Effect of boron on the ductility of ordered Ni-Ni4Mo alloys

Boron was identified to have a beneficial effect on the room temperature ductility of ordered Ni-Ni₄Mo alloys. An addition of 0.01 to 0.03 wt % boron to hypostoichiometric alloys was found to increase the tensile ductility from about 5% to 30% in the ordered state produced by exposure at 600 to 800 °C, of the annealed material. The boron effect was, however, found to diminish with exposure time at a given temperature and not to be maintained at elevated temperatures. The beneficial effect of boron on room temperature ductility was also found to be considerably less pronounced in stoichiometric Ni₄Mo alloy. Both electron energy loss spectroscopy and Auger electron spectroscopy techniques revealed no preferential segregation of boron to grain boundaries. Experimental results suggested that boron decelerates the kinetics of heterogeneous grain boundary ordering which leads to an improvement in ductility.     

NaCl-induced accelerated oxidation of 304 stainless steel and Fe-Mn-Al alloy at 900°C

The high-temperature corrosion behavior of cold-rolled and annealed 304 stainless steel (304SS) and Fe-29Mn-8Al-2.5Si-2Cr-0.74C alloy coated with 0.002 g cm^–2 NaCl initially were studied at 900°C in air. The corrosion kinetics of the two alloys follow the parabolic rate law. The initial NaCl coating accelerates oxidation of these alloys by oxychlorination and chlorination/oxidation cyclic reactions, and catalytic actions of chloride or chlorine are thought to be the principal causes. A bulky, layered scale as well as some intergranular attack is noted on the annealed 304SS, and intergranular attack distributes over the alloy substrate of the cold-rolled 304SS during a 144 h exposure. With the formation of a compact Al₂O₃ scale to decrease further chlorine attack, the corrosion resistance of Fe-Mn-Al alloy is superior to that of 304SS in this study.     

Influence of precipitation and segregation on hydrogen embrittlement in aged 9Cr–1Mo steel

Abstract

A study is reported of temper embrittlement and hydrogen embrittlement in a series of model 9Cr–1Mo steel alloys in which the levels of silicon and phosphorus have been varied to separate the formation of the brittle intermetallic (Laves) phase from the segregation of phosphorus during aging. Phosphorus segregation was mildly detrimental to ductility properties, Laves phase formation was more detrimental, and their effects combined produced the most severe loss in ductility. Hydrogen effects were additive to those of aging. In unaged material without silicon enrichment, only M₂₃C₆ precipitates were detected, with little phosphorus segregation. With silicon enrichment, phosphorus segregation to lath and grain boundaries was enhanced. This enhancement increased the susceptibility of the materials to hydrogen embrittlement, promoting transgranular cleavage and chisel fracture. In aged material, the high phosphorus alloys showed some grain boundary segregation, but only limited interaction with hydrogen. In the high silicon alloys, the formation of Laves phase was most evident. This enhanced hydrogen embrittlement resulted in extensive chisel, transgranular cleavage, and some intergranular fracture. In the high silicon high phosphorus alloy, both Laves phase formation and phosphorus segregation were evident. This resulted in enhanced susceptibility to hydrogen embrittlement, producing intergranular fracture. Thus, silicon controls the susceptibility to hydrogen embrittlement in unaged alloy by promoting phosphorus segregation and in aged alloy by promoting Laves phase formation. In the aged alloy, segregation of phosphorus can enhance the effect of silicon.

MST/1785     

Effect of long-range ordering on the corrosion properties of an Ni-Mo alloy

A correlation was made between long-range ordering to Ni₄Mo (A₄ D1_a superlattice) in a Ni-28wt % Mo alloy and its corrosion properties. Emphasis was placed upon the effects of ordering on: (i) microchemical composition including grain-boundary chemistry, and (ii) plastic deformation behaviour. Analytical electron microscopy and Auger electron spectroscopy were utilized for microstructural characterization and microchemical analysis. Corrosion testing in the ordered state revealed a considerable increase in the corrosion rate in boiling 20% HCl and also less resistance to stress corrosion cracking in boiling 10% HI, all relative to the disordered state. Molybdenum-depleted zones were detected in the ordered microstructure near the Ni₄Mo-matrix interfaces, at antiphase boundaries and alongside grain boundaries. Examination of the tensile deformation substructure indicated that ordering had lowered the stacking-fault energy of the alloy. It was concluded that the observed degradation in the chemical stability of the alloy in the ordered state was due to inhomogeneities in microchemical composition and a low stacking-fault energy.     

Interfacial properties of a SiC fibre-reinforced Ti alloy after long-term high-temperature exposure

The effect of long-term high-temperature exposure on the interfacial properties of a typical SiC continuous fibre-reinforced titanium alloy was investigated. Specimens were annealed in air and in vacuum under simulated operating conditions using temperatures up to 500 °C for times up to 700 h. The interfacial chemistry and fracture morphology of the specimens were determined using Auger and scanning electron microscopy. It was shown that high-temperature long-term exposure to air resulted in embrittlement of the fibre-matrix interface through combined oxidation and attack of the Ti matrix on the fibres and fibre coating. Exposure to operating temperatures in vacuum did not lead to a significant degradation of the interfaces. The degradation of the interfaces in high temperature service can lead to a reduction of the service life of components made of reactive system metal-matrix composites such as SiC-reinforced Ti alloys.     

Evidence on the corrosion-induced hydrogen embrittlement of the 2024 aluminium alloy

The present work aims to provide evidence of corrosion‐induced hydrogen embrittlement of the aircraft aluminium alloy 2024. An extensive experimental investigation involving metallographic and fractographic analyses as well as mechanical testing was performed. The corrosion exposure led to a moderate reduction in yield and ultimate tensile stress and a dramatic reduction in tensile ductility. Metallographic investigation of the specimens revealed a hydrogen‐rich embrittled zone just below the corrosion layer. Furthermore, fractographic analyses showed an intergranular fracture at the specimen surface followed by a zone of quasi‐cleavage fracture and further below an entirely ductile fracture. Mechanical removal of the corroded layers restored the yield and ultimate stress almost to their initial values but not the tensile ductility. The tensile ductility was restored to the level of the uncorroded material only after heat treatment at 495°C. Measurement of hydrogen evolution with temperature showed that by heating the corroded alloy at 495°C, the trapped hydrogen is released.     

Hot tensile properties and microstructural evolution of as cast NiTi and NiTiCu shape memory alloys

Hot tensile properties of as cast NiTi and NiTiCu shape memory alloys were investigated by hot tensile test at temperature range of 700–1100 °C using the strain rate of 0.1 s⁻¹. The NiTi alloy exhibited a maximum hot ductility at temperature range of 750–1000 °C, while the NiTiCu alloy showed it at temperature range of 800–1000 °C. It was found that at temperatures less than 750 °C, diffusion-assisted deformation mechanism was inactive leading to semi-brittle type of failure and limited ductility in both alloys. Also it was found that at temperature range of 800–1000 °C, dynamic recrystallization is dominant leading to high ductility. Likewise, the fracture surface of the specimens presenting the maximum hot ductility showed an ideal type of ductile rupture in which they gradually pulled out to a fine point. On the other hand, the decline in ductility occurred at the temperatures above 1000 °C was attributed to the liquid phase formation leading to interdendritic and intergranular type of fracture.     

热暴露对TiAl合金室温拉伸性能影响的研究进展

室温拉伸塑性是TiAl合金这类低损伤容限材料工程化应用必须考虑的重要指标之一。高温热暴露后TiAl合金的室温塑性显著降低,这不利于TiAl合金部件的应用可靠性。概述了高温热暴露对TiAl合金室温拉伸性能特别是室温拉伸塑性的影响,重点归纳了高温热暴露造成的表面性质变化对该合金室温塑性不利影响的可能机制,并提出了抑制热暴露所致室温塑性降低的可能途径。分析表明,热暴露导致TiAl合金室温拉伸塑性降低,是表面氧化致脆、表面残余应力以及环境潮气等因素综合作用的结果,其中表面氧化致脆是直接原因。改善基体的抗氧化性或者形成可抑制表面裂纹向基体扩展的有利组织,是保持TiAl合金高温使用后室温拉塑性水平的有效途径。     

Characteristics of intermediate temperature dynamic embrittlement of age hardenable copper-chromium alloys

Abstract

Copper-chromium alloys exhibit a severe intermediate temperature loss in ductility during uniaxial tensile loading that has the characteristics of dynamic embrittlement. The dynamic embrittlement is characterised by intergranular fracture caused by stress induced ingress of sulphur to the grain boundaries followed by decohesion.     

Mn和Zr对Al-Zn-Mg-Cu铝合金各向异性的影响

采用光学显微镜(OM)、扫描电子显微镜(SEM)、透射电子显微镜(TEM)以及室温拉伸、剥落腐蚀、晶间腐蚀等测试方法,研究了微量的Mn和Zr对Al-Zn-Mg-Cu铝合金的组织和性能各向异性的影响。结果表明,在Al-Zn-Mg-Cu-Ti合金中,分别添加微量的Mn和Zr,合金中对应析出细小弥散的Al6Mn和Al3Zr相,这两相均能抑制基体再结晶,促使合金的晶粒纵横比增大。合金的力学性能、抗晶间腐蚀和剥落腐蚀性能提高,但性能各向异性增大。同时,结果显示Zr对合金的组织和性能各向异性的影响显著大于Mn。     

Mechanism of carburization of high-temperature alloys

An investigation of the mechanism of gaseous carburization in a reducing environment was conducted for selected Fe- and Ni-base alloys. Carburization kinetics were measured as functions of temperature in the range 870–980 °C. Scanning electron microscopy, analytical electron microscopy and X-ray diffractometry were employed for microstructural characterization and microchemical analysis. Changes in mechanical strength produced by carburization were determined from microhardness and tensile property measurements. Kinetic studies indicated that the carburization reaction followed a parabolic rate law. Depending upon the nature of surface scale formed in the presence of a carburizing environment, the rate-determining step of the reaction varied from C diffusion into the alloy in the presence of a carbide scale to that in the presence of an oxide scale. Under reducing carburizing conditions, alloys inherently protected by Cr₂O₃-base scale were found to develop a surface carbide scale which allowed C to penetrate into the alloy with relative ease and, thus, the carburization kinetics was accelerated. In contrast, an alloy capable of forming Al₂O₃ developed and maintained a protective surface oxide scale which acted as an effective barrier to C diffusion into the alloy. Degradation of mechanical strength due to precipitation of carbides in the alloy was correlated with the rate of attack and consequently the nature of the surface scale.     

Long-term ageing characteristics of some commercial nickel-chromium-molybdenum alloys

The long-term ageing characteristics of some commercial Ni-Mo-Cr alloys (the high-temperature HASTELLOY alloy S and the corrosion resistant HASTELLOY alloys C-4 and C-276) at 810 K were investigated. It was found that the three alloys undergo the following long-range ordering reaction: disordered f c c lattice ordered orthorhombic, Pt₂Mo-type superlattice. Ordering was found to cause considerable strengthening without severe loss of tensile elongation. Deformation in the ordered state occurred predominantly by twinning. The corrosion rates of alloys C-4 and C-276 in boiling sulphuric-ferric sulphate solution did not seem to be greatly affected by the long-range ordering reaction. In addition to ordering, the three alloys were also found to undergo grain boundary reactions. The resulting phase in alloys S and C-4 assumed a dispersed morphology and was identified as carbide, probably of the Type M₁₂C. In alloy C-276, however, which contains higher amounts of iron and tungsten, the boundary precipitate was in the form of a continuous layer consisting of M₁₂C and Mu-phase. This could account for the reduced tensile elongation of alloy C-276 relative to alloys S and C-4 and also to its high corrosion rate.     

Intergranular corrosion in electrochemical machining

In a study of electrochemical drilling, the influence of the electrolyte composition on intergranular corrosion of various nickel alloys and of hardenable stainless steel was investigated. The nickel-base alloys remained unaffected in a solution of 15 % NaNO₃ + 20 % NaClO₃ + 65 % H₂O but suffered intergranular corrosion in solutions of NaNO₃ and NaNO₃ + NaCl. Stainless steel showed no signs of intergranular attack in any of the electrolyte solutions used.     

Mechanical properties of the Ni3(Si,Ti) alloys doped with carbon and beryllium

The Ni₃(Si, Ti) alloys doped with small amounts of carbon and beryllium were tensile tested in two environments, vacuum and air, over a wide range of test temperatures. The yield stresses of the carbon-doped alloys were almost identical to the undoped alloys while those of the beryllium-doped alloys were slightly higher than the undoped Ni₃(Si, Ti) alloys. The doping with carbon enhanced the elongation and ultimate tensile strength (UTS) whereas doping with beryllium reduced the elongation over the entire temperature range tested. The fracture patterns were primarily associated with the ductility behaviour. As the elongation (or UTS) increased, the fracture pattern changed from the intergranular to the transgranular fracture patterns. No environmental embrittlement of the ductility of the carbon-doped alloys was found at ambient temperatures but it was evident at elevated temperatures. Ductilities were reduced at high temperatures when the carbon-doped alloys were tensile tested in air. At high temperatures the environmental embrittlement observed is suggested to be due to the penetration of (free) oxygen into the grain boundaries causing the ductility loss in the carbondoped alloys.     

Intergranular corrosion resistance of nanostructured austenitic stainless steel

Nanostructured metals and alloys possess very high strength but exhibit limited plasticity. Enhancement of the strength/ductility balance is of prime importance to achieve wide industrial applications. However, post-deformation heat treatment, which is usually used to improve plasticity, can lead to a decrease in other properties. In the case of austenitic stainless steels, heat treatment in the range from 480 to 815 °C can increase their susceptibility to intergranular corrosion. The aim of the work reported in this paper was to determine if nanostructured austenitic stainless steel is susceptible to intergranular corrosion if heat treated for 1 h at 700 °C. Samples of 316LVM austenitic stainless steel were hydrostatically extruded, in a multi-step process with the total true strain of 1.84 to produce a uniform microstructure consisting of nanotwins. These nanotwins averaged 21 nm in width and 197 nm in length. Subsequent annealing at 700 °C produced a recrystallised structure of 68-nm-diameter nanograins. The heat treatment improved the ductility from 7.8 to 9.2 % while maintaining the ultimate tensile strength at the high level of 1485 MPa. Corrosion tests were performed in an aqueous solution consisting of 450 ml concentrated HNO₃ and 9 g NaF/dm³ (according to ASTM A262-77a). The evaluation of the corrosion resistance was based on transmission and scanning electron microscopic observation of the microstructure and chemical analyses. The results revealed that both the as-received and HE-processed samples are slightly susceptible to the intergranular corrosion after annealing at 700 °C for 1 h.     

Effect of carbon on mechanical properties in Fe0·5Co0·5 alloys

Effects of carbon addition on brittleness and ductility in equiatomic iron-cobalt alloys have been investigated. In alloys containing 0.5 to 2 at % C, which are ductile when quenched from 800° C, it is found that brittleness occurs due to decarburization, but ductility is reobtained by the recarburizing of the specimens when preceded by decarburization. This is shown to be independent of factors, such as coarsening of the grains occurring during heat treatments, existence of impurities and the formation of texture produced during cold rolling, and the existence of carbon is an important factor. In addition, it is described that the alloys, having generally been very brittle in the ordered state, have not always shown embrittlement even after ordering, if only cold rolled severely. The effectiveness of carbon in improving the ductility has been discussed under a proposal that host atoms, iron or cobalt, react with carbon atoms to precipitate carbides; around the individual carbides, zones, with the deviation from the equiatomic composition, are formed, thus rendering the ordering difficult; in other words, since production of the resultant unequiatomic zones corresponds to that of disordered ones, it would be expected that the ductility of such specimens depends on the morphology of distribution of the zones, such as volume fraction, uniformity and density.     

Effect of refining techniques on stress corrosion cracking behaviour of Inconel X-750

High-strength age-hardenable nickel-base superalloy Inconel X-750, is susceptible to severe intergranular stress corrosion cracking (IGSCC) when used in the triple heat-treated condition. In this research, the slow strain-rate technique has been employed to evaluate the stress corrosion cracking susceptibility of alloy X-750 under simulated nuclear pressurized water reactor (PWR) conditions, using an automated autoclave system at 8 × 10⁶Nm^–2 pressure and 289° C temperature. The alloys produced via electroslag refining (ESR) or vacuum arc refining (VAR) processing routes containing 0.004% and 0.011% sulphur, respectively, were solution annealed at either 1075 or 1240° C for 2h and water quenched followed by ageing in the 704 to 871° C temperature range for up to 200h, followed by air cooling or furnace cooling. The scanning electron microscopy performed on fractured surfaces revealed that Inconel X-750 processed through the ESR route, solution annealed at 1240° C for 2h and water quenched, aged at 871° C for 200 h and furnace cooled provided the best combination of strength, ductility and resistance to SCC. A less sensitized area adjacent to the grain boundary was responsible for the improvement in properties and the alloy X-750 is recommended for PWR applications in the above conditions of processing and heat treatment.     

Mechanical behaviour of Zr doped Ni3Al alloy: effects of heat treatment,environment, and temperature

Abstract

The mechanical behaviour of polycrystalline Ni₇₆Al₂₃Zr alloy was studied as a function of heat treatment, environment, and temperature. It was found that the tensile ductility was very sensitive to temperature, the alloy showing low ductility at temperatures from 700 to 1000°C both in air and vacuum. Environmental embrittlement could be alleviated for those specimens with elongated grains. The ductile transgranular fracture was explained by stress concentration at the intersection of slip bands and grain boundaries. It was also found that an oxide layer, formed during tensile testing at elevated temperature, affected the environmental embrittlement of Ni₃Al(Zr) alloy. An adherent Al rich oxide film was effective in protecting the underlying alloy from oxygen penetration.     

Simulation of road salt corrosion in austenitic alloys for automotive exhaust systems

Abstract

Cyclic oxidation tests in air with intermittent salt spraying have been performed to simulate the conditions of road salt (NaCl–CaCl₂) enhanced corrosion in automotive exhaust systems.

Tests were carried out at 600 and 700°C on three austenitic alloys, including two stainless steels currently employed for exhaust components (AISI 316 Ti and AISI 302B) and a higher nickel heat resisting alloy.

The presence of salt causes internal corrosion, both along a regular front beneath an outer oxide scale and down alloy grain boundaries. An increase in temperature accelerates the corrosion rate and particularly enhances intergranular penetration.

The results of micrographic and microanalytical investigations are in general agreement with an active oxidation mechanism in which Na_xCl_x vapour species, and not only chlorine, appear to play an important role. The regions directly affected are depleted in chromium and iron and enriched in nickel.

Although internal oxidation of silicon is observed, a high silicon content (2%) does not necessarily ensure effective protection against this type of attack.     

Crystallization embrittlement of Ni-Si-B alloys

Changes in the mechanical properties, and in particular the loss of ductility, have been examined during the crystallization of a series of Ni-Si-B amorphous alloys and an Ni-Fe-Si-B alloy. In all cases crystallization takes place by the formation of large numbers of small crystals and at some stage during the crystallization process embrittlement occurs. The geometrical characteristics of the crystal phases, and the mechanical properties of the crystalline and amorphous components are assessed in an attempt to define the factors leading to embrittlement. Sensitivity to embrittlement does not seem to be related to the particular phase nature, geometry nor distribution of the crystals formed. Rather, the mechanical properties of the crystals seem to be the determining factors. Hard and brittle crystals lead to alloy embrittlement; soft, ductile crystals can lead to enhanced toughness of the alloys.