Age-hardening Behavior Research Articles

Age-hardening behaviour and microstructure of a silver alloy with high Cu content for dental application

Age-hardening behaviour and the related microstructural changes of a silver alloy with relatively high Cu content were elucidated by means of hardness test, X-ray diffraction (XRD), scanning electron microscopic (SEM) observations and electron probe microanalysis (EPMA). The microstructure of the solution-treated specimen was composed of the Ag-rich matrix, the Cu-rich particle-like structures containing Pd, and the lamellar structure of both phases. By the age-hardening heat-treatment, the Cu element began to precipitate from the Ag-rich matrix by the solubility limit, and the very fine Cu-rich precipitates became coarsened by further aging. The silver alloy with relatively high Cu content showed apparent age-hardenability. The hardness of the solution-treated specimen began to increase and reached a maximum value with increasing aging time, and then the hardness decreased gradually after maintaining the maximum value for short periods of time. The early stage of precipitation of the Cu-rich phase from the Ag-rich matrix seemed to have caused the increase in hardness. The decrease in hardness was attributed to the coarsening of the Cu-rich precipitates in the later stage of the age-hardening process.

Age-hardening and related phase transformation in an experimental Ag–Cu–Pd–Au alloy

The age-hardening behaviour, phase transformation and related microstructural changes of an experimental Ag–Cu–Pd–Au alloy were examined by means of hardness test, X-ray diffraction (XRD), scanning electron microscopic (SEM) observations and electron probe microanalysis (EPMA). The specimen alloy showed apparent age-hardenability at the aging temperatures of 350 °C and 400 °C. By aging the solution-treated specimen at 400 °C, two phases of the Ag-rich α 1 phase and the Pd-containing Cu-rich α 2 phase were transformed into four phases of the Ag-rich α 1 ′ phase, the Cu-rich α 2 ′ phase, the CsCl-type CuPd phase and the AuCu(I) ordered phase. Microstructure of the solution-treated specimen consisted of the Ag-rich α 1 matrix, Cu-rich α 2 particle-like structures of various sizes and the lamellar structure of the α 1 and α 2 phases. When the peak hardness was obtained, the very fine lamellar structure consisting of the Ag-rich α 1 ′ and Cu-rich α 2 ′ phases was newly formed in the matrix. By further aging, the very fine lamellar structure grew and coarsened apparently, and the matrix was covered with the coarsened lamellar structure. The hardness increase was considered to be caused mainly by the diffusion and precipitation of Cu from the Ag-rich α 1 matrix, and the hardness decrease in the latter stage of age-hardening process was caused by the coarsening of the very fine lamellar structure. The CsCl-type CuPd phase and the AuCu(I) ordered phase did not contribute to the hardness increase.

High Strength FeCo–V Intermetallic Alloy: Electrical and Magnetic Properties

Age-hardening behavior of a new generation of FeCo alloys [Fe–40Co–5V–0.005B–0.015C–0.5Mo–0.5Nb (at.%)] is characterized by microhardness, tensile testing, electrical resistivity, and magnetic properties. The alloy exhibits maximum hardening when aged at 600 °C. Precipitation of γ2(V-rich face-centered cubic phase) during aging appears to be responsible for the observed hardening behavior. The alloy exhibits superior creep resistance when subjected to solutionizing in γ phase field and aged at 600 °C. On the other hand, the room temperature tensile ductility of the aged alloy depends on the grain size, which in turn can be controlled by varying the solutionizing condition. The age-hardened alloy exhibits a room temperature electrical resistivity of 70–75 μΩ cm. The higher resistivity of the present alloy as opposed to the commercial FeCo–2V alloys is attributed to the high V content of the alloy. Structure-sensitive magnetic properties like coercivity and core losses of the alloy are affected by the aging treatment, and the maximum coercivity is observed when the alloy is aged at 600 °C. High coercivity of the alloy is attributed to the fine distribution of paramagnetic γ2precipitate, fine grain size, and internal stress arising from phase transformation.

Microstructure, properties, and age hardening behavior of a thermomechanically processed ultralow-carbon cu-bearing high-strength steel

An ultralow-carbon steel alloyed with Ni, Mn, Mo, and Cu and microalloyed with Nb and Ti was subjected to a three-stage controlled rolling operation followed by water quenching. The effect of thermomechanical processing on the microstructure, mechanical properties, and age-hardening behavior of the steel was evaluated. The precipitation behavior of Cu at different aging temperatures was studied by transmission electron microscopy (TEM) and differential scanning calorimetry (DSC). The high strength values obtained in the present steel are due to the fine-lath martensite structure along with tiny precipitates of microalloying carbide and carbonitride of Ti and Nb at all finish rolling temperatures (FRTs). The increased strength value at the lower FRT is due to the finer lath width and packet size of martensite. The large TiN particles and the coarse martensite-austenite (MA) constituents impaired the impact-toughness value of the steel at subambient temperature. On aging at different temperatures, a wide variation in structure and properties has been obtained. At low aging temperatures, coherent Cu particles form and a peak strength is obtained due to the formation of fine e-Cu precipitates. On increasing aging temperatures, the Cu particle size increases, with a simultaneous decrease in dislocation density in the matrix resulting in a continuous decrease in strength.

Characterization of prior cold worked and age hardened Cu–3Ti–1Cd alloy

The influence of cold deformation by 50%, 75% and 90% on the age-hardening behavior of a Cu–3Ti–1Cd alloy has been investigated by hardness, tensile tests and light optical as well as transmission electron microscopy. The hardness of Cu–3Ti–1Cd alloy increased from 111 Hv in the solution-treated condition to 355 Hv in 90% cold worked and peak aged condition. The yield and ultimate tensile strengths of Cu–3Ti–1Cd alloy reached maxima of 922 MPa and 1035 MPa, respectively, on 90% deformation and peak aging. The microstructure of the deformed alloy exhibited elongated grains and deformation bands. The maximum strength on peak aging was brought about by the precipitation of ordered, metastable, coherent β′ Cu 4Ti phase, in addition to high dislocation density and deformation twins. Both the hardness and the strength of the alloy decreased on overaging due to the development of the incoherent equilibrium phase β Cu 3Ti in a cellular structure form. However, the morphology of the discontinuous precipitation was changed to globular form at high deformation levels.

Aging behavior of sub-micron Al2O3P/2024Al composites

Abstract Age-hardening behaviors of nearly spherical 0.3 μm-Al 2 O 3 particles reinforced 2024Al composites under the temperature of 160, 175 and 190 °C were studied by means of Brinell-hardness measurement, TEM and DSC. Results show that due to the addition of sub-micro Al 2 O 3 particles, hardness of the composite was obviously improved. While the age-hardening efficiency of Al 2 O 3 p/Al composite was lower than that of the monolithic 2024 alloy. Formation of G.P. regions was suppressed and thermal-diffusion activation energy of s′phase was increased which made the precipitation of s′phase more difficult. And the number and size of precipitates in the composite were relatively less and smaller.

Hardening mechanism of an Ag–Pd–Cu–Au dental casting alloy

Age-hardening behaviour and the related microstructural changes were studied to elucidate the hardening mechanism of an Ag–Pd–Cu–Au dental casting alloy by means of hardness test, X-ray diffraction (XRD), scanning electron microscopic (SEM) observations and electron probe microanalysis (EPMA). By considering hardness test and XRD results together, it was revealed that the hardness increased during the early stage of phase transformation of α into α 1. In the SEM photographs, two phases of matrix and particle-like structures were observed, and the precipitation of element from the matrix progressed during isothermal aging. By SEM observations and EPMA analysis, it could be supposed that the increase in hardness was caused by the diffusion and aggregation of Cu atoms from the Ag-rich α matrix containing Au and Cu in the early stage of age-hardening process, and that the decrease in hardness was caused by the progress of coarsening of Cu-rich lamellar precipitates in the later stage of the age-hardening process. The changes in the Ag-rich matrix caused both the increase and decrease in hardness, and the CuPd phase containing small amounts of Zn and Sn did not contribute to the hardness changes.

Aging behavior of Ti–Mo alloys heavily compressed in ultra-high strain rate mode

Abstract A high density of vacancy clusters was observed in an impact compressed Ti–20Mo specimen to 100% strain. Age-hardening behavior of the specimen at 623 K for 28.8 ks was compared with that of as-quenched specimen. The highest hardness value of both specimens appeared at the aging time of 28.8 ks. The increment of hardness value of the impact compressed specimen due to aging was one-third of that of the as-quenched specimen. At that time, the size of aged ω-particles in the impact compressed specimen were smaller, however, the particle density was higher compared with those in the as-quenched specimen. These results suggest that vacancy clusters introduced by the impact compression become nucleation-sites of aged ω-particles, and a dominant age-hardening mechanism is considered as the pass-through mechanism. Aging behavior of hydrostatically pressurized Ti–15Mo–5Zr specimens at 623 K was investigated, and the maximum hardness value was lower than that of an as-grown Ti–15Mo–5Zr specimen. The results on size and density of aged ω-particles in the hydrostatically pressurized Ti–15Mo–5Zr specimens also suggests that the dominant age-hardening mechanism may be the pass-through one.

Enhanced age-hardening behaviour in Mg?4 wt.% Zn micro-alloyed with Ca

In the Mg–Zn system it has been shown that the addition of trace amounts of Ca can have a substantial positive influence on the precipitation process during artificial ageing. The microstructure of the modified alloy is more complex and has a much refined precipitate structure that is very stable for prolonged periods at elevated temperatures (⩽450 K), leading to a significant improvement in creep resistance.

Modeling the age-hardening behavior of Al-Si-Cu alloys

We describe a new approach for modeling the age-hardening behavior of Al-Si-Cu cast alloys, that utilizes recently proposed micromechanical models of precipitation strengthening which connect key microstructural parameters for realistic precipitate morphologies (e.g., {100} plates) with the age-hardening response. This approach is illustrated and tested for a series of 319-type Al alloys (which we refer to as W319), where the microstructural parameters of ϑ′ plates measured by transmission electron microscopy and a first-principles/computational-thermodynamics model of ϑ′ volume fraction are used in the micromechanical model to predict precipitation strengthening. Thus, the precipitation-strengthening contribution contains no free parameters in our approach. An aging temperature- and time-dependent component that describes the strengthening of the Guinier-Preston (GP) zones and solid-solution copper, as well as a constant intrinsic strength, is combined with the calculated precipitation strength to predict the yield strength with a minimum of fitting parameters. This yield-strength model provides a good predictor of the yield strength of W319, and the methodology should be more generally applicable to all industrial cast alloys strengthened primarily by ϑ′. We also discuss limitations of the present approach and point to areas for improvement in future studies.

Enhanced age-hardening behaviour in Mg–4 wt.% Zn micro-alloyed with Ca

In the Mg–Zn system it has been shown that the addition of trace amounts of Ca can have a substantial positive influence on the precipitation process during artificial ageing. The microstructure of the modified alloy is more complex and has a much refined precipitate structure that is very stable for prolonged periods at elevated temperatures (⩽450 K), leading to a significant improvement in creep resistance.

The effect of Cu on mechanical and precipitation properties of Al–Zn–Mg alloys

The effect of Cu on the mechanical and precipitation properties of a high strength Al-2.4 at.% Zn-2.1 at.% Mg alloy was investigated by compression and indentation tests, as well as by differential scanning calorimetry (DSC), transmission electron microscopy (TEM) and three-dimensional atom probe field ion microscopy (3DAPFIM). The addition of 0.5 at.% Cu introduces significant changes in the precipitation process and consequently in the age-hardening behavior of the alloy. Microstructural measurements reveal that the addition of Cu changes the density of GP zones, but it also changes partly the shape and composition of the particles. Mechanical and microstructural results together lead to the conclusion that clustering of solute atoms and vacancies during or immediately after water quenching plays an important role in the nucleation of intermediate phase precipitates in one-step aging and the addition of Cu to ternary Al–Zn–Mg leads to changes also in the initial clustering process.

Cu precipitation in a prestrained Fe-1.5 wt pct Cu alloy during isothermal aging

The control of Cu precipitation at low temperatures, e.g., bake hardening of Cu bearing steels, has recently attracted considerable attention due to the potential of achieving good formability and high strength. An Fe-1.5 wt pct Cu alloy, solution treated and 10 pct prestrained, exhibits a two-step age-hardening behavior, i.e., a smaller, but substantial hardening around 200 °C to 300 °C and a major hardening around 500 °C, while only the latter hardening occurs in undeformed specimens. The precipitation behavior of nanoscale Cu particles or bcc Cu clusters that plays a major role in age hardening was simulated by Cahn-Hilliard nonclassical nucleation theory and the Langer-Schwartz model. Simulation results are compared with the distribution of Cu particles observed under three-dimensional atom probe field ion microscope (3-D APFIM) and transmission electron microscope (TEM), and age hardening behavior as well. The increase in hardness in prestrained specimens at low temperatures (≤400 °C) can be ascribed to Cu particles nucleated preferentially at dislocations or to Cu particles that were formed in the matrix as early as at dislocations presumably due to excess vacancies introduced by prestraining.

Effect of Mg on age hardening and precipitation behavior of an AlSiCuMg cast alloy

Effect of Mg on age hardening and precipitation behavior of an AlSiCuMg cast alloy

Effect of aging and deformation on the microstructure and properties of Fe−Ni−Ti maraging steel

The age-hardening behavior of Fe-25.3Ni-1.7 Ti (wt pct) alloy both in undeformed specimens and in specimens cold deformed by 10 or 20 pct prior to aging was studied. The microstructural changes during aging were observed using transmission electron microscopy (TEM) and atom probe analysis and there were related to the mechanical properties as measured by microhardness and shear punch testing. An excellent combination of hardness, strength, and ductility was achieved after only 5 seconds aging at 550 °C. We propose that this rapid strengthening is due to a dislocation friction effect arising from the formation of a fine dispersion of Ni-Ti atomic co-clusters during this short aging time. The concomitant effects of a reverse transformation of martensite to austenite during aging and a gradual increase in both size of the clusters and distance between them contributed to a decrease in strength after aging for 15 seconds. This decline proceeded until aging for 300 seconds and was followed by a secondary hardening reaction toward peak hardness (at 10,800 seconds) and subsequent overaging. This secondary hardening was associated with fine-scale precipitation of Ni3Ti and this process was accelerated by deformation prior to aging, leading to a reduction or elimination of hardness decline after the initial cluster hardening.

Effect of aging and deformation on the microstructure and properties of Fe-Ni-Ti maraging steel

The age-hardening behavior of Fe-25.3Ni-1.7 Ti (wt pct) alloy both in undeformed specimens and in specimens cold deformed by 10 or 20 pct prior to aging was studied. The microstructural changes during aging were observed using transmission electron microscopy (TEM) and atom probe analysis and there were related to the mechanical properties as measured by microhardness and shear punch testing. An excellent combination of hardness, strength, and ductility was achieved after only 5 seconds aging at 550 °C. We propose that this rapid strengthening is due to a dislocation friction effect arising from the formation of a fine dispersion of Ni-Ti atomic co-clusters during this short aging time. The concomitant effects of a reverse transformation of martensite to austenite during aging and a gradual increase in both size of the clusters and distance between them contributed to a decrease in strength after aging for 15 seconds. This decline proceeded until aging for 300 seconds and was followed by a secondary hardening reaction toward peak hardness (at 10,800 seconds) and subsequent overaging. This secondary hardening was associated with fine-scale precipitation of Ni3Ti and this process was accelerated by deformation prior to aging, leading to a reduction or elimination of hardness decline after the initial cluster hardening.

Age-hardening behavior of cast Al–Si base alloy

In this paper, age-hardening behavior of cast Al–Si base alloys was investigated. It is found that double aging peaks are present in the age-hardening curves of Al–Si–Cu–Mg alloys and the first peak is higher than the second one when aged at temperature above a certain temperature, whereas a hardness plateau and single hardness peak are present in age-hardening curves of Al–Si–Mg and Al–Si–Cu alloys. The results of differential scanning calorimetry (DSC) analyses and TEM observation reveal that the age-hardening behavior is related to the precipitation sequence of alloys. For the Al–Si–Cu–Mg alloy, the first aging peak is the result from the high-density GP zone (especially GP II zones), while the second one is from metastable phases. Obvious interval during the transition from GP zones to metastable phases in DSC curve, which caused by dissolution of GP zones and the nucleation of metastable phases on dislocation, may be the main reason of the formation of double aging peaks. The aging plateau of Al–Si–Mg alloys is corresponding to the continuous transition from GP zones to metastable phases, while single aging peak of Al–Si–Cu alloys is caused by metastable phases.

Ａｌ‐Ｓｉ‐Ｍｇ‐Ｃｕ系鋳造合金の熱疲労寿命に及ぼす析出硬化および共晶Ｓｉ組織の影響

The relationship between age-hardening and thermo-mechanical fatigue life was investigated for Al–Si–Mg–Cu cast alloys. The age-hardening behavior of the alloys was almost similar for the Cu-free and Cu containing alloys up to the peak aging, however, the softening behavior by over-aging was different depending on Cu contents. The behavior of decreasing in the loop-edge stress during thermo-mechanical fatigue cycles was also different depending on Cu contents. The Cu-free alloy exhibiting a long fatigue life shows significant decrease in hardness by over-aging and pronounced decrease in the loop-edge stress with increasing fatigue cycles. The Q′ or Q phase is formed in the alloys containing higher Cu contents and retards the hardness decrease during over-aging and the decrease in the loop-edge stress during the initial stage of thermo-mechanical fatigue cycles. These behaviors of the alloys containing higher Cu contents result in the decreased fatigue life. This is explained that the plastic strain localization occurs around the eutectic Si particles when the stress decrease in the initial stage of the fatigue cycles is small and causes micro cracks at around eutectic Si particles. The increase in the calculated area for an age-hardening curve is related to the increase in the fatigue life. Therefore, the thermo-mechanical fatigue life can be predicted based on the calculated area for an age-hardening curve.

Quantitative characterization of precipitate free zones in Al–Zn–Mg(–Ag) alloys by microchemical analysis and nanoindentation measurement

To correlate quantitatively the mechanical properties of precipitate free zones (PFZ) with the corresponding microstructural and compositional characteristics, TEM observation, EDX analysis and nanoindentation measurement have been performed in the vicinity of grain boundaries in Al–4.9 mass%Zn–1.8 mass%Mg (–0.28 mass%Ag) alloys. The remarkable decreases in hardness and solute concentrations were observed towards grain boundaries even in the regions just outside PFZ. With increasing aging time, it is firstly revealed that the hardness inside PFZ monotonously decreases although the hardness inside grains increases in the earlier stage of aging. Three distinct regions of “PFZ”, “Transition-area” and “Grain-region” were therefore proposed to explain the origins of such age-hardening behavior observed in this work. In the Ag-added alloy, on the other hand, the hardness could be maintained up to closer regions to grain boundaries at the same level as that inside grains.

Effect of Copper on Tensile Properties and Grain-Refinement of Steel and its Relation to Precipitation Behavior

Progress of Ferrous Nano-Metal Project is introduced in the present paper. In the project, maximum use of copper clusters and precipitates is pursued for achieving better strength-ductility balance than that of conventional high-tensile strength steels. Fundamental aspect of clustering and precipitation of Cu in Fe-Cu alloys was studied using Optical Tomographic Atom-Probe (OTAP). It was found that Cu precipitation during aging was enhanced by plastic deformation. The observed Cu precipitation behavior was well related to the age-hardening behavior, that is, aging started at lower temperature and maximum hardness was higher for plastically deformed and aged ferrite. Aging behavior and associated tensile properties were further examined for Fe-C-Mn-Cu martensitic steel. Higher value of tensile strength times elongation was achieved in FeC-Mn-Cu steel than Fe-C-Mn steel. Finally, effect of Cu precipitation on grain-refinement of ferrite was studied for Fe-C-Mn-Cu steel. Ferrite grain smaller than 1 mm was obtained in both processes of strain-assisted ferrite transformation from heavily deformed austenite and dynamic recrystallization of heavily deformed ferrite. Ferrite grain size was found to decrease by addition of Cu at the both processes. It was suggested that a simple additivity rule does not hold in terms of the strengthening by grain-refinement and that by precipitation, especially at grain-size range less than 1 mm.