Microstructural features of cracking in autogenous beryllium weldments

An investigation was conducted to determine the causes of centerline cracking in autogenous Be weldments. In agreement with earlier studies, features on the fracture surfaces consistent with hot shortness cracking were observed. However, the present study reveals a more complicated solidification path than previously described. Analytical transmission electron microscopy (TEM) studies of the weld centerline revealed low-melting constituents consisting of a MBe₁₂ grain boundary film and nodules containing elemental Al, elemental Si, and AlFeBe₄ phases. Possible solidifcation pathways to produce this structure are discussed in terms of available binary and ternary phase diagrams.     

Microstructural evolution and mechanical properties of Cr-Ru alloys

In an effort to enhance ductility and strength of Cr-base alloys, a series of Cr-Ru alloys with Ru contents ranging from 3 to 30 at. pct were made to study their microstructure evolution and mechanical properties. The microstructure of the alloys with 6 to 20 at. pct Ru showed signs of a eutectic structure. However, no corresponding eutectic reaction is indicated in the published Cr-Ru phase diagram. The yield strength of the Cr-Ru alloys increased with increasing Ru content at both room temperature and 1200 °C. The tensile ductility of Cr-3 at. pct Ru is about 1.5 pct at room temperature, while the alloys containing 6 at. pct or more Ru showed zero tensile elongation. The deformation mechanisms of the Cr-Ru alloys are discussed in terms of the microstructure and fracture behavior. This article is based on a presentation made in the symposium entitled “Beyond Nickel-Base Superalloys,” which took place March 14–18, 2004, at the TMS Spring meeting in Charlotte, NC, under the auspices of the SMD-Corrosion and Environmental Effects Committee, the SMD-High Temperature Alloys Committee, the SMD-Mechanical Behavior of Materials Committee, and the SMD-Refractory Metals Committee.     

Microstructural and mechanical properties of injection moulded gas and water atomised 17-4 PH stainless steel powder

Abstract

This paper describes the microstructural and mechanical properties of injection moulded 17-4 PH stainless steel gas and water atomised powder. Gas and water atomised stainless steel powders were injection moulded with wax based binder. The critical powder loading for injection moulding were 62·5 and 55 vol.-% for gas and water atomised powders respectively. Binder debinding was performed using solvent and thermal method. After dedinding the samples were sintered at different temperatures for 1 h in pure H₂. Metallographic studies were conducted to determine to extend densification and the corresponding microstructural changes. The results show that gas atomised powder could be sintered to a maximum (98·7%) of theoretical density, and water atomised powder could be sintered to a maximum (97·08%) of theoretical density. Maximum tensile strength was obtained for gas atomised powder sintered at 1350°C. The tensile strength of the water atomised powder sintered at the same temperature was lower owing to higher porosity. Finally, mechanical tests show that the water atomised powder has lower mechanical properties than gas atomised powder.     

Microstructural development and mechanical properties of interrupted aged Al-Mg-Si-Cu alloy

The effects of a recently developed interrupted aging procedure on the microstructural development and mechanical properties of the commercial Al-Mg-Si-Cu alloy 6061 have been studied using transmission electron microscopy (TEM), differential scanning calorimetry (DSC), and mechanical testing. This so-called T6I6 temper involves partially aging the alloy at a typical T6 temperature (the underaging stage), quenching, then holding at a reduced temperature (in this case 65 °C) to facilitate further hardening (the secondary aging stage), prior to final aging to peak properties at, or close to, the initial aging (T6) temperature (the reaging stage). The T6I6 aging treatment produces simultaneous increases in tensile properties, hardness, and toughness, as compared with conventional T6. The overall improvement in the mechanical properties of 6061 T6I6 is associated with the formation of a greater number of finer, and more densely dispersed, β″ precipitates in the final microstructure. Secondary precipitation took place during the interrupted aging stage of the T6I6 temper, resulting in the formation of a large number of Guinier-Preston (GP) zones that served as precursors to the needlelike β″ precipitates when elevated temperature aging was resumed.     

反挤压Zn-Mn二元合金的微观组织与力学性能

采用电子背散射衍射(EBSD)和室温拉伸试验,研究了不同挤压速度对Zn-Mn二元合金的微观组织与力学性能的影响.研究结果表明,挤压态Zn-Mn二元合金发生完全动态再结晶.在相同的Mn质量分数下,随着挤压速度的提高,Zn-Mn二元合金的强度上升、塑性下降,平均晶粒尺寸增加.晶粒尺寸和第二相是影响Zn-0.3Mn和Zn-0.7Mn力学性能的主要因素.在相同的挤压速度下,随着Mn质量分数的增加,第二相尺寸和数量增加,平均晶粒尺寸减小,但Zn-Mn二元合金的强度下降,伸长率显著上升.     

Microstructural and mechanical properties investigation of electrodeposited and annealed LIGA nickel structures

Lithographic, Galvanoformung, Abformung (LIGA) component fabrication is a process in which structural material is deposited into a patterned polymethyl-methacrylate (PMMA) mold realized through deep X-ray lithography. The process permits fabrication of metal microelectrome chanical systems (MEMS) components with representative dimensions that range from a few microns to several millimeters. This investigation characterizes the microstructure and mechanical properties of LIGA-fabricated nickel (LIGA Ni), electrodeposited using Watts bath and sulfamate bath chemistries. As a prelude to studying high-temperature joining processes in LIGA Ni components, an annealing investigation was conducted on samples fabricated from both bath chemistries. Mechanical properties and microstructural analyses on as-deposited and annealed samples were conducted using a mini servohydraulic load frame and the electron backscatter diffraction (EBSD) microtexture measurement technique. The deposits were found to have fine-grain, highly textured microstructures oriented with an acicular or columnar morphology relative to the plating direction. Previously uncharacterized, anomalous, local spatial variations in the crystallographic texture of the as-deposited microstructures were identified by EBSD analyses. Microstructural evolution during annealing seemed to follow a recovery, recrystallization, rapid grain-growth microstructural-evolution mechanism in LIGA Ni deposited from the Sulfamate bath chemistry and simply a recovery and grain-growth microstructural-evolution mechanism in LIGA Ni deposited from the Watts bath chemistry. The evolution of microstructure in the annealed samples corresponded with a dramatic drop in their strength and determined the limiting diffusion-bonding temperature for LIGA Ni components.     

Microstructural modelling of electrical conductivity and mechanical properties of sintered ferrous materials

Abstract

The role of microstructure on mechanical properties of sintered ferrous materials was studied using a method based on electrical conductivity measurement. The method was accompanied by quantitative fractography to evaluate the dewaxing and sintering process in iron compacts. The effects of manufacturing parameters, such as compacting pressure in the range of 150–800 MPa, sintering temperature from 400 to 1300°C, sintering time up to 8 h, and lubrication mode were investigated. Several mathematical models were checked to obtain the best one for prediction of electrical conductivity changes as a function of manufacturing parameters. The mechanical properties of the sintered compacts were also evaluated to establish a relationship between conductivity, total porosity, pore morphology, and mechanical behaviour. The results show that the electrical conductivity/resistivity of sintered materials is closely related to its microstructure, so that measuring these properties can replace destructive test methods for prediction of mechanical strength of sintered materials with homogeneous matrix microstructure. The application of the method is shown for sintered Fe, Fe–0·8%C, and Fe–1·5%Mo–0·7%C compacts.     

Microstructural evolution and mechanical properties of laser-welded joints of medium manganese steel

The cold-rolled 5% medium Mn steel was butt-welded using a fiber laser.The microstructure,distribution of microhardness,and tensile properties of the base metal(BM)and welded joint were investigated.The results showed that the fusion zone of the welded joint had the highest microhardness due to the formatio n of 100%marten site.A finely mixed microstructure of martensite,ferrite,and austenite was formed in the heat-affected zone,and there was no softened zone in this area.The tensile test results indicated that the ultimate tensile sirength and yield strength were higher for the joint than for BM.The joint efficiency was approximately 100%.All samples of the welded joirn failed at the location of BM during tensile deformation.The fracture surfaces of the BM and welded joint were mainly ductile fractures.The BM and welded joint exhibited strain rate independence of the tensile strength and yield strength at strain rates of 0.01-1 s_1,while the yield strength of the BM and welded joint increased rapidly when the strain rate reached 5 s_1 due to changes in the dislocation moveme nt mechanisms.The uniform elongation of the BM and welded joint decreased with in creasing strain rate.     

Sheet metal forming behaviour and mechanical properties of TRIP steels

Recently various kinds of high-strength sheet steels have been developed to meet the requirements of the automotive industry such as passive safety, weight reduction and saving energy. Usually the main problem of high-strength steels is their inferior ductility. Multiphase steels however show a very good combination of strength and formability so that the applicable region of high-strength steels has been widely enlarged. Multiphase steels have been developed for various purposes because of their ability to tailor properties by adjusting the type, the amount, and the distribution of different phases. Especially new developed triple-phase steels which make use of the TRIP effect (transformation induced plasticity) can further improve formability as well as strength due to the transformation of retained austenite to martensite during the deformation. In this work the transformation behaviour and the mechanical properties of low alloyed TRIP steels were investigated. The influence of the annealing parameters on transformation behaviour and on the amount of retained austenite were determined. In addition the temperature dependence of the mechanical properties and the effect of testing speed on the formability were studied. The investigation was carried out on seven different TRIP steels with different chemical compositions, especially the influence of the microalloying element niobium was considered. For reasons of comparison various mild and high-strength steels were tested parallel to the TRIP steels. It was found that the investigated TRIP steels offer very attractive combinations of elongation and strength values. An interesting temperature dependence of the mechanical properties can be observed, in such a way that the elongation values of the TRIP steels possess a maximum between +50 and +100°C. Due to its effect on grain size and on precipitation behaviour the addition of niobium leads to higher strength values without a strong decrease in ductility. In general, the mechanical properties are strongly affected by the type and the distribution of the different phases. The most important parameters, however, to influence the mechanical behaviour are the amount and the stability of the retained austenite, which are mainly controlled by the heat treatment and the chemical composition.     

Microstructural engineering applied to the controlled cooling of steel wire rod: Part II. Microstructural evolution and mechanical properties correlations

In the second part of this paper, the microstructural evolution and mechanical properties of plain-carbon steel rods which have been subjected to known cooling conditions are described. Specifically, the isothermal phase transformation kinetics for the decomposition of austenite into ferrite and pearlite have been determined with a diametral dilatometer and characterized in terms of empirical coefficients in the Avrami equation. The continuous cooling transformation (CCT) start time, fraction ferrite, ferrite grain diameter, and pearlite interlamellar spacing have been quantified and correlated with steel composition and cooling rate. Tensile tests have been conducted to obtain yield strength (YS) and ultimate tensile strength (UTS), which, with literature data, have been related to the microstructure and composition of the steels. These correlations, which apply to both hypoeutectoid and eutectoid steels, have been incorporated in a mathematical model of the Stelmor process, to be described in Part III of this article.^[441] Formerly Graduate Student, The University of British Columbia.     

Microstructural evolution and mechanical properties of the AA8011 alloy during the accumulative roll-bonding process

The investigation of the microstructure and mechanical properties has been conducted on an AA8011 alloy produced by a novel intense plastic straining process named accumulative roll bonding. The results show that an ultrafine-grained 8011 alloy, having a mean grain (or subgrain) size less than 1 μm, was successfully accumulative roll-bonded (ARB) at room temperature (RT-ARB) and at 200 °C (HT-ARB). The average grain (or subgrain) sizes of the RT-ARB and HT-ARB samples were reduced greatly from about 25.8 μm initially to 650 to 700 nm and 800 to 900 nm, respectively. After several cycles of accumulative roll bonding, most regions of this material were filled with ultrafine grains with high-angle boundaries. The ambient tensile strengths of the RT-ARB and HT-ARB samples increased with equivalent strain only up to the strain of 2.4. After that, the strengths of the RT-ARB samples nearly leveled off, and the strengths of the HT-ARB samples decreased with equivalent strain above the strain of 2.4. Furthermore, the elongation in both the RT-ARB and HT-ARB samples decreased greatly after the first cycle and then increased continuously with strain. The softening behavior happened in HT-ARB samples above a strain of 2.4, which is mainly attributed to the continuous recrystallization, dynamic recovery, and static recovery during and/or after the accumulative roll-bonding process.     

Microstructural evolution and mechanical properties of AlCrFeNiCoC high entropy alloy produced via spark plasma sintering

AlCrFeCoC high entropy alloy was synthesised through mechanical alloying and spark plasma sintering. The milling time had a strong influence on the particles shape and structure and consequently on microstructural and mechanical evolution of the material after sintering. The material's microstructure after spark plasma sintering contained FCC and BCC phases as well as ordered BCC and C₂₃C₆ carbide. The material's strength increased with increasing the milling time because of the finer microstructure and phases formation evolution.     

Influence of residual stresses and loading frequencies on corrosion fatigue crack growth behavior of weldments

The effect of residual stresses and loading frequencies on corrosion fatigue crack growth behavior under synthetic seawater with a free corrosion potential was examined using center-cracked tension (CCT) and single edge-cracked tension (SECT) specimens machined from mild steel butt-welded joints and the parent material. A series of fatigue crack growth tests were carried out with a sinusoidal loading wave form at a stress ratio of 0.05 with a loading frequency of 0.017 to 6.7 Hz. The results show that the crack growth resistance of a weld metal in the SECT specimen is higher than that in the CCT specimen regardless of testing conditions. The discrepancy is attributed to the differences in residual stress distribution at the crack tip in the two specimen geometries. The crack growth rate of the weld metal in the CCT specimen in seawater increased with decreasing loading frequency. The acceleration of the crack growth rate may be related to the occurrence of brittle striation or cleavage due to hydrogen embrittlement. It was found that the corrosion fatigue crack growth rate of a welded joint with tensile residual stress can be predicted using the effective stress intensity factor range, which takes into account both the residual stress and the loading frequency effects.     

金属纤维毡在气体/液体中过滤性能的差异与关系

金属纤维毡的过滤性能会因通过流体的差异产生变化,因此不能统一规定其性能参数。本文采用不同牌号的金属纤维毡分别在气体和液体流体下测试了渗透系数和过滤精度,结果表明同一牌号纤维毡在两种流体下的渗透系数和过滤精度有明显差异,气体流体下的渗透性和过滤精度远高于液体流体下的参数,但不同牌号的纤维毡的渗透性和过滤精度在两种流体下均存在着相近的比例关系,两者在一定范围内可以通过近似换算方便使用。     

Thermal and mechanical properties of aluminized fabrics for use in ferrous metal handling operations

Protective garments are normally worn in molten handling operations to provide some protection against molten metal splashes. These garments are also intended to provide protection against radiant heat, and they should be as heat resistant and comfortable as possible. Asbestos-based fabrics have been employed for many years, but recently some concern has been expressed over possible asbestos exposure. This program was undertaken to explore the ability of several types of fabrics to resist heat transfer during molten metal impact. A molten metal splash test, along with standard methods for determining tensile strength, flame resistance, and abrasion-flexing resistance were used to evaluate several classes of protective fabrics. The results indicate that there are materials available that offer equal or better mechanical properties and thermal protection compared to aluminized asbestos.     

Elevated temperature mechanical properties and residual tensile properties of two cast superalloys and several nickel-base oxide dispersion strengthened alloys

The elevated temperature tensile, stress-rupture and creep properties and residual tensile properties after creep straining have been determined for two cast superalloys and several wrought Ni-16Cr-4Al-yttria oxide dispersion strengthened (ODS) alloys. The creep behavior of the ODS alloys is similar to that of previously studied ODS nickel alloys. In general, the longitudinal direction is stronger than the long transverse direction, and creep is at least partially due to a diffusional creep mechanism as dispersoid-free zones were observed after creep-rupture testing. The tensile properties of the nickel-base superalloy B-1900 and cobalt-base superalloy MAR-M509 are not degraded by prior elevated temperature creep straining (at least up to 1 pct) between 1144 and 1366 K. On the other hand, the room temperature tensile properties of ODS nickel-base alloys can be reduced by prior creep strains of 0.5 pct or less between 1144 and 1477 K, with the long transverse direction being more susceptible to degradation than the longitudinal direction.     

Microstructural instabilities in an industrial gas turbine engine vane

Microstructural changes occurring in a Udimet 500 turbine vane during service have been examined. The gamma prime (γ’) content and carbide contents determined experimentally were in close agreement with numerical estimates obtained by Decker’s⁴ method. The method of Barrows and Newkirk¹⁰ for predicting sigma phase formation was examined and numerical predictions compared with experimental observations. The γ matrix was correctly predicted to be sigma prone when these calculations were applied to experimentally determined γ phase compositions. When the calculations were applied to computationally determined compositions the γ phase was predicted to be stable if residual aluminum in the γ phase was considered to act as a stabilizing element, or unstable if aluminum was considered to promote the formation of sigma. These discrepancies are discussed in terms of the effects of carbide and sigma formation on matrix chemistry and the interdependence of these reactions.     

Microstructural instabilities in an industrial gas turbine engine vane

Microstructural changes occurring in a Udimet 500 turbine vane during service have been examined. The gamma prime (γ’) content and carbide contents determined experimentally were in close agreement with numerical estimates obtained by Decker’s⁴ method. The method of Barrows and Newkirk¹⁰ for predicting sigma phase formation was examined and numerical predictions compared with experimental observations. The γ matrix was correctly predicted to be sigma prone when these calculations were applied to experimentally determined γ phase compositions. When the calculations were applied to computationally determined compositions the γ phase was predicted to be stable if residual aluminum in the γ phase was considered to act as a stabilizing element, or unstable if aluminum was considered to promote the formation of sigma. These discrepancies are discussed in terms of the effects of carbide and sigma formation on matrix chemistry and the interdependence of these reactions.