Drop In Ductility Research Articles

Research on the Hot Ductility of Fe-36Ni Invar Alloy

Abstract The hot ductility of Fe-36Ni invar alloy was studied by a Gleeble-3800 thermal-mechanical simulator and a microcomputer control universal material testing machine. The results show that the alloy has a good hot ductility (RA above 70%) at 1050∼1200 °C, which illustrates that the alloy might be suitable for the rolling provided the finish rolling temperature does not go below 1050 °C. Dynamic recrystallization occurred and was found to be responsible for the better hot ductility. However, the alloy exhibits a hot ductility drop (RA below 50%) at 800∼1000 °C, which is mainly attributed to the presence of the grain boundary sliding. Serious subscales, which had a typical substructure of the intragranular subscale and the intergranular subscale, occurred in the matrix after oxidation. The compression test indicates that the intergranular subscale will lead to the cracks and significantly deteriorate the hot ductility during deformation process.

Strength and ductility optimization of Mg–Y–Nd–Zr alloy by microstructural design

Understanding of essential microstructural features necessary to develop an optimum combination of strength and ductility in magnesium alloys is needed for materials by the microstructural design approach. Microstructural features required to optimize both strength and ductility of Mg–Y–Nd–Zr alloy were investigated by examining different thermo-mechanical processing conditions. Different microstructural states were obtained by either isothermally aging the hot-rolled base alloy or by friction stir processing (FSP) the base alloy and then aging the FSP microstructure. The plastic deformation behavior of different microstructural states was characterized by uniaxial tensile testing, whereas microstructural features were probed using scanning electron microscope, electron backscatter diffraction, and transmission electron microscope. As expected, precipitate formation within α-Mg matrix increased the alloy strength irrespective of processing condition. Concomitantly, the base alloy experienced a drastic drop in ductility. The presence of twins, coupled with a loss of toughness due to aging heat treatment, resulted in very poor ductility of the alloy. A good combination of strength and ductility was achieved by altering the base Mg–Y–Nd–Zr microstructure through FSP and subsequent aging. In addition to producing refined grains, FSP resulted in twin-free microstructure. The refined microstructure had a near-random texture, where a large fraction of grains was favorably oriented for plastic deformation. In this condition, intragranular precipitation increased the strength while retaining good ductility by activating additional slip systems. These results indicate that an optimum balance of strength and ductility is achieved by engineering a microstructure containing randomly-oriented fine grains, intragranular precipitation and minimal twins within α-Mg matrix. An empirical correlation among ductility, work hardening exponent, and slope of work hardening vs. flow stress curve was established, and will serve as a useful guideline for designing microstructure for the optimization of strength and ductility in magnesium alloys.

Effects of boron additions and solutionizing treatments on microstructures and ductility of forged Ti–6Al–4V alloys

The effects of boron additions on the microstructure and mechanical properties of forged Ti–6Al–4V alloys in different heat-treatment conditions have been characterized by both experimental studies and thermodynamic calculations. The results indicate a combination of proper post-forging treatments and B additions are helpful for control of the prior-β grain size and the volume fraction of α phase, thereby tuning the ductility of the forged Ti–6Al–4V alloys. However, the B-containing alloys exhibit a significant drop in ductility if the solutionizing temperature is too high, and this embrittlement is mainly due to the coarsening of brittle TiB borides. The mechanism in this case is due to the cleavage fracture of TiB rather than its debonding with the matrix, as indicated by the observation of the aligned TiB borides on the matching areas of both halves of the fracture surfaces. Thus, the TiB size and orientation, the prior-β grain size, and the volume fraction of the α phase all play important roles in controlling the mechanical properties of the forged Ti–6Al–4V alloys. The current findings shed light on the composition–microstructure–ductility relationship in the forged Ti–6Al–4V alloys.

Influence of the processing methods on the properties of poly(lactic acid)/halloysite nanocomposites

The influence of processing methods on the thermo‐mechanical properties of poly (lactic acid) (PLA) nanocomposites were investigated by preparing nanocomposites reinforced by halloysite nanotubes (HNTs) (from 0 to 10 [w/w%]) using solution casting (SC) and melt compounding (MC) methods. Statistical analysis revealed that the processing methods have a significant influence on the tensile properties, where nanocomposites prepared by MC have higher tensile properties compared to those by SC. Experimental results illustrated higher tensile strength and a drop in ductility under the higher strain rate as compared to the low strain rate for PLA/HNTs nanocomposites. At lower concentrations micrographs revealed that, HNTs dispersion was better for SC films as compared to MC, but more prominent HNTs aggregation at higher loadings. MC nanocomposites exhibited a high crystallinity as compared to SC, due to the recrystallization and nucleation effects. The thermal stability and activation energy increased with addition of HNTs, regardless of the processing methods. POLYM. COMPOS., 37:861–869, 2016. © 2014 Society of Plastics Engineers

Analysis of solidification microstructure and hot ductility of Fe–22Mn–0·7C TWIP steel

The characteristics and evolution of the solidification microstructure of a Fe–0·7C–22Mn (wt-%) TWIP steel was experimentally analysed. The primary dendrite arm space was about 1–2 mm while the diameter of equiaxed grains was about 2–4 mm. Decreasing the Mn concentration was proved beneficial to grains refinement and interdendrite soundness improvement. The tensile strength and yield strength of the concerning TWIP steel were found to be higher than that of C–Mn HSLA steel under as cast conditions above 1073 K, according to the comparison of hot tensile experiment results. It is inferred the solution strengthening effect should be the main explanation for that. The hot ductility of this TWIP steel was observed to be not very favourable, since most of the reduction rates of area were lower than 40%. Obvious drop of the hot ductility within the temperature range from 1073 to 1223 K of this Fe–0·7C–22Mn TWIP steel was determined. Intragranular MnS inclusions and grain boundaries sliding at elevated temperatures should be the main reason, according to the experimental results of optical microscope (OM), scanning electron microscope (SEM) and X-ray diffraction (XRD). Besides, both strength and ductility increased linearly as logarithmic strain rate increased, attributed to the combined effect of strain hardening, dynamic strain aging and dynamic recrystallisation.

Quantum-to-continuum prediction of ductility loss in aluminium-magnesium alloys due to dynamic strain aging.

Negative strain-rate sensitivity due to dynamic strain aging in Aluminium-5XXX alloys leads to reduced ductility and plastic instabilities at room temperature, inhibiting application of these alloys in many forming processes. Here a hierarchical multiscale model is presented that uses (i) quantum and atomic information on solute energies and motion around a dislocation core, (ii) dislocation models to predict the effects of solutes on dislocation motion through a dislocation forest, (iii) a thermo-kinetic constitutive model that faithfully includes the atomistic and dislocation scale mechanisms and (iv) a finite-element implementation, to predict the ductility as a function of temperature and strain rate in AA5182. The model, which contains no significant adjustable parameters, predicts the observed steep drop in ductility at room temperature, which can be directly attributed to the atomistic aging mechanism. On the basis of quantum inputs, this multiscale theory can be used in the future to design new alloys with higher ductility.

Hot Ductility and Deformation Behavior of C-Mn/Nb-Microalloyed Steel Related to Cracking During Continuous Casting

Hot ductility studies have been performed on C-Mn and C-Mn-Nb steels with an approach to simulate the effect of cooling conditions experienced by steel in secondary cooling zone during continuous casting. Thermal oscillations prior to tensile straining deteriorate hot ductility of steel by deepening and widening the hot ductility trough. C-Mn steels are found to exhibit ductility troughs in three distinct zones whereas C-Mn-Nb steel shows drop in ductility only at low temperature in the vicinity of ferrite transformation temperatures. Start of ferrite transformation in steels causes yield ratio to increase while work hardening rates and strength coefficient decrease with decrease in test temperature in presence of thermal oscillation prior to tensile testing. Inhibition of recrystallization due to build-up of AlN particles along with the presence of MnS particles in structure and low work hardening rates causes embrittlement of steel in austenitic range. Alloying elements enhancing work hardening rates in austenitic range can be promoted to improve hot ductility. The presence of low melting phase saturated with impurities along the austenitic grain boundaries causes intergranular fracture at high temperature in C-Mn steels.

Effects of Aluminum and Strontium Content on the Microstructures and Mechanical Properties of Mg-Al-Ca-Sr Alloys

The effects of aluminum and strontium content on an Mg-Al-Ca-Sr alloy were investigated in terms of microstructural and mechanical properties. The addition of up to 2 wt% of strontium to the Mg-5Al-2Ca alloy caused the major interdendritic intermetallic phase to change from Al2Ca to a combination of Mg2Ca, Al4Sr, and Mg17Sr2. Moreover, the addition of 2 wt% of aluminum to the Mg- 5Al-2Ca-xSr alloys caused the formation of β-Mg17Al12 phase suppressing the formation of Mg17Sr2. The creep resistance was significantly improved by the addition of strontium due to the formation of a thermally stable Al4Sr phase in the interdendritic region. However, the ductility deteriorated as the amount of second phases along the interdendritic boundary increased. Furthermore, an increase in aluminum content resulted in a drop of ductility and creep resistance at elevated temperature due to the poor metallurgical stability by the formation of β-Mg17Al12 phase.

Tensile deformation and fracture behavior of CuZn5 brass alloy at high temperature

Alpha brass alloys are widely used for production of rectangular waveguides because of their low bulk resistivity. In this paper, the microstructure, tensile deformation and fracture behavior of CuZn5 brass alloy were investigated. The strain rate sensitivity and its relation to post-uniform deformation in tensile test and correlation between strain hardening exponent (n) and temperature were examined. The results show that strain hardening exponent decreases from 0.5 to 0.4 with increase in test temperature from 250 to 450°C. Tensile fracture mechanisms of as-extruded CuZn5 brass alloy were studied over a range of temperatures from 300 to 450°C and range of strain rates from 0.01 to 0.41/s by means of scanning electron microscope (SEM) and Atomic Force Microscope (AFM). The results show that different fracture mechanisms operate in different temperature and strain rate ranges. While transgranular dimple fracture is dominant at 300°C and 0.41/s, the dominant fracture mechanism at 450°C and 0.011/s is cleavage facets. Precipitations and grain boundary sliding at high temperature may be the mechanism of ductility drop. Dynamic strain ageing (DSA) did not occur since none of the manifestations of DSA are observed.

Research on Hot Ductility Behavior of 800 MPa Grade Ultra High Strength Weathering Steel

The hot ductility behavior of test steel was investigated by Gleeble-3500 thermo-mechanical simulator through high temperature tensile test. The reduction in area (RA) and tensile strength (TS) were acquired to draw hot ductility curve and hot strength curve. SEM fractograph and microstructure of the tensile samples were analyzed. The results show that the third brittle zone of test steel is between 700°C and 800°C and the occurrence of the third brittle zone is mainly related to the formation of film-like ferrite along the prior austenite grain boundary and the precipitation of second phase. Moreover, the drop of hot ductility at 900°C is rooted in the reduction of grain boundary strength owing to the precipitation of sulfides. Therefore, it is advised that the straightening temperature of test steel should be kept over 900°C.

Mechanical and thermal behaviour of isotactic polypropylene reinforced with inorganic fullerene-like WS2 nanoparticles: Effect of filler loading and temperature

The thermal and mechanical behaviour of isotactic polypropylene (iPP) nanocomposites reinforced with different loadings of inorganic fullerene-like tungsten disulfide (IF-WS2) nanoparticles was investigated. The IF-WS2 noticeably enhanced the polymer stiffness and strength, ascribed to their uniform dispersion, the formation of a large nanoparticle–matrix interface combined with a nucleating effect on iPP crystallization. Their reinforcement effect was more pronounced at high temperatures. However, a drop in ductility and toughness was found at higher IF-WS2 concentrations. The tensile behaviour of the nanocomposites was extremely sensitive to the strain rate and temperature, and their yield strength was properly described by the Eyring's equation. The activation energy increased while the activation volume decreased with increasing nanoparticle loading, indicating a reduction in polymer chain motion. The nanoparticles improved the thermomechanical properties of iPP: raised the glass transition and heat deflection temperatures while decreased the coefficient of thermal expansion. The nanocomposites also displayed superior flame retardancy with longer ignition time and reduced peak heat release rate. Further, a gradual rise in thermal conductivity was found with increasing IF-WS2 loading both in the glassy and rubbery states. The results presented herein highlight the benefits and high potential of using IF-nanoparticles for enhancing the thermomechanical properties of thermoplastic polymers compared to other nanoscale fillers.

Local approach to ductile fracture and dynamic strain aging

Experiments on smooth and notched round specimens on a C–Mn steel used in nuclear industry are performed at different temperatures under quasi-static loadings, revealing dynamic strain aging (DSA). The behavior is highly dependent on temperature and strain rate, and a drop in fracture strain is observed. Fracture surface observation on notched tensile specimens shows classical ductile fracture mechanisms with growth and coalescence of voids. The apparent strain hardening behavior at each temperature and strain rate is taken into account to compute the void growth with the Rice and Tracey model and with a damage law developed from unit cell computations. It is shown that the apparent strain hardening at large strains is of major importance to correctly predict fracture with the Rice and Tracey model, but its influence on the void growth law is of minor importance. In particular, the stress triaxiality ratio within the notch is increased due to the negative strain rate sensitivity. The ductility drop observed in DSA domain is then partly explained, but void nucleation and void growth in presence of strain bands should be included in the fracture modeling of such materials.

Effect of Orientation, Thickness, and Composition on Properties of Ductile Iron Castings

In this work, the effects of casting orientation (horizontal, side, and vertical), section thickness (4–16 mm) and composition (Cu, Mn) were investigated on the cooling rate, microstructure, and mechanical properties (tensile strength, yield strength, elongation, hardness) of hypereutectic ductile iron castings. Overall, horizontal castings were found to cool faster than side and vertical castings. Thermal analysis (using cooling curves) showed a wide difference among the four sections. Thinner sections exhibited significant undercooling and thereby carbide formation, leading to poor ductility. The combined effect of Cu and Mn showed an increase in the amount of pearlite to 82% and nodularity to 94% along with a reduction in nodule count to 323 and the amount of ferrite. Also, increased tensile strength (659 MPa) and hardness (264 BHN) were observed along with a drop in ductility to 2.5% in 4 mm thin section, helping offset carbide formation. Thermal analysis was found to be a useful tool in understanding the combined effect of orientation, thickness variations and processing parameters.

Creep Rupture Ductility of Creep Strength Enhanced Ferritic Steels

Creep rupture strength and ductility of creep strength enhanced ferritic steels of Grades 23, 91, 92, and 122 was investigated with particular emphasis on remarkable drop in the long-term. Large difference in creep rupture strength and ductility was observed on three heats of Grade 23 steels. Remarkable drop of creep rupture strength in the long-term of T91 was comparable to those of Grades 92 and 122. Remarkable drop in creep rupture ductility in a stress regime below 50% of 0.2% offset yield stress was observed on Grade T23 steel, however, that of Grade P23 steel did not indicate any degradation of creep rupture ductility. Higher creep rupture ductility of Grade P23 steel was considered to be caused by its lower creep strength than that of T23 steels. Creep rupture ductility of Grades 92 and 122 steels indicated rapid and drastic decrease with decrease in stress at 50% of 0.2% offset yield stress. Stress dependence of creep rupture ductility of Grades 92 and 122 steels was well described by a ratio of stress to 0.2% offset yield stress, regardless of temperature. On the other hand, large drop in creep rupture ductility of Grade 91 steel was observed only in the very low-stress regime at 650 °C. Alloying elements including impurities and changes in precipitates may influence on creep rupture ductility, however, remarkable drop in ductility of the steels cannot be explained by chemical composition and precipitates. High ductility in the high-stress regime above 50% of 0.2% offset yield stress should be provided by easy plastic deformation, and it has been concluded that a remarkable drop in ductility in the low-stress regime is derived from a concentration of creep deformation into a tiny recovered region formed at the vicinity of grain boundary.

Effect of NbO Additions on the Grain Size of 1010 Steel Castings

This paper describes a set of experiments that used NbO powder additions to decrease the final as-cast microstructure of 1010 steel castings. NbO has excellent crystallographic matching between its (002) plane and the (001) plane in ferrite, making it a possible heterogeneous nucleation site for delta ferrite. A set of 1010 steel plates were cast in green sand molds and then sectioned into tensile bars. NbO powder was added via the downsprue during pouring. The yield strength and ultimate tensile strength increased with the amount of NbO added. Percent elongation was unaffected in the samples with 6 ppm of NbO powder. Specimens with higher NbO powder additions had a significantly lower elongation than the baseline material. The microstructure of the NbO containing samples was finer than the baseline castings. The increase in strength and drop in ductility was theorized to be more consistent with a grain pinning strengthening mechanism than heterogeneous nucleation.

An investigation into the hot ductility behavior of AZ81 magnesium alloy

► Investigation of the hot ductility behavior in AZ81 alloy to characterize the ductility drops. ► Detrimental effect of grain boundary sliding on ductility values due to the resulting W-type cracks. ► DRX and CGBS as the main factor increasing the ductility at higher temperatures. ► Significant effect of γ precipitates on crack propagation in AZ81 alloy. ► The sharp ductility drop at very low strain rates due to the incipient melting of γ phase. The hot ductility behavior of AZ81 magnesium alloy, as a typical Mg–Al–Zn base cast alloy with high Al content, has been studied through applying a set of low strain rate hot tensile tests in the temperature range of 250–450 °C. The results indicated a ductility drop at 400 °C under the strain rates of 0.01 and 0.001 s −1 . This has been attributed to the occurrence of grain boundary sliding and the resulting W-type and R-type cracks at this deformation conditions. The dynamic recrystallization in large scales has been occurred and considered as the main factor increasing the ductility at higher temperatures. Under lower strain rate (0.0001 s −1 ), the ductility trough has been shifted to lower temperature (350 °C). This has been followed by an enhanced ductility regime up to 30% at 400 °C. The latter is attributed to the cooperative grain boundary sliding (CGBS) mechanism and the active grain boundary sliding of γ phase. By increasing the temperature to 450 °C the material exhibits a sharp drop in ductility variation which is explained considering the occurrence of incipient melting in this temperature range.

Structural and Mechanical Behaviour of Severe Plastically Deformed OFE Copper Processed by Constrained Groove Pressing Technique

Oxygen free electrolytic (OFE) grade copper sheets (5mm thick) are severe plastically deformed by constrained groove pressing technique (CGP) at room temperature up to three passes thereby imparting a total plastic strain of 3.48 throughout the sheet material. The revelation of significant grain refinement investigated by light microscopy in constrained groove pressed sheets markedly signifies the ability of CGP technique for processing fine grained sheet materials. Relatively higher grain refinement rates are observed during initial passes. Deformation homogeneity evaluated from Vickers microhardness measurements indicated inhomogeneous deformation behaviour during early stages of straining; however enhancement in deformation homogeneity is observed later. CGP processed sheets subjected to room temperature tensile tests showed significant increase in strength associated with drop in ductility. Marginal gain in ductility coupled with slight decrease in strength attributed to the dominance of dislocation recovery is observed after third pass.

Effect of thermal annealing on microstructure and mechanical properties of a gradient structured tantalum prepared by plasma activated sintering

Fully dense bulk tantalum compacts of two different nitrogen contents with gradient structure within an individual particle were successfully synthesized by means of plasma activated sintering (PAS). Annealing of the compacts induces disappearing of the gradient structure due to the promoted diffusion of nitrogen in Ta matrix accompanied by enhancement of the strength. For the compact with low nitrogen content of 0.09 wt.%, a drop in ductility was observed. While for the compact with nitrogen content of 0.215 wt.%, improved strength was obtained without sacrificing its ductility.

Hot ductility of a Fe–Ni–Co alloy in cast and wrought conditions

The hot ductility of Fe–29Ni–17Co alloy was studied in both cast and wrought conditions by hot tensile tests over temperature range of 900–1250 °C and at strain rates of 0.001–1 s −1. Over the studied temperature range, the wrought alloy represented higher elongation and reduction in area as compared to the cast alloy. Dynamic recrystallization was found responsible for the higher hot ductility of the wrought alloy and the improvement of hot ductility of the cast alloy at high temperatures. At temperature range of 1000–1150 °C the wrought alloy exhibited a hot ductility drop while a similar trough was not observed in case of the cast alloy. It was also found that at temperatures of 1150–1250 °C the best hot ductility is achieved in both cases of cast and wrought alloy. The experimental data of flow stress were constitutively analyzed and the apparent activation energy of deformation was estimated to be 344 kJ/mol.

Strain gradient plasticity analysis of the grain-size-dependent strength and ductility of polycrystals with evolving grain boundary confinement

The loss of uniform elongation with decreasing grain size in polycrystals is analysed using a strain gradient plasticity-based finite-element model involving an enhanced description of the grain boundaries. The grain interiors are modelled by the finite strain version of the isotropic Fleck–Hutchinson theory with one internal length parameter. The grain boundaries are modelled using cohesive zone elements imposing higher-order boundary conditions at the interface between the grain interior and grain boundary layer. The plastic strain rate is initially set to zero at these interfaces to account for their impenetrability to dislocations. With increasing stress levels, the higher-order constraint can be suppressed in order to mimic grain boundary relaxation mechanisms. The model is validated towards experimental data on ferritic steels with grain sizes varying between 100nm and 10μm. The model reproduces not only the yield stress evolution, but also the drop of ductility taking place around 1μm grain size while using a single constant internal length. Relaxation of the grain boundary constraint is needed to correctly predict the response at the smallest grain sizes. The back stress increases with decreasing grain sizes. Additional analysis of bimodal grain size distributions is provided showing a large potential for ductility enhancement.