Wear resistance of NiTi alloy after surface mechanical attrition treatment

An ultrafine grain layer consisting of nanocrystallites as well as submicrometer grains is produced on NiTi shape memory alloy by surface mechanical attrition treatment (SMAT) and the effects of the ultrafine grain layer on the tribological properties are investigated under dry sliding conditions. Compared to the coarse grain (CG) NiTi, the SMAT NiTi has smaller friction coefficients and improved wear resistance at applied loads from 5 to 15 N due to the grain refinement effect. Examination of the worn surfaces indicates that materials delamination and particles co-exist on both the CG and SMAT NiTi samples. Our results indicate that delamination is the main wear mechanism on CG NiTi whereas abrasive particles dominate the wear process on SMAT NiTi.     

Surface mechanical attrition treatment induced phase transformation behavior in NiTi shape memory alloy

The phase constituents and transformation behavior of the martensite B19′ NiTi shape memory alloy after undergoing surface mechanical attrition treatment (SMAT) are investigated. SMAT is found to induce the formation of a parent B2 phase from the martensite B19′ in the top surface layer. By removing the surface layer-by-layer, X-ray diffraction reveals that the amount of the B2 phase decreases with depth. Differential scanning calorimetry (DSC) further indicates that the deformed martensite in the sub-surface layer up to 300 μm deep exhibits the martensite stabilization effect. The graded phase structure and transformation behavior in the SMATed NiTi specimen can be attributed to the gradient change in strain with depth.     

表面机械研磨处理对Cu-4Ti合金组织及力学性能的影响

对Cu-4Ti合金分别进行45 min和60 min的表面机械研磨处理(SMAT),采用X射线衍射仪,光学显微镜,扫描电镜,显微硬度计等对处理不同时间的样品组织及力学性能进行了分析。结果表明,Cu-4Ti合金经表面机械研磨处理45 min和60 min后,表层晶粒尺寸分别达到了40.72 nm和17.12 nm,并且都出现了分界层,这与前人研究结果出现的过渡层截然不同。表面机械研磨方法可以强化金属表面,Cu-4Ti合金经SMAT 45 min和60 min后,表面硬度比基体分别提高了51%和56%。     

High-temperature surface alloying of nanocrystalline nickel produced by surface mechanical attrition treatment

A nanocrystalline surface layer is synthesized in Ni by surface mechanical attrition treatment (SMAT). The characterization of microstructure and composition indicates that foreign elements from hardened balls can permeate and diffuse into the surface layer during SMAT, which can further diffuse and homogenize in the surface layer when annealing at high temperature, leading to formation of new alloy. Foreign elements permeating into the surface layer during SMAT followed by diffusing and homogenizing at high temperature provide an alternative effective technology for surface modification.     

表面机械研磨对304不锈钢渗氮组织性能的影响

对304不锈钢表面进行表面机械研磨处理(SMAT),再进行不同温度下的低温等离子渗氮。利用光学显微镜、XRD、SEM和EDS,分析渗氮层的物相、显微组织和元素;采用显微硬度计检测渗氮后硬度的变化;采用电化学工作站测试渗氮后试样的腐蚀性能。结果表明,经过1800 s的表面机械研磨处理,材料的渗氮组织性能达到最好,样品表面生成一层晶粒细化层,可以明显促进304不锈钢的低温渗氮。1800 s的表面机械研磨处理后,在350℃下进行渗氮,可以获得一层厚度约3μm的渗氮层,其硬度高达925 HV0.05。和未处理的试样对比,自腐蚀电位升高了0.2 V,自腐蚀电流降低了4.22×10-4A·cm-2。     

Low-temperature nitriding of 38CrMoAl steel with a nanostructured surface layer induced by surface mechanical attrition treatment

A nanocrystalline surface layer was fabricated on a 38CrMoAl steel plate by means of a surface mechanical attrition treatment (SMAT). The average grain size in the top surface layer (10 μm thick) is about 10 nm, and the grain size stability can be maintained up to 450 °C. The effect of the surface nanocrystalline layer on the gas nitriding process at a lower temperature was investigated by using structural analysis and wear property measurements. The surface nanocrystallization evidently enhances nitriding kinetics and promotes the formation of an ultrafine polycrystalline compound layer. The results of the investigation showed that this new gas nitriding technique can effectively increase the hardness and wear resistance of the resulting surface layer in comparison with conventional nitriding, demonstrating a significant advancement for materials processing.     

Fretting wear behaviour of surface mechanical attrition treated alloy 718

Alloy 718 was subjected to surface mechanical attrition treatment (SMAT) using SAE 52100 steel balls of a 5 mm diameter for four different treatment durations (15, 30, 45 and 60 min). Fretting wear tests were conducted at different normal loads on untreated and treated samples for 25,000 cycles using alumina as a counterbody material. Microstructural features of the surface layer of samples treated by SMAT were characterized by cross-sectional optical microscopy and transmission electron microscopy. Hardness, surface roughness and residual stress were determined using a nano-indenter, surface roughness tester and X-ray residual stress analyzer respectively. SMAT resulted in the formation of nanocrystallites on the surface and near surface regions, increased hardness, increased surface roughness and compressive residual stress at the surface. Treated samples exhibited lower tangential force coefficient (TFC) compared to untreated samples. Samples treated for 60 min exhibited higher grain refinement, higher hardness, lower surface roughness and higher TFC compared to the samples treated for 30 min. The wear volume and wear rate of samples treated for 30 min were lower compared to those of the untreated samples, which may be attributed to an optimum combination of hardness and toughness and a low work hardening rate of the nanocrystalline structure at the surface of the treated samples. In contrast, the wear volume and wear rate of the samples treated for 60 min were higher than those of untreated samples, presumably due to the higher hardness and reduced toughness of the samples treated for 60 min.     

Wear behaviour of AZ91D magnesium alloy with a nanocrystalline surface layer

The tribological behaviour of nanocrystalline surface layer with an average grain size of 30 ± 5 nm generated by surface mechanical attrition treatment on AZ91D Mg alloy samples has been investigated under dry sliding conditions. Compared with the alloy without SMAT, nano-grained surface layer showed lower friction coefficient. An improved wear resistance of the NC layer was found at the load ranges from 3 N to 9 N due to the grain refinement strengthening effect. Examination of the worn surface and the wear debris indicated that the wear mechanism of NC layer is similar to that of the coarse-grained alloy. Cooperative effects of cutting, plowing and oxidation govern the tribological behaviour of nanocrystalline AZ91D Mg alloys.     

Effect of surface mechanical attrition treatment (SMAT) on zinc phosphating of steel

The effects of surface mechanical attrition treatment (SMAT) of EN8 steel on the growth of phosphate coatings, morphological features and corrosion resistance of the resultant coatings have been studied. SMAT enabled the formation of a uniform surface profile although the average surface roughness is increased after treatment. SMAT increased the extent of metal dissolution and the rate of growth of phosphate coating. In spite of the similarity in phase composition, the phosphate crystallite size is relatively high for samples treated by SMAT. Compared to the untreated one, a cathodic shift in E_corr with a corresponding decrease in i_corr is observed for SMAT treated EN8 steel after phosphating. For all tested samples, zinc phosphate coatings deposited on EN8 steel after SMAT using 8 mm Ø balls for 30 min offers the highest corrosion resistance. The increase in surface roughness by SMAT is partly compensated by the expected improvement in corrosion resistance.     

表面机械研磨处理Mg-Gd-Y合金不同应变层的时效硬化行为和机理

采用表面机械研磨处理(SMAT)在Mg-Gd-Y合金中获得了梯度组织结构,通过维氏硬度计和透射电子显微镜对试样中不同应变层的时效硬化行为和机理进行了研究。结果表明,SMAT后合金表面梯度组织可以分为3层:剧烈应变层、中等应变层和无影响层。在225℃时效,不同应变层的时效硬化行为表现出明显差异。出现峰时效的时间由剧烈应变层、中等应变层到无影响层依次延长;而峰时效时的硬度增量则依次增大。这与不同应变层中沉淀相的形态、分布以及与位错等缺陷的相互作用有关。剧烈应变层显示了最短的峰时效时间和最高的峰时效硬度,说明SMAT表面纳米化对促进Mg-Gd-Y合金的时效硬化有显著效果。     

Mg70Al20Ca10合金机械合金化过程中的结构变化

以纯金属Mg、Al和Ca为原料,采用机械合金化的方法制备了Mg70Al20Ca10非晶态合金,分析了球磨过程中粉末试样的晶粒尺寸变化和相结构变化情况.结果表明,球磨过程中形成纳米晶和两种非晶相.Mg-Al-Ca合金在化学驱动力较小的条件下,可以通过机械驱动力的作用提高Ca、Al在Mg中的固溶度,导致非晶转变的发生.     

Surface nanocrystallization by surface mechanical attrition treatment and its effect on structure and properties of plasma nitrided AISI 321 stainless steel

A plastic deformation surface layer with nanocrystalline grains was produced on AISI 321 austenitic stainless steel by means of surface mechanical attrition treatment (SMAT). Low-temperature nitriding of SMAT and un-SMAT AISI 321 stainless steel was carried out in pulsed-DC glow discharge. The effect of SMAT pretreatment on the microstructure and properties of the stainless steel were investigated using X-ray diffraction, scanning electron microscopy, transmission electron microscopy, Vickers hardness tester and UMT-2MT tribometer. The results show that the plasma nitriding of AISI 321 steel can be enhanced considerably by means of SMAT process before nitriding, and a much thicker nitrogen diffusion layer with higher hardness was obtained for the SMAT samples when compared with un-SMAT samples. In addition, the wear resistance and load capacity of the nitrided layers on the SMAT samples was much higher than that of the un-SMAT samples due to the thicker S phase case and the gradient nitrogen diffusion layer.     

Effects of scandium and zirconium combination alloying on as-cast microstructure and mechanical properties of Al-4Cu-1.5Mg alloy

The influences of minor scandium and zirconium combination alloying on the as-cast microstructure and mechanical properties of Al-4Cu-1.5Mg alloy have been experimentally investigated.The experimental results show that when the minor elements of scandium and zirconium are simultaneously added into the Al-4Cu-1.5Mg alloy,the as-cast microstructure of the alloy is effectively modified and the grains of the alloy are greatly refined.The coarse dendrites in the microstructure of the alloy without Sc and Zr additions are refined to the uniform and fine equiaxed grains.As the additions of Sc and Zr are 0.4% and 0.2%,respectively,the tensile strength,yield strength and elongation of the alloy are relatively better,which are 275.0 MPa,176.0 MPa and 8.0% respectively.The tensile strength is increased by 55.3%,and the elongation is nearly raised three times,compared with those of the alloy without Sc and Zr additions.     

机械合金化法制备Al—Cu—Fe纳米非晶合金

采用行星式高能球磨机制备了Al80-xCuxFe20(x=20-40）三元非晶纳米合金粉末,分析了不同球磨时间及热处理工艺对粉末结构、颗粒大小等的影响。结果表明：成分为Al40Cu40Fe20的粉末球磨时逐步非晶化,球磨33h后,非晶化程度最大,最小颗粒尺寸达到5．6nm;进一步球磨,非晶晶化,颗粒尺寸增大;成分为Al80-xCuxFe20(x=20,25,30)的粉末球磨90h后,得到非晶,最小颗粒尺寸为3．4nm。球磨制备的Al-Cu-Fe非晶粉末具有铁磁性。用DSC测量了其晶化温度（Tc),Tc≈873℃。     

Enhanced reactive diffusion of Zn in a nanostructured Fe produced by means of surface mechanical attrition treatment

Nanostructured pure Fe was produced on a coarse-grained (CG) sample using surface mechanical attrition treatment (SMAT). The formation behaviors of Fe-Zn compound layer were studied in nanostructured Fe electroplated with a Zn layer. In comparison with the CG sample, the Fe-Zn reaction in the nanostructured Fe showed an onset temperature decrease of ∼21 °C and an increased enthalpy change of ∼70%. The activation energy for the growth of Fe-Zn compound layer decreased from ∼167.1 kJ mol⁻¹ in the CG sample to ∼108.0 kJ mol⁻¹ in the SMAT sample. The grain size of the formed FeZn₁₃ phase in the Zn/SMAT-Fe sample was much smaller than that in the Zn/CG-Fe sample after the same diffusion treatment. The enhanced reactive diffusion behaviors in the SMAT sample are attributed to the existence of a large number of grain boundaries in the nanostructured Fe.     

机械研磨对AZ31镁合金化学镀镍磷镀层性能的影响

在AZ31镁合金化学镀镍磷的过程中施加机械研磨作用,获得了具有新特性的镍磷镀层。扫描电镜观察表明,施加机械研磨获得的镍磷镀层的晶粒细化、致密性提高、消除了镀层中的孔隙。镀层截面的形貌结果显示,与传统化学镀镍磷层相比,施加机械研磨后镀层与基体的结合处形成了一个合金化的过渡区域,使镀层与基体的结合力得到显著的提高。由动电位极化曲线测试结果可知,施加机械研磨后镀层的自腐蚀电位提高到-0.2 V,相对传统化学镀镍磷层的电位提高了0.9 V,耐蚀性能显著提高。     

表面机械研磨处理后H13钢的等离子渗硼研究

为了提高热作模具钢H13的性能，延长模具的使用寿命，本试验将表面机械研磨（SMA）处理与等离子渗硼相结合。通过显微金相技术、扫描电子显微分析技术（SEM）、X射线衍射分析技术（XRD）以及辉光放电发射光谱技术（GDOS）分别对H13钢渗硼后的渗层厚度、表面相组成以及元素逐层分布情况进行了探讨。研究结果表明在650℃×3h等离子渗硼处理后，纳米化表面便有F02B相出现，次表面为扩散层，总渗入深度为30μm左右；在700℃×3h条件下，出现了F02B和FeB的双相组织。而对于原始表面，在650℃×3h处理后表面没有任何硼化物出现，只产生了一定的扩散层。最终提出了表面纳米化之后实施低温渗硼的新型工艺。     

机械研磨处理Zr-4合金表面纳米化研究

选用表面机械研磨技术(SMAT)处理密排六方结构Zr-4合金的表面,实现Zr-4表面纳米化,并利用X射线衍射(XRD)对比分析不同时间SMAT处理Zr-4合金表面平均晶粒尺寸。SMAT处理15min时Zr-4合金表面平均晶粒尺寸最小,约为20nm。利用光学显微镜(OM)、透射电镜(TEM)、高分辨透射电镜(HRTEM)对其纳米化表层结构进行表征,并研究了其表面纳米化机制。     

机械合金化法制备金锡合金

采用高能球磨机械合金化法制备了Au-20%Sn合金,分析了合金物相、组织和硬度随球磨时间的变化规律,探讨了合金塑性与合金组织及制备工艺的关系。结果表明:采用高能球磨机械合金化法可以制备Au-20%Sn合金;随球磨时间的增加,Au-20%Sn的合金化程度增加,组织中的金属间化合物逐渐增多,最终基本上为δ相和ζ′相;合金的硬度随球磨时间的延长逐渐升高,并在球磨60min后获得最高硬度104.2HV,然后开始下降;球磨后的合金粉末在190℃×2h的烧结过程中发生了不同程度的再结晶和晶粒长大,再结晶程度随球磨时间的延长而增加,导致烧结后合金硬度在球磨时间超过60min后反而下降。     

Microstructure evolution in nanocrystalline Cu-Nb alloy during mechanical alloying

The microstructural evolution of nanocrystalline Cu-10%Nb（mass fraction） alloy during mechanical alloying （MA） was investigated by using X-ray diffraction, optical microscopy（OM）, scanning electron microscopy （SEM） and transmission electron microscopy （TEM） observation. Upon milling of Cu-Nb powders with coarse grains, the grain size is found to decrease gradually with lengthening milling time, and reach the minimum value （about 9 nm） after 100 h milling. The microstrain and the microhardness of the powders increase during the grain refinement. And Cu lattice parameter increases steadily over 100 h milling. The mechanisms of solid solution extension during milling were discussed. The results show that up to 10%Nb can be brought into solid solution by MA. The extension of solid solution is found to relate closely with the formation of nanocrystalline.