Contributions Of Different Strengthening Mechanisms Research Articles

Aging Behavior of Al 6061 Alloy Processed by High-Pressure Torsion and Subsequent Aging

A process to achieve strengthening in an Al 6061 alloy by grain refinement to ~200 nm using high-pressure torsion (HPT) and fine precipitation using aging treatment is studied. It is shown that although aging of the HPT-processed sample is effective for extra strengthening of the alloy, the imposed shear strain and the aging temperature should be selected carefully. The HPT processing after 5 turns leads high saturation hardness and tensile strength of 163 Hv and 470 MPa, respectively. The hardness at the saturation level remains the same during aging at 373 K (100 °C), while the hardness decreases by aging at 423 K (150 °C). When the disks are processed for 0.75 turns (lower shear strains) and aged at 373 K (100 °C), the hardness increases above the hardness level at the saturation because of the formation of B′ and β′ precipitates. Quantitative analyses indicate that three major hardening mechanisms contribute to the total hardening: grain boundary hardening through the Hall–Petch relationship, dislocation hardening through the Bailey–Hirsch relationship and precipitation hardening through the Orowan relationship. This study shows that the contribution of different strengthening mechanisms can be estimated using a linear additive relationship in ultrafine-grained aluminum alloys.

On the nature of high-strength state of carbon steel produced by severe plastic deformation

Contributions of different strengthening mechanisms to yield strength of carbon steels C 10 (0.1 % C) and C 45 (0.45 % C) with ultrafine-grained microstructures have been analyzed in this work based on the precision investigation of the microstructure by electron microscopy, 3D atom probe tomography, and mechanical properties. Estimated values of yield stress showed satisfactory agreement with the experimental results. It is shown that significant contribution to strengthening is made by carbides. In particular, as the carbon content increases in carbon steels, the volume fraction of cementite increases and, accordingly, there is a bigger deviation of the yield strength value from the classical Hall-Petch relationship. The relative contribution of grain boundary strengthening into the yield stress is reduced due to an increased proportion of precipitate strengthening.

Microstructural Features Controlling Mechanical Properties in Nb-Mo Microalloyed Steels. Part I: Yield Strength

Low carbon Nb-Mo microalloyed steels show interesting synergies between the “micro”-alloying elements when high strength–high toughness properties are required. Strain accumulation in austenite is enhanced, and therefore grain sizes are refined in the final microstructures. The presence of Mo facilitates the presence of non-polygonal phases, and this constituent modification induces an increment in strength through a substructure formation as well as through an increase in the dislocation density. Regarding fine precipitation and its strengthening effect, the mean size of NbC is reduced in the presence of Mo and their fraction increased, thus enhancing their contribution to yield strength. In this paper, a detailed characterization of the microstructural features of a series of microalloyed steels is described using the electron-backscattered diffraction technique. Mean crystallographic unit sizes, a grain boundary misorientation analysis, and dislocation density measurements are performed. Transmission electron microscopy is carried out to analyze the chemical composition of the precipitates and to estimate their volume fraction. In this first part, the contribution of different strengthening mechanisms to yield strength is evaluated and the calculated value is compared to tensile test results for different coiling temperatures and compositions.

Hall-Petch Relationship in an Al-Mg-Sc Alloy Subjected to ECAP

Effect of extensive grain refinement on mechanical properties of an Al-Mg-Sc alloy subjected to equal-channel angular pressing (ECAP) at 300°C is considered in detail. It was shown that the Hall-Petch relationship with the coefficient, ky, of 0.2 MPa×m1/2 is valid in a wide strain range despite a great difference in deformation structures. Volume fraction of fine grains with an average size of ∼1 μm gradually increases with strain. It is caused by the fact that additive contributions of grain size strengthening and dislocation strengthening to the overall strengthening take place in this alloy. Upon ECAP the extensive grain refinement is accompanied by increasing dislocation density. Superposition of deformation and structural strengthening mechanisms provides achieving very high strength in the alloy. It was shown that ECAP at 300°C has no remarkable effect on a dispersion of coherent dispersoids. Al3(Sc,Zr), which gives a significant contribution to overall strength through dispersion strengthening. Contributions of different strengthening mechanisms to overall strength of the material are analyzed.

Improving the strength and ductility of Al–Mg–Si–Cu alloys by a novel thermo-mechanical treatment

A novel thermo-mechanical treatment, composed of under-aging, cold-rolling and re-aging, has been developed to improve the strength and ductility of Al–Mg–Si–Cu alloys pronouncedly. The thermo-mechanically treated 6061 Al sheets exhibit much higher ultimate tensile strength and yield strength (being 560MPa and 542MPa, respectively) than those of the conventional peak-aged sheets (being 365MPa and 348MPa, respectively), while maintaining rather good elongation to failure and uniform elongation (being 8.5% and 7%, respectively). The microstructure analyses show that the high strength is attributed to a combination of precipitation strengthening, dislocation strengthening, sub-structure strengthening and high Taylor factor value and the good ductility is attributed to both the recovery process and the re-precipitation of the GP zones. Furthermore, the contributions of different strengthening mechanisms are quantitatively estimated.

Study on cryorolled Al–Cu alloy using X-ray diffraction line profile analysis and evaluation of strengthening mechanisms

Al–Cu alloy sheets were rolled at room temperature and at liquid nitrogen temperature to true strain values of 0.69 and 1.38. The microstructural changes and variation in mechanical properties due to the effect of rolling temperature was evaluated. X-ray diffraction line profile analysis was performed to calculate crystallite size, microstrain and dislocation density. Irrespective of the amount of reduction, cryorolled samples exhibited reduced crystallite size along with enhanced lattice strain and dislocation density than room temperature rolled samples. At both the reductions, cryorolled samples exhibited better hardness and strength compared to the room temperature rolled samples and the enhanced properties are attributed to the higher dislocation density of the cryorolled samples. The electron backscatter diffraction results indicated that the grains are submicron in size in rolled conditions. It also revealed that majority of the fraction of special grain boundaries are Σ1 boundaries with decreasing tendency of Σ3 boundaries. Quantitative contributions of different strengthening mechanisms were performed on the rolled samples and it was observed that significant contribution comes from grain boundary strengthening.

Simulation of Precipitation Kinetics and Precipitation Strengthening of B2-precipitates in Martensitic PH 13–8 Mo Steel

The strength of quenched Co-free martensitic PH 13–8 Mo steel increases significantly during isothermal ageing at 575°C due to precipitation of nanometer-sized particles. The evolution of these ordered B2 type precipitates is simulated by thermo-kinetic calculations and compared with experimental results. The composition of the B2-nuclei is determined from minimization of the critical nucleation free energy. The results of the simulations are utilized for the assessment of contributions of different strengthening mechanisms to the total lower yield strength evolution. We observe good qualitative agreement between the simulations and the experimental data.

Thermal Stability of Microstructure and Hardness of Cold-Sprayed cBN/NiCrAl Nanocomposite Coating

cBN/NiCrAl nanocomposite coatings were deposited by cold spraying using mechanically alloyed composite powders. To examine their thermal stability, the nanocomposite coatings were annealed at different temperatures up to 1000 °C. The microstructure of composite coatings was characterized by x-ray diffraction, scanning electron microscopy, and transmission electron microscopy. The results showed that the nanostructure can be retained when the annealing temperature is not higher than 825 °C, which is 0.7 times of the melting point of the NiCrAl matrix. The dislocation density was significantly reduced when the annealing temperature was higher than 750 °C. The reaction between cBN particles and the NiCrAl matrix became noticeable when the annealing temperature was higher than 825 °C. The effects of grain refinement and work-hardening strengthening mechanisms were quantitatively estimated as a function of annealing temperature. The influence of annealing temperature on the contribution of different strengthening mechanisms to coating hardness was discussed.

On the strengthening mechanisms of 18 carat yellow gold and its mechanical behaviour

In order to contribute to a better understanding of the mechanical properties of gold alloys this paper investigates the mechanical behaviour of an 18 carat yellow alloy; the strain hardening behaviour during a tensile test was measured in the annealed as well as in different cold-work conditions. In addition, the contribution of different strengthening mechanisms is discussed on the basis of current results and of data reported in literature. In particular, the contributions of grain size refinement and copper solid solution alloying in improving the mechanical strength of gold alloys were quantified.

An Internal State Variable Constitutive Description for the Deformation of Metals at Elevated Temperatures

In the present investigation, an internal state variable constitutive model for the description of metals that are deformed under hot-working conditions in the range of thermal recovery, where positive work hardening is displayed, is proposed. The flow stress of the material is expressed in terms of the deformation temperature and strain rate, besides the contribution of the mechanical strength of the different obstacles that oppose dislocation motion. The constitutive description is validated employing the flow stress, temperature, strain rate, and strain increment data obtained from tests carried out both on Al-5.5 pct Mg and C-Mn steel. The model thus advanced assumes that the main obstacles to dislocation motion are solute atoms either in substitutional or interstitial solid solution and forest dislocations, although other contributions to the flow stress can also be readily incorporated. The formulation developed describes quite accurately the flow stress and work-hardening rate of the material under the temperature and strain rate conditions investigated. Also, it is able predict the occurrence of significant strain transients when deformation conditions are suddenly changed. Appropriate parameters involved in the model allow the comparison of the contribution of different strengthening mechanisms to the flow stress of different materials