Instable Microdeformation and Strain Recovery in Amorphous LiPON Thin Layer.

Investigation of cutting depth and contact area in nanoindenter scratching

In this work, we study the indenter geometrical effects on the nanoindentation and scratch behaviors of polymers (epoxy resin and its based nanocomposites) through both experimental and numerical approaches. Two different types of indenter commonly used in nanoindentation tests, Berkovich (i.e., three-sided pyramid-shaped) and conical-shaped, are studied. An indenter (tip) geometry reconstruction procedure is presented and the results are input into finite element models to better understand the mechanisms of the geometrical effects on nanoindentation. The effect of indenter geometry on nanoscratch behavior is also investigated experimentally. Overall, our results show that the corrected Oliver and Pharr's method is applicable to nanoindentation of polymeric surfaces and the indenter geometry plays significant roles in both the nanoindentation and nanoscratch tests. The conical-shaped indenter provides slightly overestimated mechanical parameters for the polymers tested due to the material pile-up. Therefore, for nanoindentation and scratch tests, the type of indenter should always be mentioned and comparisons between tests using different types of indenter should be taken with caution.

Effective prediction of springback during sheet metal forming is critically important for automotive and aerospace industries, especially when forming metals with high strength-to-weight ratio such as titanium. This requires materials mechanical data during plastic deformation and their dependencies on parameters like strain, strain rate and sample orientation. In this study, springback is quantified experimentally as elastic strain recovery, degradation in Young’s modulus and inelastic strain recovery on unloading in a commercially pure titanium type 50A (CP-Ti-50A). The results show strain rate-dependent anisotropic mechanical behaviours and a degradation in Young’s modulus with increased level of plastic deformation. The level of degradation in Young’s modules increases gradually from 13% for samples parallel to the rolling direction (RD) to 20% for those perpendicular to the RD. A measurable nonlinear strain recovery was also observed on unloading that is orientation dependent. The level of springback is characterised as the sum of elastic recovery and the contributions from both the degradation in Young’s modulus and anelastic strain recovery. It is shown that the Chord modulus can estimate springback with a reasonable accuracy taking into consideration the elastic strain recovery, degradation in Young’s modulus and anelastic strain recovery.

Small scale, grain size and substrate effects in nano-indentation experiment of film–substrate systems

Indentation testing including spherical and three-sided (Berkovich) indenter tips were conducted to evaluate the strain rate effect on the plastic properties in the weld zone of a popular structural steel, SS400, in this study. Two types of indentation test were under taken at a number of strain rate values from 0.03 to 0.2 s−1 in the weld metal (WM), heat-affected zone (HAZ), and base metal (BM) regions of the weld zone. The plastic properties including strain hardening exponent (n), yield strength (σy), and α value that represents the yielding part of structural steel, in WM, HAZ, and BM were then extracted from the combination of spherical and Berkovich load-depth curves by utilizing a proposed method. As the result, the indentation strain rate effects on plastic properties in all regions of the weld zone were evaluated. The results revealed that plastic properties in the weld zone’s components are influenced by the strain rate, where σy and n decrease with the decreasing strain rate, whereas α value decreases when the strain rate increases.

The hot deformation behavior and mechanism of Ti65 alloy with a bimodal microstructure were investigated by isothermal compression experiments conducted on the Thermecmastor-Z simulator equipment at temperatures ranging from 950 to 1110 °C and strain rates ranging from 0.01 to 10.0 s-1. The Arrhenius constitutive model, based on strain compensation, and Grey Wolf optimization-neural network with back propagation model (GWO-BP), were both established. The differences between the experimental and predicted value of flow stress were compared and analyzed using the two models. The results show that the prediction accuracy of GWO-BP in the two-phase region is higher than that of Arrhenius model. In the single-phase region, both methods demonstrated high prediction accuracy. Compared to the single-phase region, the flow stress of Ti65 alloy shows a higher degree of softening in the two-phase region. During deformation in the two-phase region, the initial lamellar α phase transformed from a kinked and elongated morphology to a globularized topography as the strain rate decreased. Boundary-splitting was the primary mechanism leading to the spheroidization process. The degree of recrystallization increased with the increase in strain rate during the deformation in the single-phase region, while dynamic recovery and strain-induced grain boundary migration were the main deformation mechanisms at a lower strain rate. Discontinuous dynamic recrystallization may be the dominant recrystallization mechanism under a high strain rate of 10 s-1.

This paper tracks the progress in research regarding the use of twinning induced plasticity (TWIP) steel in the automobile industry. The chemical composition of TWIP steel ensures that it has stable austenite and proper stacking fault energy at room temperature, allowing the main deformation mechanism (twinning) to work. The effects of alloying elements on the microstructure and deformation mechanism of TWIP steel are explained in detail, and their properties deformed under static and dynamic conditions are examined. The TWIP steel deformed at a low strain rate shows higher total elongation and strength. When the TWIP steel deforms under dynamic strain conditions, the stress, microhardness and the work hardening rate, all increase along with the increase in strain and the strain rate. The twin characteristics of TWIP steels deformed at various strain rates vary also and the twins generated under a high strain rate exhibit thinner widths and smaller interspaces compared with those formed under a low strain rate. It has also been observed that multisystem twins are able to generate and develop together. The mechanisms of toughening and strengthening in TWIP steels are noted. Finally, some potential application fields have been found for the promising material.

Constitutive modeling and processing map for hot deformation of Ti–15Mo–3Al-2.7Nb-0.2Si

Hot tensile deformation behaviors and constitutive model of an Al–Zn–Mg–Cu alloy

Optimized hot working parameters of Fe2.5Ni2.5CrAl multi-principal element alloys

Correlation of nanoindentation measurements of nickel made using geometrically different indenter tips

An in situ electrical measurement technique for the investigation of nanoindentation using a Hysitron Triboindenter is described, together with details of experiments to address some technical issues associated with the technique. Pressure-induced phase transformations in silicon during indentation are of particular interest but are not fully understood. The current in situ electrical characterization method makes use of differences in electrical properties of the phase-transformed silicon to better understand the sequence of transformations that occur during loading and unloading. Here, electric current is measured through the sample/indenter tip during indentation, with a fixed or variable voltage applied to the sample. This method allows both current monitoring during indentation and the extraction of current-voltage (I-V) characteristics at various stages of loading. The work presented here focuses on experimental issues that must be understood for a full interpretation of results from nanoindentation experiments in silicon. The tip/sample contact and subsurface electrical resistivity changes dominate the resultant current measurement. Extracting the component of contact resistance provides an extremely sensitive method for measuring the electrical properties of the material immediately below the indenter tip, with initial results from indentation in silicon showing that this is a very sensitive probe of subsurface structural and electrical changes.

Nickel-base single crystal (SX) blades, oriented along the [0 0 1] crystallographic direction, were introduced for gas turbine engines in 1980 [1, 2]. Creep resistance of SX materials is derived from the large volume fraction, about 70%, of ordered cubical γ ′ precipitates in a disordered γ matrix. Since the γ /γ ′ morphology changes during creep, considerable attention has been paid to this topic [1, 3]. Because SX materials exhibit high stress sensitivity and large elongation, the increase in stress, due to the reduction in the cross-sectional area during creep, must control the strain rate and the time to rupture for constant load tests. The objective of this letter is to present our study on this subject. Both experimental and analytical techniques presented here are applicable in general to other materials exhibiting high ductility. The “third generation” CMSX-10 with a nominal composition (wt.%) of 2Cr, 3Co, 0.4Mo, 6Re, 5W, 5.7Al, 0.2Ti, 8Ta, 0.1Nb, 0.03Hf, balance Ni [2] was used. Creep specimens, with a nominal diameter of 4.0 mm and a uniform gauge section of 25 mm, were prepared from fully heat-treated [0 0 1]-oriented bars. The γ ′ precipitates had average edge dimensions of about 0.5 μm and there were rows of spherical pores, up to 25 μm in diameter, aligned parallel to the growth direction. Six constant load tests, for an initial stress σ i of 700 MPa, were performed in air at 1073 to 1273 K (800– 1000◦C) ± 1 K using a single lever dead-load system, a three-zone furnace and an extensometer. Out of these six tests, two rupture tests were performed first at 1123 K and 1173 K to determine the fracture times and strains (19% and 18% at 1123 K and 1173 K respectively). Four strain relaxation (creep) and recovery tests (SRRT) [4] for relaxation times, trlx required to reach a total strain of 15% (close to fracture), were then performed at 1073 K, 1148 K, 1223 K and 1273 K. SRRT involves the application of the load as rapidly as possible, deforming a specimen for trlx, and then removing the load fully and rapidly, but keep monitoring the strain for a long time until no further strain recovery is noticed. SRRTs allow determination of elastic, delayed elastic and viscous components of strain as functions of trlx. Only the strain relaxation results of these tests will be presented and discussed here. Fig. 1 shows all the strain relaxation curves. The average value of the bulk Young’s modulus, E = 110 GPa, obtained from the initial elastic strain, ee, agreed with the expected range for the [0 0 1]-orientation at these temperatures [3, 5]. This allowed the estimation of creep strain, ec (= e − ee), where e is the total strain. The high stress of 700 MPa, equivalent to 6.4 × 10−3 E, was chosen to avoid complexities of γ ′ rafting [6, 7]. The fracture strains for the two tests bear strong similarities with those of the “second generation” alloy CMSX-4 for lower stresses, 130 to 400 MPa, at 1273 K [7]. Assuming incompressibility during creep, the actual stress, σA was estimated from σA = σ i (1 + ec). This gives, for example, the fracture stress, σAf of 833 MPa for the test at 1123 K. It would be appropriate to mention here that the specimen cross-sectional shape after the tests was found to be slightly elliptical. For example, the majorand the minor-axis of the specimen tested at 1148 K was 3.76 mm and 3.70 mm respectively near the middle of the specimen – giving a small but measurable eccentricity of 0.183. This does not necessarily mean asymmetry in the load axis and, in fact, is expected due to a number of active slip systems in cubic materials, as reported for SX alloy SRR99 in [8]. Fig. 2 shows the strain rate-time, e − t and σA − t curves for the test at 1223 K (950 ◦C). It shows that e changes with time in a complex manner similar to observations in CMSX-4 [9, 10]. However, the monotonically increasing σA suggests that the conventional approach of plotting eec curves for analytical purposes (assuming constant stress) violates the basic principles of experimental physics. The relationship between e and σA for the results in Fig. 2 and plotted in Fig. 3 clearly shows that the ‘tertiary’ or the ’time-wise accelerating’ stage represents a “stresswise dynamic steady state” (broken line in Fig. 3). Note the high regression coefficient, R2 of 0.997. Similar observations were also made at other temperatures (Fig. 4). The test at 1173 K shows some inconsistency due to some

Various creep tests of Zircaloy cladding tubes under variable conditions were conducted to investigate which hardening rule can be applicable for the creep behavior associated with condition changes. The results show that the strain-hardening rule is applicable in general when either the stress or temperature conditions change, provided that a certain amount of creep strain recovery is observed in case of stress drop. In stress reversal conditions, however, “softening” of the material was observed. Strain rate after stress reversal is much higher than that predicted by the strain-hardening rule. Tn this case, the modified strainhardening model, considering a recoverable creep-hardening range together with the strain recovery, predicts the creep behavior well. The applicability of the model is ascertained through a verification test that includes stress reversal, strain recovery, stress changes, and temperature changes.

The influence of strain rate changes in the range from 10 −3 to 10 −6 1/s on the zero‐to‐tension loading and unloading behavior as well as short term relaxation properties is investigated using cylindrical specimens of circular cross section. A clip‐on extensometer measures and controls the axial strain in an MTS servohydraulic, computer‐controlled mechanical testing machine. Strains do not exceed twenty percent and all deformation is macroscopically homogeneous. An increase in strain rate causes an increase in stress level. Surprisingly, the total stress drop in a 20 min relaxation period increases with prior strain rate. When the relaxation test is started in the inelastic region with low tangent modulus the total stress drop is nearly independent of the stress and strain at which relaxation commences. Unloading to zero load is not linear but curved and the strain recovery at zero stress is significant. It occurs at an ever decreasing rate and does not exceed three percent in a 12 h period. Like the relaxation behavior the recovery rate increases with prior strain rate. Repeated relaxation periods during zero‐to‐tension cycling can show a stress magnitude decrease during loading but a stress magnitude increase during unloading. The results suggest that a unified model with an overstress dependence of the inelastic rate of deformation could be useful in modeling.