Growth Kinetics Of Precipitates Research Articles

The effect of cobalt alloying on the phase transformation kinetics of Ni-Ti alloys

This study investigates the influence of cobalt (Co) alloying addition and heat treatment temperature on the phase transformation behaviour controlling the superelasticity and shape memory effect (SME) of Nickel-Titanium (Ni-Ti) alloys, commonly known as nitinol. The microstructural evolution upon heat treatment conducted at a temperature ranging from 440 to 560 °C was thoroughly analyzed via Differential Scanning Calorimetry (DSC), X-ray Diffraction (XRD), and Scanning Electron Microscopy/Energy Dispersive Spectroscopy (SEM/EDS). Increase in heat treatment temperatures from 470 °C up to 530 °C led to the dissolution of particles present in as-received (cold-worked) condition. It was determined that Co addition into the Ni-Ti alloy system resulted in a change in the nucleation and growth kinetics of Ti-rich precipitates, leading to the formation of larger and fewer particles during processing. Both binary and ternary alloys showed a decrease in austenite finish temperature (Af) with increasing heat treatment temperatures, however, the rate of decrease was found to be higher for Co containing ternary alloys. This is linked with faster structural relaxation when Co is present and evidenced by lattice size variation during heat treatment. It is highlighted that heat treatment methodology needs to be tailored to the specific alloy composition for controlling superelasticity and SME via alloy design.

Solidification precipitation behaviour and growth kinetics of AlN inclusions in FeCrAl alloys

AlN inclusions are the major inclusions in FeCrAl alloys and precipitate during solidification. The precipitation behaviour and growth kinetics of AlN inclusions in FeCrAl alloys were theoretically calculated based on the Clyne–Kurz model and a diffusion-controlled growth model. The results indicated that AlN inclusions precipitated at a solid fraction of 0.45. Following precipitation, the Al concentration exhibited a slight decrease compared to scenarios where AlN inclusion precipitation was not considered, although it still showed an increasing trend. Conversely, the N concentration exhibited a significant turnaround, gradually dropping below its initial value. In addition, varying the cooling rate to 0.17, 0.83, 1.67, and 16.67 K/s, the final sizes of AlN inclusions were 46.5, 21.1, 14.8, and 4.7 μm, respectively. Fixing other factors, the solid fractions for AlN precipitation at initial Al concentrations of 4.19, 4.69, and 5.19 wt% were 0.535, 0.45, and 0.365, respectively, and the final sizes of AlN inclusions were 16.2, 14.8, and 13.6 μm, respectively. Similarly fixing the other factors and varying the initial N concentrations to 0.0045, 0.0053, and 0.0060 wt%, the solid fractions of AlN were 0.515, 0.45, and 0.40, respectively, and the final sizes of AlN inclusions were 14.6, 14.8, and 15.0 μm, respectively. This indicates that increasing the cooling rate, increasing the initial Al concentration, and decreasing the initial N concentration could effectively reduce the precipitation size of AlN inclusions. This work provides guidance for the investigation of the precipitation and growth kinetics of AlN inclusions in FeCrAl alloys.

Inhibition of precipitation structure transformation and enhancement of hardness by trace La addition in Cu-1Cr-0.1Zr alloy sheets

Cu-1Cr-0.1Zr and Cu-1Cr-0.1Zr-0.07La alloy sheets were prepared by the following process: vacuum casting– homogenization treatment– hot forging– solid solution treatment– cold rolling– aging treatment. The effect of trace La on the aging behavior and mechanical properties of Cu-1Cr-0.1Zr alloy was investigated. Compared to the Cu-1Cr-0.1Zr (0La) alloy, the Cu-1Cr-0.1Zr-0.07La (0.07La) alloy exhibited a significant increase in hardness and a slight decrease in electrical conductivity due to larger number of finer and coherent f.c.c Cr-rich precipitates. It was found that trace La can affects the evolution of precipitation phases. The thermodynamics calculation indicates that with La addition, the Cr-rich precipitates were much more inclined to nucleate with f.c.c structure, and the precipitation growth kinetics results indicate that the addition of La can reduce the activation energy of precipitation, promote the precipitation of f.c.c Cr phase and form Cr-poor zones, making it more difficult to form bcc nuclei. Thus, the transformation of nanoscale Cr-rich precipitates from f.c.c structure to bcc structure was inhibited due to trace La addition, causing higher hardness with appropriate conductivity.

Effect of rare earth Er on discontinuous precipitation and mechanical properties of Cu-15Ni-8Sn-0.3Fe alloy

The Cu-15Ni-8Sn-0.3Fe-xEr (wt%, x = 0.1–0.4) alloy was prepared by vacuum induction melting. The microstructure evolution, mechanical properties and discontinuous precipitation (DP) growth kinetics during solidification and heat treatment were systematically investigated. The results show that the added Er leads to the formation of micrometer-sized ErNiSn2 phase and nanometer-sized ErNi phase in the Cu-15Ni-8Sn-0.3Fe alloy, which suppresses the formation of the discontinuous precipitation phases. Compared to alloys with other compositions, the Cu-15Ni-8Sn-0.3Fe-0.3Er alloy exhibits superior mechanical properties during aging at different temperatures. Specifically, after aging for 13 h at 723 K, the alloy achieved a peak hardness of 383.5 HV. In addition, the growth kinetics of the DP in Cu-15Ni-8Sn-0.3Fe-xEr (wt%, x = 0.1–0.4) alloys were evaluated using the Johnson-Mehl-Avrami-Kolmogorov equation. The activation energy (Q) of DP in the Cu-15Ni-8Sn-0.3Fe-0.3Er alloy is 178.4 kJ/mol, which is 39.2 kJ/mol higher than that of the Cu-15Ni-8Sn alloy.

Modification of MnS inclusion with trace tellurium to improve the machinability of medium-carbon low-sulfur steel

ABSTRACT Under the industrial process conditions, trace tellurium (13 ppm and 22 ppm) was added to medium-carbon low-sulfur steel to optimize the morphology of MnS inclusion and thereby enhance the machinability of the steel. Voller-Beckermann microsegregation model was used to calculate the precipitation thermodynamics and growth kinetics of MnS inclusion. The morphology of observed MnS inclusion is mainly polygonal (type III), the observed microstructures of the steel are bulk-, acicular-ferrite, and pearlite, the inclusions are mainly present in the bulk-ferrite. The calculation results of inclusion relative plasticity show that 13 ppm Te reduces the relative plasticity of inclusions, while at a Te content of 22 ppm, the relative plasticity of inclusions increases by approximately 70%. Compared with the steel without Te, when containing 22 ppm Te, the material has the best machinability, the average cutting force and average surface roughness of steel are reduced by 3.47% and 38.75%, respectively.

Monitoring the precipitation of the hardening phase in the new VDM® Alloy 780 by in-situ high-temperature small-angle neutron scattering, neutron diffraction and complementary microscopy techniques

The hardening phase precipitation process plays an important role in the development of new Ni-base superalloys. In the present work, we apply a powerful combination of complementary characterization techniques to characterize in-situ the γ’ precipitation in the new VDM® Alloy 780. During the whole heat treatment process, in-situ time-of-flight (TOF) neutron diffraction (ND) unambiguously identified the γ’ phase as well as its weight fraction and the misfit with the matrix while in-situ small-angle neutron scattering (SANS) provided precise precipitates’ size analysis. Atom probe tomography (APT) and scanning electron microscopy (SEM) provided detailed microstructural characterization and chemical composition of the phases necessary for a proper neutron scattering data evaluation. This contribution reveals more detailed information on the nucleation, growth, and Ostwald ripening processes starting from the early precipitation stage in bulk samples using the complementary microstructure investigation methods. The nucleation and growth kinetics of precipitates at 720 °C depend on heating rates and the size distribution obtained in the pre-heating history of the sample. A subsequent heat treatment step at 620 °C, typically used in Ni-base superalloys, does not lead to similar progressive precipitation or growth. The expected matrix-diffusion-controlled Ostwald ripening process of the γ’ precipitates was in-situ monitored by SANS on a full precipitation hardened sample at expected operating temperatures (750 °C) showing slower coarsening kinetics than other reported Ni-based superalloys.

Modelling dynamic precipitation in pre-aged aluminium alloys under warm forming conditions

This work presents a model for deformation enhanced precipitate growth and coarsening kinetics in pre-aged 7xxx aluminium alloys under warm forming conditions (100 to 200 °C). A multi-class Kampmann–Wagner framework is used to describe the evolution of precipitate size distribution, where the effect of deformation is incorporated through enhanced solute diffusivity resulting from deformation induced excess vacancies. A classical phenomenological model is used to describe the accumulation of excess vacancies. The model is validated with experimental warm deformation data from the literature, and applied to investigate a wide range of deformation conditions to predict the effect of strain, strain rate and temperature on precipitate growth and coarsening kinetics. It is demonstrated that the interaction of solute supersaturation and excess vacancy concentration can lead to complex non-monotonic precipitate growth rate variation for certain regimes of strain rate and temperature.

Growth kinetics of second phase particles in N36 zirconium alloy: Zr–Sn–Nb–Fe

Analyzing the diffusion kinetics behavior that leads to precipitates formation can assist in understanding the properties of Zr alloy materials. In this study, coarsening growth kinetics of precipitates in N36 zirconium alloy with Nb dominant are experimentally calculated during the isothermal and isochronal aging process (580 °C–700 °C, 0.5–30 h respectively). Microstructure observation and the second phase particles (SPPs) have been investigated and numerically extracted through several processes, including the use of an optical microscope (OM), scanning electron microscopy (SEM), and transmission electron microscopy (TEM), in addition to image software tools. Conclusions confirm that as the temperature or aging period increases, the SPPs begin to lose fine and linear distribution properties ranging from α-plate grain boundary to the random distributions. Moreover, the small particles dissolve, whereas the large ones grow, accurately pursuing the Ostwald ripening property. Besides, most SPPs are found to be in the form of Zr(Nb, Fe)2, and some of them are (Zr,Nb)2Fe. The activation energy of kinetic growth of precipitates has been experimentally calculated using Kahlwiet second-order kinetics and found to be 206 kJ/mol. Compared to other documented Zr-alloy series, we have noticed that the different types of SPPs and the Nb element content in most contribute greatly to the slow diffusion rate, explaining the higher value in the activation energy of the precipitates growth process.

Precipitate growth kinetics under inhomogeneous concentration fields using a phase-field model

We investigate precipitation dynamics in the presence of a local solute gradient using phase-field simulations. During the homogenization heat treatment of the solidified Inconel 718 alloy, high Nb concentration within the Laves phases or at the core of the secondary arms results in Nb diffusion into the $\ensuremath{\gamma}$ matrix. The volume fraction and spatial distribution of precipitation during subsequent annealing can be controlled by tailoring the Nb concentration gradient in the matrix during homogenization. We use a surrogate Ni-Fe-Nb alloy for Inconel 718 to explore the growth dynamics of $\ensuremath{\delta}$ precipitates related to the local Nb concentration levels. The simulations indicate that in the presence of a Nb concentration gradient the growth rate of $\ensuremath{\delta}$ precipitates is higher than in a matrix of uniform average Nb concentration. The higher growth rate is a result of the higher local thermodynamic driving force at the interface between the solute-rich matrix and the $\ensuremath{\delta}$ interface. We propose a phenomenological model to describe the diffusion-controlled growth kinetics of the $\ensuremath{\delta}$ phase under a solute concentration gradient.

Integrated Computational Materials Engineering-Based Simulation of Detrimental Precipitates in Power Plant Steel Weld

The microstructure stability of welded power plant steel components is typically assessed using the precipitate growth kinetics. The long-term exposure of welded structures at high temperatures results in coarsening of precipitates which may lead to the failure of the components. The dilution of base and filler material in the weldment could give rise to detrimental precipitates. The present work proposes an Integrated Computational Materials Engineering (ICME) methodology to identify optimum dilution levels for welded components used during high-temperature aging to obtain best possible lifetime for the component. For this study, the TC-PRISMA® module of ThermoCalc® software was used for precipitation calculations after appropriate benchmarking. Weld dilution study was performed to predict dilution effect influence on Laves phase precipitates in a welded component of CB2 and P92 steels during aging for up to 100,000 h. From the dilution study, it was found that the composition of the weld affects the size and coarsening of the Laves phase precipitate. It was also observed that the size of Laves phase precipitates for all dilution ratios remains within the critical size limit for CB2 steel and it exceeds the critical size limit in P92 steel during aging. Based on the study, an ICME workflow was proposed to determine the optimum dilution ratio for welded power plant steel components used in high-temperature applications for obtaining the best possible lifetime for the component and to reduce the associated cost of experimental trials.

Multiphase modelling of the growth kinetics of precipitates in Al-Cu alloys during artificial aging

ABSTRACT In the aluminum industry, the needs of predictability of the kinetics of precipitation during the artificial aging processes increase as the targeted applications require the maximisation of properties at the lowest costs possible. In this regard, kinetics modelling can be helpful to design the heat treatment processes. Despite using many fitting parameters, available models show a lack of fitting with experimental data, especially for the apparent heat capacity measured at high temperatures by a differential scanning calorimeter (DSC). To address this issue, a mixed-mode model was recently developed for isothermal heat treatment, whereas non-isothermal heat treatment must be considered to compare the calculated results with those measured by isochronal heating in a DSC. In this contribution, the model is extended to non-isothermal heat treatments. To this end, the growth kinetics pathway and sequence of precipitation in a binary Al-Cu alloy have been simulated, optimising the pre-exponential factor and the activation energy of the interfacial mobility of the secondary phases. This calibration of the interfacial mobilities allowed a very good reproduction of the evolution of the apparent heat capacity with temperature. The model and calibrated interfacial mobilities were then used to compute the size evolution of θ′ precipitates in an Al-4 wt%Cu. The isothermal growth rates calculated at 4 temperatures were in good agreement with those measured and reported by independent researchers. The good predictability of the model indicates that the assumptions made were suitable and well funded, especially regarding the growth rates of embryos emerging from the subcritical growth regime.

The effect of Co addition on the modulated structure coarsening and discontinuous precipitation growth kinetics of Cu–15Ni–8Sn alloy

To understand the effect of Co addition on the modulated structure coarsening and discontinuous precipitation (DP) growth kinetics of Cu–15Ni–8Sn alloy, the experiments are carried out using optical microscope (OM), scanning electron microscope (SEM) and transmission electron microscopy (TEM). Our results showed that the addition of Co in Cu–15Ni–8Sn alloy did not recover with a new second-phase associated with Co. During the aging treatment process, the addition of Co reduced the coarsening kinetics of the modulated structure, which was related to the equilibrium solid solubility (Xe) of Ni and Sn in Cu, diffusion coefficient of Ni and Sn in Cu (D), and to the interfacial energy (σ) of α-Cu matrix/L12. The decrease in values of Xe, D and σ were resulted in a slowdown of the coarsening kinetics. Further, the suitable addition of Co significantly inhibited the nucleation and growth of discontinuous precipitation (DP). The growth kinetics of DP for Cu–15Ni–8Sn-xCo alloy were evaluated using the Johnson-Mehl-Avrami-Kolmogorov equation. With the combination of the Arrhenius equation, the activation energies (Q) of the samples were 105.45 kJ/mol, 150.76 kJ/mol, 178.44 kJ/mol, 251.53 kJ/mol, and 148.89 kJ. Compared with cobalt-free alloy, cobalt-containing alloys exhibited greater activation energy, which suggested that the addition of Co can greatly inhibit the formation of DP. Furthermore, the addition of Co led to decrease in the thickness of the grain boundary and increase in the interlamellar spacing of the DP, which were considered to be the main reasons for suppressing the growth of DP.

The effect of particle size and phosphorous content in biomineralization media on in vitro bioactivity of monticellite based ceramic powders obtained from boron derivative waste

The effect of particle size and phosphorous content in biomineralization media on in vitro bioactivity of monticellite based ceramic powders was investigated. Monticellite based ceramic powder was synthesized at 800°C for 4h using boron derivative waste. Monticellite based ceramic powder, comprising monticellite, akermanite, diopside, calcium magnesium borate and zeolite LTA crystalline phases, was crushed and then ball-milled for optimized time to obtain lowest average particle size and the narrowest particle size distribution. In vitro bioactivity of both coarse (d10: 0.5μm, d50: 3.0μm, d90: 42μm) and fine (d10: 0.5μm, d50: 1.4μm, d90: 4.8μm) wafers was determined by incubation in Lactated Ringer's Solution and Human Blood Plasma for 1, 3, 5, 7, 14, 21 and 28 days at 36.5±0.5°C. The obtained results exhibited that calcite (CaCO3) layer after immersion in Lactated Ringer's Solution and bone-like apatite layer after immersion in Human Blood Plasma were formed on the surface of coarse and fine wafers. The presence of phosphorus in biomineralization media is necessary for apatite formation. The increment of surface roughness favors homogeneous nucleation, and fasten nucleation and growth kinetics of precipitation. As a result, the bioactive characteristic of monticellite based ceramic powder could be governed by the particle size.

Thermodynamics and kinetics of discontinuous precipitation in Cu–9Ni–xSn alloy

In this work, the hardness during aging, the, morphology and the thermodynamics and growth kinetics of discontinuous precipitation (DP) in Cu–9Ni–xSn alloy were investigated by mechanical testing, metallographic optical microscopy, scanning electron microscopy, and transmission electron microscopy. The results show that Cu–9Ni–xSn alloy overages faster with increasing Sn content and aging temperature. Also, thermodynamic analysis shows that the γ-DO3 phase precipitates more easily with increasing Sn content. The growth kinetics of DP were described by using the Johnson–Mehl–Avrami–Kolmogorov (JMAK) equation. The diffusion activation energy (Q) values of DP in Cu–9Ni–xSn (x = 3,4,6) alloy are calculated to be 45.5 kJ/mol, 54.6 kJ/mol, and 97.7 kJ/mol, respectively, indicating that the DP reaction was controlled by grain boundary diffusion. The thermodynamics and growth kinetics analyses of DP support the concept that the diffusion of Cu–9Ni–xSn alloy becomes easier with increasing Sn content. Moreover, the results show that the aging temperature and Sn content greatly affect the Interlamellar spacing of DP in Cu–9Ni–xSn alloy.

Study of precipitation and growth processes of Ti-bearing inclusions in tire cord steel

Precipitation behavior and growth kinetics of Ti-bearing inclusions in SWRH 92A tire cord steel were analyzed in the present work. The calculated results showed that higher cooling rate would lead to a higher precipitation ratio of solution elements and make the precipitation time of inclusions earlier. However, both TiCyN1−y (a continuous solid solution between TiC and TiN, y denotes the molar ratio of TiC in TiCyN1−y) and TiN could only precipitate in the two-phase region at the very late stage during the solidification processes, with the solid fraction exceeds 0.99. The precipitation time of TiCyN1−y was a little earlier than that of TiN as well as the value of y in the range of 0.1577–0.1617 (cooling rate: 0.5–2 K/s) was also obtained. Particle size of Ti-bearing inclusions decreased with the increasing cooling rate and were within the range of 1.5–6.5 μm under the calculated conditions, which accorded well with the size observed by the FE-SEM (field emission scanning electronic microscope). Increasing the contents of both N and Ti could lead to an earlier precipitation time of Ti-bearing inclusions and make the particle size bigger, while the effect of N is much larger than that of Ti. The results obtained by the adopted model were in well consistent with those of the statistical analysis. This work may give an important guidance on the understanding of the precipitation and growth processes of Ti-bearing inclusions in tire cord steel.

Suppression of discontinuous precipitation in age-hardening Cu-15Ni-8Sn alloy by addition of V

In the present study, the morphology and growth kinetics of discontinuous precipitation (DP) in Cu-15Ni-8Sn-xV alloys were investigated by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The results indicate that the DP colony presents a lamellar structure consisting of alternating lamellae of α-Cu matrix and γ-DO3 phase ((CuxNi1-x)3Sn). In addition, the growth kinetics of DP in Cu-15Ni-8Sn alloy was evaluated using the Johnson–Mehl–Avrami–Kolmogorov (JMAK) equation and the time exponent can be ascertained between n = 2 and n = 1. It also demonstrates that the initiation of DP in Cu-15Ni-8Sn alloy occurs at grain edges and boundaries. Moreover, the activation energy (Q) of DP is determined to be about 75 kJ/mol by using the Arrhenius equation, thus DP reaction is believed to be controlled by grain boundary diffusion. Interestingly, the addition of V has an obvious effect on the suppression of DP in Cu-15Ni-8Sn alloy. It can be determined that the activation energy (Q) of DP in Cu-15Ni-8Sn-0.2V alloy is about 350 kJ/mol, which is much larger than that of in Cu-15Ni-8Sn alloy. The increasing activation energy of DP after adding V can certainly suppress the formation of DP greatly. Additionally, it is found that a larger number of Ni3V particles generate accompanied by addition of V in Cu-15Ni-8Sn-0.4V alloy. Therefore, the considerable precipitated Ni3V particles impose a strong pinning effect on grain boundary, which also should be responsible for the resultant suppressing effect. Besides, the interlamellar spacing of DP increases and the growth rate of lamellae decreases with increasing V content when aging at 673 K.

Growth and coarsening kinetics of gamma prime precipitates in CMSX-4 under simulated additive manufacturing conditions

Additive manufacturing of superalloys offers new opportunities for alloy design but also poses significant processing difficulties. While the γ′ phase is responsible for the excellent high-temperature resistance of these alloys, it also induces cracking by precipitation hardening during manufacturing. Using small-angle X-ray scattering, we characterized the dynamic precipitation, dissolution, coarsening, and morphological evolution of the γ′ phase in situ during simulated additive manufacturing conditions. For this purpose, a CMSX-4 cylinder was subjected to cyclic heat treatment with heating and quenching rates up to 300 K/s. A specialized setup employing aluminum lenses to focus the X-ray beam was utilized to extend the q-range to small scattering vectors up to 0.035 nm−1. It was shown that the γ′ phase precipitates extremely fast without any measurable undercooling but remains below the equilibrium fraction throughout the process. Coarsening is readily measurable over timespans of only several seconds. A fraction of the γ′ phase that was dissolved during heating reprecipitated by forming new particles instead of growing on already existing precipitates. The findings provide new insight into the dynamic behavior of the γ′ phase during additive manufacturing and may prove valuable in designing new superalloys and processing strategies for additive manufacturing.

Study on Precipitation and Growth of TiN in GCr15 Bearing Steel during Solidification.

In this paper, the precipitation thermodynamics and growth kinetics of TiN inclusions in GCr15 bearing steel during solidification were calculated in more detail. A more reasonable formula for calculating the segregation of the solute elements was adopted and the stability diagram of TiN precipitation considering solidification segregation was given. By solving equations, the change of the solute element content before and after TiN inclusion precipitation was calculated, and the results were substituted into the kinetic formula of the inclusion growth, which made the kinetic calculation more accurate. Results showed that the most effective way to reduce the precipitation of TiN is to increase the cooling rate and decrease the contents of Ti and N in steel. The effect of Ti content on the size of TiN inclusions is greater than that of N content.

Analysis of β´(Cu4Ti) Precipitation During Isothermal Aging of a Cu-4wt.%Ti Alloy

This work analyzed experimentally and numerically the growth kinetics of β′ precipitation of a Cu–4 wt%Ti alloy after aging at 400, 500, and 600 °C for times from 0.0166 to 200 h. Results indicated that the precipitation process is almost controlled by nucleation and growth during aging at 400 °C, originating a slow growth kinetics of precipitation. In contrast, the coarsening of precipitates dominates the precipitation process during aging at 500 and 600 °C. The interfacial energy of interface between the α matrix phase and β′ precipitates was determined to be about 0.1135, 0.0980, and 0.0725 Jm−2 for aging at 400, 500, and 600 °C, respectively. These values suggest a coherent interface, which is in agreement with the flat faces of β′ cuboid precipitates. Calculated Time–Temperature–Precipitation diagram for the β′ precipitation indicated good agreement with experimental results. Precipitation hardening was higher for the slower growth kinetics of precipitation.

Precipitation analysis of as-cast HK40 steel after isothermal aging

As-cast HK40 steel was aged at 700, 800, or 900°C for times as long as 2000 h. Microstructural characterization showed that the primary M7C3 carbide network contained a substantial content of manganese, in agreement with the microsegregation of manganese calculated by Thermo-Calc using the Scheil–Gulliver module. The dissolution of primary carbides caused the solute supersaturation of austenite and subsequent precipitation of fine M23C6 carbides in the austenite matrix for aged specimens. During prolonged aging, the carbide size increased with increasing time because of the coarsening process. A time–temperature–precipitation diagram for M23C6 carbides was calculated using the Thermo-Calc PRISMA software; this diagram showed good agreement with the experimental growth kinetics of precipitation. The fine carbide precipitation caused an increase in hardness; however, the coarsening process of carbides promoted a decrease in hardness. Nanoindentation tests of the austenite matrix indicated an increase in ductility with increasing aging time.