Diffusion Bonding of 17-4 Precipitation Hardening Stainless Steel to Ti Alloy With and Without Ni Alloy Interlayer: Interface Microstructure and Mechanical Properties

In the present study, the diffusion bonding of 17-4 precipitation hardening stainless steel to Ti alloy with and without nickel alloy as intermediate material was carried out in the temperature range of 1073 K to 1223 K (800 °C to 950 °C) in steps of 298 K (25 °C) for 60 minutes in vacuum. The effects of bonding temperature on interfaces microstructures of bonded joint were analyzed by light optical and scanning electron microscopy. In the case of directly bonded stainless steel and titanium alloy, the layerwise α-Fe + χ, χ, FeTi + λ, FeTi + β-Ti phase, and phase mixture were observed at the bond interface. However, when nickel alloy was used as an interlayer, the interfaces indicate that Ni₃Ti, NiTi, and NiTi₂ are formed at the nickel alloy-titanium alloy interface and the PHSS-nickel alloy interface is free from intermetallics up to 1148 K (875 °C) and above this temperature, intermetallics were formed. The irregular-shaped particles of Fe₅Cr₃₅Ni₄₀Ti₁₅ have been observed within the Ni₃Ti intermetallic layer. The joint tensile and shear strength were measured; a maximum tensile strength of ~477 MPa and shear strength of ~356.9 MPa along with ~4.2 pct elongation were obtained for the direct bonded joint when processed at 1173 K (900 °C). However, when nickel base alloy was used as an interlayer in the same materials at the bonding temperature of 1148 K (875 °C), the bond tensile and shear strengths increase to ~523.6 and ~389.6 MPa, respectively, along with 6.2 pct elongation.     

Effect of Bonding Temperature on Microstructure and Mechanical Properties of WC–Co/Steel Diffusion Brazed Joint

In this work, the diffusion brazing of AISI 4145 steel to WC–Co cemented carbide using RBCuZn-D interlayer with bonding temperature values of 930, 960, 990 and 1020 °C was studied. The microstructure of the joint zone was evaluated by scanning electron microscope (SEM) and X-ray diffraction (XRD). Vickers microhardness and shear strength tests were performed to investigate mechanical behaviors of the brazed joints. The XRD and SEM results indicated that with increase of bonding temperature, the elements readily diffused along the interface and formed various compounds such as γ, α and β and Co₃W₃C. The results also showed that with the increase of bonding temperature from 930 to 960 °C, a sound metallurgical bond was produced, however in higher bonding temperatures (990 and 1020 °C) a decrease in mechanical properties of the joints was observed which could be due to the excessive zinc evaporation, interface heterogeneity and voids formation. The maximum shear strength of 425 MPa was obtained for the bond made at 960 °C.     

Effect of Bonding Temperature on Interfacial Reaction and Mechanical Properties of Diffusion-Bonded Joint Between Ti-6Al-4V and 304 Stainless Steel Using Nickel as an Intermediate Material

An investigation was carried out on the solid-state diffusion bonding between Ti-6Al-4V (TiA) and 304 stainless steel (SS) using pure nickel (Ni) of 200-μm thickness as an intermediate material prepared in vacuum in the temperature range from 973 K to 1073 K (700 °C to 800 °C) in steps of 298 K (25 °C) using uniaxial compressive pressure of 3 MPa and 60 minutes as bonding time. Scanning electron microscopy images, in backscattered electron mode, had revealed existence of layerwise Ti-Ni-based intermetallics such as either Ni₃Ti or both Ni₃Ti and NiTi at titanium alloy-nickel (TiA/Ni) interface, whereas nickel-stainless steel (Ni/SS) diffusion zone was free from intermetallic phases for all joints processed. Chemical composition of the reaction layers was determined in atomic percentage by energy dispersive spectroscopy and confirmed by X-ray diffraction study. Room-temperature properties of the bonded joints were characterized using microhardness evaluation and tensile testing. The maximum hardness value of ~800 HV was observed at TiA/Ni interface for the bond processed at 1073 K (800 °C). The hardness value at Ni/SS interface for all the bonds was found to be ~330 HV. Maximum tensile strength of ~206 MPa along with ~2.9 pct ductility was obtained for the joint processed at 1023 K (750 °C). It was observed from the activation study that the diffusion rate at TiA/Ni interface is lesser than that at the Ni/SS interface. From microhardness profile, fractured surfaces and fracture path, it was demonstrated that failure of the joints was initiated and propagated apparently at the TiA/Ni interface near Ni₃Ti intermetallic phase.     

Interfacial microstructure and mechanical properties of diffusion-bonded joints of titanium TC4 (Ti-6Al-4V) and Kovar (Fe-29Ni-17Co) alloys

Diffusion bonding is a near net shape forming process that can join dissimilar materials through atomic diffusion under a high pressure at a high temperature.Titanium alloy TC4(Ti-6 Al-4 V)and 4 J29 Kovar alloy(Fe-29 Ni-17 Co)were diffusely bonded by a vacuum hot-press sintering process in the temperature range of 700-850°C and bonding time of 120 min,under a pressure of 34.66 MPa.Interfacial microstructures and intermetallic compounds of the diffusion-bonded joints were characterized by optical microscopy,scanning electron microscopy,X-ray diffraction(XRD)and energy dispersive spectroscopy(EDS).The elemental diffusion across the interface was revealed by electron probe microanalysis.Mechanical properties of joints were investigated by micro Vickers hardness and tensile strength.Results of EDS and XRD indicated that(Fe,Co,Ni)-Ti,TiNi,Ti_2Ni,TiNi_2,Fe_2 Ti,Ti_(17) Mn_3 and Al_6 Ti_(19) were formed at the interface.When the bonding temperature was raised from 700 to 850°C,the voids of interface were reduced and intermetallic layers were widened.Maximum tensile strength of joints at 53.5 MPa was recorded by the sintering process at 850°C for 120 min.Fracture surface of the joint indicated brittle nature,and failure took place through interface of intermetallic compounds.Based on the mechanical properties and microstructure of the diffusion-bonded joints,diffusion mechanisms between Ti-6 Al-4 Vtitanium and Fe-29 Ni-17 Co Kovar alloys were analyzed in terms of elemental diffusion,nucleation and growth of grains,plastic deformation and formation of intermetallic compounds near the interface.     

Diffusion Bonding of Microduplex Stainless Steel and Ti Alloy with and without Interlayer: Interface Microstructure and Strength Properties

The interface microstructure and strength properties of solid state diffusion bonding of microduplex stainless steel (MDSS) to Ti alloy (TiA) with and without a Ni alloy (NiA) intermediate material were investigated at 1173 K (900 °C) for 0.9 to 5.4 ks in steps of 0.9 ks in vacuum. The effects of bonding time on the microstructure of the bonded joint have been analyzed by light optical microscopy and scanning electron microscopy in the backscattered mode. In the direct bonded joints of MDSS and TiA, the layer-wise σ phase and the λ + FeTi phase mixture were observed at the bond interface when the joint was processed for 2.7 ks and above holding times. However, when NiA was used as an intermediate material, the results indicated that TiNi₃, TiNi, and Ti₂Ni are formed at the NiA-TiA interface, and the irregular shaped particles of Fe₂₂Mo₂₀Ni₄₅Ti₁₃ have been observed within the TiNi₃ intermetallic layer. The stainless steel-NiA interface is free from intermetallics and the layer of austenitic phase was observed at the stainless steel side. A maximum tensile strength of ~520 MPa, shear strength of ~405 MPa, and impact toughness of ~18 J were obtained for the directly bonded joint when processed for 2.7 ks. However, when nickel base alloy was used as an intermediate material in the same materials, the bond tensile and shear strengths increase to ~640 and ~479 MPa, respectively, and the impact toughness to ~21 J when bonding was processed for 4.5 ks. Fracture surface observations in scanning electron microscopy using energy dispersive spectroscopy demonstrate that in MDSS-TiA joints, failure takes place through the FeTi + λ phase when bonding was processed for 2.7 ks; however, failure takes place through σ phase for the diffusion joints processed for 3.6 ks and above processing times. However, in MDSS-NiA-TiA joints, the fracture takes place through NiTi₂ layer at the NiA-TiA interface for all bonding times.     

Self-joining of Si3N4 using metal interlayers

This work focuses on various aspects of diffusion bonding of Ti-foil and Nb-foil interlayers during the self-joining of Si₃N₄. Joints were diffusion joined by hot-uniaxial pressing at temperatures ranging from 1200 °C to 1600 °C using different holding times. The microstructural characterization of the resulting interfaces was carried out by scanning electron microscopy, electron-probe microanalysis (EPMA), and X-ray diffraction. The results showed that Si₃N₄ could not be bonded to Ti at temperatures lower than 1400 °C; however joining was successful at higher temperatures. On the other hand, Si₃N₄ was solid-state bonded to Nb at temperatures ranging from 1200 °C to 1600 °C. Joining occurred by the formation of a reactive interface on the metal side of the joint. Ti₅Si₃, TiSi, and TiN were detected at the Si₃N₄/Ti interface, and Nb₅Si₃ and NbSi₂ at the Si₃N₄/Nb interface, resulting from high-temperature reaction between Ti or Nb and Si₃N₄. Four-point bending testing gave a maximum joint strength of 147 MPa for Si₃N₄/Ti/Si₃N₄ samples hot pressed at 1500 °C and 120 minutes. However, strong joints were obtained above 1450 °C (>100 MPa). These results indicated that there is a strong relationship between the thickness of the interface and the mechanical strength of the preceding joints. This article is based on a presentation made in the symposium entitled “Processing and Properties of Structural Materials,” which occurred during the Fall TMS meeting in Chicago, Illinois, November 9–12, 2003, under the auspices of the Structural Materials Committee.     

Interfacial Microstructure and Mechanical Strength of 93W/Ta Diffusion-Bonded Joints with Ni Interlayer

93W alloy and Ta metal were successfully diffusion bonded using a Ni interlayer. Ni₄W was found at the W-Ni interface, and Ni₃Ta and Ni₂Ta were formed at the Ni-Ta interface. The shear strength of the joints increases with increasing holding time, reaching a value of 202 MPa for a joint prepared using a 90-minute holding time at 1103 K (830 °C) and 20 MPa. The fracture of this joint occurred within the Ni/Ta interface.     

Role of Microstructure and Temperature on the Tensile Fracture Behavior of Oxide Dispersion Strengthened 18Cr Ferritic Steel

The tensile fracture behavior of oxide dispersion strengthened 18Cr (ODS-18Cr) ferritic steels milled for varying times was studied along with the oxide-free 18Cr steel (NODS) at 25, 200, 400, 600, and 800 °C. At all the test temperatures, the strengths of ODS–18Cr steels increased and total elongation decreased with the duration of milling time. Oxide dispersed 18Cr steel with optimum milling exhibited enhanced yield strength of 156 pct at room temperature and 300 pct at 800 °C when compared to oxide-free 18Cr steel. The ductility values of ODS-18Cr steels are in the range 20 to 35 pct for a temperature range 25 to 800 °C, whereas NODS alloy exhibited higher ductility of 37 to 82 pct. The enhanced strength of ODS steels when compared to oxide-free steel is due to the development of ultrafine grained structure along with nanosized dispersion of complex oxide particles. While the pre-necking elongation decreased with increasing temperature and milling time, post-necking elongation showed no change with the test temperature. Fractographic examination of both ODS and NODS 18Cr steel fractured tensile samples, revealed that the failure was in ductile fracture mode with distinct neck and shear lip formation for all milling times and at all test temperatures. The fracture mechanism is in general followed the sequence; microvoid nucleation at second phase particles, void growth and coalescence. The quantified dimple sizes and numbers per unit area were found to be in linear relation with the size and number density of dispersoids. It is clearly evident that even nanosized dispersoids acted as sites for microvoid nucleation at larger strains and assisted in dimple rupture.

     

Effect of Bonding Time on Interfacial Reaction and Mechanical Properties of Diffusion-Bonded Joint Between Ti-6Al-4V and 304 Stainless Steel Using Nickel as an Intermediate Material

In the current study, solid-state diffusion bonding between Ti-6Al-4V (TiA) and 304 stainless steel (SS) using pure nickel (Ni) of 200-μm thickness as an intermediate material was carried out in vacuum. Uniaxial compressive pressure and temperature were kept at 4 MPa and 1023 K (750 °C), respectively, and the bonding time was varied from 30 to 120 minutes in steps of 15 minutes. Scanning electron microscopy images, in backscattered electron mode, revealed the layerwise Ti-Ni-based intermetallics like either Ni₃Ti or both Ni₃Ti and NiTi at titanium alloy-nickel (TiA/Ni) interface, whereas nickel-stainless steel (Ni/SS) interface was free from intermetallic phases for all the joints. Chemical composition of the reaction layers was determined by energy dispersive spectroscopy (SEM–EDS) and confirmed by X-ray diffraction study. Maximum tensile strength of ~382 MPa along with ~3.7 pct ductility was observed for the joints processed for 60 minutes. It was found that the extent of diffusion zone at Ni/SS interface was greater than that of TiA/Ni interface. From the microhardness profile, fractured surfaces, and fracture path, it was demonstrated that the failure of the joints was initiated and propagated apparently at TiA/Ni interface near Ni₃Ti intermetallic for bonding time less than 90 minutes, and through Ni for bonding time 90 minutes and greater.     

Influence of interface microstructure on the strength of the transition joint between Ti-6Al-4V and stainless steel

Solid-state diffusion bonding of Ti-6Al-4V and type 304 SS was investigated in the temperature range of 750 °C to 950 °C, under a uniaxial load for 5.4 ks in vacuum. The diffusion bonds were characterized using light and scanning electron microscopy. The scanning electron microscopic images in backscattered mode show the existence of different reaction layers in the diffusion zone. The composition of these layers was determined by energy-dispersive X-ray spectroscopy (EDS) to contain the α-Fe, χ, λ, FeTi, β-Ti, and Fe₂Ti₄O phases. The presence of these intermetallics was confirmed by X-ray diffraction. The bond strength was evaluated, and the maximum tensile strength of ∼342 MPa and the maximum shear strength of ∼237 MPa were obtained for the diffusion couple processed at 800 °C due to the finer width of the brittle intermetallic layers. With a rise in joining temperature, the bond strength drops owing to an increase in the width of the reaction layers. The activation energy and growth constant were calculated in the temperature range of 750 °C to 950 °C for the reaction products. The χ phase showed the fastest growth rate. A fracture-surface observation in a scanning electron microscope (SEM) using EDS demonstrates that failure takes place mainly through the β-Ti phase for the diffusion couples processed in the aforementioned temperature range.     

一种阻燃隔热复合功能涂层性能研究

为提高航空发动机推重比、效率以及有效载荷,钛合金工程应用技术在发动机多部件的设计与研制中得到重视。针对钛合金极易发生的"钛火"问题及承受高温特性不能满足特殊零部件设计要求,开展具兼具阻燃与隔热性能的复合功能涂层十分必要,德国MTU航空发动机公司(MTU Aero Engines GmbH)开发出具有阻燃、封严性能的复合功能涂层,已经应用于航空发动机的压气机叶片和机匣,并取得了良好的效果。本文采用微弧脉冲离子表面改性技术与高能等离子喷涂工艺制备了具有阻燃特性的Ti_(40)Zr_(25)Ni_3非晶材料层和具有隔热特性的YSZ隔热一体化复合功能涂层,重点研究了该复合功能涂层的隔热能力、阻燃特性以及结合强度等关键性能。试验表明,采用微弧脉冲离子表面改性技术与高能等离子喷涂工艺制备的Ti_(40)Zr_(25)Ni_3非晶材料阻燃层与YSZ隔热复合功能涂层具有较高的结合强度,平均结合强度达到36.3MPa;阻燃改性层与基体间的界面存在微冶金结合的特征大大提高了涂层使用的可靠性;当正面火焰温度为达到600℃时,制备有复合功能涂层试样平均隔热温度达到了70℃;无复合功能涂层的试样在350℃条件下即发生燃烧现象,而制备有阻燃、隔热复合功能涂层试样在750℃条件下仍然具有良好的阻燃能力。     

Oxidation of mechanically alloyed powders during processing: origins and control

Abstract

Oxide dispersion strengthened (ODS) alloys prepared by mechanical alloying (MA) and subsequent consolidation are usually subjected to a series of heat treatments during production, typically comprising a degassing process at ?600°C and a preconsolidation high temperature 'soak' at ?1000°C, both under vacuum. In the current work, the oxidation behaviour of a prototype ODS Fe₃Al alloy and a commercial FeCrAl alloy has been studied during simulation of these temperature and pressure regimes. After the high temperature 'soak' simulation, oxidation had taken place on both alloys with a significantly thicker scale forming on the ODS Fe₃Al. This scale is believed to be the source of much of the high alumina content found in fully consolidated ODS Fe₃Al. Variation in the amount of particulate alumina found in different batches of commercially consolidated powder is discussed. Novel processes involving hydrogen purging and powder precompaction have been employed to decrease oxidation and thereby increase sintering efficiency.     

Hydrogen trapping at spheroidized and elongated sulphidic inclusions-matrix interfaces in mild steel

The present work is concerned with those factors which determine the hydrogen trapping at the interfaces between spheroidized and elongated sulphidic inclusions, and matrix in mild steel by using gas-phase charging and electrochemical detection techniques. Three kinds of specimens A, B and C were prepared from the calcium-treated mild steel by water quench from 950°C, and from the ordinary mild steel by water quench from 950 and 1150°C, respectively. Specimen A was characterized by the interface between the spheroidized sulphidic inclusions and matrix, but the specimens B and C were characterized by the elongated sulphide-matrix interface. The values of time-lag decreased with increasing hydrogen input pressure for the specimens A, B and C. The results indicated that the defects produced at the interfaces act as saturable trap sites for hydrogen. The hydrogen trap density and binding energy were obtained from the plot of [(t_T/t_L)–1] vs. . The trap densities for the specimens A, B and C were found to be about 5.0 × 10^?8, 2.1 × 10^?7 and 5.0 × 10^?7 mol cm^?3, respectively. The trap-binding energy was determined to be ?(56.4 ± 1.1) kJ mol^?1 for the specimens A, B and C as well. The experimental results indicated that the nature of the interfaces is determined by the number of defects produced in the interfaces per unit volume, regardless of the inclusion shape. The defects distributed in the interfaces included namely microvoids and water-quench-created dislocations which act as deep trap sites for hydrogen.     

Taguchi Analysis of Relation Between Tensile Strength and Interfacial Phases Quantified via Image Processing

Quantitative analysis was performed via image processing to identify the relationship between the tensile strength and the thickness of the Cr_xB_y phase layer at the interface of brazed 304 stainless steel with Ni-based filler metal (MBF20). The experimental design was based on the Taguchi method to determine the relative contributions of the processing conditions, including the brazing temperature, heating rate, holding time, and filler metal thickness. The Cr_xB_y phase at the brazement interface was extracted by an image-processing method from backscattered electron imaging; the numerically defined thicknesses of the Cr_xB_y phase layers developed under varied processing conditions were calculated. The tensile strengths at temperatures of 25 °C and 650 °C were measured for specimens brazed under identical experimental conditions based on the Taguchi method and the post-tensile testing fracture surfaces were analyzed. Regarding the relationship between the thickness of the Cr_xB_y phase layer, as determined through the image processing of the microstructure, and the tensile strength at 25 °C, thicker Cr_xB_y layers deteriorated the tensile strength of the brazement interfaces. Although a slight discrepancy occurred in the brazement tensile strengths between the testing temperatures of 25 °C and 650 °C, the elevated temperature during tensile testing affected the brazed interface microstructure; with this consideration, the overall results for both tensile strength tests corresponded to the quantitatively analyzed Cr_xB_y phase layer thicknesses. From the relationship between Cr_xB_y layer thickness and tensile strength, the heating rate is the most effective processing condition to achieve high bonding strength, because changes in heating rate compared to those of other processing conditions have the greatest effect in changing the Cr_xB_y phase layer thicknesses and tensile strength. Therefore, the results confirmed that the image-processing method enabled accurate quantitative analysis of the microstructure, permitting prediction of the mechanical strength of the joint.

     

Effects of sulfur on the fatigue and fracture resistance of interfaces between γ-Ni(Cr) and α-Al2O3

Dramatic effects of S on the adhesion and fatigue resistance of interfaces between γ-Ni(Cr) and α-Al₂O₃, have been explicitly demonstrated and quantified. This has been achieved by using two bonding conditions: one involving solid-state diffusion (SSDB) and another through a liquid phase (LPB) that forms at temperatures above a eutectic that releases a S-rich liquid. Upon SSDB, there is no significant S excess at the interface, whereas LPB forms a thin interphase with a high local concentration of S. The SSDB materials have interfaces with such high toughness (above 300 J m⁻²) and fatigue resistance that mode I cracks divert into the Al₂O₃, rather than propagate at the interface. Conversely, the LPB materials delaminate at the interface, solely as a result of the residual stresses from thermal expansion misfit, with an interface toughness in the range 2 to 7 J m⁻².     

A Novel Bonding Method of Pure Aluminum and SUS304 Stainless Steel Using Barrel Nitriding

A great deal of research is being carried out on welding or bonding methods between iron and aluminum. However, it is not so easy to make Fe-Al bonding materials with both high strength and light weight. Recently, a new nitriding process has been proposed to produce aluminum nitride on an aluminum surface using a barrel. This study proposes a new concept in the production of a multilayer which has an AlN and Fe-Al intermetallic compound layer between the aluminum and steel using a barrel nitriding process. The bonding process was carried out from 893 K to 913 K (620 °C to 640 °C) for 18, 25.2, and 36 ks with Al₂O₃ powder and Al-Mg alloy powder. After the process, an aluminum nitride (AlN) layer and a Fe-Al intermetallic compound (Fe₂Al_5.4) layer were formed at the interface between the pure aluminum and SUS304 austenitic stainless steel. The thicknesses of the AlN layer and the intermetallic compound layer increased with increasing treatment temperature and time. The maximum hardnesses of the AlN layer and Fe₂Al_5.4 layers were found to be 377HV and 910HV, respectively, after barrel nitriding at 893 K (620 °C) for 18 ks.     

Interface nanochemistry effects on stainless steel diffusion bonding

The diffusion-bonding behavior of single-phase austenitic stainless steel depends strongly on the chemistry of the surfaces to be bounded. We found that very smooth (0.5 nm root-mean-square (RMS) roughness), mechanically polished and lapped substrates would bond completely in ultrahigh vacuum (UHV) in 1 hour at 1000 °C under 3.5 MPa uniaxial pressure, if the native oxide on the substrates was removed by ion-beam cleaning, as shown by in-situ Auger analysis. No voids were observed in these bonded interfaces by transmission electron microscopy (TEM), and the strength was equal to that of the unbounded bare material. No bond formed between the substrates if in-situ ion cleaning was not used. The rougher cleaned substrates partially bonded, indicating that roughness, as well as native oxides, reduced the bonding kinetics.     

Effect of reaction products on mechanical properties of diffusion bonded of titanium to 304 stainless steel with Cu interlayer joints

In present study, pure copper was used as an interlayer of the diffusion bonded joints between commercially pure titanium and 304 stainless steel. The process was carried out at 900°C for 30–150 minutes in steps of 30 minutes under 3MPa uniaxial load in vacuum. Microstructures of the bonded assemblies were observed in optical and scanning electron microscopes. The study exhibits the presence of different reaction layers in the diffusion zone and their chemical compositions were determined by energy dispersive spectroscopy. Formation of reaction products in diffusion interfaces were confirmed by xray diffraction technique. The maximum tensile strength of ∼322MPa (∼101% of Ti) and shear strength of ∼254MPa (∼86% of Ti) along with 8.4 % ductility were obtained for the couple bonded for 60 minutes and due to the rise in bonding time up to 90 minutes, bond strength drops marginally. With a further increase in joining time, the bond strength drops gradually due to the increase in the width of Fe-Ti base reaction products. Observation of fracture surfaces in SEM using EDS indicated that fracture takes place through the SS-Cu interface for all bonded samples.     

Microstructural evolution of modified 9Cr-1Mo steel

The tempering and subsequent annealing of modified 9Cr-lMo steel have been investigated to determine the influence of trace amounts of V and Nb on the sequence of precipitation processes and to identify the basis for the enhanced high-temperature strength compared to the standard 9Cr-lMo composition. Air cooling (normalizing) from 1045 °C results in the precipitation of fine (Fe, Cr)₃C particles within the martensite laths. Additional carbide precipitation and changes in the dislocation structure occur during the tempering of martensite at 700 °C and 760 °C after normalizing. The precipitation of M₂₃C₆ carbides occurs preferentially at lath interfaces and dislocations. The formation of Cr₂C was detected during the first hour of tempering over the range of 650 °C to 760 °C but was replaced by V₄C₃ within 1 hour at 760 °C. During prolonged annealing at 550 °C to 650 °C, following tempering, the lath morphology remains relatively stable; partitioning of the laths into subgrains and some carbide coarsening are evident after 400 hours of annealing at 650 °C, but the lath morphology persists. The enhanced martensite lath stability is attributed primarily to the V₄C₃ precipitates distributed along the lath interfaces and is suggested as the basis for the improved performance of the modified 9Cr-lMo alloy under elevated temperature tensile and creep conditions.