Showing papers on "Hardening (metallurgy) published in 2005"

Mechanical properties and machining performance of Ti1−xAlxN-coated cutting tools

Abstract: The mechanical properties and machining performance of Ti 1− x Al x N-coated cutting tools have been investigated. Processing by arc evaporation using cathodes with a range of compositions was performed to obtain coatings with compositions x =0, x =0.25, x =0.33, x =0.50, x =0.66 and x =0.74. As-deposited coatings with x ≤0.66 had metastable cubic structures, whereas x =0.74 yielded two-phase coatings consisting of cubic and hexagonal structures. The as-deposited and isothermally annealed coatings were characterised by nanoindentation, scanning electron microscopy (SEM) and X-ray diffraction (XRD). Cutting tests revealing tool wear mechanisms were also performed. Results show that the Al content, x , promotes a (200) preferred crystallographic orientation and has a large influence on the hardness of as-deposited coatings. The high hardness (∼37 GPa) and texture of the as-deposited Ti 1− x Al x N coatings are retained for annealing temperatures up to 950 °C, which indicates a superior stability of this system compared to TiN and Ti(C,N) coatings. We propose that competing mechanisms are responsible for the effectively constant hardness: softening by residual stress relaxation through lattice defect annihilation is balanced by hardening from formation of a coherent nanocomposite structure of c-TiN and c-AlN domains by spinodal decomposition. This example of secondary-phase transformation (age-) hardening is proposed as a new route for advanced surface engineering, and for the development of future generation hard coatings.

Microstructures and microhardness of an aluminum alloy and pure copper after processing by high-pressure torsion

Abstract: Experiments were conducted on an Al–3 wt.% Mg–0.2 wt.% Sc alloy and pure Cu to evaluate the effect of high-pressure torsion (HPT). The results show that very substantial grain refinement is achieved in both materials with as-strained grain sizes of ∼150 and ∼140 nm, respectively. Microhardness measurements demonstrate increases in the hardness near the edges of the disks by factors of approximately three and two over the solution-treated conditions for these two materials, respectively. It is shown in experiments on the Al–Mg–Sc alloy that no additional hardening is achieved by reversing the direction of the torsional straining during the HPT processing. In addition, experiments on pure Cu confirm that the high values of hardness are not retained when samples are subjected to short-term anneals for 15 s or 3 min at a temperature of 473 K.

A one-dimensional theory of strain-gradient plasticity : Formulation, analysis, numerical results

Abstract: This study develops a one-dimensional theory of strain-gradient plasticity based on: (i) a system of microstresses consistent with a microforce balance; (ii) a mechanical version of the second law that includes, via microstresses, work performed during viscoplastic flow; (iii) a constitutive theory that allows • the free-energy to depend on the gradient of the plastic strain, and • the microstresses to depend on the gradient of the plastic strain-rate. The constitutive equations, whose rate-dependence is of power-law form, are endowed with energetic and dissipative gradient length-scales L and l, respectively, and allow for a gradient-dependent generalization of standard internal-variable hardening. The microforce balance when augmented by the constitutive relations for the microstresses results in a nonlocal flow rule in the form of a partial differential equation for the plastic strain. Typical macroscopic boundary conditions are supplemented by nonstandard microscopic boundary conditions associated with flow, and properties of the resulting boundary-value problem are studied both analytically and numerically. The resulting solutions are shown to exhibit three distinct physical phenomena: (i) standard (isotropic) internal-variable hardening; (ii) energetic hardening, with concomitant back stress, associated with plastic-strain gradients and resulting in boundary layer effects; (iii) dissipative strengthening associated with plastic strain-rate gradients and resulting in a size-dependent increase in yield strength.

On the effect of dose rate on irradiation hardening of RPV steels

Abstract: The effect of dose rate (DR), or neutron flux (ϕ), on irradiation hardening (Δσy) and embrittlement of reactor pressure vessel (RPV) steels is a key unresolved issue. We report a rigorous evaluation of DR effects based on a very large Δσy database we developed for RPV steels with a wide range of compositions, including a set of split-melt alloys with controlled and systematic variation in Cu, Ni and Mn content. The steels were irradiated at 290°C in three ϕ-regimes to a wide range of overlapping fluences (ϕt). The contribution of copper-rich precipitates (CRPs) to Δσy increases up to a plateau hardening that is a strong function of the alloy Cu, Ni and Mn content, but is relatively independent of DR. However, the pre-plateau region is shifted to higher ϕt with increasing DR. The shift can be approximately accounted for by defining an effective fluence (ϕte) as ϕte ≈ ϕt(ϕr /ϕ)1/2, where ϕr is a reference flux. The ϕ −1/2 scaling is consistent with a vacancy plus self-interstitial-atom (SIA) recombination r...

Room temperature precipitation in quenched Al–Cu–Mg alloys: a model for the reaction kinetics and yield strength development

Abstract: The microstructural evolution during low temperature ageing of two commercial purity alloys (Al–1.2Cu–1.2Mg–0.2Mn and Al–1.9Cu–1.6Mg–0.2Mn at.%) was investigated. The initial stage of hardening in these alloys is very rapid, with the alloys nearly doubling in hardness during 20 h ageing at room temperature. The microstructural evolution during this stage of hardening was investigated using differential scanning calorimetry (DSC), isothermal calorimetry and three–dimensional atom probe analysis (3DAP). It is found that, during the hardening, a substantial exothermic heat evolution occurs and that the only microstructural change involves the formation of Cu–Mg co–clusters. The kinetics of cluster formation is analysed and the magnitude of the hardening is discussed on the basis of a model incorporating solid solution hardening and modulus hardening originating from the difference in modulus between Al and clusters.

Influence of Si on the microstructure of arc evaporated (Ti,Si)N thin films; evidence for cubic solid solutions and their thermal stability

Abstract: Ti1−xSixN (0 ≤ x ≤ 0.14) thin solid films were deposited onto cemented carbide (WC-Co) substrates by arc evaporation. X-ray diffraction and transmission electron microscopy showed that all films were of NaCl-structure type phase. The as-deposited films exhibited a competitive columnar growth mode where the structure transits to a feather-like nanostructure with increasing Si content. Films with 0 ≤ x ≤ 0.01 had a 〈111〉 crystallographic preferred orientation which changed to an exclusive 〈200〉 texture for 0.05 ≤ x ≤ 0.14. X-ray photoelectron spectroscopy revealed the presence of Si–N bonding, but no amorphous Si3N4. Band structure calculations performed using a full potential linear muffin tin orbital method showed that for a given NaCl-structure Ti1−xSixN solid solution, a phase separation into cubic SiN and TiN is energetically favorable. The microstructure was maintained for the Ti0.86Si0.14N film annealed at 900 °C, while recrystallization in the cubic state took place at 1100 °C annealing during 2 h. The Si content influenced the film hardness close to linearly, by combination of solid-solution hardening in the cubic state and defect hardening. For x = 0 and x = 0.14, nanoindentation gave a hardness of 31.3 ± 1.3 GPa and 44.7 ± 1.9 GPa, respectively. The hardness was retained after annealing at 900 °C, while it decreased to below 30 GPa for 1100 °C following recrystallization and W and Co interdiffusion.

Research on aging precipitation in a Cu–Cr–Zr–Mg alloy

Abstract: The effects of aging processes on the properties and microstructure of Cu–0.3Cr–0.15Zr–0.05Mg lead frame alloy were investigated. Aging precipitation phase was dealt with by transmission electronic microscope (TEM). After solid solution was treated at 920 °C and aged at 470 °C for 4 h, the fine precipitation of an ordered compound CrCu 2 (Zr, Mg) is found in copper matrix as well as fine Cr and Cu 4 Zr. Along the grain boundary, there are larger chromium. The hardness and electrical conductivity can reach 109 HV and 80% IACS, respectively. Sixty percent cold-rolled deformation prior to aging at 470 °C enhances the hardness of the alloy. The coherent precipitates Cr in copper matrix and the dislocations pinned by the fine precipitates are responsible for maximum strengthening of the alloy. So the hardness 165 HV and electrical conductivity 79.2% IACS are available.

Plastic flow localization in bulk tungsten with ultrafine microstructure

Abstract: Shear localization is demonstrated in bulk tungsten (W) of commercial purity under dynamic uniaxial compression. Microstructure refinement via severe plastic deformation was the strategy used to induce this unusual deformation mode for W. The ultrafine microstructure achieved in bcc materials leads to elevated strength and ductility, as well as reduced strain hardening and strain rate hardening, thus enhancing the propensity for adiabatic plastic flow localization.

Influence of silicon content on the precipitation of secondary carbides and fatigue properties of a 5%Cr tempered martensitic steel

Abstract: In comparison with the conventional AISI H11 tool steel, which contains approximately 1 wt.% silicon, the modified steel AISI H11 (∼0.35 wt.% silicon) exhibits improved tensile and fatigue properties at 550 °C – the estimated tool surface temperature during the high-pressure injection of aluminium alloys. The effect of silicon on the stability of secondary carbides was studied using transmission electron microscopy and small-angle neutron scattering. Silicon has a considerable influence on the precipitation of secondary carbides. A higher volume fraction and density of small particles were observed in the low-silicon-grade steel, both after heat treatment and after fatigue testing. The final discussion focuses on the influence of silicon in the precipitation sequence. It is concluded that silicon has a detrimental effect as it shifts the secondary hardening peak towards lower tempering temperatures.

Strain softening and hardening of amorphous polymers: Atomistic simulation of bulk mechanics and local dynamics

Abstract: Molecular-dynamics (MD) simulations have been performed for two amorphous polymers with extremely different mechanical properties, atactic polystyrene (PS) and bisphenol A polycarbonate (PC), in the isotropic state and under load. The glass transition temperatures, Young moduli, yield stresses and strain-hardening moduli are calculated and compared to the experimental data. Both chemistry-specific and mode-coupling aspects of the segmental mobility in the isotropic case and under the uniaxial deformation have been identified. The mobility of the PS segments in the deformation direction is increased drastically beyond the yield point. A weaker increase is observed for PC.

Plasticity-improved Zr–Cu–Al bulk metallic glass matrix composites containing martensite phase

Abstract: Zr48.5Cu46.5Al5 bulk metallic glass matrix composites with diameters of 3 and 4mm were produced through water-cooled copper mold casting. Micrometer-sized bcc based B2 structured CuZr phase containing martensite plate, together with some densely distributed nanocrystalline Zr2Cu and plate-like Cu10Zr7 compound, was found embedded in a glassy matrix. The microstructure formation strongly depends on the composition and cooling rate. Room temperature compression tests reveal significant strain hardening and plastic strains of 7.7% and 6.4% before failure are obtained for the 3-mm- and 4-mm-diam samples, respectively. The formation of the martensite phase is proposed to contribute to the strain hardening and plastic deformation of the materials.

Influence of severe plastic deformations on secondary phase precipitation in a 6082 Al-Mg-Si alloy

Abstract: The role of severe plastic deformation on the second-phase stability in a 6082 Al-Mg-Si alloy was studied using differential scanning calorimetry (DSC) and transmission electron microscopy (TEM) techniques. The alloy was fully annealed prior to undergoing up to six equal channel angular pressing (ECAP) passes using route C. The Orowan strengthening mechanism was calculated on the basis of TEM inspections for the two hardening second-phase precipitates: Mg2Si and Si. The former had a major tendency to be cut and fragmented by dislocations, while in the latter, a dissolution process was induced by severe plastic deformation. Accordingly, the second-phase Si particles became progressively less effective with increasing deformation (i.e., additional ECAP passes). The increase in hardness with the ECAP passes was mostly due to the grain refining mechanism and to dislocation tangles within the newly formed grains. The expected, though if limited, contribution of second-phase hardening was prevalently accounted for by the Mg2Si particles.

Numerical Simulation for Three Dimensional Elastic-Plastic Contact with Hardening Behavior

Abstract: An elastic-plastic contact (EPC) solution and code is developed using a modified semi-analytical method. The indentation tests with different hardening behavior are simulated by using the developed EPC code. The distributions of contact pressure, residual stress and plastic strain are obtained and compared with the results of the finite element method models without hardening. Some techniques, such as fast Fourier transform and fast convergence method, are used to increase the computation speed.

Ultra grain refinement and hardening of IF-steel during accumulative roll-bonding

Abstract: A strip of commercial Ti–IF-steel was processed by accumulative roll-bonding (ARB) up to three cycles. The mean grain size and hardness distribution profiles along the thickness were obtained using atomic force microscopy and instrumented indentation tests. The grain refinement occurred mainly at the subsurface regions where the shear strain is high. The stacking of the strips before the subsequent rolling passes places the ultra-fine grains in the center region, leading to a complete ultra grain refinement along the thickness after only three ARB cycles. A parabolic grain size distribution profile results along the thickness that is a direct consequence of the shear strain during ARB so that the number of superimposed parabolas is 2 n −1 , where n is the number of ARB cycles. For all samples the hardness distribution along the thickness bears a direct correspondence to the local grain size following the well-known Hall–Petch relation. It is concluded that there is a direct relationship among shear strain, grain refinement and hardening of ARB processed materials with the shear strain being the main instrument for grain refinement in the present process.