Interface characteristics in cemented carbides with alternative binders

Cemented tungsten carbides with Co, Ni or Fe binders were studied by transmission electron microscopy. Particular attention was paid to phase and grain boundaries. A striking feature is the high frequency of coherent carbide/binder interfaces with Fe. The Fe rich binder adopts an epitaxy orientation relationship with prismatic facets of WC, with a parametric misfit of about 1.5%. A special orientation relationship with basal facets of WC grains is sometimes observed with Ni binder, as already noticed for Co. It is associated with a parametric misfit of about 15%. Binder segregation in WC grain boundaries was studied taking into account the effect of carbon content in the alloys. Whatever the binder, no influence of the carbon content could be pointed out. The analyses performed in random grain boundaries in WC-Co alloys agree with the literature value of 0.5 monolayer of segregated Co while slightly larger values are obtained for Ni and Fe binder. ∑ = 2 special grain boundaries were studied in WC-Co and WC-Ni alloys and no segregation was detected. The higher grain boundary segregation as well as the occurrence of coherent interfaces should influence the mechanical properties of WC-Fe alloys.     

On the microstructure and properties of 100Cr6 steel processed in the semi-solid state

After semi-solid treatment and rapid quenching, bearing steel 100Cr6 exhibits a martensitic structure with a large amount of retained austenite along the grain boundaries. The toughness values are significantly lower than after conventional hardening, even when the alloys are post-processed by an additional heat treatment step. Slow cooling from the freezing range results in a reduced amount of retained austenite and improved toughness properties. The low impact toughness values of the rapidly cooled conditions are associated with the appearance of intergranular fracture, while the slowly cooled condition fails in a transgranular manner. The structural development and the mechanical behaviour are explained by severe segregation of the main alloying elements during solidification, but also by sulphur enrichment in the remaining liquid phase. Chain-like precipitates of MnS are formed along the grain boundaries, similar to the well-known ‘burning’ phenomenon of low-alloyed steels.     

退火对Ni-W合金药型罩组织与性能的影响

用电沉积方法制备了含钨1.76%(质量分数)的Ni-W合金药型罩。通过拉伸试验对原始电铸态和热处理后样品的力学性能进行了评价,并利用X射线衍射、透射电子显微镜、俄歇电子能谱等技术对原始电铸态和热处理后样品的微观结构进行了分析。结果表明,通过电铸技术制备的Ni-W合金药型罩材料强度较高,晶粒细小并具有(111)织构。制备的Ni-W合金仍具有面心立方结构,W固溶在Ni的晶格中。Ni-W合金药型罩材料经300℃退火后,晶粒发生异常长大,(111)织构转变为(220)织构。经500℃退火后,由于硫在晶界的偏析,发生沿晶断裂,严重影响了材料的力学性能。     

Evidence for excess vacancies at sliding grain boundaries during superplastic deformation

Rapid quenching of Al–Mg alloys during superplastic deformation has revealed the presence of nano-scale cavities along many grain boundaries. They were observed only under deformation conditions where grain boundary sliding was the dominant mechanism. Fine-probe compositional measurements revealed that the cavity surface is enriched in Mg, and in situ heating in the transmission electron microscope demonstrated that they are not stable above 175°C. Kinetic analysis of cavity formation during a quench concludes that the cavities did not exist during deformation but were formed as the sample cooled. It is proposed that these cavities are evidence for a localized excess of vacancies during grain boundary sliding.     

INTERGRANULAR EMBRITTLEMENT IN PLASMA CARBURIZED LAYER

This paper deals with the cause of intergranular fracture occurred in the retained austeniticregion in plasma carburized layer.The results show that the presence of retained austenite,which has a good effect on the impact toughness,has no relation to this embrittlement.Analy-sis by Auger electron spectroscopy shows that the impurities S and P segregate at the grainboundaries is the main reason of the intergranular embrittlement in carburized layer.However,the segregation of P and S can be removed by reheating and quenching treatment.     

THE THERMAL STABILITY OF PdTa_1 Si_(16.5) ALLOY OF DIFFERENT GLASSY STATES

以示差扫描量热法(DSC)研究了处于不同玻璃态PdTa_1Si_(16.5)合金的热稳定性和结晶过程。非晶态合金处于不同玻璃态时,其玻璃转变和结晶过程中的行为及其淬火态与退火态的热稳定性表现出差异。这是制备非晶态材料保证性能均匀性必须注意的一个问题。     

Stabilization and strengthening of nanocrystalline copper by alloying with tantalum

Nanocrystalline Cu-Ta alloys belong to an emerging class of immiscible high-strength materials with a significant potential for high-temperature applications. Using molecular dynamics simulations with an angular-dependent interatomic potential, we study the effect of Ta on the resistance to grain growth and mechanical strength of nanocrystalline Cu-6.5 at.% Ta alloys. Ta segregation at grain boundaries greatly increases structural stability and strength in comparison with pure copper and alloys with a uniform distribution of the same amount of Ta. At high temperatures, the segregated Ta atoms agglomerate and form a set of nanoclusters located at grain boundaries. These nanoclusters are capable of pinning grain boundaries and effectively preventing grain growth. It is suggested that the nanoclusters are precursors to the formation of larger Ta particles found in Cu-Ta alloys experimentally.     

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高强度23MnNiMoCr54链环钢是煤矿设备主要部件,在进行生产测试过程中发生断裂。对该链条钢裂纹部位进行了金相检测、化学元素成分检测、扫描电镜和断口检验和俄歇电子能谱仪分析。结果表明,链条钢的化学元素组成、气体含量、低倍组织和夹杂物含量符合相关技术标准要求;其金相组织为回火屈氏体,实际晶粒度为4级,断口为典型的脆性断口,断裂方式为沿晶脆性断裂。由于链环在生产过程中热处理不当,淬火加热温度过高,产生过热组织,造成链环晶粒粗大,在随后的回火过程中由于加热温度或者加热时间不当,导致晶界处杂质元素偏析,从而形成回火脆性。在晶粒粗大的情况下同时出现回火脆性,导致了链环钢在试验检查时发生沿晶脆性断裂。     

ANTIMONY GRAIN BOUNDARY SEGREGATION AND ITS SUPPRESSION BY CERIUM IN Fe—2Mn—Sb STRUCTURAL STEELS

Antimony grain boundary segregation in Fe-2%Mn-Sb structure steels has been studied through measurements of the ductile-brittle transition temperature in conjunction with scanning electron microscopy, Auger electron spectroscopy and secondary ion mass spectroscopy. The research result reveals that during tempering or ageing after quenching at 980℃, Sb segregates to grain boundaries with both equilibrium and non-equilibrium natures and brings about temper embrittlement in the steels. Cerium can relieve temper embrittlement of the steels and its segregation to grain boundaries may play an important role in reducing this embrittlement.     

Influence of Fe on Microstructure and Mechanical Properties in Pdoped Ni–Cr–Fe Alloys

The influence of Fe on the microstructure and mechanical properties of P-doped Ni–Cr–Fe alloys has been investigated.Results showed that increasing Fe content refined the dendrite microstructure and enhanced the solubility of P in as-cast alloys. The change of microhardness in different dendrite regions was attributed to the segregation of P atoms in solid solution state, which had strengthening effects. Increasing Fe contents from 15.2 to 60.7 wt% reduced the yield strength and tensile strength but had little influence on the elongation of alloys. The stress rupture life of alloys after heat treatment decreased with the increment of Fe contents, and the failure fracture modes transferred from transgranular to intergranular fracture mode. The change of fracture modes was due to the weakness of grain boundaries caused by the increment of Fe.In addition, the precipitation of M_(23)C_6 was believed to be related to the segregation of P toward grain boundaries, which led to the fluctuation of carbon and chromium atoms near the grain boundaries in alloys with low Fe contents. Consequently, the increment of Fe decreased the strength of matrix and changed the existence of P atoms and the precipitates at grain boundaries.     

Tailoring and patterning the grain size of nanocrystalline alloys

Nanocrystalline alloys that exhibit grain boundary segregation can access thermodynamically stable or metastable states with the average grain size dictated by the alloying addition. Here we consider nanocrystalline Ni–W alloys and demonstrate that the W content controls the grain size over a very broad range: ∼2–140 nm as compared with ∼2–20 nm in previous work on strongly segregating systems. This trend is attributed to a relatively weak tendency for W segregation to the grain boundaries. Based upon this observation, we introduce a new synthesis technique allowing for precise composition control during the electrodeposition of Ni–W alloys, which, in turn, leads to precise control of the nanocrystalline grain size. This technique offers new possibilities for understanding the structure–property relationships of nanocrystalline solids, such as the breakdown of Hall–Petch strength scaling, and also opens the door to a new class of customizable materials incorporating patterned nanostructures.     

Grain boundary segregation and intergranular fracture in Ferrite containing Mo,Si or Al

Roles of molybdenum, silicon or aluminum in ferrite on grain boundary segregation and hence on intergranular fracture have been investigated by using Auger electron spectroscopy (AES) and tensile test. Competitive segregation between sulfur and carbon or nitrogen, which caused the decrease below 700°C of sulfur content at the grain boundaries, was observed in the pure iron. The intergranular brittleness of the pure iron was caused by sulfur at the grain boundaries. When molybdenum was added to the pure iron, the sulfur contents at the grain boundaries were lowered in comparison to those in the pure iron. The molybdenum-bearing alloy showed higher fracture strength than that of the pure iron, and fractured mostly in the transgranular mode. This arises from the intrinsic effect of molybdenum on the grain boundaries as well as the decrease in sulfur content. Tn the 3.37 wt.%Si alloy, silicon and carbon or nitrogen competitively segregated to the grain boundaries, and such a competitive segregation was also observed between sulfur and carbon or nitrogen. The sulfur content at the grain boundaries decreased with increasing silicon content. The fracture modes in the 3.37- and 4.26 wt.%Si alloys were transgranular in the rolling direction, but were mostly intergranular in the transverse direction and in the as-rolled condition. The intergranular characteristic in the fracture behavior may be attributed to the detrimental effect of silicon as well as sulfur on the intergranular cohesion. Carbon and aluminum only were found at the grain boundaries of the aluminum-bearing alloy. This suggests that aluminum is a strong repulser of sulfur or nitrogen at the grain boundaries. Additionally, it was found that aluminum has a detrimental effect on grain boundary strength of ferrite.     

THE NON-EQUILIBRIUM SEGREGATION OF BORON TO AUSTENITE GRAIN BOUNDARIES

采用高分辨率的径迹显微照相技术,研究了淬火硼钢中硼向奥氏体晶界偏聚的规律,定量地测定出跨过奥氏体晶界的硼的成分剖面图以及非平衡晶界偏聚的特征参量(晶界贫硼区宽度、晶界富集程度和富集带宽度)。试验表明,这种偏聚具有如下特征: 在偏聚晶界的两侧存在有一定宽度的贫硼区,晶界偏聚的硼是在冷却过程中由该区富集而来;这种偏聚对冷却速度很敏感,急速冷却可以抑制这种偏聚。冷却速度降低,晶界偏聚由连续的偏聚带,逐步发展为不连续的聚集直至明显地析出硼相,贫硼区宽度与冷却速度的平方根成反比;它的温度关系与晶界平衡偏聚预言相反,随淬火温度升高,晶界偏聚程度与贫硼区宽度增加。 通过试验,论证了淬火钢中硼向奥氏体晶界的偏聚,是在冷却过程中发生的一种非平衡的晶界偏聚现象。     

Microstructure and Mechanical Properties of Nb- and Nb+ Ti-Stabilised 18Cr-2Mo Ferritic Stainless Steels

To explore the optimum use of stabilised elements and study the influences of stabilisation in 18 Cr-2 Mo grades,the Nb and Nb+Ti microalloying investigation focused on the relationships of the microstructure and mechanical properties of the microalloyed 18 Cr-2 Mo ferritic stainless steel thick plates.Thermo-Calc calculation was performed to predict the equilibrium phase diagrams.Afterwards,the microstructure,i.e.grain size and precipitation,of as-annealed specimens was analysed by means of optical microscopy,scanning electron microscopy and transmission electron microscopy,X-ray diffraction and energydispersive spectroscopy.Also,electron backscatter diffraction mapping was constructed to characterise grain boundary.The mechanical properties,including tensile strength and impact toughness,were tested to correlate with the microstructure.The results show that the grain sizes of Nb-stabilised steel are comparatively smaller,which is related to the fine precipitation at the grain boundaries and beneficial to the impact toughness.The increase in its strength is not apparent due to the inhomogeneous grain sizes.The grain boundary characters are similar,which is not the main factor related to their mechanical properties.When Ti is added,TiN forms above the liquidus,and large TiN particles evidently impair impact toughness.     

退火孪晶对hcp结构二元镁锂合金腐蚀行为的影响

采用浸泡实验、电化学实验、腐蚀形貌观察以及扫描开尔文探针测试等实验方法对挤压态和退火态二元Mg-xLi（x=1,3,5,质量分数,%）合金的耐腐蚀性能进行研究。结果表明：挤压态Mg-Li合金经350 ℃/4 h退火处理后,可在基体中形成退火孪晶。在 0.1 mol/L NaCl溶液中,退火态合金的耐腐蚀性能明显劣于挤压态合金。通过扫描电子显微镜和激光共聚焦三维形貌分析,发现挤压态合金在腐蚀初期具有丝状腐蚀特征,并且随着腐蚀时间的延长形成了腐蚀坑;退火态合金表面发生快速腐蚀损伤,同时形成较大且深的腐蚀坑形貌。另外,退火态Mg-3Li合金的腐蚀等势线分布表明该合金退火孪晶区域优先发生了腐蚀溶解现象。因此,退火孪晶与母体晶粒之间可形成微电偶腐蚀效应,且退火孪晶区域充当阳极,母体晶粒作为阴极,从而降低了退火态Mg-Li合金的耐腐蚀性能。     

分级均匀化对7055铝合金组织和力学性能的影响

采用差热分析、X射线衍射分析、光学显微镜、扫描电镜、透射电镜和常温拉伸等方法研究分级均匀化工艺对7055铝合金显微组织、枝晶偏析、Al3Zr粒子析出行为和力学性能的影响。结果表明:合金经(468℃、24h)+(473℃、4 h)的分级均匀化处理后,消除了铸锭晶界上的非平衡凝固共晶组织;铸锭先于264℃保温最有利于获得尺寸细小、均匀弥散分布的Al3Zr粒子;在单级均匀化基础上增加473℃的短时高温均匀化,能够提高合金基体中溶质原子的固溶度,增强时效强化效果,提高综合性能。     

工程机械用Q1300E超高强钢的淬火工艺

利用全自动拉力试验机、全自动冲击试验机、光学显微镜和扫描电镜研究不同淬火工艺下Q1300E钢板的力学性能和显微组织。结果表明,当淬火加热时间为60 min,淬火温度为840 ℃时,强度和低温冲击性能最好,回火态力学性能满足GB/T 28909—2012要求,屈服强度1302 MPa,抗拉强度1505 MPa,-40 ℃纵向和横向冲击吸收能量分别为74 J和61 J;淬火温度为870、900和930 ℃时,抗拉强度和低温冲击吸收能量满足GB/T 28909—2012要求,但屈服强度低于1300 MPa;淬火温度的变化对晶粒尺寸的影响较为明显,淬火温度840 ℃时,平均晶粒尺寸最小,为5.7 μm,淬火温度930 ℃时,平均晶粒尺寸为15.9 μm。淬火加热时间对力学性能和晶粒尺寸的影响相对较小,当淬火温度为840 ℃,淬火加热时间为40~80 min时,回火态力学性能满足GB/T 28909—2012要求,晶粒尺寸为4.5~6.5 μm。     

The role of oxygen content and cooling rate on transformations in TiAl-based alloys

Samples of Ti46Al8Nb (at.%) with oxygen contents of 500 or 1500 wt ppm have been cooled at various rates in order to further our understanding of the factors which influence the phase transformations in these alloys. In agreement with earlier work it has been found that increase of oxygen can suppress the massive transformation. At high cooling rates retained alpha (ordered alpha 2) is found, with a very limited amount of massively transformed grain boundary gamma in both alloys. At lower cooling rates the response to quenching is very different in the different oxygen content samples. Retained alpha, heavily faulted alpha or fine lamellae are formed at grain boundaries as the cooling rate is decreased in the higher oxygen samples with limited massive gamma in grain centres, whereas in the 500 ppm sample the microstructure is virtually fully massive at these cooling rates. The amount of massive gamma which appears to be nucleated away from the original alpha boundaries is also strongly influenced by cooling rate and oxygen content. These results are interpreted in terms of the overall oxygen level, the extent of the segregation of oxygen during quenching, the nucleation of massive gamma in grain centres and the instability of retained alpha during slow cooling and at saltbath temperatures.     

Microstructure and mechanical properties of laser beam welding 6061 aluminum alloy with different states

With the diversification of manufacture methods,joining the same materials with different states becomes indispensable in practical application.In present work,6061 aluminum alloys with different states were welded by laser beam welding (LBW).The microstructures of welded joint,before and after heat treating,were investigated.The mechanical properties,such as the tensile properties and microhardness,were tested.And the fracture characteristic was observed by means of scanning electron microscope (SEM).The results show that the 6061 aluminum alloys have superior weldability and the microstructures are different significantly in different states.Besides,the grain boundaries of the joint microstructures become unclear after the heat treating.The strength and the elongations of welded joints could reach to those of the base metal.The tensile fracture occurs in the fusion zone and near 6061-0 alloy.And the fracture presents ductile rupture.Therefore,the LBW is an effective method for 6061 aluminum alloy.     

BEHAVIOUR OF BORON SEGREGATION AT GRAIN BOUNDARIES IN BCC Fe-3% Si ALLOY

The behaviour of baron segregation at grain boundaries in Fe-3% Si has been studied bymeans of particle tracking autoradiography(PTA).The results indicate that the tendency ofequilibrium segregation of B at grain boundaries in Fe-3% Si is higher than that in austeniticalloys.No observable nonequilibrium segregation of B at grain boundaries was revealed inFe-3% Si alloy during cooling and isothermal holding.The binding energy between boron at-oms and grain boundaries is 55.7±1.7 kJ/mol.Based on the complex mechanism of B segre-gation of quenching-induce-nonequilibrium segregation,different behaviours of B segrega-tion in γ-Fe and bcc Fe-3% Si alloy were discussed.