The effect of phosphorus on the ductility of 9Cr-1Mo steels

The ductile fracture characteristics of three special casts of 9Cr-1Mo steel with different phosphorus contents have been studied using plain tensile, notched tensile and Charpy impact specimens. A commercial cast of this steel has also been investigated for comparative purposes. The materials have been tested in the normalised and tempered condition and after ageing at 550°C for 1000 h and 5000 h. The results show that the ductility, DBTT and upper shelf energy (USE) of the low phosphorus material is unaffected by the 1000 h age, in contrast to the higher phosphorus casts which experience a deterioration in these properties. All the materials exhibit a deterioration in these properties after the 5000 h ageing treatment. The results support the view that the decrease in ductility associated with the 1000 h age in the higher P materials is caused by the segregation of phosphorus to the carbide/matrix interfaces and that the decrease in USE also arises from this source. The decrease in ductility and USE and the increase in DBTT after long term ageing is associated principally with the precipitation of Laves phase. Each material exhibits a decrease in ductility with increasing stress triaxiality and it is shown that a contribution to this effect may arise from the dependence of void nucleation on stress state.     

The influence of strength and phosphorus segregation on the ductile fracture mechanism in a Ni-Cr steel

Upper-shelf toughness and its degradation through ageing at 500°C have been investigated in quenched-and-tempered 300 M steel as a function of tempering treatment at 650°C. Material having a 0.2% proof stress in the range 550–1000 MPa was examined. Toughness was assessed using the parameters J_Ic and reduction-in-area, and was related to the ductile fracture mechanism through measurements of the strain required for void nucleation and void growth to coalescence. Changes in matrix rheology, void-nucleating precipitate morphology, bulk chemistry and interfacial chemistry were monitored during tempering and ageing, and the associated fractography was quantitatively assessed. The ductile fracture process in unaged material was dominated by the strain required for void nucleation on carbide precipitates. Nucleation strain increased with tempering time at 650°C causing a rise in ductility. Ageing at 500°C produced a loss of ductility for all temper conditions, and the sole cause of this effect was the segregation of phosphorus to carbide-matrix interfaces, identified by high resolution Auger spectroscopy. Both the strain required for void nucleation at carbides and that for void growth to coalescence were suppressed by ageing, through a reduction in interfacial cohesion consistent with the embrittling effect of segregated phosphorus.     

Age hardening and mechanical properties of a 2400 MPa grade cobalt-free maraging steel

The age hardening kinetics in the temperature range of 713 to 813 K of a 2400 MPa grade cobalt-free maraging steel (Fe-(18.8 ∼ 19.1) pct Ni-(4.4 ∼ 5.4) pct Mo-2.6 pct Ti, wt pct) has been studied. Study of microstructure and mechanical properties showed that a high number of Ni₃Ti and Fe₂(Mo,Ti) precipitates were formed during the ageing process, which resulted in high strength and relatively low fracture toughness. Ni₃Ti was the main precipitation phase. Fractography has shown ductile failure of tensile and fracture toughness specimens. Thermodynamic calculations showed that the equilibrium phases are Ni₃Ti, Fe₂(Mo,Ti), ferrite, and austenite.     

The grain size dependence of flow and fracture in a Cr-Mn-N austenitic steel from 300 to 1300K

The influence of polycrystal grain size in the range 18 μm to 184 μm on the tensile behavior of an austenitic stainless steel containing by wt pct 21 Cr, 14 Mn, 0.68 N and 0.12 C has been investigated over the temperature range 298 to 1273 K. Decreasing grain size has been shown to increase the flow stress at small strains in accordance with the Hall-Petch relationship at temperatures up to 873 K. The variation of the Hall-Petch constants with temperature is influenced by dynamic strain ageing between 575 and 775 K. Above 875 K, especially at low strain-rates a reversal of the Hall-Petch correlation occurs and the flow stress decreases with decreasing grain size. The relationship between ductility and temperature is marked by a minimum ductility at about half the absolute melting temperature and intergranular cavitation is observed. A decrease in grain size generally enhanced the ductility in this temperature regime whilst at fine grain sizes this trend was reversed. These results are explained in terms of a combination of a Griffith-Orowan type fracture criterion and an intergranular void sheet mechanism of fracture.     

时效处理和氮含量对316L超低碳奥氏体不锈钢冲击韧性的影响

316L超低碳不锈钢(/%:0.010～0.013C、17.50～17.67Cr、10.10～10.60Ni、1.89～2.02Mo、0.020～0.201N)由200 kg真空感应炉冶炼,并经550 mm轧机热轧成15 mm钢板。研究了1 050℃40 min水冷,750℃25～100 h空冷后316L钢的组织和冲击韧性。结果表明,随时效时间增加,316L钢的冲击功下降;同样时效时间,0.201%N的高氮316L钢的冲击功低于0.020%N较低氮316L钢,750℃100 h时效后两者的冲击功分别为40 J和150 J。低氮316L钢主要析出物为碳化物,在晶界和晶内呈细小弥散分布,尺寸为3～5μm;高氮316L钢析出物为100～200μm氮碳化物和σ-相,沿晶界分布。     

Improved ductility of Ni3Si by microalloying with boron or carbon

The effects of boron and carbon additions on the tendency for intergranular fracture in trinickel silicide intermetallics are reported. Melt spinning of Ni₇₇Si₂₃ alloyed with 0.1 at. pet boron results in full bend ductility and complete transgranular fracture compared with brittle intergranular fracture for the unmodified compound. Alloying with 0.1 at. pet carbon also produces full bend ductility but a mixed mode failure (≈30 pct transgranular). For both carbon and boron additions, reducing the Ni concentration of the base compound results in a greater percentage of intergranular fracture. The boron solubility limit depends on the Ni concentration of the base compound. For Ni₇₇Si₂₃, the solubility limit is between 0.1 and 0.2 at. pet boron. For compounds with silicon concentrations of 23.5 and 24.0 at. pct, the solubility limit is less than 0.1 at. pct boron. Boron additions above the solubility limit result in Ni₃B precipitates which degrade the bend ductility and increase the percentage of integranular fracture. Alloying with carbon above the solubility limit (<0.1 at. pct) produces graphite precipitation. For Ni₇₇Si₂₃, increasing the carbon concentration from 0.1 to 1.0 at. pct resulted in no change in the ductility. Auger examination of the grain boundary composition showed strong segregation of both boron and carbon. Enrichment in silicon concentration was also observed.     

The effect of the precipitation of coherent and incoherent precipitates on the ductility and toughness of high-strength steel

The effect of the coexistence of coherent and incoherent precipitates, such as M₂C and NiAl, on the ductility and plane strain fracture toughness of 5 wt pct Ni-2 wt pct Al-based high-strength steels was studied. In order to disperse coherent and incoherent precipitates, the heat treatments were carried out as follows: (a) austenitizing at 1373 K, (b) tempering at 1023 or 923 K for dispersing the incoherent precipitates of M₂C and NiAl, and then (c) aging at 843 K for 2.4 ks to disperse the coherent precipitate of NiAl into the matrix, which contains incoherent precipitates, such as M₂C and NiAl. The results were obtained as follows: (a) when the strengthening precipitates consist of coherent ones, such as M₂C and/or NiAl, the ductility and toughness are extremely low, and (b) when the strengthening precipitates consist of coherent and incoherent precipitates, such as M₂C and NiAl, the ductility and fracture toughness significantly increase with no loss in strength. It is shown that the coexistence of coherent and incoherent precipitates increases homogeneous deformation, thus preventing local strain concentration and early cleavage cracking. Accordingly, the actions of coherent precipitates in strengthening the matrix and of incoherent precipitates in promoting homogeneous deformation can be expected to increase both the strength and toughness of the material.     

The mechanism of void formation,void growth,and tensile fracture in an alloy consisting of two ductile phases

An investigation has shown that it is possible to relate void formation, void growth, and tensile ductility to microstructural features in an α-β titanium alloy, Ti-5.25A1-5.5V-0.9Fe-0.5Cu, heat treated to a constant yield strength. Equations relating tensile void growth rates to microstructure for both equiaxed,E, and Widmanstätten plus grain boundaryα, W + ITG. B.,in aged β morphologies have been derived. A mechanism for void formation at α-β interfaces is presented which accounts for the observed fact that voids do not form at Widmanstätten α platelets. Tensile fracture is shown to be intergranular in nature and occurs when a critical crack length-stress relationship is satisfied. The amount of ductility achievable in a specimen depends upon the rate of void growth. If the rate is large, the void reaches a critical size for fracture at a lower applied stress and strain and hence the ductility is less.     

The fracture of ordered Zr3AI polycrystals

Metallographic studies of Zr₃Al polycrystals deformed in tension have established that failure occurs by grain boundary fracture at low temperatures (<200 K), by microvoid coalescence at intermediate temperatures (≃295 to ≃875 K) and by grain boundary sliding following dynamic recrystallization at high temperatures C>875 K). The ductile fracture mode is composed of three separate processes —the nucleation, the growth and the coalescence of microvoids. Microvoid nucleation occurs by the cleavage of hard, secondphase Zr₂Al particles and by particle/matrix separation, while void growth occurs by the concentration of plastic strain at the crack tip. Both void nucleation and growth occur throughout plastic deformation and generate the damage which eventually causes ductile fracture.     

Acoustic emission from particulate-reinforced metal matrix composites

A systematic study of the effect of microstructural parameters on the fracture behaviour of silicon carbide particle reinforced aluminium matrix composites has been carried out. Acoustic emissions have been monitored during tensile testing, giving the size and number of emmissions as a function of strain. This has been shown to be simply related to the rate of void nucleation at the reinforcing phase. Both particle fracture and particle/matrix decohesion mechanisms can be detected. Void nucleation was observed from the onset of plastic deformation and a linear relationship between damage initiation rate and strain was found. The rate of emission increased with reiforcing particle size and volume fraction but was independent of matrix alloy composition and heat treatment. These results show that the failure strain of particulate metal matrix composites is not controlled solely by the onset of void nucleation at the reinforcing phase. Local failure processes in the matrix are shown to promote void coalescence and dominate the ductility. However, suppression of void nucleation at the particles increases the ductility. It is suggested that a critical number of fractured particles is required before failure.     

An investigation of the plastic fracture of AISI 4340 and 18 Nickel-200 grade maraging steels

The mechanisms of plastic fracture (dimpled rupture) in high-purity and commercial 18 Ni, 200 grade maraging steels and quenched and tempered AISI 4340 steels have been studied. Plastic fracture takes place in the maraging alloys through void initiation by fracture of titanium carbo-nitride inclusions and the growth of these voids until impingement results in coalescence and final fracture. The fracture of AISI 4340 steel at a yield strength of 200 ksi (1378 MN/mm²) occurs by nucleation and subsequent growth of voids formed by fracture of the interface between manganese sulfide inclusions and the matrix. The growth of these inclusion-nucleated voids is interrupted long before coalescence by impingement, by the formation of void sheets which connect neighboring sulfide-nucleated voids. These sheets are composed of small voids nucleated by the cementite precipitates in the quenched and tempered structures. The sizes of non-metallic inclusions are an important aspect of the fracture resistance of these alloys since the investigation demonstrates that void nuclea-tion occurs more readily at the larger inclusions and that void growth also proceeds more rapidly from the larger inclusions. Using both notched and smooth round tensile specimens, it was demonstrated that the level of tensile stress triaxiality does not effect the void nu-cleation process in these alloys but that increased levels of triaxial tension do result in greatly increased rates of void growth and a concomitant reduction in the resistance to plastic fracture.     

Ductile fracture by the growth and coalescence of microvoids of non-uniform size and spacing

A two-dimensional plane strain model of void growth and coalescence in a rigid/plastic solid, containing void sizes and spacings which can be highly non-uniform, is developed to investigate the effects of non-uniform distributions of void-nucleating particles on the ductility of a metal. The theoretical void-growth strains to ductile fracture for a wide variation in void diameters and spacings show that, for a given volume fraction of voids, the minimum ductile-fracture strain occurs when the voids are of uniform size and spacing. For the same volume fraction of voids, greatly increased ductility is likely to be achieved when the void sizes and spacings are highly non-uniform and the sub-cell volume fractions are also non-uniform.     

Monotonic and cyclic damage in metallic sheets

This paper aims to provide an account of some interesting features of damage in metallic sheets under monotonic and cyclic loading using the information and understanding developed through a series of experimental investigations conducted on interstitial free steels. The experiments primarily consisted of damage evaluation in un-notched and notched sheets vis-à-vis that in thick specimens under monotonic loading, and that on sheets by interrupted or continuous cyclic loading. Some salient observations indicate that: (i) void nucleation occurs in two different stages, originating from non-metallic inclusions and precipitates. The critical strain for void nucleation at precipitates (?_c) is lower for sheet metals than that in thick specimens. (ii) ?_c is a function of notch length in sheets, and the function assists to estimate the strength of particle/matrix interface, (iii) under cyclic loading, steel sheets exhibit non propagating microcracks below the endurance limit. Above the endurance limit slip bands promote formation of larger fatigue cracks primarily at ferrite grain boundaries. A series of grain boundary cracks link up to form meso-cracks, one of which grows to cause final failure and (iv) the growth of the macro-crack initially occurs in opening mode followed by its propagation in mixed mode through striations, intergranular cracking and through thickness necking prior to failure.     

The effect of molybdenum addition on properties of iron aluminides

The effect of the addition of up to 10 pct molybdenum on several metallurgical properties of Fe-28Al (at. pct) to which 1 pct TiB₂ was added for grain refinement has been studied. It was determined that the addition of molybdenum results in a decrease in grain size, an increase in the recrystallization temperature, and an increase in the DO₃ to B2 ordering transformation temperature. The solubility limit of molybdenum in the matrix of the base alloy was found to be about 6 pct. At this concentration, another 1 pct is dissolved in the TiB₂ precipitates. Tensile strengths were increased slightly by adding up to 2 pct Mo, but ductility decreased, even though grain sizes were reduced. The fracture mode in tension did not change with addition of molybdenum up to 2 pct.     

Microstructure, mechanical properties, and fracture mechanism of As-cast (Ti0.5Cu0.25Ni0.15Sn0.05Zr0.05)100−x Mo x composites

Copper mold cast cylinders of (Ti_0.5Cu_0.25Ni_0.15Sn_0.05Zr_0.05)_100−x Mo _x composites are prepared. Addition of Mo in the bulk glass-forming alloy induces the formation of a dendrite/matrix composite. For 3-mm-diameter cylinders, the matrix exhibits a homogenous ultrafine microstructure for Mo content of 2.5 at. pct, and a fine eutectic microstructure for 5 at. pct Mo. For 5-mm-diameter cylinders, the matrix exhibits a dendritic microstructure for 2.5 at. pct Mo, and exhibits a coarser eutectic microstructure for 5 at. pct Mo. Despite the formation of a dendrite/nanostructured matrix composite in the cylinders, the quenched surface layer with a nanoscale grain size dominates the deformation and fracture of the 3-mm-diameter cylinders. More than 56 vol pct quenched layer leads to a distensile fracture mode and the samples exhibit high fracture strength and high Young’s modulus but low ductility. For 5-mm-diameter cylinders, the composite microstructure becomes dominant due to its more than 64 vol pct volume fraction leading to a cone-shaped fracture surface. The samples exhibit lower yield strength and lower Young’s modulus but better ductility compared to the 3-mm-diameter cylinders. The mechanical behavior of the Mo-bearing composites strongly depends on the microstructural homogeneity and casting defects formed upon solidification.     

Hydrogen-Induced subcritical crack growth of a 12 Cr-1 Mo ferritic stainless steel

Subcritical crack growth and tensile ductility measurements have been made on a 12 Cr-1 Mo ferritic stainless steel at cathodic potentials in a 1 N H₂SO₄ solution at 25 °C. The tensile ductility was found to be a minimum at −600 mV (SCE) and both the subcritical crack growth behavior and tensile ductility were similar for material in the tempered (760 °C/2.5 h) or tempered-plus-segregated (540 °C/240 h) condition. A rising-load crack growth threshold of 20 MPa √m was measured and a rising-load fracture toughness of 110 MPa √m was determined from extrapolation of the stage III crack growth curve. A K-independent stage II was observed and a stage II crack growth rate of about 1 × 10⁻⁵ mm/s was measured. The fracture mode was a mixture of intergranular and quasi-cleavage for both heat treatments and for subcritical and tensile fracture tests. Impact fracture properties were independent of heat treatment and grain boundary composition with the fracture mode predominantly transgranular. The difference in the fracture mode for hydrogen-induced crack growth and dynamic crack growth was explained by a difference in the relationship between their stress profiles and the maximum grain boundary segregation distribution.     

Reinforcement shape effects on the fracture behavior and ductility of particulate-reinforced 6061-Al matrix composites

Particle shape effects on the fracture and ductility of a spherical and an angular particulate-reinforced 6061-Al composite containing 20 pct vol Al₂O₃ were studied using scanning electron microscopy (SEM) fractography and modeled using the finite element method (FEM). The spherical particulate composite exhibited a slightly lower yield strength and work hardening rate but a considerably higher ductility than the angular counterpart. The SEM fractographic examination showed that during tensile deformation, the spherical composite failed through void nucleation and linking in the matrix near the reinforcement/matrix interface, whereas the angular composite failed through particle fracture and matrix ligament rupture. The FEM results indicate that the distinction between the failure modes for these two composites can be attributed to the differences in the development of internal stresses and strains within the composites due to particle shape.     

The effects of superimposed hydrostatic pressure on deformation and fracture: Part II. Particulate-reinforced 6061 composites

The effects of various levels of superimposed hydrostatic pressure on the tensile ductility and fracture micromechanisms were determined for 6061 + 15 pct Al₂0₃ composites heat-treated to underaged (UA) and overaged (OA) conditions of equivalent yield strength. Superimposed pressures of 0.1, 150, and 300 MPa were selected, while the ductility significantly increased with each increment in pressure. At 300 MPa pressure, the monolithic 6061 and 6061 composite exhibited nearly identical ductility. It is shown that the levels of pressure chosen significantly inhibit void growth and coalescence in the composite. Void nucleation in the composites occurredvia the fracture of the reinforcement, followed by void growth and coalescence in the matrix. Tests conducted with 500 MPa pressure additionally provided evidence for suppression of void nucleation. Neither the ductility nor the pressure response was significantly affected by the heat treatments chosen. This article is based on a presentation made in the symposium “Quasi-Brittle Fracture” presented during the TMS fall meeting, Cincinnati, OH, October 21–24, 1991, under the auspices of the TMS Mechanical Metallurgy Committee and the ASM/MSD Flow and Fracture Committee.     

Fracture toughness: A rationalization of the role of microstructure in an α-β titanium alloy

The role of microstructure in affecting fracture toughness is examined by considering how microstructure affects the formation of a critical increment of crack extension leading to catastrophic fracture. It is proposed that this critical increment of crack extension occurs by void formation ahead of the main crack and growth back to it. Factors affecting void nucleation and void growth are, therefore, examined in this connection. Published data on the Ti-5.25Al-5.5V-0.9Fe-0.5Cu alloy are used for this purpose. In equiaxed(E) α structure voids nucleate at Eα/agedβ matrix interfaces for both tensile and fracture toughness tests. Although the interparticle spacing, A, is four times more effective than priorβ grain size,D _β, in controlling void growth rate,G _L, in a tensile test,D _β is at least five times more effective in controlling fracture toughness. For Widmanstätten plus grain boundary (W + GB) α structures there are marked similarities betweenG _Lbehavior as a function of GBa thickness, J, and the contribution of J to fracture toughness. These similarities have led to the proposal that the increase in fracture toughness, ΔK_Q, with increasingl is due to blunting of the crack tip, and the plateau in ΔK_Q which follows, with increasingl, is due to a balance between blunting and sharpening processes. Blunting occurs by crack penetration into GBα. The sharpening occurs by void formation and growth along GBα/agedβ interfaces back to the main crack.     

Tensile Properties and Fracture Behavior of ATI 718Plus Alloy at Room and Elevated Temperatures

The effect of temperature over the range of ambient to 704 °C and strain rate from 10⁻⁴ to 10⁻² s⁻¹ on the tensile properties and fracture behavior of ATI 718Plus was investigated. The results showed that with increase in temperature at a strain rate 10⁻⁴ s⁻¹, there is a small reduction in the yield strength, but a large drop in ductility at 704 °C. This reduction was accompanied by a change in fracture mode from ductile transgranular to brittle intergranular cracking. Detailed analysis of the microstructure and microchemistry of the areas around the crack using electron microscopy showed that the driving mechanism behind the failure at elevated temperatures and slow strain rates is oxygen-induced intergranular cracking, a dynamic embrittlement mechanism. In addition, the results suggest that the δ precipitates on the grain boundaries tend to oxidize and may facilitate the oxygen-induced intergranular cracking. Finally, an increase in strain rate at 704 °C caused a small increase in the yield strength and a huge increase in ductility. This increase in ductility was accompanied by a change in fracture mode from brittle-to-ductile failure. Possible mechanisms for the deformation, failure mechanisms, and strain rate dependence are discussed.