Mechanical Properties and Microstructure Evolution of Mg-Gd Alloy during Aging Treatment

Rare-earth-containing Mg alloys are a group of widely investigated alloys due to the disperse nano-sized precipitations formed during heat treatment. The underlying formation and strengthening mechanisms of precipitation is critical for their industrial applications. In this work, we systematically studied the evolution of precipitations in a Mg-10Gd alloy, based on the atomic-scaled TEM and HAADF-STEM observations. Especially, the in-depth transition mechanism from G.P. Zone to β”, β’, βT and βM is proposed, as well as their relationships with mechanical properties. It is found that blocking effect of precipitations improves the strength significantly, according to the Orowan mechanism. The elliptic cylinder shaped β’ phase, with a base-centered orthorhombic lattice structure, provides significant strengthening effects, which enhance the hardness and ultimate tensile strength from 72 HV and 170 MPa to 120 HV and 300 MPa.

Effect of V Addition on Microstructure and Properties of Cu-1.6Ni-1.2Co-0.65Si Alloys

To obtain high strength and high electrical conductivity at the same time, the microstructure and properties of 0.2 wt.% V-added, 0.1 wt.% V-added and V-free Cu-1.6Ni-1.2Co-0.65Si(-V) alloys were investigated. We examined with electrical conductivity and hardness measurements, tensile test, optical microscope and transmission electron microscope (TEM). The results show that Cu-1.6Ni-1.2Co-0.65Si-0.1V alloy obtains excellent combination properties: electrical conductivity is 46.12% IACS, hardness is 293.88 Hv, and tensile strength is 782 MPa, which are produced by 65% cold rolling + aging at 500 °C for 480 min. The addition of vanadium (V) can accelerate the precipitation of solute atoms from the copper matrix, improve the hardness and electrical conductivity of Cu-1.6Ni-1.2Co-0.65Si alloys, and greatly accelerated the aging response. δ-(Co,Ni)2Si and β-Ni3Si phases are detected in Cu-1.6Ni-1.2Co-0.65Si-0.1V alloy. The Orowan mechanism and grain boundary strengthening play a major role in the yield strength strengthening due to δ-(Co,Ni)2Si phase.

Microstructure and Mechanical Properties of Mg–6Al–1Sn–0.3Mn Alloy Sheet Fabricated through Extrusion Combined with Rolling

Hot rolling was carried out in this study to modify the microstructures of an extruded Mg–6Al–1Sn–0.3Mn alloy sheet and investigate its effects on mechanical properties. After hot rolling, the grains and second phase of the extruded alloy sheet were remarkably refined, and the c-axis of a few grains was parallel to the transverse direction. The strength improvement was mainly attributed to the grain and Mg17Al12 particle refinement due to the Hall–Petch effect and the Orowan mechanism. The random orientation of the fine grains resulted in improving ductility and anisotropy.

Comparative Assessment of Strength Models for AA6063 Alloy

This report presents a comparative assessment of the suitability of two existing physical models for predicting yield strength of artificially aged AA6063 Al-alloy. One model is based on the modified Orowan mechanism of dislocation by-pass for non-shearable rod shaped precipitates rejuvenated by Zhu and Stark, and the other model is based on classical dislocation-particle interaction that incorporates both cutting and by-pass mechanisms for spherical shaped precipitates developed by Deschamps and Brechet. Using these models, simulation of yield strength values have been performed considering nucleation-growth as well as nucleation-growth-coarsening of precipitates during the entire period of ageing. Comparison of experimental and simulated results reveals that the model by Deschamps and Brechet predicts yield strength more accurately when nucleation-growth-coarsening of precipitates are incorporated.

The Development of Interphase Precipitated Nanometre-Sized Carbides in the Advanced Low-Alloy Steels

In this work, the investigation of transmission electron microscopy has elucidated the morphologies of the interphase precipitated carbides in an experimental Ti-Mo-bearing steel into three types: (1) planar interphase precipitation with regular sheet spacing (designated as PIP), (2) curved interphase precipitation with regular sheet spacing (designated as Regular CIP), and (3) curved interphase precipitation with irregular sheet spacing (designated as Irregular CIP). The planar sheets of carbides have also been analyzed and found to be oriented close to ferrite planes {211}, {210} and {111}; the results of transmission electron microscopy provide strong evidence to suggest that the development of interphase-precipitated carbides can be associated with the growth of incoherent ferrite/austenite interface by the ledge mechanism. The sheet spacing and inter-carbide spacing in the sheet have been measured and estimated in this work. The sheet spacing is found to be finer than the inter-carbide spacing in the sheet for all samples investigated. The result reflects that the distribution of interphase-precipitated carbides is anisotropic and cannot be considered random distribution. The relevance of the Orowan mechanism to the non-random distribution of interphase-precipitated carbides has been considered. The contribution of the dispersion of interphase-precipitated carbides to the yield strength of the steel studied has been estimated. It is revealed that an optimum component about 400 MPa contributed by interphase-precipitated carbides can be achieved, and the finding is consistent with the hardness data. Other examples of the different alloy steels are also addressed.

The Ageing Characteristics and Strengthening Mechanism of a Cu-2.1Ni-0.5Si-0.2Zr-0.05Cr Alloy

The ageing process of a Cu-2.1Ni-0.5Si-0.2Zr-0.05Cr alloy in the temperature range from 673 K to 773 K is investigated in this research. The phase transformation process, electric conductivities and mechanical properties during the ageing process are also studied by experimental methods. The precipitation sequence during the aging process of Cu-Ni-Si alloy is GP zone→ (Cu,Ni)3Si→ δNi2Si phase. The conductivity of the aged specimen increases from 20% IACS to 40-45% with prolonging time during aging process in the temperature range from 673 K to 773 K. The dynamic curve of precipitation is obtained by Avrami-equations, based on which the increase of yield strength can be estimated according to Orowan mechanism. Experimental data have been compared with the calculated results in this paper, indicating this method is reasonable, suggesting that strengthening is mainly attributed to Orowan by passing mechanism.

The Precipitation and Strengthening Mechanism of Cu-Ni-Si-Co Alloy

The precipitates, mechanical properties and strengthening effect of Cu-Ni-Si,Cu-Ni-Si-Co0.8 and Cu-Ni-Si-Co1.9 alloys are investigated. It is concluded from the TEM and XRD analysis that the strengthening of Cu-Ni-Si-Co alloy is mainly attributed to precipitates of both Ni2Si and Co2Si phases, with the same structures and very close lattice parameters. The addition of 1.9 wt% Co barely affects the precipitation process of Ni2Si or Co2Si, and may not be beneficial for enhancing the strength. The strengthening of Cu-Ni-Si-Co alloy is determined by the Orowan mechanism, and a critical precipitate radii rc about 1.5 nm corresponding to the peak strength is obtained through the theoretical analysis. It can be deduced that the peak strength of aged Cu-Ni-Si-Co alloy is obtained with the microstructures containing metastable phase and Ni2Si (or Co2Si) precipitates.

Modeling of Thermal Exposure Effect on the Microstructure Evolution and Properties of High Cu/Mg Ratio Al-Cu-Mg-Ag Alloys

This work aims to develop a physically based numerical model to predict the microstructure evolution and residual yield strength of high Cu/Mg ratio Al-Cu-Mg-Ag alloys after thermal exposure. Based on the established interfacial energy model and the classical nucleation and growth theories, a thermodynamically based precipitation model had been established to describe competitive precipitation of two phases, being of different stability, in under-aged and the subsequent thermally exposed Al-Cu-Mg-Ag alloys. And the strengthening model based on Orowan mechanism was deduced to predict the residual yield strength after thermal exposure. The microstructural evolution and yield strength predictions of the model in this work are generally in good agreement with the experimental result.