The effects of gas nitriding on fatigue behavior in titanium and titanium alloys

Fatigue behavior has been studied on gas-nitrided smooth specimens of commercial pure titanium, an alpha/beta Ti-6Al-4V alloy, and a beta Ti-15Mo-5Zr-3Al alloy under rotating bending, and the obtained results were compared with the fatigue behavior of annealed or untreated specimens. It was found that the role of the nitrided layer on fatigue behavior depended on the strength of the materials. Fatigue strength was increased by nitriding in pure titanium, while it was decreased in the Ti-6Al-4V and Ti-15Mo-5Zr-3Al alloys. Based on detailed observations of fatigue crack initiation, growth, and fracture surfaces, the improvement and the reduction in fatigue strength by nitriding in pure titanium and both alloys were primarily attributed to enhanced crack initiation resistance and to premature crack initiation of the nitrided layer, respectively.     

油气开采用钛合金石油管材料耐腐蚀性能研究

选取5种油气开发常用钛合金材料(Ti-6Al-4V、Ti-6Al-4V-0.1Ru、Ti-6Al-2Sn-4Zr-6Mo、Ti-3Al-8V-6Cr-4Zr-4Mo和Ti-5.5Al-4.5V-2Zr-1Mo)为研究对象,使用高温高压釜模拟国内典型严酷服役工况环境,研究了不同钛合金材料耐均匀腐蚀、局部腐蚀、点蚀、应力腐蚀开裂(SCC)及缝隙腐蚀的性能,通过使用扫描电镜和能谱分析等手段对腐蚀形貌和腐蚀产物进行了分析,并使用电化学方法对不同合金的耐腐蚀机理进行了研究。结果显示,在所测试工况条件下,所有钛合金材料腐蚀反应均为阳极控制过程,均匀腐蚀速率均低于0.001mm/a,并且对应力腐蚀开裂均有良好的抗力。Ti-6Al-4V和Ti-5.5Al-4.5V-2Zr-1Mo合金出现明显的点蚀和缝隙腐蚀问题。对腐蚀机理研究表明,在工况条件温度下,随着pH值的降低,所有钛合金均发生自腐蚀电位降低,极化电阻减小,腐蚀电流增大,耐腐蚀性能下降,其中Ti-6Al-4V耐腐蚀性能下降的最为明显,研究结果为油气开发工况下钛合金石油管的选材和缝隙腐蚀问题防治提供理论基础。     

Studying and modeling surface gas nitriding for titanium alloys

In this article, experimental studies for the nitriding of four titanium alloys at different temperatures and for different periods of time are summarized. The studies focused on microstructural changes in relation to the alloy composition and processing parameters; microindentation hardness testing on the nitrided titanium alloys to analyze their hardness evolution; and the corrosion behavior of titanium alloys before and after gas nitriding in response to the corrosive condition. In addition, models were developed to simulate and monitor the evolution of surface layers during the gas nitriding of titanium alloys.     

氢对Ti-6Al-4V合金微观组织的影响

采用累计流量法对供应态Ti-6Al-4V合金进行了固态置氢,运用OM、XRD、TEM分析等方法研究了Ti-6Al-4V合金固态置氢后的微观组织状态及演变过程。结果表明:供应态Ti-6Al-4V合金的置氢量低于0.30%(质量分数,下同)时,置氢使得Ti-6Al-4V合金中的α相减少、β相增加;置氢量达到0.30%时,置氢Ti-6Al-4V合金中有δ氢化物(TiH2相)形成;β-Ti(H)共析转变生成α-Ti和δ氢化物时主要以切变方式进行;置氢Ti-6Al-4V合金的相变温度最多下降了180°C,与Ti-6Al-4V合金在置氢过程中的相体积比变化和共析转变有密切关系。     

Double Glow Plasma Surface Alloyed Burn-resistant Titanium Alloy

TITANIUM and its alloys have been extensively usedin aeronautics and astronautics industries because oftheir low specific gravities,high strength,stiffness,good toughness and high corrosion resistance.However,conventional titanium alloys may be ignited and burntunder high temperature,high pressure and high gasflow velocity conditions.Titanium fire took placeseveral times in engines.This shortcoming limitsapplication of titanium alloys.In order to avoid thistechnical problem,Ti-Cr-V,Ti-Cu-…     

Influence of heat treatments on microstructure and mechanical properties of laser additive manufacturing Ti-5Al-2Sn-2Zr-4Mo-4Cr titanium alloy

The effect of heat treatments on laser additive manufacturing (LAM) Ti-5Al-2Sn-2Zr-4Mo-4Cr titanium alloy (TC17) was studied aiming to optimize its microstructure and mechanical properties. The as-deposited sample exhibits features of a mixed prior β grain structure consisting of equiaxed and columnar grains, intragranular ultra-fine α laths and numerous continuous grain boundary α (α_GB). After being pre-annealed in α+β region (840 °C) and standard solution and aging treated, the continuous α_GB becomes coarser and the precipitate free zone (PFZ) nearby the α_GB transforms into a zone filled with ultra-fine secondary α (α_S) but no primary α (α_P). When pre-annealed in single β region (910 °C), all α phases transform into β phase and the alloying elements distribute uniformly near the grain boundary. Discontinuous α_GB and uniform mixture of α_P and α_S near grain boundary form after subsequent solution and aging treatment. The two heat treatments can improve the tensile mechanical properties of LAM TC17 to satisfy the aviation standard for TC17.     

冷轧预处理对TC4合金540 ℃渗氮组织的影响

TC4钛合金经820 ℃固溶处理及室温下冷轧得到10%、20%、30%、40%、50%不同变形量的试样,之后在540 ℃进行渗氮复合时效处理8 h。利用光学显微镜、X射线衍射仪、扫描电镜等分析了不同变形量下合金渗氮组织和时效组织的变化特点以及渗氮试样表面的相结构和形貌的变化。试验结果显示,渗氮处理后,试样表面生成了TiN、Ti₂N等高硬的氮化物颗粒,变形量越大,试样表面生成的氮化物颗粒越多且尺寸越大;经固溶处理后,TC4合金的室温组织为α相+亚稳β相,经渗氮复合时效处理后,亚稳β相分解为α相,同时α相向β相转化,变形量越大,析出的β相越多,且这种趋势不如单纯的时效处理明显。     

固溶时效处理对Ti-5.43Al-3.11Mo-1.41V合金显微组织和力学性能的影响

对BT14钛合金(Ti-5.43Al-3.11Mo-1.41V)进行不同温度固溶+时效热处理,研究了固溶温度对合金的显微组织、元素分布和硬度和压缩性能的影响。结果表明,在β相转变温度以下固溶后,随固溶温度上升,初生α相含量不断减少,初生α相和基体相(α′、α″或亚稳β相)中的Al含量均增加,Mo和V含量均下降,显微硬度上升。890、940、990 ℃固溶+540 ℃×6 h时效处理后,基体相分解形成弥散细小的α+β相,起到显著的强化作用,导致显微硬度整体提高,且随着固溶温度的升高,显微硬度和压缩屈服强度提高。     

Thermal expansion and lattice parameters of shaped metal deposited Ti-6Al-4V

Thermal expansion and lattice parameters are investigated up to 1100 °C for Ti-6Al-4V components, fabricated by shaped metal deposition. This is a novel additive layer manufacturing technique where near net-shape components are built by tungsten inert gas welding.The as-fabricated SMD Ti-6Al-4V components exhibit a constant coefficient of thermal expansion of 1.17 × 10⁻⁵ K⁻¹ during heating up to 1100 °C, not reflecting the α to β phase transformation. During cooling a stalling of the contraction is observed starting at the β transus temperature. These high temperature experiments denude the α phase of V and enrich the β phase.The development of the lattice parameters in dependence on temperature are observed with high temperature X-ray diffraction. The unit cell volumes derived from these parameters are at room temperature larger for the α than for the β phase. With increasing temperature the unit cell volume of the β phase increases stronger than the one of the α phase resulting in a similar unit cell volume at the β transus temperature.These observations are interpreted as an indication for as-fabricated the SMD components being in a non-equilibrium state and reaching equilibrium during the slow heating and cooling during of the two different high temperature experiments.     

Influence of melt hydrogenation during induction skull melting process on the solidification microstructure of Ti-6Al-4V alloy

A new method to modify the solidification microstructure of titanium alloys, named melt hydrogenation, by adding TiH2 as additive into the melt of titanium alloys during induction skull melting process (ISM), is put forward and the refining effect of this method on the solidification microstructure of Ti-6Al-4V alloy was studied experimentally. After melt hydrogenation, the grain sizes of as-cast Ti-6Al-4V alloy decreased to 612 μm from 1,072 μm, lath-shaped α phase was also refined and fine α/β lamellar microstructure was formed when 1.0 wt.% TiH2 was added. δ-hydride was found in the X-ray diffraction (XRD) spectra of Ti-6Al-4V alloy that prepared with 1.0 wt.% TiH2 added and the δ-hydride distributes in α phase as acicular precipitations.     

中国钛工业概览

采用弯曲和循环拉伸实验研究了新型Ti-3Zr-2Sn-3Mo-15Nb(TLM)钛合金的形状记忆和超弹性性能.探讨了变形温度、总应变和热处理等对TLM钛合金形状记忆和超弹性的影响规律.结果表明:新合金在热轧态和在α β两相区固溶处理后空冷比从β相区固溶处理后空冷条件下具有较高的形变恢复率,最大恢复应变可达1.8%.随着总变形应变增加,形变恢复率降低.从β相区固溶处理后空冷后TLM合金具有较好的超弹性,优于热轧态或时效处理后TLM合金的超弹性能.     

Design for Ti-Al-V-Mo-Nb alloys for laser additive manufacturing based on a cluster model and on their microstructure and properties

In this study, α+β Ti-Al-V-Mo-Nb alloys with the addition of multiple elements that are suitable for laser additive manufacturing (LAM) were designed according to a Ti-6Al-4V cluster formula. This formula can be expressed as 12[Al-Ti₁₂](AlTi₂)+5[Al-Ti₁₄]((Mo, V, Nb)₂Ti), in which Mo and Nb were added into the alloys partially instead of V to give alloys with nominal compositions of Ti-6.01Al-3.13V-1.43Nb, Ti-5.97Al-2.33V-2.93Mo, and Ti-5.97Al-2.33V-2.20Mo-0.71Nb (wt.%). The microstructures and mechanical properties of the as-deposited and heat-treated samples prepared via LAM were examined. The sizes of the β columnar grains and α laths in the Nb-containing samples are found to be larger than those of the Ti-6Al-4V alloy, whereas Mo- or Mo/Nb-added alloys contain finer grains. It indicates that Nb gives rise to coarsened β columnar grains and α laths, while Mo significantly refines them. Furthermore, the single addition of Nb improves the elongation, whereas the single addition of Mo enhances the strength of the alloys. The simultaneous addition of Mo/Nb significantly improves the comprehensive mechanical properties of the alloys, leading to the best properties with an ultimate tensile strength of 1,070 MPa, a yield strength of 1,004 MPa, an elongation of 9%, and micro-hardness of 355 HV. The fracture modes of all the alloys are ductile-brittle mixed fracture.     

Low cost synthesis and property evaluation of P/M Ti alloys and ti-based particulate composites

Ti-6A1-4V alloy and Ti-6Al-2Sn-4Zr-2Mo/10TiB particulate composites were synthesized using a hlended elemental powder metallurgy processing route. Microstructures and mechanical properties, such as tensile, creep, Young’s modulus and high-cycle fatigue-strength were evaluated. Special attention was paid to improve the high-cycle fatigue-strength of these two materials by modifying the matrix microstructure or process innovation. The fatiguecrackinitiation mechanism, relationship between α-phase morphology and high-cycle fatigue-strength, and other factors contributing to the increase in fatigue strength were analyzed.     

Effect of hot deformation on α→β phase transformation in 47Zr-45Ti-5Al-3V alloy

The effect of strain rate and deformation temperature on the α→β phase transformation in 47Zr-45Ti-5Al-3V alloy with an initial widmanstatten α structure was investigated. At the deformation temperature of 550 °C, the volume fraction of α phase decreased with increasing strain rate. At 600 and 650 °C, the volume fraction of α phase firstly increased to a maximum value with increasing strain rate from 1×10^?3 to 1×10^?2 s^?1, and then decreased. At 700 °C, the microstructure consisted of single β phase. At a given strain rate, the volume fraction of α phase decreased with increasing deformation temperature. With decreasing strain rate and increasing deformation temperature, the volume fraction and size of globular α phase increased. At 650 °C and 1×10^?3 s^?1, the lamellar α phase was fully globularized. The variation in the volume fraction and morphology of α phase with strain rate and deformation temperature significantly affected the hardness of 47Zr-45Ti-5Al-3V alloy.     

Sintering properties of Ti-6Al-4V-xMo powder prepared by addition of Mo to Ti-6Al-4V scraps and subsequent pulverization

In this study, the sintering properties of Ti-6Al-4V-xMo powder prepared by an addition of Mo to Ti-6Al-4V scraps and subsequent pulverization were investigated. As the content of Mo added to Ti-6Al-4V scraps as a β stabilizer increased, the weight ratio of the α and β stabilizers in the Ti-6Al-4V-xMo changed and the original weight ratio of 6:4 varied to 5.71:8.57 when 5 wt% xMo was added. In order to compare the difference in properties of Ti-6Al-4V-xMo ingots with sintered bodies of the Ti-6Al-4V-xMo powder, we prepared sintered bodies from Ti-6Al-4V-xMo powder with an O content of about 5000 ppm and 325 mesh size. As a result, it was found that the sintered bodies of Ti-6Al-4V-xMo powder showed different properties of density and micro hardness compared to the Ti-6Al-4V-xMo ingots. These differences can be explained by the larger specific surface area of the sintered bodies, which formed a porous oxide layer on the surface due to the increase of Mo in the β zone of the Ti-6Al-4V alloys.     

Using Ti-5111 for marine fastener applications

Titanium offers virtually unsurpassed resistance to crevice, pitting, and stress-corrosion cracking in seawater environments. Ti-5Al-1Sn-1Zr-1V-0.8Mo (Ti-5111) is a near-alpha titanium alloy of intermediate strength, excellent toughness, and roomtemperature creep resistance designed in a U.S. Navy-sponsored titanium alloy development program for structural submarine applications. Ti-5111’s desirable mechanical properties and excellent corrosion resistance render this titanium alloy an ideal fastener material from both a technical and economic perspective. For more information, contact J. Been, Titanium Metals Corporation, Henderson Technical Laboratory, 8000 West Lake Mead Drive, Henderson, Nevada 89015; (702) 566-4413; fax (702) 564-9038; e-mail jenny.been@timet.com.     

工业纯钛TA2激光气体氮化表面硬度的研究

激光气体氮化可以有效地提高钛合金表面硬度，从而改善钛合金耐磨损性能。通过金相组织观察和硬度测试分析了钛合金激光气体氮化时．激光气体氮化参量和激光光束分布对钛合金氮化后表面硬度的影响。结果表明，氮化区域内生成硬质相TiN是TA2钛合金表面硬度得到提高的主要原因，有利于硬质相TiN形成的激光气体氮化参量都会提高钛合金表面的硬度。而采用带式积分镜进行激光光束变换可以有效提高激光氮化处理的效率，钛合金平均表面硬度是原来的1.3倍。     

Ti-6Al-4V合金渗氮层的生物腐蚀行为

采用真空感应快速渗氮技术对Ti-6Al-4V合金进行表面渗氮,将渗氮前后的试样置于SBF溶液中探究其耐蚀性。利用维氏硬度计、光学显微镜、电化学工作站、光学3D表面轮廓仪、扫描电镜(SEM)和X射线衍射(XRD)等技术对渗氮层进行测量和观察。试验结果表明:820 ℃下对Ti-6Al-4V钛合金进行真空感应渗氮处理后,渗氮层厚度达20 μm,其氮化物层硬度得到明显提高。同时,经过感应渗氮处理后样品的表面粗糙度增加。电化学试验表明经感应渗氮后Ti-6Al-4V钛合金的耐蚀性明显增强。     

Ti-6Al-4V合金的轧制与组织性能

采用光学显微镜、透射电镜和拉伸试验等手段,研究了多道次两向轧制和单向轧制对不同原始状态(热轧态、水淬态和空冷态)Ti-6Al-4V合金显微组织和力学性能的影响。结果表明,热轧态Ti-6Al-4V合金的组织为片状α相+β相+少量等轴α相,水淬态Ti-6Al-4V合金形成了针状马氏体组织,空冷态Ti-6Al-4V合金形成了网状组织。Ti-6Al-4V合金适宜的两向轧制温度为700 ℃,此时合金中可见颗粒状β相弥散分布在α基体上。两向轧制Ti-6Al-4V合金的抗拉强度和屈服强度从高至低顺序为:水淬态>热轧态>空冷态,且轧向强度要高于横向;相较于单向轧制,两向轧制明显降低了Ti-6Al-4V合金板材拉伸性能的各向异性,且水淬态Ti-6Al-4V合金的轧向和横向强度差异最小,700 ℃轧制Ti-6Al-4V合金的主要细化机制为位错细化。     

Microstructure and mechanical performance of new Al0.5CrFe1.5MnNi0.5 high-entropy alloys improved by plasma nitriding

Effects of plasma nitriding at 525 °C on microstructure and mechanical performance of a brand-new Al_0.5CrFe_1.5MnNi_0.5 high-entropy alloy (HEA) were investigated. This alloy exhibits a large age hardening effect at temperatures from 600 to 800 °C and can be well-nitrided in the as-cast condition or the homogenized and furnace-cooled state. The nitrided layer has a thickness around 75 μm and a peak hardness level of Hv 1250 near the surface. The nitrided Al_0.5CrFe_1.5MnNi_0.5 alloys exhibit superior adhesive wear resistance to conventional nitrided steels by 25-54 times due to their much thicker highly-hardened layer and higher peak hardness than that of conventional steels.