Age-hardening Mechanism Research Articles

Twofold age-hardening mechanism of Al-added high-Cr ODS ferritic steels

Age-hardening of oxide dispersion strengthened (ODS) ferritic steels with various contents of Cr and Al was investigated by Vickers hardness (HV) measurement and tensile test after ageing at 475°C for up to 9000 h. The correlation between age-hardening and the evolution of nanometer-scale structure and chemistry studied by atom probe tomography (APT) reveals that the age-hardening could be interpreted in terms of twofold hardening mechanism involving α–α′ phase separation and/or the precipitation of (Al, Ti)-enriched β′ phases. The results of APT characterization indicate that the Cr concentration and volume fraction of α′ phase increase significantly while the chemical compositions and volume fraction of β′ phases don't changed considerably during ageing from 300 h to 9000 h. At the early stage of ageing, the age-hardening due to α–α′ phase separation is very limited because of the low Cr concentration of α′ phase, which results in very weak modulus strengthening. With the increasing of ageing time, the age-hardening is increased because of the increasing of Cr content in the α′ phase, resulting in considerably enhanced modulus mismatch strengthening. However, age-hardening due to β′ phase precipitation is significant once the β′ phases are formed and, thereafter, it doesn't change apparently. The metal/oxide interfaces in ODS steels provide nucleation sites for α′ and β′ phases. Based on the comprehensive data developed, the roles of Cr and Al in the age-hardening of ODS ferritic steels were elucidated.

Early-stage clustering and precipitation behavior in the age-hardened Al–Mg–Zn(-Cu) alloys

T-Mg32(AlZnCu)49 phase-strengthened Al–Mg–Zn–Cu alloys have shown a promising age-hardening response. However, the age-hardening mechanism underlying the different heat treatments is not clear during the early-stage precipitation. Additionally, the lack of information on the early-stage precipitation impedes the development of this series alloys. In this study, the formation of the Guinier-Preston (GP) zone and T′-phase of the novel Al–Mg–Zn(-Cu) alloys during the early-stage precipitation has been investigated using transmission electron microscopy and atom probe tomography analysis. Investigation revealed that the Cu content and thermal treatment had significant effects on the microstructure evolution of the alloys. GP zones were observed to act as sites for the formation of the T′-phase. The higher number density of Cu-incorporated GP zones led to higher strength and bake-hardening response after pre-aging. Occasionally, some GPI zones were observed to dissolve into the Al matrix without further transforming into T′ precipitates during natural aging (NA). Pre-aging prior to NA facilitated stable GP zone formation and mitigated the negative effect of NA. Furthermore, the precipitates were grown to an appreciable size after single-step aging at 180 °C, which adversely affected the mechanical properties of the novel alloys. The present findings provide an efficient guideline for designing the T-phase in the crossover Al–Mg–Zn-(Cu) alloys.

Natural Aging Effect of Al-20Zn-3Cu Alloy on Mechanical Properties and Its Relation to Microstructural Change

We investigate the effect of the natural age-hardening response of the Al-20Zn-3Cu alloy with natural aging times up to 12 months. The ultimate tensile strength of the Al-20Zn-3Cu alloy is drastically enhanced from 308 to 320 MPa after 2 months and from 320 to 346 MPa after 9 months. Then, natural age hardening becomes saturated after 9 months. A microstructural investigation reveals that the natural age-hardening mechanism is mainly induced by the diffusion of the Zn element. First, a rapid decrease in the volume fraction of the eutectoid lamellae (α-Al+η-Zn) is observed at the early stage of natural aging, leading to an increase in the tensile strength. This originates from the relatively high diffusivity of Zn due to its low melting temperature. Then, the diffusion of Zn into the Al matrix induces clusters of solute atoms that enhance the growth rate of the nanoprecipitates formed in the Al matrix. As a consequence, the tensile strength of the natural-aged Al-20Zn-3Cu alloy increases drastically after 9 months, whereas the ductility is significantly degraded.

Understanding the age-hardening mechanism of CrWN coating

A microstructure and age-hardening investigation was performed on a chromium nitride and tungsten nitride (CrWN) transition metal nitride coating for optical glass molds using electron microscopy and nano-indentation. The as-deposited coating had a non-uniformly distributed coherent solid solution matrix causing limited strengthening and unsatisfactory mechanical performance. Annealing of the coating induced spinodal decomposition of supersaturated solid solution and promoted diffusion within and homogenization of the solid solution matrix. Large areas of homogenized solid solution greatly strengthened the coherent matrix resulting in significant age-hardening of the annealed coating. These findings provide supporting data for understanding the age-hardening mechanism of transition metal nitride coatings.

The effect of age-hardening mechanism on hydrogen embrittlement in high-nitrogen steels

The effect of age-hardening regime on peculiarities of hydrogen-assisted fracture and tensile properties in two high-nitrogen Fe-23Cr-17Mn-0.1C-0.6N and Fe-19Cr-22Mn-1.5V-0.3C-0.9N steels was studied. A large number of intergranular (austenite/austenite) and interphase boundaries (austenite/coarse particle) provides high fraction of trapping sites for hydrogen atoms in V-alloyed steel. This leads to a change in fracture regime from transgranular brittle mode in coarse-grained V-free steel to intergranular brittle fracture of hydrogen-assisted surface layers in fine-grained V-alloyed steel with coarse (V,Cr)(N,C) particles. The formation of cells (Cr2(N,C) particles and austenite) along the grain boundaries due to discontinuous precipitate-hardening reaction facilitates predominantly interphase hydrogen-assisted fracture for both steels. The complex reaction of the particle-strengthening mechanisms including discontinuous precipitation with formation of austenite/Cr2(N,C)-plates interfaces or homogeneous nucleation of coherent (V,Cr)(N,C) particles in austenite (age-hardening regime 700 °C, 10 h) promotes mainly transgranular cleavage-like fracture mode under hydrogen-charging. The structural scheme is proposed to describe a change in hydrogen-assisted fracture micromechanisms and tensile properties of the steels with different density and distribution of interphase and intergranular boundaries.

Effect of Nitrogen on Age-Hardening of Metastable Austenitic Stainless Steel after Cold Drawing

Metastable austenitic stainless steels transform to the deformation-induced martensite by cold working. Especially, metastable stainless steel with high nitrogen content has high age-hardening property after aging treatment. In this work, effect of nitrogen on age-hardening of metastable austenitic stainless steel (SUS304: 0.04% N, type-SUS201: 0.18% N) after cold drawing was investigated, and age-hardening mechanism was elucidated. Strength after cold drawing of SUS201 containing high N is higher than that of SUS304, and the age-hardening of SUS201 is significantly higher than that of SUS304 at the aging temperature of 200 ~ 500°C. It is suggested that strengthening mechanism of SUS201 is caused by aging products of N, because exothermal reaction in SUS201 is clearly recognized at low aging temperature by DSC analysis.

Lamellar-forming grain boundary reaction related to age-hardening mechanism in an Au-Pt-Pd-In metal-ceramic alloy

This study examined the relationship between the lamellar-forming grain boundary reaction and the change in hardness during the aging process of an Au-Pt-based metal-ceramic alloy composed of 76.6 Au–9.9 Pt–9.3 Pd–1.7 In–1.2 Ag–0.56 Sn (wt%) with minor ingredients. The phase decomposition of the parent Au-rich α phase occurred within a very short time (30 s) by aging at 550 °C after a solution-treatment at 950 °C due to the solubility limits of Au and Pt in each other, which initiated a lamellar-forming grain boundary reaction. The observed hardening was attributed to both the grain interior and grain boundary precipitate. On the other hand, the grain boundary precipitate comprising the fine lamellar structure caused more powerful hardening from the early stage of the aging process. The alternate layer of the grain boundary lamellar structure was composed of Pt-, In- and Sn-depleted Au-rich α1 phase as well as precipitated Pt-, In- and Sn-concentrated β1 phase. The extremely fine nature of the grain boundary lamellar structure supplied large amounts of inter-phase boundaries, which contained lattice strain by the difference in the lattice parameter between the α1 and β1 phases, resulting in hardening.

Age-hardening mechanism for nanocrystalline Ni–P alloys synthesized by electrodeposition

Strengthening mechanism of age-hardenable electrodeposited Ni–P alloy was investigated focusing on the role of Ni3P precipitates in nanocrystalline structures. Specimens of P content ranging from 3.0 to 12.5wt.% were synthesized by electrodeposition followed by aging treatment at a temperature between 473 and 773K. The specimens became harder with higher P content after the electrodeposition and aging treatment. As-electrodeposited structures comprised only Ni phase, and Ni3P precipitated by the subsequent aging treatment. Vickers hardness of electrodeposits showed the maximum after aging at 673K, and was higher than that predicted by the rule of mixture of Ni and Ni3P phases. Distribution of parent Ni grain size and Ni3P precipitate size was examined in detail by field-emission type transmission electron microscopy. The maximum hardness was obtained when the grain size of Ni matrix and Ni3P particle size were comparable. The hard interphase boundary to the dislocation slip is included as a possible mechanism which is responsible for the hardening by the aging treatment rather than the classical precipitation hardening.

The Age-Hardening Mechanism of Nanocrystalline Ni-P Alloys Synthesized by Electrodeposition

Strengthening mechanism of age-hardenable electrodeposited Ni-P alloy was investigated focusing on the role of Ni3P precipitates in nanocrystalline structures. Specimens were synthesized by electrodeposition followed by aging treatment at temperatures raging from 473 to 773 K. As-electrodeposited structures became only Ni phase and Ni3P precipitated by the subsequent aging treatment. Distribution of grain size of Ni matrix was examined in detail by field-emission type transmission electron microscopy. The specimens became harder with higher P content after the electrodeposion and aging treatment. After the aging treatment 573 K or bellow, average grain size of Ni matrix was under 10 nm. On the other hand, after aging at 673 and 773 K both the grain size of Ni matrix and Ni3P particle size grow to be comparable. It is considered the hard interphase boundary to the dislocation slip is responsible to the hardening rather than the classical precipitation hardening.

Age-hardening mechanism by precipitation in a zinc- and indium-added silver-palladium alloy

The age-hardening mechanism of a copper-free silver-palladium alloy is unclear yet because the age-hardening mechanism is variable even with slight compositional changes. The aim of present study is to elucidate the age-hardening mechanism and relation of age-hardenability with the phase transformation and microstructural changes of a Zn- and In-added Ag-Pd alloy by means of the hardness test, X-ray diffraction study, field emission-scanning electron microscopic observation and energy dispersive spectrometer analysis. The matrix was composed of the Ag-rich α1 phase containing small amounts of solute elements, and the particle-like structures were composed of the PdZn-based β1 phase containing relatively large amounts of In. The lamellar-forming grain boundary reaction which is usually observed in Ag-Pd alloys containing Cu was not observed in the Zn- and In-added Ag-Pd alloy. The grain interior precipitate composed of the Pd-Zn-rich phase containing In resulted in apparent hardening from the early stage of aging process, and its coarsening by prolonged aging resulted in softening. In the specimen alloy, Zn and In formed extra phase preferentially with Pd, and the amount of Zn and In which took part in the precipitation hardening was very small.

TEM study on the microstructural evolution in an Mg–Y–Gd–Zn alloy during ageing

The multi-phase microstructural evolution in an Mg–Y–Gd–Zn alloy during ageing at 225 °C was studied in TEM and the age-hardening mechanism is elucidated in this paper. Two independent precipitation reactions were observed in this alloy, i.e. α-Mg(SSS) + β(Mg24(Y,Gd,Zn)5) → α + β + 18R and α-Mg(SSS) → βʺ(D019) → β′(cbco) → β1(fcc) → β. The age-hardening contribution from precipitates is discussed based on their size, shape, orientation and distribution at corresponding ageing stages. The major hardening precipitate is found to be β' phase with a c-axis base-centered orthogonal (cbco) structure, rather than the long period stacking ordered (LPSO) phase that prevailed in some earlier studies.

관교의치용 Au-Ag-Cu-Pt-Zn 합금의 시효경화성과 관련된 상변태와 입계석출

Purpose: The age-hardening mechanism of an Au-Ag-Cu-Pt-Zn alloy for crown and bridge fabrication was investigated by means of hardness test, X-ray diffraction study and field emission scanning electron microscopic observation. Methods: Before hardness testing, the specimens were solution treated and then were rapidly quenched into ice brine, and were subsequently aged isothermally at <TEX>$400-450^{\circ}C$</TEX> for various periods of time in a molten salt bath and then quenched into ice brain. Hardness measurements were made using a Vickers microhardness tester. The specimens were examined at 15 kV using a field emission scanning electron microscope. Results: By the isothermal aging of the solution-treated specimen at <TEX>$450^{\circ}C$</TEX>, the hardness increased rapidly in the early stage of aging process and reached a maximum hardness value. After that, the hardness decreased slowly with prolonged aging. However, the relatively high hardness value was obtained even with 20,000 min aging. By aging the solution-treated specimen, the f.c.c. Au-Ag-rich <TEX>${\alpha}_0$</TEX> phase was transformed into the Au-Ag-rich <TEX>${\alpha}_1$</TEX> phase and the AuCu I ordered phase. Conclusion: The hardness increase in the early stage of aging process was attributed to the formation of lattice strains by the precipitation of the Cu-rich phase and then subsequent ordering into the AuCu I-type phase. The decrease in hardness in the later stage of aging process was due to the release of coherency strains by the coarsening of tweed structure in the grain interior and by the growth and coarsening of the lamellar structure in the grain boundary. The increase of inter-lamellar space contributed slightly to the softening compared to the growth of lamellar structure toward the grain interior.

Investigation of the age hardening and operative deformation mechanism of 7075 aluminum alloy under creep forming

Creep forming is a process where plastic deformation is applied at the material's ageing temperature. In order to study the effect of microstructure changes on the hardening and creep deformation behavior of 7075 aluminum alloys, creep forming experiments have been performed at temperatures of 150°C and 190°C for 6–72h. Results indicated that with increasing of forming time and temperature the size of globular shaped phase (known as GP zones) increased and transformed into η′ precipitates. Meanwhile, the η-MgZn2 phase with almost the same size formed along the grain boundaries with quite narrow precipitate free zones. Further increasing of forming time, led to a decrease in the number of η′ precipitates and an increase in precipitate spacing, where the large equilibrium η phase formed and the precipitate free zones became wider. Moreover and creep curves can be divided into two stages. During the first stage, higher creep rate occurred, which increased with temperature but decreased with forming time. While during the second stage, the creep rate reached its lowest value and kept constant. The variations of hardness and creep strain have been discussed in detail by taking account of the evolution of precipitate microstructure.

Phase transformation and microstructural changes during ageing process of an Ag–Pd–Cu–Au alloy

Age-hardening behaviour and the related phase transformation and microstructural changes during isothermal ageing process were studied to elucidate the age-hardening mechanism of an Ag-based dental casting alloy composed of Ag–Pd–Cu–Au–Zn, Ir and In by means of hardness test, X-ray diffraction (XRD), scanning electron microscopic (SEM) observations and energy dispersive spectroscopic microanalysis (EDS). In the hardness test at 350 and 400 °C, the hardness of the solution-treated specimen began to increase and reached a maximum value with increasing ageing time, and subsequently the hardness decreased gradually. By considering XRD results and SEM observations together, the solution-treated specimen consisted of three phases, the Ag-rich α 1 phase as a matrix, the Cu–Pd α 2 phase and the CuPd β phase with a CsCl-type as particle-like structures. By ageing the solution-treated specimen, the Ag-rich α 1 and Cu–Pd α 2 phases were transformed into the Ag-rich α ′ 1 and Cu 3Pd α ′ 2 phases, respectively. The CuPd β phase with a CsCl-type was not changed apparently during the ageing process. From the results of the hardness test, XRD study, SEM observations and EDS analysis, it could be derived that the hardness increased by the diffusion and precipitation of the Cu-rich phase from the Ag-rich matrix during the early stage of phase transformation of α 1 into α ′ 1 and that the progress of coarsening of the Cu-rich precipitates with an entanglement structure caused the hardness decrease during the later stage of phase transformation of α 1 into α ′ 1 . The particle-like structures composed of the Cu–Pd α 2 and the CuPd β phase with a CsCl-type contributed little to the hardness increase which occurred in the early stage of aging process.

Phase Transformation and Microstructural Changes during Ageing Process of an Ag-Pd-Cu-Au Alloy

Abstract Age-hardening behaviour and the related phase transformation and microstructural changes during isothermal ageing process were studied to elucidate the age-hardening mechanism of an Ag-based dental casting alloy composed of Ag–Pd–Cu–Au–Zn, Ir and In by means of hardness test, X-ray diffraction (XRD), scanning electron microscopic (SEM) observations and energy dispersive spectroscopic microanalysis (EDS). In the hardness test at 350 and 400 °C, the hardness of the solution-treated specimen began to increase and reached a maximum value with increasing ageing time, and subsequently the hardness decreased gradually. By considering XRD results and SEM observations together, the solution-treated specimen consisted of three phases, the Ag-rich α1 phase as a matrix, the Cu–Pd α2 phase and the CuPd β phase with a CsCl-type as particle-like structures. By ageing the solution-treated specimen, the Ag-rich α1 and Cu–Pd α2 phases were transformed into the Ag-rich α ′ 1 and Cu3Pd α ′ 2 phases, respectively. The CuPd β phase with a CsCl-type was not changed apparently during the ageing process. From the results of the hardness test, XRD study, SEM observations and EDS analysis, it could be derived that the hardness increased by the diffusion and precipitation of the Cu-rich phase from the Ag-rich matrix during the early stage of phase transformation of α1 into α ′ 1 and that the progress of coarsening of the Cu-rich precipitates with an entanglement structure caused the hardness decrease during the later stage of phase transformation of α1 into α ′ 1 . The particle-like structures composed of the Cu–Pd α2 and the CuPd β phase with a CsCl-type contributed little to the hardness increase which occurred in the early stage of aging process.

Hardening and overaging mechanism of a commercial Au–Ag–Cu–Pd dental alloy

To reveal the aging behaviour and age-hardening mechanism of an Au–Ag–Cu–Pd alloy with small amounts of Pt, Zn and Ir, the hardness changes, phase transformation and changes in microstructure and element distribution during the aging process were observed. The specimen alloy showed apparent age-hardenability at the aging temperature of 400 °C. By aging, the hardness of the solution-treated specimen began to increase and reached a maximum value, and then, the hardness decreased continuously by further aging. The solution-treated specimen was composed of the Ag–Au-based α 1 matrix containing Cu and the PdZn particle-like structures containing Cu and Pt. By aging the solution-treated specimen at 400 °C, the Ag–Au-based α 1 matrix was replaced by the lamellar structures of the solute-depleted α ′ 1 matrix and the AuCu I precipitates. The hardness increase in the early stage of the aging process seemed to be caused by the nucleation of the AuCu I phase from the matrix which induced the formation of lattice strains in the interface between the matrix and the AuCu I phase. The hardness decrease in the later stage of the aging process was caused by the coarsening and granulation of the AuCu I precipitates which released the strains that formed in the early stage of the AuCu I precipitation from the matrix. The PdZn particle-like structures did not contribute to the hardness increase.

Age-hardening by metastable phases in an experimental Au–Ag–Cu–Pd alloy

The age-hardening mechanism and phase transformation during the age-hardening process were examined for a low carat gold alloy with a composition of 30 wt.% Au–30 wt.% Ag–20 wt.% Cu–20 wt.% Pd. By aging the solution-treated specimen, the fcc α 0 phase was transformed into the Au-containing Ag-rich phase and the Pd-containing AuCu I phase through the metastable state. The phase transformation of the α 0 phase into the metastable Ag-rich α ′ 1 and AuCu I′ phases caused the apparent hardness increase. And the phase transformation of the Ag-rich α ′ 1 and AuCu I′ phases into the Ag-rich α 1 and AuCu I phases, caused the hardness decrease. The hardness increase in the early stage of the age-hardening process seemed to be caused by an introduction of the coherency strains at the interface between the metastable Ag-rich α ′ 1 and AuCu I′ phases during the phase transformation of α 0 into Ag-rich α ′ 1 and AuCu I′. In the later stage of aging process, the alternative lamellar structure composed of the Au-containing Ag-rich phase and the Pd-containing AuCu I phase grew from the grain boundary consuming the Ag-rich α ′ 1 and AuCu I′ phases in the grain interior, which caused the hardness decrease by releasing the coherency strains.

Hardening of Low-Alloyed Gold

The development of strengthened high carat Au alloys for jewellery applications is an interesting but challenging task, with significant potential for further development of new age-hardenable alloys. The paper first presents an overview on the possibilities and limitations to harden 22ct Au by conventional means, i.e. by alloying with conventional base metals (solid solution hardening) and in combination with heat treatment processes (age-hardening). The paper then refers to 22ct up to 24ct Au and reports on the enhancement of the age-hardening effect of the primary base metals by additions of selected secondary elements. It is suggested that the prevailing age-hardening mechanism in these types of alloys consists in precipitation of intermetallic phases formed by the primary and secondary additions.

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.

Age hardening by dendrite growth in a low-gold dental casting alloy

Commercial low-gold dental casting alloy composed of Ag–Pd–In–Au–Zn was studied to clarify the age-hardening mechanism and related microstructural changes. The hardness of solution-treated specimen began to increase and reached the maximum value with ageing time, and then the maximum hardness value decreased by further ageing. The changes of X-ray diffraction (XRD) pattern during isothermal ageing revealed that the age hardening was not caused by phase transformation. By comparing the age-hardening curve with the changes in full-width at half-maximum of the XRD peaks at each ageing time, it was revealed that the coherency strains were formed in the Ag-rich matrix, which contributed to the hardness increase during ageing. From scanning electron microscopic observation and electron probe microanalysis, it was clarified that fine particle-like structures composed of InPd containing small amount of Zn gathered by diffusion in the Ag-rich matrix, and the coherency strains which formed during that time caused the hardness increase in the early stage of age-hardening process. The coherency strains were released by the progress of coarsening of Zn-containing InPd dendrite during further ageing, which caused the overaging in the later stage of age-hardening process.