnetic alloys. The alloys investigated were of nominal composition 0.058 wt % Si in Ni, 0.48 wt % Si in Ni and 0.17 wt % Si in Co 66.2%-Ni 33.7%. The alloys were in polycrystalline form and in addition single crystals of 0.058 wt % Si in Ni were examined. The diffusion and oxidation rate constants are calculated and the state of the oxide product as a function of (i) depth in the alloy, (ii) oxidising temperature and (iii) alloy composition is considered. The possibility of using such systems as device material and as test material for theories of coercivity and approach to magnetic saturation is considered.     

Low temperature peculiarities of plastic deformation in titanium and its alloys

A number of unexpected phenomena of the mechanical behaviour of metals, due to the specific character of dislocation motion and interaction are analysed using the strength and plasticity characteristics of titanium of different grades of purity and its binary alloys with aluminium, vanadium, niobium and oxygen at 4.2–300 K. The non-monotonic temperature dependence of deforming stresses, which is inconsistent with the conventional thermal fluctuation concepts of the plastic deformation process, is considered along with the plastic flow instability appearing as drops of load in the stress-strain curve, and a low temperature increase in plasticity of the metals studied.The analysis of the effects of alloying, preliminary strain and sample sizes on the plastic flow instability parameters (onset temperature, amplitude and number of load drops) has shown that this may be accounted for by a combination of hypotheses of the dislocation array overcoming a barrier and the following local heating which results in a drop of load in the stress-strain curve.It has been found that substitution elements at concentrations corresponding to a homogeneous solid solution do not affect the usual behaviour of temperature dependence of yield stress O_0.2 typical of high purity titanium. The presence of the second phase due to high concentrations of the above elements reverses the sign of the do_0.2/dT derivative. Abov certain concentrations of oxygen atoms forming an interstitial solid solution, it becomes temperature independent. The comparison between the derived experimental results and theoretical concepts available shows that the former make allowance for increasing importance of the inertial mechanism of overcoming pinning centres by dislocations at low temperatures.The observed increase in plasticity of titanium and its alloys based on solid solutions at 40–120 K is attributed to a combined action of two deformation mechanisms - slip and lower temperature activated twinning.     

An experimental investigation of failure behavior of conducting polythiophene coating films

Polymeric films with different conductivities are widely used as coatings on electronic devices. For the reliability of their application, evaluation of mechanical properties is essential. In this paper, mechanical properties of conducting polythiophene (Pth) films are investigated with Scanning Electron Microscopy (SEM) to understand the relation between the mechanical properties and the electrochemical polymerization (ECP) deposition process and microstructure as well as film thickness. An attempt to explain the special properties exhibited by films with different thickness is made. The fatigue microcrack initiation and growth around a cross indent was observed in low cycle tension-tension fatigue tests. Three typical fatigue fracture models (short micro crack propagation, spallation and fragmentation) for films deforming coherently with the stainless steel substrate have been directly observed with SEM and conditions for their occurrences are discussed.     

Stress-dependent deformation behaviour in bulk nanocrystalline titanium–nickel alloys

In this study, the deformation mechanisms operating with stress in bulk nanocrystalline (NC) titanium–nickel with an average grain size below a critical size of 10–20?nm have been investigated. We demonstrate a sequential variation of the deformation mechanism from grain boundary (GB) sliding and grain rotation to grain growth and dislocation activity with the increase of the deformation stress. These deformation mechanisms are different from the previous understanding that below a critical grain size of 10–20?nm, GB sliding and grain rotation govern plastic deformation of NC materials.     

An experimental investigation of fatigue behavior and deformation of double spot friction welded joints

Fatigue behavior of double spot friction welded joints in aluminum alloy 7075-T6 plates is investigated by conducting monotonic tensile and fatigue tests. The spot friction welding procedures are carried out by a milling machine with a designed fixture at the best preliminary welding parameter set. The fatigue tests are performed in a constant amplitude load control servo-hydraulic fatigue testing machine with a load ratio of (R = P_min/P_max) 0.1 at room temperature. It is observed that the failure mode in cyclic loading (low-cycle and high-cycle) resembles that of the quasi-static loading conditions i.e. pure shearing. Primary fatigue crack is initiated in the vicinity of the original notch tip and then propagated along the circumference of the weld’s nugget.     

Plastic deformation of titanium alloys under simple loading and its mathematical modeling

Results of an experimental investigation of the regularities of plastic deformation of titanium alloys in a plane stress state are analyzed. Tests were performed by loading thin-walled tubular specimens by an axial force and internal pressure under conditions of a proportional increase in the loads. The alloys are found to be transversely isotropic materials whose isotropic surface coincides with the cylindrical surface of the specimen. The process of plastic deformation of the alloys under simple loading is shown to be described well by equations of a previously proposed deformation theory of the plasticity of transversely isotropic media. Translated from Problemy Prochnosti, No. 5, pp. 27–35, September–October, 1999.     

工业纯铝L2应变率相关的拉伸力学性能研究

对工业纯铝L2进行了不同应变率下（0.001,0.02,200,500和1300s^-1)拉伸力学性能的试验，得到各应变率下完整的应力-应变曲线，试验结果表明，在应变率很低时，其屈服应力（σs)，拉伸强度（σb)和失稳应变（εb)随着应变率的增加变化不大；当应变率很高时，σs和σb随着应变率对数的增加而增大，而εb随着应变率的增加而减小；弹性模量则对应变率不敏感，采用修正的幂次强化模型及其简化模型来描述L2的应变率敏感行为，并采用Ramberg-Osgood模型拟合各个应变率下的应力应变曲线，结果表明，此两种模型能够很好地描述L2应变率相关的力学行为。     

An experimental investigation of phase transformation superplastic diffusion bonding of titanium alloy to stainless steel

The solid-state direct diffusion bonding of a near α-phase titanium alloy to an austenitic stainless steel by means of the phase transformation superplasticity (PTSP) caused by the cycles of heating and cooling has been carried out. The test results showed that, under the conditions of T_max = 890°C, T_min = 800°C, cyclic number of heating and cooling N = 10 cycles, specific pressure P = 5 MPa, heating rate V_h = 30°C/s and cooling rate V_c = 10°C/s, the ultimate tensile strength of the joint reached its maximum value (307 MPa), and the bonding time was only 120 s. In the phase transformation superplastic state, the deformation of titanium alloy has a character of ratcheting effect and it accumulates with the cycles of heating and cooling. The observations of tensile fracture interface showed that both the brittle intermetallic compound (FeTi) and the solid solution based on β-Ti were formed on the interface, and the more in quantity and the smaller in size the solid solutions are, the higher the ultimate tensile strength is.     

Computation and experimental investigation of the resistance of steels to plastic deformation and failure under a non-stationary repeating-static load

The structural model of a material and the corresponding equation of mechanical states for strain calculation in a complex cyclic stress state are presented. Present computational and theoretical data are compared with those obtained by authors of other studies. An energy-type damage equation is presented. The results of computation in accordance with this equation are compared with some experimental data obtained on tubular specimens in a complex stress state.Translated from Problemy Prochnosti, No. 4, pp. 3–7, April, 1991.