Microstructural development of brass alloys with various Bi and Pb additions

In the study, using the gravity casting method, adding 1.52%Pb, 0.5%Bi, 1%Bi and 1.5%Bi into the brass (Cu-40%Zn) alloy. The microstructural changes from the Widmanstätten into the networked structures when Pb was added to 1.5%. The microstructure was an acicular Widmanstätten when Bi contents were 0.5% and 1% and it was a plate Widmanstätten when Bi contents were 1.5%. There were four kinds of precipitation morphologies of Bi particles. The precipitation morphologies of Bi particles can be divided into a globular (<1 μm), a disc (=1 μm), discontinuous massive (>1 μm), and continuous block structures (about 20～30 μm). The Pb particles were embedded in the networked α phase and the Bi particles precipitated at the α/α and the α/β′ grain boundaries. The XRD analysis showed the high proportion of β′ phase with 0.5% Bi-brass and 1% Bi-brass and indicated a lower one with Pb-brass and 1.5% Bi-brass.     

Effect of titanium addition on the microstructure and inclusion formation in submerged arc welded HSLA pipeline steel

The effect of titanium addition on the SAW weld metal microstructure of API 5L-X70 pipeline steel was investigated. The relationship between microstructure and toughness of the weld deposit was studied by means of full metallographic, longitudinal tensile, Charpy-V notch and HIC tests on the specimens cut transversely to the weld beads. The best combination of microstructure and impact properties was obtained in the range of 0.02–0.05% titanium. By further increasing of titanium content, the microstructure was changed from a mixture of acicular ferrite, grain-boundary ferrite and Widmanstätten ferrite to a mixture of acicular ferrite, grain-boundary ferrite, bainite and ferrite with M/A microconstituent. Therefore, the mode of fracture also changed from dimpled ductile to quasi-cleavage. The results showed an increase in the titanium content of inclusions with increased titanium levels of weld metal. Titanium-base inclusions improve impact toughness by increasing the formation of acicular ferrite in the microstructure. No HIC susceptibility was found in the weld metals with titanium contents less than 0.09%.     

Structure formed in two-phase (α + γ) field and mechanical properties of a cryogenic alloy 10N7

The effect of different types of structures produced by quenching from the intercritical temperature range on the strength, plasticity, and impact toughness of the Fe-6.9Ni-0.1C alloy has been studied. Two structures—ferrite + globular cementite and lath martensite—have been used as the initial state. The rate of heating into the two-phase (α + γ) field has been selected such that four morphological types of twophase structures, namely, ferritic-martensitic (Widmanstäten or globular) and duplex (lamellar or lamellarglobular), could be formed in a single alloy as a result of partial α → γ and γ → α transformations. It has been found that the mechanical properties of the alloy depend on the type of the initial structure and on the rate and temperature of heating to the intercritical temperature range. It has been shown that the alloy with a Widmanstätten ferritic-martensitic structure has a more favorable combination of the strength and plasticity properties than the alloy with a globular structure. The alloy with a lamellar duplex structure offers a much higher level of the impact toughness, plasticity, and strength at low temperatures than the alloy with a Widmanstätten ferritic-martensitic structure.     

Effects of temperature and initial microstructure on the equal channel angular pressing of Ti–6Al–4V alloy

Equal channel angular pressing of Ti–6Al–4V alloy was successfully carried out isothermally above 600 °C. The equiaxed microstructure presented more uniform material flow than the Widmanstätten microstructure, which was discussed in relation to flow softening behavior of the two microstructures.     

Orientation relations between austenite, Widmanstätten carbides, and martensite in 150G4 high-carbon steel

The crystallographic tie between structural components in high-carbon steel 150G4 after isothermal γ → α transformation is investigated. The method of electron microscopic analysis is used for experimental determination of the orientation relation between Widmanstätten carbides and martensite, which is a consequence of the crystallographic relation between austenite and Widmanstätten carbides and between austenite and martensite. The causes of implementation of specific variants of orientation relations in high-carbon steels are discussed.     

Crystallographic variant selection in α–β brass

The transformation texture of α/β brass with a diffusional Widmanstätten α growth morphology has been investigated. Electron micrographs and electron backscattered diffraction was used to determine that the orientation relationship between the β phase and the α associated with nucleation at β grain boundaries was 44.3° 〈1 1 6〉. Crystallographic variant selection was observed across those prior β/β grain boundaries, but this has little effect on the transformation texture due to the crystal symmetry. The effect of the crystallographic variant selection on texture is further weakened by nucleation of diffusional transformed α in the grain interior.     

Crystallography of Widmanstätten austenite in duplex stainless steel weld metal

Abstract

Two kinds of austenite grow from δ-ferrite during the cooling of the duplex stainless steel weld deposits studied here, Widmanstätten plates and allotriomorphs which precipitate at δ–δ grain boundaries. It is found using microtexture measurements that the preferred crystallographic orientation of the Widmanstätten austenite can be estimated using established theory if it is assumed that there is an interaction between external stress and the growing plates. It is also demonstrated that the Widmanstätten and allotriomorphic forms of austenite may not be identically oriented even when the former appears to originate from the latter; this may be a consequence of differences in the transformation mechanisms of these two forms of austenite.     

Microstructure and mechanical properties of B<Subscript>4</Subscript>C-TiB<Subscript>2</Subscript>-Al composites fabricated by vacuum infiltration

B_4C-TiB_2-Al composites were fabricated by infiltrating aluminum into porous B_4C-TiB_2 preforms in vacuum. The microstucture and mechanical properties of the B_4C-TiB_2-Al composites were investigated. The hardness decreased, the flexurai strength increased, and the fracture toughness first increased and then decreased slightly with an increase in TiB_2 content. The B_4C-TiB_2-Al composite with 40wt.% TiB_2 showed the optimized properties. The infiltrated aluminum addition was the leading reason for the fracture toughness improvement of the composites. The tear ridges and dimples on the fracture surface of the composites increased gradually with the increase of infdtrated alu-minum content showing inter/transgranular fracture mode. The relationships between the microstructures and the mechanical properties of the composites were discussed.     

Weldability of X100 linepipe

Abstract

Research has been carried out to identify weld metal compositions and microstructures capable of meeting high strength and toughness requirements for X100 seam welded linepipe. Single pass, multiwire submerged arc welds were made in experimental, high strength low alloy steel plates using consumables to give a wide range of weld metal alloying. Work has shown that the optimum strength and toughness are obtained in Mo–B–Ti alloyed weld metals with P _cm values between 0.218 and 0.250. Weld metal microstructures were almost fully acicular ferrite with an ultrafine grain size (1–2 µm). Dilatometric studies demonstrated that at typical weld cooling rates the optimised welds transformed at significantly lower temperatures than those reported for X65 plate deposits, which contain acicular ferrite in the form of idiomorphic primary ferrite and intragranular Widmanstätten ferrite. The maximum rate of transformation in the optimised welds occurred between 515 and 570°C, which indicates that the acicular ferrite in this case consisted of intragranular Widmanstätten ferrite and/or bainite. The ferrite would appear to have a fine plate morphology growing from large as well as small inclusions, but not very far before the onset of hard impingement, thereby ensuring an ultrafine grain size. Tensile strengths of 708–784 MPa were achieved with an 80 J Charpy impact transition temperature toughness between -68 and -115°C. More highly alloyed weld metals containing 2–3%Mn and 1.5%Si transformed at lower temperatures and showed increased strength, but there was a substantial loss of toughness, attributed to the relatively unimpeded growth of large ferrite plates from larger inclusions, and the replacement of ultrafine acicular ferrite between these plates by blocks of martensite–austenite. One pass per side, multiwire submerged arc welds manufactured to the optimum weld metal chemistry confirmed their applicability for thin section X100 linepipe.     

Microstructure and mechanical properties of B_4C-TiB_2-Al composites fabricated by vacuum infiltration

B4C-TiB2-Al composites were fabricated by infiltrating aluminum into porous B4C-TiB2 preforms in vacuum. The microstucture and mechanical properties of the B4C-TiB2-Al composites were investigated. The hardness decreased, the flexural strength increased, and the fracture toughness first increased and then decreased slightly with an increase in TiB2 content. The B4C-TiB2-Al composite with 40wt.% TiB2 showed the optimized properties. The infiltrated aluminum addition was the leading reason for the fracture toughness improvement of the composites. The tear ridges and dimples on the fracture surface of the composites increased gradually with the increase of infiltrated aluminum content showing inter/transgranular fracture mode. The relationships between the microstructures and the mechanical properties of the composites were discussed.     

Study on dynamic fracture behavior of TA15ELI alloy under mode-Ⅱ loading by caustics method

The dynamic fracture behavior of TA15ELI alloy with lath-like microstructure was studied by caustics method.Specimens with double-side pre-notch were tested under the plane-stress condition at mode-Ⅱ loading with a drop hammer system.Caustics information recorded in films illustrated the histories of both crack length and stress intensity factor.The dynamic fracture toughness and crack growth velocity of TA15ELI with lath-like microstructure were determined to be 279 MPa·m1/2 and 32.6 m/s,respectively.SEM fractograph analysis showed a mixed feature of mainly plastic mode for TA15ELI alloy in dynamic mode-Ⅱ fracture.Shear localization was observed in the vicinity of the crack initiation area.     

Three-dimensional observations of proeutectoid cementite precipitates at short isothermal transformation times

Three-dimensional (3D) analysis techniques were used to examine a model Fe–1.34% C–13.0% Mn alloy to reveal connectivities of proeutectoid cementite precipitates whose growth was arrested at an early stage. The present results indicate that grain boundary cementite precipitates nucleate at austenite grain boundary edges and corners, then grow and spread on the grain boundary faces. No cementite precipitates were found to connect solely to austenite twin boundaries, which appear to act as barriers to precipitate growth rather than as potential nucleation sites. Cementite precipitates were all connected to austenite grain boundaries or cementite grain boundary precipitates, confirming 3D observations made earlier on a specimen transformed at a longer isothermal transformation time. Widmanstätten lath precipitates appear to emanate only from grain boundary cementite precipitates. While edges of several Widmanstätten plate precipitates were observed to intersect with areas of ‘clean’ austenite grain boundary, they may or may not nucleate directly on austenite grain boundaries.     

Fabrication and characteristics of magnesium alloys produced via powder metallurgy

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Ti-6.5Al-3.5Mo-1.5Zr-0.3Si合金中α′相和α″相的组织演变与力学性能

对Ti-6.5Al-3.5Mo-1.5Zr-0.3Si合金进行固溶时效处理,随后使用光学显微镜、扫描电镜、XRD衍射仪、拉伸试验以及冲击性能试验,分析固溶时效对合金中α′相和α″相的组织演变与力学性能的影响。结果表明,固溶处理后的微观组织中发生初生α相尺寸变小并趋于等轴化,尺寸较小的初生α相发生溶解并消失,其β转变组织变得不明显,经时效后的微观组织中析出大量α_s相,β转变组织更加明显。经固溶处理后,组织均由α+α′+α″相构成,经时效处理后,组织由α相和β相构成。合金经固溶处理后,其抗拉强度为1336 MPa,屈服强度为1070 MPa,断后伸长率为6%,断面收缩率为22%,冲击吸收能量为16 J。经时效处理后,强度随时效温度升高而升高,塑性趋势与之相反,其冲击性能几乎没有变化。合金经固溶处理后的拉伸与冲击断口微观形貌均由韧窝构成,为典型的韧性断裂。经时效处理后,拉伸和冲击断口的微观形貌有明显的高低起伏,随着时效温度的升高,韧窝的尺寸和数量减少,并出现撕裂棱以及空洞,断裂类型有向脆性断裂转变的趋势,但仍以韧性断裂为主。     

电子束熔丝成形Ti-6Al-3Nb-2Zr-Mo合金的组织与力学性能

试验采用电子束熔丝快速成形方法(EBRM)制备了Ti-6Al-3Nb-2Zr-Mo合金试样,研究了EBRM Ti6321合金化学成分、显微组织、力学性能及冲击韧性. 结果表明,该合金在熔丝成形中Al元素烧损1.0%左右,且合金内部没有元素偏析. EBRM Ti6321合金显微组织为沿沉积高度方向生长、晶贯穿多个沉积层的粗大柱状晶,柱状晶内部以α片层为主. EBRM Ti6321合金x向和z向的室温抗拉强度各项异性系数为2.6%,断裂方式均为韧性断裂. x向和z向冲击韧性均不低于80 J,各向异性系数为3.1%;冲击断口有大量的韧窝,为典型的韧性断裂.     

Tempering of Widmanstätten ferrite in cooling steel castings

The formation of Widmanstätten ferrite is not the goal of a technological process. On the contrary, it should be eliminated in order to avoid a decrease in the ductility of the steel. A detailed study of this structural component, which has no practical use, is often assumed to be unnecessary. At the same time, coarse-grain Widmanstätten ferrite makes it possible to use the range of magnification of a light microscope more efficiently for studying the mechanism of a shift γ→α transformation. The present paper is devoted to the variation of the properties and structure of Widmanstätten ferrite in the process of self-induced tempering that occurs in cooling of cast specimens of steel 20L.     

On the mechanism of the formation of widmanstätten graphite in flake graphite cast irons

The mechanism whereby Widmanstätten graphite develops during the solidification of flake graphite cast irons has been found to involve the preferential segregation and a complex interaction of specific elements at the surface of the graphite flake during solidification and the development of the plate like appendages in the solid austenite adjacent to the graphite flake. The literature has suggested that lead, calcium and hydrogen may be causal to the formation of Widmanstätten graphite, but has the interaction of these elements has not been effectively documented. While the formation of this degraded graphite is often attributed to the presence of a sufficient amount of lead alone, it has been observed that Widmanstätten graphite develops only in conjunction with a combination of factors operative at the graphite-austenite interface. Commercial flake graphite cast irons may exhibit Widmanstätten graphite as a function of lead and calcium content in the iron, moisture content in the molding media, solidification cooling rate and the rate of cooling immediately after solidification, etc. Lead contamination of cast irons was also observed to increase the chilling tendency of the iron. The detrimental effects of lead can be counteracted by the presence of rare earths in the iron, where rare earth elements react with lead to form stable, high melting point compounds.     

TiB2含量对B4C-TiB2-Al复合材料组织与力学性能的影响

通过在B4C-TiB2预烧体中真空熔渗Al制备了B4C-TiB2-Al复合材料,研究了TiB2含量对复合材料显微组织和力学性能的影响.结果表明: B4C-TiB2-Al复合材料主要由B4C,TiB2,Al和Al3BC等相组成;随着TiB2含量的增加,复合材料的HRA硬度逐渐降低,抗弯强度逐渐增大,断裂韧性先增大后稍微降低,当TiB2含量为40%(质量分数)时,复合材料的气孔率、硬度HRA、抗弯强度和断裂韧性分别为1.32%,80.3,559.4 MPa和7.83 MPa·m1/2;延性Al的加入,裂纹的偏转和分叉、B4C和TiB2晶粒的细化以及B4C基体和TiB2晶粒热膨胀的不匹配,是造成材料断裂韧性提高的主要原因;随着Al渗入量的增加,复合材料断口中金属撕裂棱及韧窝的比例增加.     

低C含Cu NV-F690特厚钢板的精细组织和强韧性

本文通过板坯连铸、钢板控轧控冷(TMCP)、固溶淬火回火(QT)工业生产流程,开发低C含Cu高强韧NV-F690特厚(厚度t为80 mm)船体和海洋平台用钢板.使用SEM,EBSD和TEM分别研究了淬火(Q)态和QT态钢板的精细组织,测试了距离钢板表面t/4处(高冷却速率)和芯部t/2处(低冷却速率)的室温硬度和拉伸性能,在-60和-80℃下进行了Charpy冲击(Charpy V notch,CVN)示波实验.结果表明,淬火速率较大有利于板条组织形成和提高大角度晶界比例,t/4处的组织为板条状贝氏体(LB),板条间存在细小片状马氏体/奥氏体(M/A)组元,晶粒间大角度晶界(>15°)体积分数为67.5%;t/2处的组织为粒状贝氏体(GB)+LB,大角度晶界体积分数为63.0%;Q态下的LB具有高位错密度,但晶粒内不存在Cu析出相.经过650℃回火150 min,钢板的强韧性匹配优良,低温下呈韧性断裂,大量含Cu弥散沉淀相在基体组织内析出.t/2处的M/A组元分解为Cr-Mo碳化物,贝氏体板条宽度为0.4μm,大角度晶界分数为62.5%;t/4处的LB板条回复,板条内存在与基体取向差较大的亚晶,大角度晶界分数提高到71.7%,板条平均宽度为0.2μm.在-80℃下,NV-F690钢板t/4处的韧性高于t/2处的韧性.随着纤维断裂位移的增大,韧窝断裂区比例和韧窝尺寸逐渐增大,NV-F690钢低温Charpy冲击能量逐渐提高.     

Heat-treated microstructure and mechanical properties of laser solid forming Ti-6Al-4V alloy

The effects of heat treatment on the microstructure and mechanical properties of laser solid forming (LSF) Ti-6Al-4V alloy were investigated. The influences of the temperature and time of solution treatment and aging treatment were analyzed. The results show that the microstructure of LSFed samples consists of Widmanstätten α laths and a little acicular in columnar prior β grains with an average grain width of 300 µm, which grow epitaxially from the substrate along the deposition direction (Z). Solution treatment had an important effect on the width, aspect ratio, and volume fraction of primary and secondary α laths, and aging treatment mainly affects the aspect ratio and volume fraction of primary α laths and the width and volume fraction of secondary α laths. Globular α phase was first observed in LSFed samples when the samples were heat treated with solution treatment (950°C, 8 h/air cooling (AC)) or with solution treatment (950°C, 1 h/AC) and aging treatment (550°C, above 8 h/AC), respectively. The coarsening and globularization mechanisms of α phase in LSFed Ti-6Al-4V alloy during heat treatment were presented. To obtain good integrated mechanical properties for LSFed Ti-6Al-4V alloys, an optimized heat treatment regimen was suggested.