Peak-aged Condition Research Articles

Impact of retained austenite on the aging response of additively manufactured 17-4 PH grade stainless steel

Additively manufactured precipitation hardened (PH) martensitic grade stainless steels display wide variations in their heat treatment response and mechanical properties. Much of this variability can be attributed to the selection of the atomizing gas and the resulting variations in powder composition. Differences in nitrogen compositions present in argon and nitrogen atomized powders led to the formation of a wide range of retained austenite levels between 0 and 81 vol% and significant differences in mechanical properties in as-deposited and aged 17-4 PH grade stainless steels. Material fabricated using argon atomized powder contained little to no retained austenite and displayed mechanical properties in the peak-aged condition similar to those in wrought alloys (1359 ± 59 MPa ultimate tensile strength and 10.1 ± 0.8% elongation to failure). On the other hand, materials fabricated with nitrogen atomized powder displayed between 20% and 81% retained austenite, leading to discontinuous yielding driven by the strain-induced transformation of the retained austenite to martensite. Even with the presence of this discontinuous yielding, the materials fabricated using the nitrogen atomized powders consistently displayed strengths 300 MPa higher in the as-deposited, 200 MPa higher in the peak-aged, and 500 MPa higher in the over-aged conditions than materials fabricated using the argon atomized powders. By directly comparing the local strains and microstructures in nitrogen atomized materials, a [011]γ//[001]α relationship in line with the Bain transformation process was present between the retained austenite and transformed martensite across the as-deposited and aged conditions.

Nanoprecipitation behavior and resultant mechanical and magnetic properties in Fe–Si–Ni–Al–Mn high strength non-oriented silicon steel

Low mismatch and low energy interface between precipitates and ferrite matrix was adopted to promote the precipitation of high density of uniformly dispersed coherent NiAl phase in the silicon steel, aiming to improve strength to meet the requirement of drive motor. The precipitation mechanism of NiAl and its influence on magnetic and mechanical properties of silicon steel were studied. The theoretical lattice mismatch between precipitation and matrix was 0.72%, and a new high strength non-oriented silicon steel reinforced by B2 structure NiAl precipitation with a volume content of 1.73% and an average size of 15 nm was prepared. The uniformity of precipitation size and minimization of interface energy remarkably reduced the driving force for NiAl precipitation and hindered the Ostwald ripening. The texture of annealed silicon steel was characterized by strong α* fiber with a peak at {411}<148> and a moderate γ fiber texture with a peak at {111}<110>. With increase in aging time (from 0 h to 16 h), the strength of silicon steel increased to peak values then decreased and the magnetic properties decreased slightly. The nano-scale NiAl precipitation had little effect on coercivity and hysteresis loss of silicon steel while improving the strength of silicon steel, P10/400 and P10/1000, of silicon steel was as low as 21.7 W/kg and 93.3 W/kg, respectively in the peak-aged condition (at 550 °C for 4 h), and the yield strength was 802 MPa. Thus, it is underscored that using NiAl precipitation as strengthening phase improves the mechanical properties of silicon steel without excessively deteriorating the magnetic properties.

Synergistic strengthening by nano-sized α-Al(Mn,Fe)Si and Al3Zr dispersoids in a heat-resistant Al–Mn–Fe–Si–Zr alloy

Strengthening by dispersoids in Al alloys has been limited by either low number density or low volume fraction of dispersoids. In the present work, the influence of an addition of 0.28 wt% Zr on precipitation hardening behavior of AA3003 alloy subjected to different heat treatments has been investigated. A superior microstructure simultaneously consisting of nano-sized α-Al(Mn,Fe)Si and Al3Zr dispersoids at peak-aged state was achieved by two different heat treatment regimes, ramp heating with a speed of 50 °C/h and isothermal aging at 400 °C. As a result, a substantial increase in yield strength of 30 MPa (43%) was achieved in comparison to AA3003 alloy. Meanwhile, the AA3003-Zr alloy exhibits excellent heat-resistance with a stable yield strength from 12h (106 MPa) up to 250h (107 MPa) during isothermal aging at 400 °C. This remarkable thermal stability was ascribed to the continuously hardening from Al3Zr precipitation, which compensates the strength loss from α-dispersoid coarsening. Besides, TEM study shows that the precipitation kinetics of Al3Zr is significantly enhanced compared with the reference binary Al–Zr alloy, which is attributed to the Si content in the alloy. This work proposes an effective method to design a low-cost heat-resistant Al alloy in mass production.

Effects of microstructure on tensile properties of AA2050-T84 Al-Li alloy

The effect of microstructure evolution on the tensile properties of 2050 Al−Li alloy thick plate aged at 150 °C with 80 mm in thickness (t) was studied from a microstructural perspective. Scanning electron microscope, optical microscope, transmission electron microscope and X-ray diffractometer were used to explore the surface (t/6), interlayer (t/3) and center (t/2) thickness layer of this alloy. Results show that the secondary phases on grain boundaries, precipitates and textures vary depending on the thickness location. The precipitation strengthening has a stronger influence on the alloy along the rolling direction than the transverse direction from the under-aged to the peak-aging condition; however, its effect on the anisotropy is insignificant. The higher Taylor factor (M) value caused by stronger β fiber rolling textures and the intergranular phases is the main reason that leads to the highest strength at the t/2 position along the rolling direction. The M-value has a limited change at different thickness layers along the transverse direction, which causes the same tensile strength.

Effects of Aging on the Microstructure and Phase Transformation Behavior of Cu-Al-Mn Shape Memory Alloy

In the present work, the effects of artificial aging treatment on the transformation temperatures and hardness of Cu-Al-Mn shape memory alloy have been investigated. The aging processes have been performed on the one-time re-melted and 90% rolled samples. Differential scanning calorimetry reveals that reverse transformation is present for the re-melted sample which is aged at 400°C. However, in 90% rolled condition, this transformation takes place at 200°C and 300°C. Hardness examination shows that the aged specimens possess higher values in hardness in comparison to un-aged samples at all studied temperatures. Although, the peak-aged condition was demonstrated at 300°C for the re-melted sample, the rolled sample displayed increased hardness levels up to 500°C. Based on the DSC measurements and microstructural observations, it can be asserted that the thermo-mechanical processing including rolling plus aging at 300°C provides favorable transformation characteristics for shape memory behavior.

Effect of tempering duration on hydrogen embrittlement of vanadium-added tempered martensitic steel

This paper presents a quantification of the effect of tempering duration tTEM on hydrogen embrittlement (HE) resistance of vanadium-added tempered martensitic steel. High-carbon steel containing 0.3 wt% vanadium was tempered at 570 °C for durations 1 ≤ tTEM ≤ 16 h. With increasing tTEM, hardness and strength were well maintained up to tTEM = 4 h, then decreased afterwards at tTEM > 4 h. The notch-fracture strength of hydrogen charged specimen, a measure of HE resistance, followed the same trend as those of strength and hardness. Thermal desorption analysis indicated that tTEM = 4 h resulted in the maximum hydrogen-trapping capability, accompanied by a slight increase in the peak temperature. A hydrogen permeation test also revealed that hydrogen diffusivity was the lowest at tTEM = 4 h. Our results indicate that HE resistance of tempered martensitic steel is greatly affected by the characteristics of V-carbide such as the degree of precipitation-hardening, and that the HE resistance is greatest at the peak aging condition.

Effect of Ge and Mg additions on the aging response behavior and mechanical properties of Al-Si-Cu alloy

The influence of Ge and Mg additions on the aging response behavior, microstructural evolution and mechanical properties of Al-Si-Cu alloy was investigated. The addition of Ge significantly accelerates the aging response of Al-Si-Cu alloy with shorter aging time to reach peak-hardness. The enhanced aging response is caused by the formation of Si-Ge particles serving as heterogeneous nucleation sites for θ′-Al2Cu phases due to their high interfacial energy. The strength of Al-Si-Cu alloy slightly increases because of the accelerated precipitation of θ′ phases. The addition of Mg results in the early precipitation of Q′-Al5Cu2Mg8Si6 phases in Al-Si-Cu alloy due to the strong interaction between Cu and Mg, and both θ′ phases and Q′ phases dominate in the peak-aging state, which leads to the ultimate tensile strength of 396.3 MPa. The addition of both Ge and Mg leads to the formation of Mg2Ge phases which are undissolving in solution treatment and the quick peak-aging time (20 h) is also presented in Al-Si-Cu-Mg-Ge alloy due to the effect of Ge. Besides, the additional strengthening contribution of Q′ phases leads to a high peak-aging yield strength (300.3 MPa) and ultimate tensile strength (342.1 MPa). The results indicate that adding both Mg and Ge to Al-Si-Cu alloy has a synergetic advantage to introduce the strengthening effect and obtain the accelerated aging response.

Precipitation behavior of an Al–Cu–Li–X alloy and competing relationships among precipitates at different aging temperatures

Abstract The precipitation behavior of strengthening phases has a significant effect on the properties of Al–Cu–Li–X (X = Mg, Ag, Zr) alloys during aging treatment. In this work, the precipitation behavior and competing relationships among precipitates were studied, and the relationship between the microstructure and mechanical properties was analyzed in detail at four aging temperatures for an Al–Cu–Li–X alloy (AA2195). Scanning electron microscopy (SEM), transmission electron microscopy (TEM), and differential scanning calorimetry (DSC) were used to systematically explore the precipitation characteristics of the alloy. Then, the precipitation sequence and the interrelationship among the precipitates were determined in the under-aged and peak-aged states at various temperatures. The results show that the alloy exhibits different precipitation characteristics in various aging conditions, particularly for the under-aged state. Furthermore, the T1 phase (subject to aging temperature) preferentially multiplies to dominate the microstructure, governs the formation of other precipitates, and eventually gives rise to the multi-precipitate coexistence of the peak-aged state. Therefore, the influence of temperature on the precipitation behavior is realized by its effect on nucleation of the T1 phase. Moreover, the increase in the strength of the alloy arises from the comprehensive strengthening effect of all precipitates, in which the T1 phase plays a decisive role.

High cycle fatigue behavior and mechanical performance of a novel sand-cast Mg-Nd-Gd alloy: Effect of heat treatment

Abstract This work was primarily aimed at evaluating the effect of heat treatment on the microstructure, high cycle fatigue behavior and mechanical properties of a novel sand-cast Mg-3Nd-2.6Gd-0.2Zn-0.5Zr alloy. Results indicated that the as-cast studied alloy consisted of non-dendritic α-Mg grains and considerable eutectic phases distributed along grain boundaries, most of which could been successfully dissolved into matrix accompanied by the appearance of rod-shaped Zn–Zr phases after subjected to the optimized solution treatment (535 °C × 10 h). Examinations of microstructure revealed a finely dense dispersion of predominant β′′ and minor β′ phases in the peak-aged studied alloy (isothermally aging at 200 °C for 14 h). Tensile properties were significantly improved, and the relatively best combination of strength and ductility was obtained in peak-aged condition (YS = 220 MPa, UTS = 303 MPa and EL = 4.1%). Heat treatment conditions were found to exert a pronounced influence on the high cycle fatigue properties, and the highest fatigue strength σf (approximately 121.5 MPa), was achieved in peak-aged studied alloy, which is about 24% higher than that of commercial NZ30K-T6 alloy. Owing to the specifically significant response to heat treatments, the fatigue strength of the studied alloy was confirmed to increase in a near linear relationship with the increasing yield strength and ultimate tensile strength. This is also well consistent with the characteristic fatigue life predicted by Weibull statistics in different heat treatment conditions.

Evolution of dislocation and stacking-fault densities for a Cu-0.7Cr-0.07Zr alloy during cryogenic tensile test: An in-situ synchrotron X-ray diffraction analysis

Processing at cryogenic temperatures is one of the ways of improving mechanical properties of copper alloys. However, such processing routes may result in unstable microstructures, which make it difficult to understand the deformation mechanisms controlling microstructure evolution during cryogenic deformation. In this study, the microstructure evolution of a Cu-0.7Cr-0.07Zr alloy was analyzed as a function of the flow behavior during tensile tests performed at 123 K and 298 K, through in-situ X-ray diffraction (XRD) experiments in a synchrotron source. The tensile behavior was analyzed in terms of the dynamic recovery rate using the Kocks-Mecking model, while the evolution of stacking-fault density, dislocation density and crystallite size during the tests was estimated by the modified Williamson-Hall method. Solution-treated and peak-aged conditions were tested to assess the influence of particles distribution. At 298 K, the strain-hardening rate was controlled by dislocation glide and the flow stress was affected mainly by particle strengthening. At 123 K, a simultaneous increase in strength and ductility was attributed to the twinning induced plasticity (TWIP) effect, promoted by precipitate interfaces acting as nuclei for stacking-faults, thus delaying necking and increasing the plasticity. Based on the evolution of dislocation and stacking-fault densities with increasing plastic strain, it was concluded that, at 123 K, mechanical twinning predominated at strain levels below 0.15, mainly for aged conditions, while dislocation glide between twin boundaries, and its accumulation, started after TWIP mechanism reached its saturation. • In-situ synchrotron X-ray diffraction of Cu-0.7–0.07Zr has been performed. • Solutionized and peak-aged conditions were evaluated at 298 K and 123 K. • Both temperature and precipitates affected the plastic strain regime. • At 123 K, mechanical twinning predominated at strains below 0.15. • Twinning induced plasticity (TWIP) effect occurred during plastic flow at 123 K.

Fatigue Behavior of 18% Ni Maraging Steels: A Review

18% Ni maraging steels are based on the Fe-Ni-Co-Mo system with low-carbon content. They display an excellent combination of high strength and high toughness. However, they suffer from a low fatigue ratio, the ratio decreasing monotonically with increasing strength. Considering prospective applications for these steels involving fatigue loading, attempts were made by researchers to improve their fatigue life. The studies suggest that fatigue strengths higher than those realized in peak aged condition can be obtained through controlled overaging with a small amount of reversed austenite playing a critical role. Corrosion fatigue in different environments is a serious problem with these steels. Double aging seems to reduce the susceptibility to corrosion fatigue under high humidity conditions. Inclusion content has a strong influence on the fatigue life, inclusion size and type playing an important role. Surface treatments such as shot peening, laser peening, and nitriding were found to improve the fatigue life; however, it is important to optimize the process parameters. This paper attempts a critical review of studies reported in the published literature aiming to improve the fatigue life of 18% Ni maraging steels.

Improved stress corrosion cracking resistance of in-situ TiB2/7050Al composite by pre-precipitation treatment

A two-step aging treatment (50 L P, a peak aging following 50 ℃ pre-precipitation) has been investigated for the in-situ TiB2/7050Al composite. The 50 L P composite has the comparable mechanical properties to the composite at peak-aged (T6) state, and even better stress corrosion cracking resistance over the composite with the retrogression and re-aging (RRA) treatment. In detail, the different aging conditions lead to different precipitate morphologies and grain boundary microchemistries. According to the microstructure characteristics in the Al matrix, the 50 L P composite has considerably increased grain boundary precipitate interspace in comparison with the T6 composite, since the lower aging temperature should result in the reduced grain boundary precipitate number. Furthermore, the 50 L P composite has more Cu content in the grain boundary precipitate and reduced precipitate free zone width over the RRA composite, indicating the improved stress corrosion cracking resistance. For the reinforcement, the TiB2 particles should slightly aggravate the stress corrosion cracking susceptibility, since the grain boundary precipitates are still the preferential corrosion sites due to their lower corrosion potentials.

Effect of ageing treatment on microstructures, mechanical properties and corrosion behavior of Mg-Zn-RE-Zr alloy micro-alloyed with Ca and Sr

Effects of ageing treatment on the microstructures, mechanical properties and corrosion behavior of the Mg-4.2Zn-1.7RE-0.8Zr-xCa-ySr [x=0, 0.2 (wt.%), y=0, 0.1, 0.2, 0.4 (wt.%)] alloys were investigated. Results showed that Ca or/and Sr additions promoted the precipitation hardening behavior of Mg-4.2Zn-1.7RE-0.8Zr alloy and shortened the time to reaching peak hardness from 13 h to 12 h. The maximum hardness of 77.1±0.6 HV for the peak-aged Mg-4.2Zn-1.7RE-0.8Zr-0.2Ca-0.2Sr alloy was obtained. The microstructures of peak-aged alloys mainly consist of α-Mg phase, Mg51Zn20 phase and ternary T-phase. The Zn-Zr phase is formed within the α-Mg matrix, and the Mg2Ca phase is formed near T-phase due to the enrichment of Ca in front of the solid-liquid interface. Furthermore, fine short rod-shaped β′1 phase is precipitated within the α-Mg matrix in the peak-aged condition. The peak-aged Mg-4.2Zn-1.7RE-0.8Zr-0.2Ca-0.2Sr alloy exhibits optimal mechanical properties with an ultimate tensile strength of 208 MPa, yield strength of 150 MPa and elongation of 3.5%, which is mainly attributed to precipitation strengthening. In addition, corrosion properties of experimental alloys in the 3.5wt.% NaCl solution were studied by the electrochemical tests, weight loss, hydrogen evolution measurement and corrosion morphology observation. The results suggest that peak-aged alloys show reduced corrosion rates compared with the as-cast alloys, and minor additions of Ca and/or Sr improve the corrosion resistance of the Mg-4.2Zn-1.7RE-0.8Zr alloy. The peak-aged Mg-4.2Zn-1.7RE-0.8Zr-0.2Ca-0.2Sr alloy possesses the best corrosion resistance, which is mainly due to the continuous and compact barrier wall constructed by the homogeneous and continuous second phases.

Precipitation reactions in 12Cr–3Ni–3Mn–3Cu–0.15Nb steel

Abstract In the current study, a new precipitation hardenable stainless steel was designed with primarily Ni and Mn modification using Thermo-Calc. The solution-annealing heat treatment at 1035 °C for 60 min followed by air quenching resulted in a fully martensitic microstructure. The as-quenched steel responded to the aging heat treatment, resulting in a significant enhancement of hardness and mechanical strength. The aging heat treatment at 500 °C for 120 min provided a good combination of tensile properties. The change in mechanical properties was quite conducive to the aging parameters (i.e., temperature and time) between 400 °C and 600 °C. Transmission electron microscopy and atom probe tomography studies demonstrated that the evolution of Cu-precipitate was the prime reason for the change in hardness and mechanical properties of as-quenched steel. As a result, the newly developed PH-1 steel was comparable to a commercial Cu-precipitate strengthened 17-4 PH steel in terms of mechanical properties. The Cu-precipitate evolved with a concurrent increase of Cu and decrease of Fe contents. APT studies revealed the interfacial segregation of Ni and Mn at the matrix and the Cu-rich precipitate in a peak-aged condition, which evolved as Ni–Mn rich reverse austenite upon over-aging, accompanied by high Mn:Si ratio. Cu precipitate only acted as a heterogeneous nucleation site for reverse austenite formation as there was no Cu atoms partition.

Modeling peak-aged precipitate strengthening in Al–Mg–Si alloys

Strengthening by needle-shaped β′′ precipitates is critical in Al–Mg–Si alloys. Here, the strengthening is studied computationally at the peak-aged condition where precipitate shearing and Orowan looping are usually considered to have equal strengths. Pseudo-random precipitate microstructures are constructed based on experimental precipitate dimensions and volume fractions at peak aging. A Discrete Dislocation Dynamics method is then adapted to compute the Critical Resolved Shear Stress (CRSS) for Orowan looping of dislocations moving through the non-shearable precipitate field. The CRSS for Orowan looping is determined by a typical in-situ precipitate spacing that is smaller than the average spacing and by the dislocation core energy within a radius of ≈5b, a factor rarely considered. The matrix misfit stresses, volume fraction, and precipitate shape have small effects on the CRSS. With microstructure and property details introduced as faithfully as possible, the CRSS for Orowan looping using atomistically-calibrated core energies at room temperature is nonetheless ≈33% higher than experiments. This suggests that precipitate shearing controls strength, and analyses of (i) forces acting on the precipitates, (ii) misfit stresses inside the precipitates, (iii) first-principles results for the relevant precipitate fault energies, and (iv) simulations that mimic precipitate shearing indicate a shearing CRSS closer to experiments. Thus, Orowan looping only sets an upper bound for the CRSS even at peak aging, and further quantitative progress requires detailed modeling of precipitate shearing.

Aging behavior and precipitate characterization of Zn-rich Al–Zn–Mg–Cu alloys with various Mg and Cu contents

Four Zn-rich Al–Zn–Mg–Cu alloys with different Mg and Cu contents were fabricated by spray deposition. The effects of Mg and Cu contents on the aging behaviors of the whole precipitation process of the alloys were systematically investigated. The results show that the primary precipitates in the under-aged and peak-aged alloys are GPII and η′ phases; no GPI zones were observed. During aging, the dissolution driving force of the precipitates increases with increasing Mg content; therefore, the volume fraction of precipitates in the grain interior and the area fraction of precipitates at the grain boundary increase obviously, which contributes to a considerable improvement in yield strength and decrease in plasticity of the high Mg content alloys. Cu content has no apparent effect on the size and volume fraction of precipitates. However, a higher Cu content can effectively prevent coarsening and transformation of precipitates, which is beneficial to maintain the peak aging state of the alloys. This research provides a basis for the composition optimization of the rapid-solidified highly alloying Al–Zn–Mg–Cu alloys.

Phase Field Simulation of AA6XXX Aluminium Alloys Heat Treatment

Heat treatment has a significant impact on the microstructure and the mechanical properties of Al-Mg-Si alloys. The present study presents a first Phase-Field modelling approach on the recrystallisation and grain growth mechanism during annealing. It focuses on the precipitate fraction, radius, and Mg-Si concentration in the matrix phase, which are used as input data for the calculation of the yield strength and hardness at the end of different ageing treatments. Annealing and artificial ageing simulations have been conducted on the MultiPhase-Field based MICRESS@ software, while the ThermoCalc@ software has been used to construct the pseudo-binary Al-Mg phase-diagrams and the atomic-mobility databases of MgxSiy precipitates. Recrystallisation simulation estimates the recrystallisation kinetics, the grain growth, and the interface mobility with the presence/absence of secondary particles, selecting as annealing temperature 400 °C and a microstructure previously subjected to cold rolling. The pinning force of secondary particles decelerates the overall recrystallisation time, causing a slight decrease in the final grain radius due to the reduction of interface mobility. The ageing simulation examines different ageing temperatures (180 and 200 °C) for two distinct ternary systems (Al-0.9Mg-0.6Si/Al-1.0Mg-1.1Si wt.%) considering the interface energy and the chemical free energy as the driving force for precipitation. The combination of Phase-Field and the Deschamps–Brechet model predicted the under-ageing condition for the 180 °C ageing treatment and the peak-ageing condition for the 200 °C ageing treatment.

Influence of Retrogression and Reaging Treatment on Intergranular Corrosion Resistance and Exfoliation Corrosion Resistance in AA7010 Aluminum Alloy

Abstract The aluminum alloys with zinc as the primary alloying element (AA7XXX series) exhibit high strength and are widely used as aircraft structural materials. These categories of aluminum alloys in their peak aging condition (T6) are known to be highly susceptible to general as well as stress corrosion cracking (SCC). Retrogression and reaging (RRA) treatment of this category of alloys leads to improvement in the corrosion resistance without significant compromise in strength. SCC was found to be improving in general in these alloys, but the intergranular corrosion (IGC) and exfoliation corrosion (EXCO) need to be evaluated. The effect of RRA heat treatment on the IGC and exfoliation of AA7010 aluminum alloy has been investigated and compared with T6 and T73 temper conditions of the alloy. The results indicate that the corrosion susceptibility of AA7010 aluminum alloy decreased by subjecting the alloy to multistage (RRA) heat treatment. It is attributed to the grain boundary precipitate coarsening and is also the major factor responsible for reducing the susceptibility to IGC and EXCO.

Microstructure and mechanical properties of high-strength high-pressure die-cast Mg–4Al–3La–1Ca–0.3Mn alloy

A new high-pressure die-cast (HPDC) Mg–4Al–3La–1Ca–0.3Mn (ALaX431) alloy with high strength has successfully been fabricated. This HPDC alloy in peak-aged state exhibits tensile yield strength (TYS) of 220 MPa at room temperature and TYS of 145 MPa at 250 °C, higher than the corresponding strength of HPDC Mg alloys reported so far. These high strengths are mainly due to the formation of fine grained structure, semi-continuous reticular structure consisting of stable Al3La and (Mg,Al)2Ca particles along grain boundaries and numerous nanoscale Al2Ca precipitates within grains. Due to its higher strength than existing HPDC Mg alloys, the new developed alloy has great application potential.

Precipitation behavior and age hardening effect of the precipitates in a Fe-13Cr-4Al-2Mo-1.2Nb alloy

Precipitation and growth behavior of Fe2Nb-type Laves precipitates are crucial for the mechanical behavior of FeCrAl-based cladding alloys containing Mo and Nb. In this study, the age hardening response of a Fe-13Cr-4Al-2Mo-1.2Nb alloy at 600–800 °C and the corresponding precipitation behavior were systematically investigated. The alloy exhibited a typical two-stage hardening behavior separated by a plateau during aging at all the temperatures. For the alloy aged at 700 °C, the total hardness increment by aging reached ~102.7 HV (40%), with an aging time of 60 min to reach the peak-aged state. Based on detailed transmission electron microscopy (TEM) investigations, the precipitation sequence of the alloy was elucidated as: Supersaturated solid solution → γ″ → γ' → C14-Fe2(Nb,Mo) Laves phase. As the aging proceeded, the Fe2(Nb,Mo) precipitates also evolved from a needle-like morphology with a semi-coherent interfacial structure into a short-rod-like and finally a blocky morphology with a non-coherent interfacial structure. The JMA equation describing the aging kinetics of the alloy was established. The apparent activation energy for the precipitation of the Fe2(Nb,Mo) phase was calculated as 141.5 KJ/mol.