The effect of hydrostatic pressure on the deformation behavior of maraging and HY-80 steels and its implications for plasticity theory

Earlier results showed that the difference between the tensile and compressive strengths of tempered martensites is primarily a manifestation of the general pressure dependence of flow stress in these materials. However, the same results also showed that the volume expansion after deformation was much smaller than that predicted by the normality flow rule of plasticity theory for materials with such pressure dependence. Additional results now obtained on maraging and HY-80 steels support these conclusions. The results for all these materials exhibit a strong, but not perfect, correlation between pressure dependence, yield stress, and volume expansion. The volume expansion, however, which is believed to result primarily from the generation of new dislocations, is very small and does not appear to be essential to the pressure dependence. Most of the pressure dependence, the portion responsible for the discrepancy with the normality flow rule, may be an effect on dislocation motion. The results suggest that an appropriate plasticity model would be one in which the octahedral shear yield stress is linearly dependent on the mean pressure, but the volume change is negligible in violation of the normality flow rule. Such a model has been proposed previously for the plastic deformation of soils. However, unlike that model, the present theory includes strain hardening.     

The influence of hydrostatic pressure on the tensile deformation and fracture of copper

An investigation has been made into the effects of hydrostatic pressures up to 600 MPa on the tensile deformation and fracture mechanisms of commercial copper. Fracture occurred by the normal tensile fracture mechanism involving void coalescence followed by shear tearing at pressures up to 300 MPa. At pressures in excess of 300 MPa fracture occurred entirely by a mechanism involving highly localized bands of intense shear deformation. This mechanism resulted in fracture at a chisel-point so that the natural strain to fracture at these pressures approached infinity.     

The influence of hydrostatic pressure on the tensile deformation and fracture of copper

An investigation has been made into the effects of hydrostatic pressures up to 600 MPa on the tensile deformation and fracture mechanisms of commercial copper. Fracture occurred by the normal tensile fracture mechanism involving void coalescence followed by shear tearing at pressures up to 300 MPa. At pressures in excess of 300 MPa fracture occurred entirely by a mechanism involving highly localized bands of intense shear deformation. This mechanism resulted in fracture at a chisel-point so that the natural strain to fracture at these pressures approached infinity.     

静水压与溶解氧耦合作用对低合金高强钢腐蚀电化学行为的影响

采用动电位极化测试和扫描电子显微镜/能谱仪表征,通过理想动电位极化曲线分析方法和微观腐蚀形貌观察研究了静水压与溶解氧耦合作用对低合金高强钢在质量分数为3. 5%Na Cl溶液中腐蚀电化学行为的影响.结果表明:随着静水压和溶解氧溶度的同时增大,腐蚀电位先增高而后逐渐降低,腐蚀电流呈非线性增长;静水压与溶解氧在腐蚀过程中存在相互竞争抑制关系,在静水压与溶解氧同时增长过程中,溶解氧首先促进阴极反应过程并抑制阳极反应过程,而后静水压逐渐加速阳极过程并对阴极反应过程有一定的抑制作用;静水压与溶解氧耦合作用加速了腐蚀产物膜的生长,增加了低合金高强钢表面点蚀坑的数量和生长尺寸.     

静水压与溶解氧耦合作用对低合金高强钢腐蚀电化学行为的影响

采用动电位极化测试和扫描电子显微镜/能谱仪表征, 通过理想动电位极化曲线分析方法和微观腐蚀形貌观察研究了静水压与溶解氧耦合作用对低合金高强钢在质量分数为3.5% NaCl溶液中腐蚀电化学行为的影响. 结果表明: 随着静水压和溶解氧溶度的同时增大, 腐蚀电位先增高而后逐渐降低, 腐蚀电流呈非线性增长; 静水压与溶解氧在腐蚀过程中存在相互竞争抑制关系, 在静水压与溶解氧同时增长过程中, 溶解氧首先促进阴极反应过程并抑制阳极反应过程, 而后静水压逐渐加速阳极过程并对阴极反应过程有一定的抑制作用; 静水压与溶解氧耦合作用加速了腐蚀产物膜的生长, 增加了低合金高强钢表面点蚀坑的数量和生长尺寸.      

Influence of sulfide inclusion on ductility and fracture behavior of resulfurized HY-80 steel

The influence of sulfide inclusions on the ductile fracture process of experimental HY-80 steels having graded sulfur levels from 50 to 500 ppm and heat-treated to different strength levels was studied with respect to mechanical properties, namely, tensile ductility and Charpy impact en-ergy. Sulfide inclusions are found to have deleterious effect on both axisymmetric ductility and Charpy impact properties, whereas the plane strain ductility was found to be less sensitive to sulfide inclusions. The effect of interaction between the inclusion and the matrix and the as-sociated stress strain distribution at the void nucleating sites, which control the fracture process by microvoid coalescence, were discussed in the light of various models to suggest a micro-mechanism of fracture. Other toughness parameters obtained from instrumented impact tests were evaluated and discussed as a function of sulfur content.     

The morphology and substructure of martensite in maraging steels

Thin foil transmission electron microscopy, X-ray diffraction and dilatometric techniques have been used to study the martensitic γ → α transformation in three steels with nominal contents of 8 pct nickel and 0.2 pct beryllium and chromium contents of 12, 14 and 16 pct. In each case the martensite formed as laths with a habit plane close to {225}_γ. With increasing chromium content and increasing cooling rate greater numbers of the laths were observed to be internally twinned. Detailed analysis of the martensitic transformation suggested that the internally twinned laths are formed by a sequence of γ→ ε or faulted γ→ ά. The orientation relationships between the three phases γ, ε and α, determined from selected area diffraction analysis, corresponded to Kurdjumov-Sachs.     

High-strain-rate behavior of low-alloy multiphase aluminum- and silicon-based transformation-induced plasticity steels

High-strength, low-alloy transformation-induced plasticity (TRIP) steels are advanced multiphase steel grades that combine high-strength levels with an excellent ductility, making them ideally suited for application in crash-relevant parts of automotive car bodies. The enhanced plastic hardening and deformability are due to a complex interaction between the microstructural phases and to the transformation of metastable austenite to martensite during plastic deformation. During high-strain-rate loading, not only the material but also the transformation will be influenced by adiabatic heating. The impact-dynamic properties of CMnAl- and CMnSi-TRIP steels were determined in the range of 500 to 2000 s⁻¹ using a split Hopkinson tensile bar (SHTB) setup. Bake-hardening treatments were applied to study the effect of strain aging. The experiments show that strain-rate hardening is superior to thermal softening: yield stresses, deformation, and energy dissipation increase with the strain rate. Phenomenological material models were investigated to describe the strain-rate and temperature-dependent behavior of TRIP steels. Both the Johnson-Cook model and an extended version of the Ludwig model were found to give good agreement with the experimental data.     

The effect of reverted austenite on the mechanical properties and toughness of 12 Ni and 18 Ni (200) maraging steels

A study has been made of the effects of reverted austenite on the mechanical properties and toughness of three maraging steels. It is found that reverted austenite has no detrimental effects on the mechanical properties and toughness and even improves these properties when precipitated along martensite lath boundaries. This occurs for underaged specimens. A detrimental effect on toughness is found when reverted austenite precipitates at prior austenitic grain boundaries which occurs for over aged specimens. Overaged precipitates are also responsible for the decrease in toughness in the overaged condition. C. A. PAMPILLO, formerly with the Department of Metallurgy and Materials Science, Carnegie-Mellon University, Pittsburgh, Pa. H. W. PAXTON, on leave from Carnegie-Mellon University     

The effects of superimposed hydrostatic pressure on deformation and fracture: Part I. Monolithic 6061 aluminum

The effects of various levels of superimposed hydrostatic pressure on the tensile ductility and fracture micromechanisms were determined for 6061 specimens heat-treated to underaged and overaged conditions of equivalent yield strength. Superimposed pressures of 0.1, 150, and 300 MPa were selected; the ductility increased between 0.1 and 150 MPa and remained constant between 150 and 300 MPa. It is shown that the levels of pressure chosen inhibit void growth and coalescence. Void nucleation occurred at nonmetallic inclusions, and neither the ductility nor pressure response were 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.     

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.     

Cyclic creep and cyclic deformation of high-strength spring steels and the evaluation of the sag effect: Part I. Cyclic plastic deformation behavior

The cyclic creep and cyclic plastic deformation behavior of two commercial suspension spring steels of high hardness levels, namely, SAE 9259 and SAE 5160, were studied under different testing conditions of cyclic peak stress and cyclic stress ratio. The experimental results indicate that both the cyclic stress ratio and cyclic peak stress have strong, but complicated, effects on the cyclic creep and cyclic plastic deformation behavior of these materials. It has also been found that the addition of silicon can increase the resistance of these steels to cyclic creep and cyclic plastic deformation, although the extent of this increase is also related to other cyclic deformation conditions. A transition in the relationship between the total plastic strain range and the cyclic stress ratio (R) has been detected at approximately R=0.5. The mechanism of such a transition is explained by the operation of cross-slip during the unloading process of cycling. Moreover, a cyclic softening behavior of these spring steels in the quench-tempered condition was also detected and is attributed to the activation and reorganization of obstacle dislocations introduced into the steels during the process of martensitic transformation. More importantly, this study has indicated that parameters such as the cyclic creep strain, the cyclic creep rate in the secondary creep stage, and the total cyclic plastic strain range can better reflect, and should be used to depict and characterize, the sag behavior of spring steels as well as other materials. Finally, the effect of silicon on sag behavior, in comparison with the results from the Bauschinger-effect test, has also been discussed through the influence of Si on carbide formation and distribution.     

Effects of martensite morphology and tempering on dynamic deformation behavior of dual-phase steels

The effects of martensite morphology and tempering on the quasistatic and dynamic deformation behavior of dual-phase steels were investigated in this study. Dynamic torsional tests were conducted on six steel specimens, which had different martensite morphologies and tempering conditions, using a torsional Kolsky bar, and then the test data were compared via microstructures, tensile properties, and fracture mode. Bulky martensites were mixed with ferrites in the step-quenched (SQ) specimens, but small martensites were well distributed in the ferrite matrix in the intermediate-annealed (IA) specimens. Under a dynamic loading condition, the fracture mode of the SQ specimens was changed from cleavage to ductile fracture as the tempering temperature increased, whereas the IA specimens showed a ductile fracture mode, irrespective of tempering. These phenomena were analyzed in terms of a rule of mixtures applied to composites, microstructural variation, martensite softening and carbon diffusion due to tempering, and adiabatic shear-band formation.     

The effect of industrial composition variations on the transformation behavior of ultralow-carbon steels

The transformation behavior and microstructural evolution of two alloys representing the extreme rich and lean compositional variations expected for production quantities of an ultralow-carbon (ULC) steel have been characterized. Dilatometry, optical microscopy, transmission electron microscopy (TEM), and microhardness measurements were used to identify the microstructures produced at various cooling rates and to develop continuous cooling transformation (CCT) diagrams for these ULC steels. It was demonstrated that the compositional variations expected from production heats made with current fabrication practices cause extensive variations in the transformation behavior, microstructure, and microhardness (as well as other mechanical properties) of these steels.     

Effect of prestrain and deformation temperature on the recrystallization behavior of steels microalloyed with niobium

The evolution of microstructure during the hot working of steels microalloyed with Nb is governed by the recrystallization kinetics of austenite and the recrystallization-precipitation interaction. The present study focuses on the effects of prestrain and deformation temperature on the rectrystallization behavior in these steels. The extent of recrystallization is characterized by a softening parameter calculated from a series of interrupted plane strain compression tests carried out at different deformation temperatures and strain levels. The results indicate that at low temperatures, softening is caused by static recovery, while at higher temperatures, static recrystallization is the predominant mechanism. The recrystallization-stop temperature (T _5pct) and the recrystallization-limit temperature (T _95pct), marking the beginning and end of recrystallization, respectively, are determined as a function of strain. In order to achieve a homogeneous microstructure, finish rolling should be carried out outside the window of partial recrystallization (T _5pct<T<T _95pct), as determined in this study. The Nb(CN) precipitation kinetics have been calculated using a model proposed in an earlier work, and these results are used to estimate the precipitate pinning force under the given processing conditions. Based on these estimations, a criterion has been proposed to predict the onset of recrystallization. The predicted results are found to be in reasonably good agreement with the experimental measurements.     

Role of nitrogen in the cyclic deformation behavior of duplex stainless steels

The role of nitrogen in the cyclic deformation behavior of duplex stainless steels (DSS) has been studied under fully reversed total-strain amplitude. The cyclic hardening-softening curves show that cyclic stress levels become lower with increasing nitrogen content. The cyclic softening becomes more evident with increasing nitrogen content. It can be attributed to the greater strength of austenite than that of ferrite as plastic strain is accumulated beyond the critical strain. This is achieved by a higher strain hardening of austenite than that of ferrite with increasing nitrogen content. In this regard, the higher austenite volume fraction is also responsible for higher cyclic softening, resulting from much stronger strain partitioning in ferrite. Dislocation-structure observations reveal that severe strain localization in ferrite causes greater cyclic softening in the alloys with higher nitrogen content. The cyclic stress-strain response can be described in terms of two regimes with low and high plastic-strain amplitudes. In the former regime, the cyclic strain-hardening rates (CSHRs) become higher with increasing nitrogen content because austenite dominantly takes part in plastic deformation, being more strain hardened due to the higher nitrogen content in austenite. On the contrary, those in the high-plastic-strain-amplitude regime hardly change because ferrite, more dominantly accommodating plastic strain, rarely shows a change of strain-hardening behavior due to the similar nitrogen content in ferrite.     

The effects of hydrostatic pressure on the compressive mechanical behavior of L12 Al3Ti-based intermetallic

A Mn-modified Ll₂ Al₃Ti-base intermetallic was subjected to compressive deformation at room temperature under superimposed hydrostatic pressures up to 1000 MPa. It is found that its yield strength is essentially unaffected by hydrostatic pressure. The apparent work-hardening rate of true stress-strain curves increases substantially with increasing hydrostatic pressure. Vickers microhardness of pressurized samples always increases with increasing compressive strain, indicating the work-hardening behavior, but it is independent of the superimposed hydrostatic pressure up to 1000 MPa. The density of microcracks (cm/cm²) observed in specimens compressed under hydrostatic pressure increases with increasing compressive strain for each level of pressure. At each constant compressive strain, the corresponding density of microcracks is higher for specimens tested under 170 MPa hydrostatic pressure than that for specimens tested in the 400 to 1000 MPa hydrostatic pressure range. This may imply that besides propagation, the nucleation stage may also be suppressed by a superimposed hydrostatic pressure. It is proposed that both the cataclastic (characteristic for deformation of some rocks) and plastic deformation occur simultaneously during compressive deformation of Ti trialuminides under hydrostatic pressure.