Effects of surface oxide layers on crack initiation and growth of HSLA steel under cyclic loading in air and in ultrahigh vacuum

Effects of the thermally grown wustite on the fatigue crack initiation and growth in HSLA steel are evaluated as a function of oxide thickness, strain amplitude, and gaseous environment in the push-pull plastic strain control mode, with special attention being given to the early stage of microcrack initiation. Specimens with a wustite surface layer thermally grown to 0.2 and 0.6 m thicknesses show predominantly intergranular cracking at plastic strain amplitudes of 5×10^–4 and 1×10^–3 both in air and in ultrahigh vacuum (UHV), in contrast to the as-polished specimens where slip band cracking is the favoured mode. The cracking mode in the oxide layer is discussed in terms of the strain amplitude and the dislocation behaviour near the oxide/metal interface. The features of microcrack initiation in the oxide layer is not affected by the gaseous environment. Once, however, the surface oxide fractures, the rate of crack growth through the base metal is greatly reduced in UHV.     

Work hardening of mild steel within dynamic strain ageing temperatures

An investigation has been performed on the plastic behaviour of a mild steel within the region of dynamic strain ageing. For this purpose tension tests have been performed on annealed XC18 steel within a range of temperatures, from 305–776 K, and a range of strain rates, from 1.0×10^–4–1.85×10^–1s^–1. An analysis of experimental results is presented using a model for plastic deformation based on dislocation multiplications.     

Effects of grain size on cyclic plasticity and fatigue crack initiation in nickel

Fatigue experiments were conducted on polycrystalline nickel of two grain sizes, 24 and 290 μm, to evaluate the effects of grain size on cyclic plasticity and fatigue crack initiation. Specimens were cycled at room temperature at plastic strain amplitudes ranging from 2.5×10⁻⁵ to 2.5×10⁻³. Analyses of the cyclic stress–strain response and evolution of hysteresis loop shape indicate that the back stress component of the cyclic stress is significantly affected by grain size and plastic strain amplitude, whereas these parameters have little effect on friction stress. A nonlinear kinematic hardening framework was used to study the evolution of back stress parameters with cumulative plastic strain. These are related to substructural evolution features. In particular, long range back stress components are related to persistent slip bands. The difference in cyclic plasticity behavior between the two grain sizes is related to the effect of grain size on persistent slip band (PSB) morphology, and the effect this has on long range back stress. Fine grain specimens had a much longer fatigue life, especially at low plastic strain amplitude, as a result of the influence of grain size on fatigue crack initiation characteristics. At low plastic strain amplitude (2.5×10⁻⁴), coarse grain specimens initiated cracks where PSBs impinged on grain boundaries. Fine grain specimens formed cracks along PSBs. At high plastic strain amplitude (2.5×10⁻³), both grain sizes initiated cracks at grain boundaries.     

Low-cycle fatigue of polycrystalline α-iron modified by mutually immiscible silver-ion implantation

Cyclic deformations of annealed pure polycrystalline -iron with and without further mutually immiscible silver-ion implantation (90 keV, 6×10¹⁶ ions cm^–2) were studied in a plastic strain-controlled tension-compression fatigue test (triangular loading waveform, frequency 0.02–0.3 Hz, and plastic strain range 3×10^–3–1.2×10^–2). The obtained plastic strain-life (_p-N _f) curves showed that the iron specimens could survive for a greater number of cycles before failure when implanted. Comparison of the cyclic stress-strain curves suggested that the implanted specimens had maintained a relatively more stable microstructural change than those unimplanted ones which had undergone a violent cyclic hardening during cyclic deformation. This is proposed to be a strong indication that the fatigue ductility has been improved and the cross slip of screw dislocations, which leads to the evolution of the persistent slip bands for fatigue damage, was hindered to some extent after ion implantation.     

Cyclic plasticity of single crystal nickel oriented for single slip

The cyclic deformation behavior of single crystal nickel was investigated by performing uniaxial fully reversed constant plastic strain amplitude fatigue experiments at plastic shear strain amplitudes ranging from 1.1 × 10⁻⁴ to 8.8 × 10⁻³. Digitally acquired stress–strain hysteresis loops were used to calculate friction and back stresses and to relate the shapes of the loops to the evolving dislocation structures and magnetomechanical effects. The results indicate that the cyclic stress–strain curve exhibits a plateau of 50 MPa between plastic strain amplitudes of 1 × 10⁻⁴ and 7 × 10⁻³. Saturation friction stress, calculated using the Cottrell method, is reasonably constant over the entire range of plastic strain amplitudes at a value of 15 MPa. The back stress is 35 MPa within the plateau and increases to 48 MPa at the highest strain amplitude. When cycled at low plastic strain amplitude, magnetomechanical effects account for a significant portion of the measured inelastic strain.     

The rate dependent response of a titanium alloy subjected to quasi-static loading in ambient environment

A dual phase Ti-6A1-4V alloy was tested in uniaxial tension over a large quasi-static loading range (10^–5–10^–1 s^–1) in ambient environment. As strain rate increases, strength of the alloy was found to increase at the expense of ductility. In the low strain-rate region, strain rate sensitivity of the material experienced a gradual decrease during plastic deformation. In the high strain-rate region, strain-rate sensitivity of the material was largely constant for most part of the plastic deformation. The different rate dependent behaviours are believed to be caused by a change of governing plastic deformation mechanism from dislocation slip at low strain rates to twinning at the highest strain rate. Strong fractographic and metallographic evidence was obtained to understand the micromechanisms of plastic deformation.     

High temperature compressive properties over a wide range of strain rates in an AZ31 magnesium alloy

High temperature compressive properties in AZ31 magnesium alloy were examined over a wide strain rate range from 10^–3 to 10³ s^–1. It was suggested that the dominant deformation mechanism in the low strain rate range below 10^–1 s^–1 was dislocation creep controlled by pipe diffusion at low temperatures, and by lattice diffusion at high temperatures. On the other hand, analysis of the flow behavior and microstructural observations indicated that the deformation at high strain rates of 10³ s^–1 proceeds by conventional plastic flow of dislocation glide and twinning even at elevated temperatures.     

Investigation of time-dependent characteristics of materials and micromechanisms of plastic deformation on a submicron scale by a new pulse indentation technique

A new pulse technique is proposed to investigate dynamics and micromechanisms of plastic deformations of materials underneath the indenter during microindentation. It is established that the process of indenter penetration under pulse loading conditions can be described by several distinct stages (as many as four in some cases) differing in kinetics and activation parameters. In each investigated material, the first stage was characterized by a high strain rate (10³ s^–1) and the high contact stresses (dynamic hardness), which exceeded static hardness by factor of 5–10. Typical values of activation volumes during the first stage were of the order of 10^–30 m³, i.e. close to the volume occupied by an atom (ion) in a lattice. During the second and the subsequent stages, the activation volume in ionic crystals increased up to about 10^–28 m³,( i.e. 10b ³, where b is Burger's vector of glide dislocations). Dynamics of initial stages of the indenter penetration was, therefore, determined by elastic and subsequently by plastic deformation, which is carried out by non-equilibrium point defects (most likely by interstitials or crowdions). A relative role of point defects in the process of mass transfer underneath the indenter and its contribution to microhardness is estimated. In soft materials (NaCl, LiF, Pb) during long indentation times (1 s) contribution of point defects can be estimated as more than 10%, and in the case of hard materials (Si, amorphous alloys) became closer to 100%. Dynamics of the final stages of the indenter penetration in soft crystals were governed by the dislocation creep. In all investigated materials for short indentation times (1 s) plastic deformation underneath the indenter occurred predominantly via generation and motion of interstitials and their clusters containing a few atoms.     

Plastic deformation behavior in dual-phase Ni–31Al intermetallics at elevated temperature

Plastic deformation behavior of dual-phase Ni–31Al intermetallics at elevated temperature was examined. It was found that the alloy exhibited good plasticity under an initial strain rate of 1.25 × 10⁻⁴ s⁻¹ to 8 × 10⁻³ s⁻¹ in a temperature range of 950–1075 °C. A maximum elongation of 281.3% was obtained under an initial strain rate of 5 × 10⁻⁴ s⁻¹ at 1000 °C. The strain rate sensitivity, m value was correlated with temperature and initial strain rate, being in the range of 0.241–0.346. During plastic deformation, both the two phases Ni₃Al and NiAl in dual-phase Ni–31Al could co-deform without any void formation or debonding, the initial coarse microstructure became much finer after plastic deformation. Dislocation played an important role during the plastic deformation in dual-phase Ni–31Al alloy, the deformation mechanism in dual-phase Ni–31Al could be explained by continuous dynamic recovery and recrystallization.     

Influence of strain rate on the behavior of constructional alloys in static tension under conditions of extreme cooling (4.2 K)

The influence of strain rate on the features of interrupted flow of alloys for cryogenic technology at the boiling point of liquid helium (4.2 K) was investigated. The specimens were tested with strain rates of 0.28·10^–3, 0.28·10^–2, 0.56·10^–2, 0.83·10^–2, 0.14·10^–1, and 0.28·10^–1 sec^–1. It was shown that an increase in rate within these limits leads to a change in the character of deformation of the investigated alloys, which is determined by the power of the heat flow formed as the result of plastic deformation and dependent upon strain rate V and the amount of the jumps in stresses _max. The relationship established of the character of spasmodic deformation to loading conditions is an indication of the necessary of taking this factor into consideration in evaluation of the strength of structures under conditions of extreme cooling.Translated from Problemy Prochnosti, No. 1, pp. 3–8, January, 1990.     

Superplastic properties of a TiAl based alloy with a duplex microstructure

The superplastic properties of a engineering TiAl based alloy with a duplex microstructure were investigated with respect to the effect of testing temperatures ranging from 950°C to 1075°C and strain rates ranging from 8 × 10^–5 s^–1 to 2 × 10^–3 s^–1. A maximum elongation of 467% was achieved at 1050°C and at a strain rate of 8 × 10^–5 s^–1. The apparent activation energy was calculated to be 345 kJ/mol. Also, the dependence of the strain rate sensitivity values on strain during superplastic deformation was examined through the jump strain rate tests, and microstructural analysis was performed after superplastic deformation. It is concluded that superplasticity of the alloy at relatively low temperature and relatively high strain rate results from dynamic recrystallization, and grain boundary sliding and associated accommodation mechanism is related to superplasticity at higher temperature and lower strain rate.     

Load-relaxation testing at elevated strain rates

The feasibility of a new mechanical testing technique is described. This technique allows the collection of load-relaxation type data at higher strain rates than is possible in a conventional load-relaxation test. This technique was used on commercial purity aluminium and 304 stainless steel at strain rates up to 5 sec^–1. Good agreement with Hart's state variable model for plastic deformation was found.     

Non-linear response of internal friction to tensile strain rate and frequency during plastic deformation of high-purity aluminium

The internal friction of high-purity aluminium during the process of plastic deformation was measured by a middle torsion pendulum on a modified tensile testing machine. The effects of tensile strain rate, , in the range of 0.73×10^–6 to 50×10^–6s^–1, and frequency of internal friction measurement, f, in the range of 0.38 to 2.6 Hz were studied. The results showed a non-linear dependence of internal friction, Q ^–1, on and f ^–1 or on (=2 f). The interrelationship between internal friction during the process of plastic deformation and dislocation motion, and the effect of non-linearity on the dynamic behaviour of dislocations are discussed.     

The temperature and strain rate dependence of the flow stress of tantalum

The temperature and strain rate dependence of the flow stress of tantalum was studied between 78 to 800 K at strain rates from 10^–5 to 2×10⁴ sec^–1. The effect of temperature and strain rate on the lower yield stress can be explained by a model incorporating the combined operation of the Peierls mechanism and dislocation drag processes. The general behaviour of the stress—strain curve at various strain rates and temperatures is analysed in terms of a rate—temperature parameter.     

Copper ductility and strength at high strain rates

The results of investigations of the mechanical properties and structural changes in M2 copper at high strain rates 5·10³ and 10⁶ sec^–1 are presented. In both cases the material is observed to have high deformability. A deformation mechanism leading to such plasticity is proposed.Translated from Problemy Prochnosti, No. 10, pp. 47–52, October, 1993.     

Dynamic strain aging effect on the fatigue resistance of type 316L stainless steel

Mechanism of dynamic strain aging (DSA) and its effect on the high-temperature low-cycle fatigue resistance in type 316L stainless steel were investigated by carrying out low-cycle fatigue tests in a wide temperature range from 20 to 650 °C with strain rates of 3.2×10⁻⁵–1×10⁻²/s. The regime of DSA was evaluated using the anomalous features of material behavior associated with DSA. The activation energies for each type of serration were about 0.57–0.74 times those for lattice diffusion indicating that a mechanism other than lattice diffusion is involved. It is reasonably concluded that the pipe diffusion of solute atoms along the dislocation core is responsible for DSA. Dynamic strain aging reduced the fatigue resistance by ways of multiple crack initiation, which comes from the DSA-induced inhomogeneity of deformation, and rapid crack propagation due to the DSA-induced hardening.     

Influence of loading rate on the mechanical properties of steels of different strength levels

The results are presented of mechanical tensile and shear tests of steels of different strength levels with plastic strain rates of 3·10^–3–10⁵ sec^–1. The changes in the characteristics of strength, plasticity, and microstructure are analyzed in relation to the strength level of the steel and loading rate. For steels of different structural classes at high deformation rates (approximately up to 10⁵ sec^–1) a significant increase in the characteristics of strength and plasticity is observed.Institute of Problems of Strength, Academy of Sciences of the Ukrainian SSR, Kiev, Leningrad. Translated from Problemy Prochnosti, No. 10, pp. 42–48, October, 1989.     

Static recrystallization kinetics of 304 stainless steels

Static restoration mechanism during hot interrupted deformation of 304 stainless steel was studied in the temperature range from 900 to 1100°C, various strain rate from 0.05 to 5/sec and pass strain of 0.25–3 times peak strain. It was clarified that the static recrystallization was happened after 3–10 seconds at first deformation. The static restoration was depended on the pass strain, deformation temperature and strain rate and fractional softening (FS) values increased with increasing strain rate, deformation temperature and pass strain. Recystallization kinetics was explained with Avrami equation and Avrami constant was 1.113. This value was independent of deformation variables significantly. The time of 5, 50, 95% recrystallization was evaluated using such equations: t _0.05 = 2.9 × 10^{–12 –1.17 –0.94} D exp(222000 J/mol/RT), t _0.5 = 2.0 × 10^{–10 –1.56 –0.81} D exp(197000 J/mol/RT), t _0.95 = 1.9 × 10^{–8–1.63 –0.76} D exp(173000J/mol/RT). The predicted values by use of upper equations had a good agreement with a measurement.     

Crystallisation of PET with strain, strain rate and temperature

The mechanical properties of Poly ethylene terephthalate (PET) were studied over several decades of strain rate and a temperature range of 263 K–453 K. Tests were carried out in the range 10^–3–10⁴ s^–1 using a conventional Hounsfield machine and two high strain rate test systems. Strain limited tests were carried out at all the strain rates and the temperature rises were estimated from the area under the stress strain curves. X-ray diffraction was used to extract interatomic plane distances and crystallite dimensions. Differential Scanning Calorimetry (DSC) was employed to estimate the degree of crystallinity of the material and the kinetics of crystallisation. PET yield stress increased with strain rate with a sharp increase at rates of 10³ s^–1 and above. It crystallised into the triclinic form at rates above 10³ s^–1 beyond 140% strain but crystallisation was not observed at lower strain rates. Increases of up to 40% in crystallinity content were found which, it is concluded, were thermally induced after the test ended. The results shed light on the development of crystallinity in PET as a function of strain, strain rate and temperature and indicate that the rapid increase in yield and flow stresses previously reported cannot be accounted for by increases in crystallinity.     

Effect of strain rate on the strength and ductility characteristics of polyamides

The main features are presented for dynamic tests in tension, compression, and shear with plastic strain rates up to 10⁵ sec^–1. An increase in strength characteristics and a reduction in ductility characteristics with an increase in strain rate are established with different forms of stressed state for the test materials. Experimental results confirm the necessity of studying the dynamic properties of polymer materials.Translated from Problemy Prochnosti, No. 9, pp. 19–22, September, 1991.